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Beverage emulsions: Recent developments in formulation, production, and applications
Accepted Manuscript Beverage Emulsions: Recent Developments in Formulation, Production, and applications Daniel T. Piorkowski, David Julian McClements PII:
S0268-005X(13)00211-7
DOI:
10.1016/j.foodhyd.2013.07.009
Reference:
FOOHYD 2311
To appear in:
Food Hydrocolloids
Received Date: 13 May 2013 Revised Date:
8 July 2013
Accepted Date: 9 July 2013
Please cite this article as: Piorkowski, D.T., McClements, D.J., Beverage Emulsions: Recent Developments in Formulation, Production, and applications, Food Hydrocolloids (2013), doi: 10.1016/ j.foodhyd.2013.07.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract “BEVERAGE EMULSIONS: RECENT DEVELOPMENTS IN FORMULATION, PRODUCTION, AND APPLICATIONS” by D.T. Piorkowski and D.J. McClements
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Food Hydrocolloids
The article provides an overview of recent research on the formation, stability, and
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properties of beverage emulsions.
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BEVERAGE EMULSIONS: RECENT DEVELOPMENTS IN
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FORMULATION, PRODUCTION, AND APPLICATIONS
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Daniel T. Piorkowski and David Julian McClements1
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Department of Food Science, University of Massachusetts, Amherst, MA 01003
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Journal: Food Hydrocolloids
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Submitted: March 2013
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Revised: July 2013
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Department of Food Science, University of Massachusetts, Amherst, MA 01003, [email protected], 413 545 1019
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Abstract Soft drinks are one of the most widely consumed and profitable beverages in the world.
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This review article focuses on the utilization of emulsion science and technology for the
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fabrication of soft drinks by the beverage industry. A brief overview of the various high and low
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energy methods available for preparing this type of beverage emulsions is given, as well as a
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discussion of the functional ingredients used to formulate these systems, including oil phases,
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emulsifiers, weighting agents, ripening inhibitors, and thickening agents. The influence of
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droplet characteristics on the physicochemical and sensory properties of beverage emulsions is
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reviewed, with special focus on their influence on product stability. Finally, we discuss recent
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developments in the soft drinks area, including fortification with vitamins, reduced calorie
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beverages, and “all-natural” products.
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Keywords: beverages; soft drinks; nutraceuticals; flavors; emulsions; nanoemulsions
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1. Introduction Globally, soft drinks are one of the most widely consumed and profitable beverages (Table
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1). Cola is the top soft drink flavor currently consumed in the United States, with lemon-lime
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and orange being the second and third. All three of these soft drink flavors contain hydrophobic
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citrus compounds extracted from fruit peels. Soft drinks may also contain a variety of other
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hydrophobic components, such as clouding agents, weighting agents, nutraceuticals, oil-soluble
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vitamins, and oil-soluble antimicrobials. The non-polar character of flavor oils and other
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hydrophobic ingredients means that these ingredients cannot simply be dispersed directly into an
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aqueous phase – they would rapidly coalesce and separate through gravitational forces leading to
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a layer of oil on top of the product (Given, 2009). Instead they first have to be converted into a
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colloidal dispersion consisting of flavor molecules encapsulated within small particles suspended
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within an aqueous medium, e.g., a microemulsion, nanoemulsion, or emulsion (McClements,
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2011; McClements & Li, 2010). These colloidal delivery systems must be carefully designed to
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provide desirable physicochemical, sensory, and biological attributes to the final product. A
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number of desirable attributes of colloidal delivery systems suitable for application in beverage
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products are highlighted below (McClements, Decker, & Weiss, 2007; McClements & Li, 2010):
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Fabrication: Ideally, the delivery systems should be fabricated using robust, reliable and inexpensive manufacturing methods that are easily implemented.
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Stability: The delivery systems should be designed to withstand all of the stresses
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Composition: Ideally, the delivery systems should be fabricated entirely from “label friendly” food-grade ingredients that are economic and easy to handle.
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that a product may experience during its production, storage, transport and utilization, such as temperature fluctuations, exposure to light and oxygen, exposure to mechanical forces (such as stirring, flow through a pipe, and vibrations), variations in aqueous phase composition (such as pH, ionic strength, buffer type,
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ingredient interactions), and exposure to microorganisms (such as yeasts, molds or
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bacteria).
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Physicochemical and sensory properties: The delivery system should not adversely
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affect the optical properties, rheology, or flavor profile (aroma, taste, and mouthfeel)
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of the beverage product into which it is incorporated. Biological activity: The delivery system should not adversely affect the biological
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activity of any encapsulated bioactive components, such as antimicrobials, vitamins,
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or nutraceuticals.
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This review article provides an overview of the current status of the design, formulation, and production of emulsion-based delivery systems suitable for utilization within the beverage
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industry.
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2. Emulsion Science and Technology in the Beverage Industry
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Hydrophobic components (such as flavor oils, clouding agents, oil-soluble vitamins, and nutraceuticals) can be incorporated into a variety of different colloidal delivery systems suitable
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for application within beverage products (McClements, 2012; McClements & Rao, 2011), with
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the most common being microemulsions, nanoemulsions, and emulsions (Figure 1). Each of
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these colloidal dispersions has particular benefits and limitations for the encapsulation of
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hydrophobic compounds. Microemulsions are thermodynamically stable systems under a
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specific set of environmental conditions (e.g., composition and temperature), and are therefore
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easy to fabricate (often by simple mixing) and tend to have good long-term stability.
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Microemulsions typically contain very small particles (r < 25 nm) and therefore tend to be
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optically transparent, which is desirable for soft drinks that should be clear. On the other hand,
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the formation of microemulsions usually requires relatively high levels of synthetic surfactants
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and sometimes the use of cosurfactants/cosolvents, which can be undesirable for cost, taste, and
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labeling reasons. Microemulsions may also become thermodynamically unstable if
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environmental conditions are altered (such as temperature or composition).
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Conventional emulsions (r > 100 nm) and nanoemulsions (r < 100 nm) are both
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thermodynamically unstable systems, and therefore tend to breakdown during storage through a
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variety of instability mechanisms (Figure 2), such as gravitational separation, flocculation,
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coalescence and Ostwald ripening (McClements, et al., 2007; McClements & Rao, 2011).
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Emulsion systems must therefore be carefully designed to inhibit these instability mechanisms
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and provide sufficient kinetic stability throughout the lifetime of the product. Emulsions usually
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contain larger droplets than microemulsions and therefore they scatter light more strongly and
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appear more turbid or cloudy. This is an advantage for soft drinks that are required to have a
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cloudy appearance, but a disadvantage for products where optical clarity is required.
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Nevertheless, recently it has been shown that emulsions with ultrafine droplets, often referred to
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as “nanoemulsions”, can be prepared that are optically transparent (McClements, 2012;
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McClements & Rao, 2011). A major advantage of emulsions and nanoemulsions is that the
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emulsifier-to-oil ratio required to formulate them is often much less than that required for
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microemulsions, and they can be formulated from all natural ingredients (such as proteins and
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polysaccharides) rather than synthetic surfactants (such as Tweens). In this article, we focus
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primarily on the utilization of emulsion systems (conventional emulsions and nanoemulsions) in
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the preparation of soft drinks but much of the material is also relevant to the formulation of
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microemulsions.
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It should be noted that the emulsions used in the beverage industry are typically divided into two groups: flavor emulsions and cloud emulsions. Flavor emulsions contain lipophilic
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compounds that are primarily present to provide taste and aroma to a beverage product (such as
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lemon, lime, or orange oils). On the other hand, cloud emulsions are used to provide specific
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optical properties to certain beverage products, i.e., to increase their turbidity (“cloudiness”).
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Cloud emulsions are typically prepared using an oil phase that is highly water-insoluble and that
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is not prone to chemical degradation, such as flavorless vegetable oils. In addition, the size of the
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droplets within cloud emulsions is designed so that they have dimensions where strong light
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scattering occurs, but are not too large to undergo gravitational separation (e.g., r = 100-200
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nm). Cloud emulsions are often added to beverages that only contain a relatively low percentage
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of juice and provide a desirable cloudy appearance that hides sedimentation and ringing.
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In this article, we will use the term “emulsion” to refer to both nanoemulsions and
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conventional emulsions because they have similar structures and properties. Generally, an
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emulsion consists of at least two immiscible liquids (usually oil and water), with one of the
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liquids being dispersed as small spherical droplets in the other (Dickinson, 1992a; Dickinson &
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Stainsby, 1982; Friberg, Larsson, & Sjoblom, 2004; McClements, 2005b). In general, emulsions
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are classified according to the relative spatial organization of the oil and water phases. A system
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that contains oil droplets dispersed within water is called an oil-in-water (O/W) emulsion,
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whereas a system that contains water droplets dispersed in oil is called a water-in-oil (W/O)
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emulsion. It is possible to prepare more complex emulsion structures, e.g., oil-in-water-in-oil
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(O/W/O), water-in-oil-in-water (W/O/W) or oil-in-water-in-water (O/W/W) emulsions
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(Benichou, Aserin, & Garti, 2004; Garti & Bisperink, 1998; van der Graaf, Schroen, & Boom,
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2005). Currently, almost all of the emulsions used in the beverage industry are of the O/W type,
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although there may be certain advantages to using other emulsion types for certain applications.
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For example, in principle it is possible to trap a hydrophilic bioactive component within the inner
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water phase of a W/O/W emulsion to protect it from chemical degradation or for taste masking.
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In practice, it is often difficult to formulate W/O/W emulsions that have sufficient stability for
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commercial applications, although this is still an active area of research.
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Emulsions are thermodynamically unfavorable systems that tend to break down over time
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though a variety of physicochemical mechanisms, including gravitational separation (creaming
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and sedimentation), droplet aggregation (flocculation and coalescence) and droplet growth
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(Ostwald ripening) (Dickinson, 1992a; Friberg, et al., 2004; McClements, 2005b). It is possible
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to form emulsions that are kinetically stable for a reasonable period of time by including
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substances known as stabilizers, e.g., emulsifiers, weighting agents, ripening inhibitors, or
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texture modifiers. It is important to clearly distinguish the different physicochemical
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mechanisms involved in promoting emulsion stability for these different categories of stabilizers.
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Emulsifiers are surface-active molecules that adsorb to the surface of freshly formed droplets
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during homogenization, forming a protective layer that prevents the droplets from aggregating.
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Weighting agents are dense hydrophobic components added to low-density oils to prevent
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gravitational separation. Ripening inhibitors are water-insoluble components added to polar oils
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to prevent Ostwald ripening. Texture modifiers are substances used to increase the viscosity or
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gel aqueous solutions, thereby retarding or preventing droplet movement. A more detailed
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description of different types of stabilizers that can be used in beverage emulsions is given in a
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later section. Selecting the most appropriate stabilizer(s) for a particular application is one of the
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most important factors determining the shelf-life and physicochemical properties of beverage
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emulsions.
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3. Controlling Droplet Characteristics The bulk physicochemical properties of beverage emulsions (such as optical properties,
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stability, rheology, molecular partitioning, and release characteristics) are largely determined by
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the properties of the droplets they contain (McClements, 2005b), such as composition,
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concentration, size, and charge (Figure 3). In this section, we discuss some of the most
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important droplet characteristics that can be controlled by beverage manufacturers in order to
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create products with specific desirable functional properties.
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3.1. Droplet composition
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The composition of the oil phase has a major influence on the formation and stability of beverage emulsions, which has often been overlooked in academic research. Beverage
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emulsions may contain a variety of different hydrophobic components, including flavor oils,
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essential oils, triacylglycerol oils, oil-soluble vitamins, nutraceuticals, weighting agents, and
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ripening inhibitors. These components vary in their molecular characteristics (such as molecular
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weight, molecular conformation, and functional groups), which leads to changes in their
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physicochemical properties (such as polarity, water-solubility, density, viscosity, refractive
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index, physical state, and melting point). Many of these molecular and physicochemical
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properties have a major influence on the formation, stability, and functionality of emulsions. For
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example, oil viscosity influences the efficiency of droplet disruption during high energy
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homogenization – the closer the ratio of dispersed phase viscosity to continuous phase viscosity
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(ηD/ ηC) is to unity, the more efficient is droplet disruption and the smaller is the particle size
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produced (Walstra, 1993, 2003). Oil density determines the rate of particle creaming or
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sedimentation within emulsions – the greater the density contrast between the droplets and
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surrounding fluid, the faster the rate of gravitational separation (McClements, 2005c). Oil
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refractive index determines the efficiency of light scattering by droplets in emulsions – the
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greater the refractive index contrast between the droplets and surrounding fluid, the stronger the
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degree of light scattering and the more turbid the appearance (Chanamai & McClements, 2002b).
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The water-solubility of an oil phase determines the stability of an emulsion to Ostwald ripening
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due to diffusion of oil molecules through the aqueous phase (Kabalnov, 2001; McClements,
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Henson, Popplewell, Decker, & Choi, 2012). Oil interfacial tension plays a number of important
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roles in determining emulsion formation and stability. First, the ease of droplet disruption during
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high energy homogenization decreases as the interfacial tension decreases (Walstra, 1993).
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Second, the rate of droplet coalescence increases as the interfacial tension decreases (Kabalnov
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& Wennerstrom, 1996). Third, the ability of emulsifiers to adhere to droplet surfaces decreases
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as the bare oil-water interfacial tension decreases (Chanamai, Horn, & McClements, 2002).
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Finally, the rate of droplet growth due to Ostwald ripening depends on the interfacial tension at
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the oil-water interface (Kabalnov, 2001).
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For flavor emulsions, it is important to control the type and concentration of the flavor
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molecules initially present in the oil phase. It is also important to be aware that the location of
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the flavor molecules within an emulsion is governed by their oil-water partition, which depends
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on carrier oil type (Choi, Decker, Henson, Popplewell, & McClements, 2009; Choi, Decker,
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Henson, Popplewell, & McClements, 2010b). The flavor profile of an emulsion may therefore
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change if the carrier oil type is altered, if the physical state of the carrier oil changes, or if an
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emulsion is diluted, since this will change the distribution of the flavor molecules in the oil,
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water and air (Choi, et al., 2009; Choi, et al., 2010b; Mei, et al., 2010).
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It is important for beverage manufacturers to understand the composition of the oil phases
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used to formulate commercial products, and to understand how specific lipophilic components
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influence the formation, stability, and properties of final products.
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3.2. Droplet concentration
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In general, the concentration of droplets in an emulsion influences its texture, stability, appearance, sensory attributes, and nutritional quality (McClements, 2005b; McClements & Rao,
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2011). Droplet concentration is usually characterized in terms of the dispersed phase volume
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fraction (φ), which is the volume of emulsion droplets (VD) divided by the total volume of
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emulsion (VE): φ = VD/VE. Practically, it is often more convenient to express the droplet
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concentration in terms of the dispersed phase mass fraction (φm), which is the mass of emulsion
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droplets (mD) divided by the total mass of emulsion (mE): φm = mD/mE. When the densities of the
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two phases are equal, the mass fraction is equivalent to the volume fraction. It is particularly
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important to convert the droplet concentration to the appropriate units when comparing
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experimental work with theoretical predictions.
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In beverage emulsions, controlling the droplet concentration is important for a number of
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reasons. Beverage emulsions are often prepared in a concentrated form (> 10% oil) because this
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facilitates handling and transport, but they are highly diluted when they are introduced into the
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final product (< 0.1% oil). The amount to which an emulsion concentrate is diluted influences
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the appearance of a final product, since emulsion turbidity or cloudiness increases with oil
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droplet concentration. Dilution also influences the total amount of flavor molecules present in
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final products, as well as their partitioning between the oil and water phases (Choi, et al., 2009).
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In the concentrate, droplet concentration has a major impact on the rheological properties of the
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system. From a practical point of view, it may be important to have a high oil loading in the
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concentrate emulsion so as to reduce transport and storage costs, but not have the oil content so
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high that the product is unstable or cannot easily be dispersed into the final product.
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3.3. Droplet size distribution
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physical stability (e.g., to gravitational separation, flocculation, coalescence and Ostwald
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ripening) and its optical properties (e.g., lightness and color) (McClements, 2005b). Beverage
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manufacturers must therefore specify the optimum droplet size distribution required for their
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particular product based on the properties required, e.g., optical clarity and shelf-life. They must
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then develop a formulation and manufacturing process that can reliably produce a beverage with
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this droplet size distribution. Immediately after the product has been manufactured it is usually
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important to measure the droplet size distribution to ensure that it has met the specified quality
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criteria, e.g., using light scattering instruments. It may also be important to measure changes in
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the droplet size distribution of the product during storage or after an accelerated storage test to
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predict its long-term stability (McClements, 2007).
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The particle size distribution (PSD) of an emulsion specifies the concentration of droplets within different size classes, and can be conveniently measured using various commercially
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available instruments (McClements, 2005b). When presenting or interpreting PSD data on a
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beverage emulsion it is important to pay particular attention to the manner in which the particle
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concentration and particle size are presented. The concentration of particles within a particular
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size class is usually presented as either the volume or number percent, whereas the size of the
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particles in a particular size class is usually presented as either the mid-point particle radius or
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diameter. The same PSD may look very different if it is plotted as volume versus particle size or
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as number versus particle size. Typically, particle volume versus particle diameter is the most
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widespread and informative way of presenting particle size data. Commercial beverage
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emulsions are always polydisperse systems that can be characterized as being "monomodal",
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"bimodal" or "multimodal" depending on whether there are one, two, or more peaks in the
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particle size distribution. Typically, beverage manufacturers would like to produce a final
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product that has a narrow monomodal distribution, as this usually provides the best long-term
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stability.
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In many practical situations it is important to have knowledge of the full PSD of a beverage emulsion since this contains information about the size characteristics of all of the particles
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present, as well as providing insights into the possible origin and nature of any instability
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mechanisms. For example, it may be possible to detect a small population of large particles that
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may cause problems with creaming during long-term storage (i.e., ringing). In addition, by
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measuring changes in the PSD overtime it is sometimes possible to distinguish between different
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instability mechanisms (e.g., coalescence versus Ostwald ripening). Nevertheless, in some
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situations it is more convenient to represent the full particle size distribution by a measure of its
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central tendency and spread. The mean, median, or modal particle sizes are often used as
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measures of the central tendency, whereas the relative standard deviation is often used as a
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measure of spread (Walstra, 2003). The mean particle size is the most widely used method of
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representing the central tendency of emulsion particle size distributions in the beverage industry.
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It is important to realize that a number of different mean particles sizes can be derived from
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a full PSD and each mean size can have a different magnitude and physical meaning
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(McClements, 2007). The three most commonly used mean particle sizes are the number-
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weighted mean diameter (dN or d10 = Σnidi/ Σni), the surface-weighted mean diameter (dS or d32 =
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Σnidi3/ Σnidi2) and the volume-weighted mean diameter (dV or d43 = Σnidi4/ Σnidi3). Generally, the
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volume-weighted mean diameter is more sensitive to the presence of large particles than the
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number-weighted mean diameter, and so it often provides the most rigorous test of the physical
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stability of a beverage emulsion, i.e., if d43 is small then the emulsion is more likely to remain
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stable. Appreciable differences between the values of d10, d32 and d43 generally indicate that the
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particle size distribution is broad or multimodal. One must therefore be very careful when
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interpreting or reporting particle size data to identify which mean particle size value (and
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corresponding relative standard deviation) is being used. It should also be noted that mean
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values should be treated with caution when used to represent highly polydisperse emulsions (e.g.,
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aggregated systems), and it is always useful to examine the full particle size distribution.
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Commercial beverage manufacturers usually develop a set of standardized particle size criteria that they use to determine whether a particular batch of product has the desired
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physicochemical characteristics, e.g., long-term stability and optical properties. For example, a
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manufacturer might specify that that mean droplet diameter (d43) of a particular class of products
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should be < 500 nm, and that > 90% of the droplets should be smaller than 800 nm. The precise
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criteria used will depend on the product being manufactured (especially whether it should be
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clear or opaque).
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3.4. Droplet charge
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ionic species to their surfaces, e.g., proteins, ionic polysaccharides, ionic surfactants,
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phospholipids, fatty acids, and some small ions (McClements, 2005b). The electrical
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characteristics of a droplet surface depend on the type, concentration and organization of the
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ionized species present, as well as the ionic composition and physical properties of the
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surrounding aqueous phase. The electrical charge on the oil droplets in a beverage may be
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important for a number of reasons: it determines the stability of the droplets to aggregation due
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to its influence of the magnitude, range and sign of electrostatic interactions; it determines the
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interactions of droplets with other charged species in an emulsion e.g., ions (such as calcium or
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iron), or polyelectrolytes (such as proteins or polysaccharides); it influences how the droplets
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interact with electrically charged surfaces, such as storage vessels, bottles, cups, and the mouth;
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it influences the behavior of the droplets in an electrical field, which is important for measuring
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their charge using electrophoresis.
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The electrical characteristics of a droplet in an emulsion are usually characterized in terms
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of its surface charge density (σ), electrical potential (Ψ0), and/or ζ−potential (ζ) (Hunter, 1986).
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The surface charge density is the amount of electrical charge per unit surface area, which
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depends on the net number of ionized groups per unit interfacial area. The electrical potential is
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the amount of energy required to increase the surface charge density from zero to σ. The
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electrical potential depends on the surface charge density, but also on the ionic composition of
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the surrounding medium due to electrostatic screening effects. At a fixed surface charge density,
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the electrical potential decreases with increasing ionic strength due to these effects. The zeta-
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potential (ζ) is the electrical potential at the "shear plane", which is defined as the distance away
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from the droplet surface below which the counter-ions remain strongly attached to the droplet
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when it moves in an electrical field. Practically, the ζ-potential is a better representation of the
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electrical characteristics of an oil droplet because it inherently accounts for the adsorption of any
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counter ions or ionic species to the droplet surface. In addition, the ζ-potential is more
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convenient to measure than the surface charge density or electrical potential (Hunter, 1986).
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Typically, the electrical characteristics of the droplets in an emulsion are determined by
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measuring the ζ-potential versus pH under appropriate measurement conditions (such as ionic
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composition).
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adsorb to the droplet surfaces and prevent them from coming close together because of
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electrostatic repulsion (Dickinson, 1992b; Friberg, et al., 2004; McClements, 2005b).
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Electrostatic repulsion plays a major role in determining the aggregation stability of fat droplets
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coated by charged emulsifiers that only form thin layers that generate short range steric
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repulsion, such as globular proteins and ionic surfactants. On the other hand, electrostatic
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repulsion is less important in systems where the fat droplets are coated by emulsifiers that form
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thick interfacial layers that generate long range steric repulsion, such as polysaccharides (gum
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arabic and modified starch). For electrostatically-stabilized emulsions, the magnitude of the ζ-
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potential should be greater than about 20 mV to produce systems that are stable during long-term
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storage. For sterically-stabilized emulsions, the droplet charge may not be important in terms of
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their physical stability, but it may still be important in systems where chemical reactions occur
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within the oil droplets that are induced by water-soluble ionic species, such as oxidation of ω-3
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fatty acids by transition metals.
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3.5. Interfacial properties
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The boundary between the oil and water phases in an emulsion consists of a narrow region
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(≈ 1 to 50 nm thick) that surrounds each oil droplet, and contains a mixture of oil, water, and
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emulsifier molecules, as well as possibly other molecular species, such as mineral ions,
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polyelectrolytes, and polar lipids. The interfacial region makes up a significant fraction of the
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volume of a droplet when the droplet diameter is less than about 1 µm (McClements & Rao,
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2011), and is therefore particularly important in beverage emulsions since they usually contain
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droplets considerably smaller than this size . The interfacial region can influence many
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important physicochemical and sensory properties of beverages emulsions, including their
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stability, rheology, mouthfeel, and flavor. For this reason, it is often important to have
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knowledge about the interfacial properties of the droplets in a beverage emulsion, and to
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establish the major factors that influence them. Some of the most important properties of the
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interfacial region are: composition; structural organization; thickness; rheology; interfacial
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tension; and charge. These properties are determined by the type, concentration and interactions
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of any surface-active species present, as well as by the events that occur before, during, and after
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emulsion formation, e.g., complexation, competitive adsorption, layer-by-layer formation
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(Dickinson, 2003). As mentioned earlier, the electrical charge on the droplet interface influences
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its interaction with other charged molecules, as well as its stability to aggregation. The thickness
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and rheology of the interfacial region influences the stability of emulsions to gravitational
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separation, coalescence and flocculation, and determines the rate at which molecules leave or
349
enter the droplets (Dickinson, 2003; McClements, 2005b). For example, the ability of interfacial
350
coatings to prevent droplet flocculation is strongly influenced by their thickness.
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Beverage manufacturers should therefore be aware of the nature of the interfacial region
352
surrounding the oil droplets in their products, and the fact that they may be able to manipulate its
353
properties to improve product performance.
354
3.6. Colloidal interactions
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The attractive and repulsive colloidal interactions that operate between the oil droplets in beverage emulsions determine their stability to flocculation and coalescence, which in turn
357
influences their creaming stability and rheology (Friberg, et al., 2004; McClements, 2005b). The
358
colloidal interactions between two oil droplets can be described in terms of an interaction
359
potential (w(h)), which is the energy required to bring two droplets from an infinite distance
360
apart to a surface-to-surface separation of h (Figure 4). The overall interaction potential is made
361
up from contributions from various types of interactions, with the most important being van der
362
Waals, steric, electrostatic, depletion, and hydrophobic interactions (Israelachvili, 2011;
363
McClements, 2005b). These individual interactions vary in their sign (attractive or repulsive),
364
magnitude (weak to strong) and range (short to long). Each of the individual interactions usually
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has a simple dependence on surface-to-surface separation, but the sum of the interactions can
366
exhibit a more complex dependence. For example, the interaction potential between two oil
367
droplets coated by a layer of charged polymer molecules would have a number of maximum and
368
minimum values at certain separations, such as short- and long-range energy barriers, and
369
primary and secondary minima (Figure 4). Generally, droplets tend to aggregate when attractive
370
interactions dominate, but remain as individual entities when repulsive interactions dominate
371
(McClements, 2005b).
372
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It is particularly important for scientists working in the beverage industry to identify and understand the major colloidal interactions operating between the droplets in their particular
374
product. This knowledge can then be used to establish the optimum approach for maintaining
375
product stability during production, transport and storage. For example, if a beverage emulsion
376
is stabilized by a protein-based emulsifier, then electrostatic repulsive interactions will play an
377
important role in preventing droplet aggregation. In this situation, the system will be sensitive to
378
environmental changes that reduce the magnitude and range of the electrostatic repulsion acting
379
between droplets, such as altering the pH or adding salts (particularly multivalent counter-ions).
380
On the other hand, if the beverage emulsion is stabilized by a polysaccharide-based emulsifier,
381
then steric repulsive interactions will be most important for preventing droplet aggregation. In
382
this case, the product will be much less sensitive to droplet aggregation when the pH or ionic
383
strength is changed. In this latter case, emulsion stability depends on the thickness and
384
hydrophilicity of the interfacial layer, which will depend on the molecular characteristics of the
385
polysaccharide molecules. A summary of the major colloidal interactions in beverage emulsions
386
is given in Table 2.
387
4. Physicochemical Properties
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The physicochemical properties of beverage emulsions play an important role during the
389
manufacturing process, as well as in determining the perceived quality attributes of the final
390
product. The most important physicochemical attributes of these systems are briefly discussed in
391
this section. Some of these properties are more important in the concentrated form of the
392
beverage, whereas others are more important in the diluted form.
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393 394
4.1. Optical properties The first cue that a consumer uses to judge the quality or desirability of a finished beverage product is its visual appearance (provided it is packaged or poured into a transparent container,
396
such as a bottle or cup). Each type of beverage product is expected to have a particular
397
appearance depending on its nature, e.g., a dark brown cola, a cloudy orange juice, or a clear
398
green lime juice. From a scientific viewpoint, emulsion appearance is categorized in terms of
399
their opacity and color, which can be quantitatively described using tristimulus color coordinates,
400
such as the L*a*b* system (McClements, 2005b). In this color system, L* represents the
401
lightness, and a* and b* are color coordinates: where +a* is the red direction, -a* is the green
402
direction; +b* is the yellow direction, -b* is the blue direction; low L* is dark and high L* is
403
light. The opacity of an emulsion can therefore by characterized by the lightness (L*), while the
404
color intensity can be characterized by the chroma: C = (a*2 + b*2)1/2. The color intensity is
405
usually inversely related to the lightness, so that the chroma decreases (fades) when the lightness
406
increases. The optical properties of emulsions are mainly determined by the relative refractive
407
index, concentration, and size distribution of the droplets they contain (Chanamai &
408
McClements, 2002b; Danviriyakul, McClements, Decker, Nawar, & Chinachoti, 2002;
409
McClements, 2005b). The lightness of an emulsion tends to increase with increasing refractive
410
index contrast and increasing droplet concentration, and has a maximum value at a particular
411
droplet size. This has important implications for the development of beverage products that
412
should be either clear or opaque. In general, the lightness of emulsions increases steeply as the
413
oil droplet concentration increases from about 0 to 5 wt%, but then increases more gradually at
414
higher droplet concentrations (Figure 5).
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As mentioned earlier, some beverages are expected to have optical clarity, whereas others
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416
are expected to be cloudy. Optimizing the initial particle size distribution of a beverage
417
emulsion, as well as inhibiting any changes in the particle size during storage, is therefore a
418
particularly important part of designing a commercial product with the desired optical properties.
419
For clear products, the majority of droplets should be less than about 50 nm in diameter so that
420
the light scattering is very weak (Wooster, Golding, & Sanguansri, 2008a). The scattering
421
efficiency of the individual oil droplets determines the maximum amount of oil phase that can be
422
incorporated into a clear beverage before it becomes noticeably cloudy. As a rule of thumb, a
423
turbidity of 0.05 cm-1 (at 600 nm) can be considered to be a rough cut-off point between
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transparent and cloudy products. For cloudy products, the majority of droplets should be
425
between about 200 and 400 nm in diameter so that the light scattering is very strong
426
(McClements, 2002). In this case, the scattering efficiency of the individual oil droplets will
427
determine the minimum amount of a clouding emulsion required to reach a particular turbidity in
428
the final product.
429
4.2. Rheology
430
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The rheological properties of beverage emulsions are also an important factor determining their manufacture and utilization. Most beverage emulsions are initially manufactured in a
432
concentrated form, which is diluted appreciably during the production of the final beverage
433
product. The droplet concentration in the beverage concentrate typically ranges from 3 to 30%,
434
while that in the final product is typically < 0.1%. Industrially, the rheology of the beverage
435
concentrate is important since it influences the ease of mixing, flow through a pipe, and
436
packaging. A manufacturer typically wants to have as high an oil loading as possible, without
437
the product becoming too viscous or gel-like to handle easily. This requires careful control of
438
the total droplet concentration in the system. The droplet concentration in the final beverage
439
concentration is usually so low that the rheology is dominated by the properties of the aqueous
440
continuous phase (see discussion below).
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The rheology of dilute colloidal dispersions is normally characterized by the shear viscosity
442
(Genovese, Lozano, & Rao, 2007; McClements, 2005b). When the droplet concentration is less
443
than about 5% (φ < 0.05), the shear viscosity can be described by Einstein’s equation:
444
446 447
η = η 0 (1 + 2.5φ )
(1)
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Here, η is the viscosity of the overall system, η0 is the viscosity of the continuous phase, and
448
φ is the disperse phase volume fraction. This equation under-predicts the viscosity of colloidal
449
dispersions with higher droplet concentrations due to the effects of droplet-droplet interactions.
450
Einstein’s equation was derived assuming that the colloidal particles were rigid isolated spheres
451
surrounded by a Newtonian fluid. Nevertheless, it still provides a good approximation of the
452
rheological properties of dilute beverage emulsions because the flow of the liquid within the oil
453
phase is inhibited by the emulsifier coating. The above equation shows that the viscosity of a
ACCEPTED MANUSCRIPT 17
dilute emulsion increases linearly with droplet concentration, but that the most important factor
455
affecting the overall rheology is the viscosity of the continuous phase. Thus the most effective
456
means of controlling the viscosity of a dilute beverage emulsion is to change the viscosity of the
457
continuous phase, e.g., by adding sugars or polymer thickening agents.
458
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The viscosity of concentrated emulsions can be described by a semi-empirical equation that
459
takes into account droplet-droplet interactions (Berli, Deiber, & Quemada, 2005; McClements,
460
2005b; Quemada & Berli, 2002): −2
461
φ η = η 0 1 − φc
462
Here, φ is the disperse phase volume fraction, and φc (≈ 0.63) is a critical disperse phase
(2)
SC
volume fraction above which the droplets are so closely packed together that they cannot easily
464
flow past each other. This equation shows that the viscosity of an emulsion increases with
465
increasing droplet concentration, gradually initially and then steeply as the droplets become more
466
closely packed (Figure 5). Around and above the droplet concentration where close packing
467
occurs, the emulsion becomes highly viscosity and may exhibit solid-like characteristics, such as
468
visco-elasticity and plasticity (Berli, et al., 2005; McClements, 2005b; Quemada & Berli, 2002).
469
In flocculated systems the critical concentration where the system becomes highly viscous or
470
solid-like may be much lower than in a non-flocculated system. It is therefore important for
471
beverage manufactures to consider the influence of droplet concentration and interactions on the
472
rheological properties of emulsion concentrates (Genovese, et al., 2007; McClements, 2005b;
473
Walstra, 2003).
474
4.3. Molecular distribution and release characteristics
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A beverage emulsion may contain a number of constituents that partition into different
476
phases within the product, e.g., oil, aqueous, interfacial, or gas phases (McClements, 2005b).
477
The physical location of some of these constituents may have a major impact on the quality
478
attributes of the final product. For example, many flavor oils contain constituents that are
479
chemically unstable and therefore prone to degradation during storage (e.g., citral). The
480
chemical stability of these constituents is often dependent on their molecular environment. For
481
example, the rate of acid-catalyzed degradation of citral has been shown to occur considerably
482
faster when it is located within an aqueous phase than when it is present within an oil phase
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(Choi, et al., 2009; Choi, et al., 2010b). This would suggest that it is better to keep citral under
484
neutral conditions or within the emulsion concentrate as long as possible before the final dilution
485
into the acid phase is carried out. The perceived flavor profile of beverage emulsions depends on
486
the distribution of volatile molecules between the liquid and gas phases. Increasing the oil
487
content of an emulsion decreases the concentration of hydrophobic (KOW > 1) volatiles in the
488
headspace and therefore reduces the perceived flavor profile (Figure 6). This phenomenon is
489
important to take into account when reformulating a beverage product so that it contains a
490
different fat concentration, e.g., fortification with a bioactive lipid such as ω-3 oils.
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The location of a constituent within a beverage emulsion is governed by its equilibrium
492
partition coefficients (e.g., oil-water, oil-air, oil-interface) and its mass transport kinetics through
493
the system (McClements, 2005b). When a beverage emulsion is placed in the mouth there is a
494
redistribution of flavor molecules, with some of the aroma compounds leaving the product and
495
entering the nasal cavity. The rate at which flavor molecules leave the droplets in beverage
496
emulsions is usually extremely quick (< 0.1 s for KOW < 1000), and therefore droplet dimensions
497
tend to have little impact on the flavor release profile (McClements, 2005b). Nevertheless, it
498
may be possible to encapsulate oil droplets within hydrogel matrices to slow down the release of
499
flavor molecules within the mouth.
500
5. Beverage Emulsion Shelf-Life
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One of the most important factors determining the commercial viability of beverage
502
emulsions is their ability to resist changes in their physical and chemical properties after their
503
production. Beverage emulsions experience a range of environmental stresses during their
504
manufacture, transport, storage, and utilization that may reduce their shelf lives: mechanical
505
forces (e.g., stirring, flow through a pipe, centrifugation, vibrations, and pouring); temperature
506
variations (e.g., freezing, chilling, warming, pasteurization, and sterilization); exposure to light
507
(e.g., natural or artificial visible or ultraviolet waves); exposure to oxygen; variations in solution
508
properties (e.g., pH and mineral composition of water). Exposure to these environmental
509
stresses may promote emulsion instability through a variety of physicochemical mechanisms:
510
loss of ingredient functionality (e.g., changes in solubility, surface activity, or stabilization
511
capacity); acceleration of chemical degradation reactions (e.g., oxidation, polymerization, or
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hydrolysis); acceleration of physical instability mechanisms, (e.g., flocculation, coalescence or
513
Ostwald ripening). In this section, a brief overview of some of the major instability mechanisms
514
in beverage emulsions is given, and some suggestions for preventing them from occurring are
515
provided.
516
5.1. Physical Stability
517
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Emulsions are thermodynamically unfavorable systems that tend to break down over time due to a variety of physicochemical mechanisms (Figure 2), including gravitational separation,
519
flocculation, coalescence and Ostwald ripening (Dickinson, 1992a; Friberg, et al., 2004;
520
McClements, 2005b). All of these instability mechanisms lead to a change in the structural
521
organization of the various components within the system, rather than in the type of molecules
522
present. Nevertheless, changes in the chemical structure of active components can lead to
523
changes in physical stability, and vice versa.
524
5.1.1. Gravitational Separation
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Gravitational separation is one of the most common forms of physical instability in commercial beverage emulsions, and it may take the form of either creaming or sedimentation
527
depending on the relative densities of the oil droplets and the surrounding aqueous phase.
528
Creaming is the upward movement of droplets when they have a lower density than the aqueous
529
phase, whereas sedimentation is the downwards movement of droplets when they have a higher
530
density than the aqueous phase. The oil phases used in beverage emulsions consist primarily of
531
triacylglycerol and/or flavor oils, which have lower densities than water and so creaming is more
532
prevalent (Table 3). However, if a beverage emulsion contained an excess of weighting agent
533
within the oil phase then it may be prone to sedimentation. A beverage emulsion is also prone to
534
sedimentation if it contains very small oil droplets covered by relatively thick and dense
535
interfacial layers (see below) (McClements, 2011).
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536
One of the most common problems reported in beverage emulsions is “ringing”, which is the
537
accumulation of a visible ring of oil droplets on the top of a product. This effect is due to droplet
538
creaming, which may have occurred because a population of droplets in the initial emulsion was
539
too large, or because some droplet growth occurred during storage, e.g., due to flocculation,
540
coalescence, or Ostwald ripening (see later sections). To a first approximation, the velocity that
541
an oil droplet moves upwards in a dilute emulsion due to gravity is given by Stokes’ law:
ACCEPTED MANUSCRIPT 20 2 2 grparticle ( ρ particle − ρ 0 )
542
v=−
543
where, v is the creaming velocity, rparticle is the particle radius, ρparticle is the particle
544
density, ρ0 is the aqueous phase density, η0 is the aqueous phase viscosity, and g is the
545
acceleration due to gravity. This equation shows that the rate of droplet creaming should
546
decrease as the droplet size decreases, the density contrast decreases, or the aqueous phase
547
viscosity increases. Gravitational forces cause droplets to move either upwards or downwards
548
depending on their density relative to the surrounding aqueous phase. Hence, if only
549
gravitational forces operated, then the droplets would accumulate at either the top or the bottom
550
of an emulsion. In practice, droplets may also move because of Brownian motion associated
551
with the thermal energy of the system. Brownian motion favors the random distribution of the
552
droplets throughout the entire volume of the emulsion, rather than their accumulation at either
553
the top or bottom.
554
containing relatively large droplets (r > 100 nm), whereas Brownian motion forces tend to
555
dominate droplet movement in emulsions containing smaller droplets (McClements, 2011).
556
Consequently, emulsions become more stable to creaming or sedimentation as the particle size
557
decreases because the creaming velocity decreases (v ∝ r2) and because Brownian motion effects
558
increase.
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Gravitational forces tend to dominate droplet movement in emulsions
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(3)
9η 0
The above calculations assume that the particles in beverage emulsions are homogeneous spheres consisting entirely of oil phase. In practice, the particles in beverage emulsions actually
561
have a core-shell structure, consisting of an oil core and an interfacial shell. In this case, the
562
overall particle radius is given by rparticle = rcore + δ, and the overall particle density (ρparticle)
563
depends on the densities of the core (ρC) and shell (ρS) materials and the volume fraction of the
564
shell (Φ S):
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565
ρ particle = Φ S ρ S + (1 − Φ S ) ρ C
566
The shell layer usually has a higher density than the oil or aqueous phases, so that an
(4)
567
increase in the volume fraction of the shell layer will tend to increase the overall particle density.
568
This has important implications for preventing gravitational separation in beverage emulsions
569
with small droplets sizes since it reduces the density contrast between the particles and aqueous
570
phase. In addition, very small particles may actually sediment rather than cream if they contain
ACCEPTED MANUSCRIPT 21
571
sufficiently thick and dense emulsifier layers. Thus, it should be possible to produce density
572
matched particles in beverage emulsions by controlling the oil core size and the thickness of the
573
adsorbed emulsifier layer. The above discussion has highlighted a number of approaches that can be used to inhibit or
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prevent gravitational separation in beverage emulsions. First, gravitational separation can be
576
prevented by matching the density of the dispersed (oil) and continuous (aqueous) phases. The
577
density of the aqueous phase typically varies from about 1000 to 1050 kg m-3, depending on the
578
amount of sugars and other solutes present (Table 3). The density of most oil phases is less than
579
this value, and therefore oil droplets will tend to move upwards. As already mentioned, the
580
density of the core-shell particles within a beverage emulsion can be matched to the surrounding
581
aqueous phase by adding a weighting agent to the oil phase, or by controlling the thickness and
582
density of the emulsifier layer. Second, gravitational separation can be inhibited by reducing the
583
size of the droplets in the emulsion, since the creaming velocity is proportional to the droplet size
584
squared (Stoke’s Law). If the droplets are sufficiently small, then Brownian motion effects will
585
dominate and the system will remain stable to creaming or sedimentation. Third, gravitational
586
separation can be inhibited by increasing the viscosity of the aqueous phase, e.g., by adding
587
thickening or gelling agents. This approach may not always be viable since it will also influence
588
the texture and mouthfeel of the final product.
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Another approach some beverage manufacturers have used to mask the undesirable effects of creaming (“ringing”) on the appearance of a product is to design the packaging so as to
591
obscure the effect, e.g., with appropriate placement of the labels or cap.
592
5.1.2. Droplet Aggregation
The aggregation state of the droplets in a beverage emulsion is important because it
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590
594
influences the stability of the product to gravitational separation. Changes in particle size during
595
storage may also influence other important quality attributes of beverage products, such as their
596
appearance (cloudiness or homogeneity). The tendency for droplet aggregation to occur in a
597
beverage emulsion depends on the balance of attractive and repulsive forces operating between
598
the droplets (see earlier). The nature of the colloidal interactions operating in a particular
599
beverage emulsion depends on the physicochemical properties of the oil, water and interfacial
600
phases (e.g., dielectric constant and refractive index), oil core characteristics (such as radius),
ACCEPTED MANUSCRIPT 22
interfacial shell characteristics (such as thickness, charge, packing, rheology and
602
hydrophobicity), and the properties of the intervening fluid (such as pH, ionic strength, osmotic
603
pressure, and temperature). To a first approximation the overall colloidal interactions between a
604
pair of droplets in a beverage emulsion can be described by the sum of the van der Waals (wVDV),
605
electrostatic (wE), and steric (wS) interactions (McClements, 2005b):
606 607
w(h) = wVDV(h) + wE(h) + wS(h)
(5)
608
The van der Waals interactions are attractive, whereas the steric and electrostatic
SC
609
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interactions are usually repulsive (Table 2). The van der Waals attraction operates between all
611
kinds of droplets and would always cause aggregation if there were no opposing repulsive forces.
612
The magnitude and range of the steric repulsion depend on the thickness and chemistry of the
613
interfacial layer, whereas the magnitude and range of the electrostatic repulsion depend on the
614
droplet charge (ζ-potential) and the ionic composition of the aqueous phase. To design a product
615
that is stable to droplet aggregation one must assure that the repulsive interactions dominate the
616
attractive interactions. This is usually achieved by using an emulsifier that generates repulsive
617
interactions between the droplets. The emulsifiers used in the beverage industry typically
618
stabilize the droplets against aggregation by generating steric and/or electrostatic repulsive
619
interactions. Emulsifiers that form relatively thick open interfaces (such as polysaccharides and
620
non-ionic surfactants with large hydrophilic head-groups) can generate a steric repulsion that is
621
sufficient strong and long range to overcome the attractive van der Waals interactions, and
622
thereby stabilize the system against aggregation. Emulsifiers that form highly charged interfaces
623
(such as proteins and ionic surfactants) can generate a strong electrostatic repulsion between
624
droplets that prevent aggregation. However, emulsifiers that can only stabilize emulsions due to
625
electrostatic interactions may be prone to instability when the pH or ionic strength is changed.
626
Some emulsifiers use a combination of electrostatic and steric repulsion to stabilize the system,
627
e.g., such as casein and whey proteins.
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628
The droplets in emulsions are in continual motion because of the effects of thermal energy,
629
gravity, or applied mechanical forces, and as they move about they frequently collide with their
630
neighbors. After a collision, emulsion droplets may either move apart or remain aggregated,
631
depending on the relative magnitude of the attractive and repulsive interactions between them.
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Droplets aggregate when there is a primary or secondary minimum in the interaction potential
633
that is sufficiently deep and accessible to the droplets (Figure 4). The two major types of
634
aggregation in beverage emulsions are flocculation and coalescence.
635 636
5.1.2.1. Flocculation Droplet flocculation is the process whereby two or more droplets come together to form an
637
aggregate in which the droplets retain their individual integrity (Figure 2). Droplet flocculation
638
is usually detrimental to beverage emulsion quality because it accelerates the rate of gravitational
639
separation thereby reducing their shelf-life. Flocculation can also cause an appreciable increase
640
in the viscosity of beverage emulsion concentrates, and may even lead to the formation of a gel.
641
This may be undesirable since it would influence the transport, handling and dispersibility of the
642
product. Flocculation may occur in beverage emulsions through a variety of different processes
643
that either increase the attractive forces or decrease the repulsive forces operating between the
644
droplets. The mechanism that is important in a particular emulsion depends largely on the nature
645
of the emulsifier used and the solution conditions (e.g., pH, ion type and concentration, and
646
functional ingredients).
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Reduced electrostatic repulsion: Electrostatically stabilized emulsions may flocculate when the electrostatic repulsion between the droplets is reduced. A number of physicochemical
649
changes may cause this reduction in electrostatic repulsion (Israelachvili, 2011): (i) the pH is
650
altered so that the net charge on the droplets is reduced; (ii) counter-ions bind to the surface of
651
the droplets and reduce their charge (“charge neutralization”); (iii) the ionic strength of the
652
aqueous phase is increased to screen the electrostatic interactions (“electrostatic screening”).
653
Protein-coated oil droplets are particularly sensitive to flocculation due to reduction in the
654
electrostatic repulsion between them when the pH or ionic composition is altered (Demetriades,
655
Coupland, & McClements, 1997a; McClements, 2004).
EP
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Increased depletion attraction: The presence of non-adsorbing colloidal entities in the
657
continuous phase of an emulsion, such as biopolymers or surfactant micelles, generates an
658
increase in the attractive force between the droplets due to an osmotic effect associated with the
659
exclusion of the colloidal entities from a narrow region surrounding each droplet (Israelachvili,
660
2011). This attractive force increases as the concentration of colloidal entities increases, until
661
eventually it becomes large enough to overcome the repulsive interactions between the droplets and
ACCEPTED MANUSCRIPT 24
causes them to flocculate. This type of droplet aggregation is usually referred to as depletion
663
flocculation. The presence of relatively high concentrations of non-adsorbed biopolymer
664
emulsifiers (gum arabic and modified starch) have been shown to induce depletion flocculation in
665
model beverage emulsions (Chanamai & McClements, 2001). Depletion flocculation may also be
666
promoted by other kinds of biopolymers that might be used in beverages, such as maltodextrin,
667
pectin, xanthan gum, and carrageenan (Cao, Dickinson, & Wedlock, 1990; Cho & McClements,
668
2009; Gu, Decker, & McClements, 2004; Gunning, Hibberd, Howe, & Robins, 1988).
669
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Increased hydrophobic interactions: This type of interaction is important in emulsions that contain droplets that have some non-polar regions exposed to the aqueous phase. A good
671
example of this type of interaction is the effect of thermal processing on the flocculation stability
672
of oil-in-water emulsions stabilized by globular proteins (Demetriades, Coupland, &
673
McClements, 1997b). At room temperature, whey protein stabilized emulsions (pH 7) are stable
674
to flocculation because of the large electrostatic repulsion between the droplets, but when they
675
are heated above 70 oC they become unstable. The globular proteins adsorbed to the surface of
676
the droplets unfold above this temperature and expose non-polar amino acids that were originally
677
located in their interior. Exposure of these non-polar amino acids increases the hydrophobic
678
character of the droplet surface and therefore leads to flocculation because of the increased
679
hydrophobic attraction between the droplets.
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Formation of biopolymer bridges: Many types of biopolymer promote flocculation by
681
forming bridges between two or more droplets. Biopolymers may adsorb either directly to the
682
bare oil surfaces of the droplets or to the adsorbed emulsifier molecules that form the interfacial
683
layer. To be able to bind to the droplets there must be a sufficiently strong attractive interaction
684
between segments of the biopolymer and the droplet surface. The most common types of
685
interaction that operate in food emulsions are hydrophobic and electrostatic (Dickinson, 2003).
686
For example, a positively charged biopolymer (such as chitosan) might adsorb to the surface of
687
two negatively charged emulsion droplets causing them to flocculate (Ogawa, Decker, &
688
McClements, 2003) or a negatively charged biopolymer (such as pectin, carrageenan or xanthan)
689
might adsorb to the surface of two positively charged droplets causing them to flocculate
690
(Dickinson, 2003; Guzey & McClements, 2006).
691 692
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The development of a suitable strategy to prevent droplet flocculation in a particular beverage emulsion therefore depends on identification of the physicochemical origin of
ACCEPTED MANUSCRIPT 25
flocculation in that system. In general, flocculation can be prevented by ensuring that the
694
repulsive forces dominate the attractive forces, and that there are no additives that can promote
695
bridging.
696 697
5.1.2.2. Coalescence Coalescence is the process whereby two or more liquid droplets merge together to form a
698
single larger droplet (Figure 2). Coalescence causes emulsion droplets to cream or sediment
699
more rapidly because of the increase in their particle size. In beverage emulsions, coalescence
700
eventually leads to the formation of a layer of oil on top of the material, which is referred to as
701
oiling off. This process is one of the main reasons for the shiny oily layers often seen on top of
702
unstable beverage emulsions.
SC
The susceptibility of a beverage emulsion to droplet coalescence is highly dependent on the
M AN U
703
RI PT
693
nature of the emulsifier used to stabilize the system, since this instability mechanism involves
705
two or more droplets fusing together. In general, the susceptibility of oil droplets to coalescence
706
is determined by the nature of the forces that act between the droplets (i.e. gravitational,
707
colloidal, hydrodynamic and mechanical forces) and the resistance of the interfacial layer to
708
rupture. The stability of emulsions to coalescence can be improved by preventing the droplets
709
from coming into close proximity for extended periods, e.g., by preventing droplet flocculation,
710
preventing the formation of a creamed layer, or having too high droplet concentrations
711
(McClements, 2005b). Alternatively, one can control the properties of the interfacial layer
712
surrounding the oil droplets to make it more resistant to rupture, e.g., by selecting an appropriate
713
emulsifier or other additives that alter surface properties.
714
5.1.3. Ostwald Ripening
EP
AC C
715
TE D
704
This susceptibility of a beverage emulsion to Ostwald ripening (OR) is mainly determined
716
by the solubility of the oil phase in the aqueous phase: the higher the solubility, the more
717
unstable the emulsion. Oil phases with very low water-solubilities (such as the vegetable oils
718
used in clouding emulsions) do not exhibit OR, but oil phases with relatively high water-
719
solubilities (such as flavor or essential oils) may be highly unstable. Mechanistically, OR is the
720
process whereby the size of the oil droplets in an oil-in-water emulsion increases over time due
721
to diffusion of oil molecules from small to large droplets through the intervening aqueous phase
722
(Kabalnov, 2001; Kabalnov & Shchukin, 1992). The driving force for this effect is the fact that
ACCEPTED MANUSCRIPT 26
the water-solubility of an oil contained within a spherical droplet increases as the radius of the
724
droplet decreases, which means that there is a higher concentration of solubilized oil molecules
725
in the aqueous phase surrounding a small droplet than surrounding a larger one (Kabalnov &
726
Shchukin, 1992; McClements, 2005b). The presence of this concentration gradient means that
727
solubilized oil molecules tend to move from the immediate vicinity of smaller droplets to that of
728
larger droplets. This leads to an increase in mean droplet size over time, which can be described
729
by the following equation once steady state conditions have been achieved (Kabalnov &
730
Shchukin, 1992):
RI PT
723
732
d (t ) 3 − d ( 0 ) 3 = ω t =
32 9
αS ∞ Dt
(6)
M AN U
733 734
SC
731
Here, d(t) is the number-weighted mean droplet diameter at time t, d0 is the initial number-
735
weighted mean droplet diameter, ω is the Ostwald ripening rate, α =2γVm/RT, S∞ is the water-
736
solubility of the oil phase in the aqueous phase, D is the translational diffusion coefficient of the
737
oil molecules through the aqueous phase, Vm is the molar volume of the oil, γ is the oil-water
738
interfacial tension, R is the gas constant, and T is the absolute temperature. The most important factor determining the stability of a beverage emulsion to OR is the
740
water-solubility of the oil phase (S∞) (Weiss, Herrmann, & McClements, 1999). For this reason
741
OR is not usually a problem for emulsions prepared using oils with a very low water-solubility,
742
such as long chain triglycerides (e.g., corn, soy, sunflower, or fish oils). On the other hand, OR
743
may occur rapidly for emulsions prepared using oils with an appreciable water-solubility, such as
744
flavor oils and essential oils (Li, Le Maux, Xiao, & McClements, 2009; McClements, et al.,
745
2012; Wooster, Golding, & Sanguansri, 2008b). OR can be retarded in these systems by adding
746
a substance known as a ripening inhibitor. A ripening inhibitor is a non-polar molecule that is
747
soluble in the oil phase but insoluble in the water phase, e.g., a long chain triacylglycerol (such
748
as corn oil). This type of molecule can inhibit OR by generating an entropy of mixing effect that
749
counter-balances the curvature effects.
AC C
EP
TE D
739
750
Consider an oil-in-water emulsion that contains droplets comprised of two different lipid
751
components: a water-insoluble component and a water-soluble component. The water-soluble
752
molecules will diffuse from the small to the large droplets due to OR. Consequently, there will
753
be a greater concentration of water-insoluble molecules in the smaller droplets than in the larger
ACCEPTED MANUSCRIPT 27
droplets after OR occurs. Differences in the composition of emulsion droplets are
755
thermodynamically unfavorable because of the entropy associated with mixing: it is more
756
favorable to have the two lipids distributed evenly throughout all of the droplets, rather than to
757
be located in particular droplets. Consequently, there is a thermodynamic driving force that
758
operates in the opposite direction to the OR effect. The change in droplet size distribution with
759
time then depends on the concentration and solubility of the two components within the oil
760
droplets. This approach has previously been used to improve the stability of food-grade
761
nanoemulsions, such as those containing short chain triglycerides, essential oils, and flavor oils
762
(Li, et al., 2009; McClements, et al., 2012; Wooster, et al., 2008b). An example of this effect is
763
shown in Figure 7 which shows that droplet growth in orange oil-in-water emulsions during
764
storage can be inhibited by adding a sufficiently high concentration of corn oil (the ripening
765
inhibitor) (McClements, et al., 2012),. Orange oil (4-fold) has a relatively high solubility in
766
water, and therefore is highly prone to OR, which leads to an appreciable increase in mean
767
droplet size during storage. On the other hand, corn oil has a very low solubility in water, and
768
therefore it can retard OR if it is incorporated into the oil phase prior to homogenization. These
769
results show that incorporating ≥ 10% corn oil into the oil phase was sufficient to inhibit OR in
770
these systems (Figure 7). OR may also be retarded by adding certain kinds of weighting agents
771
(such as ester gums) since these substances also have a very low water solubility and therefore
772
act as ripening inhibitors (Lim, et al., 2011).
773
5.2. Chemical Stability
SC
M AN U
TE D
EP
774
RI PT
754
A number of lipophilic compounds that may be present in beverage emulsions can undergo chemical degradation during storage, which leads to a loss of color, flavor and/or nutrients. A
776
few representative examples of chemical degradation of lipophilic components in oil-in-water
777
emulsions are given below.
778
AC C
775
Citrus Degradation. Several mechanisms lead to the chemical decomposition of citrus flavor
779
components (such as citral, d-limonene, and citronellal), including oxidation, hydrolytic
780
reactions, the formation of terpene alcohols, and the cyclization of terpene aldehydes (Clark,
781
Powell, & Radford, 1977; Kimura, Iwata, & Nishimura, 1982; Kimura, Nishimura, Iwata, &
782
Mizutani, 1983a, 1983b). Acid-catalyzed decomposition and oxidation reactions change the
783
desirable flavor profile of citrus oils by reducing the concentration of desirable flavor
ACCEPTED MANUSCRIPT 28
784
components and increasing the concentration of undesirable flavor components (Tan, 2004;
785
Ueno, Masuda, & Ho, 2004). The beverage industry would therefore like to identify effective
786
strategies for preventing these undesirable chemical degradation reactions. There has been a great deal of research on establishing the major factors that influence the
RI PT
787
chemical degradation of citral because this is one of the most important flavor compounds found
789
in commercial beverages. The degradation rate of citral in aqueous solutions has been shown to
790
increase with decreasing pH (Choi, et al., 2009) (Figure 8). Most commercial beverages have
791
acidic aqueous phases and are therefore highly susceptible to flavor loss during storage due to
792
this acid-catalyzed mechanism. The chemical stability of citral has been shown to be much
793
higher when it is located within an oil phase than in an aqueous phase (Choi, et al., 2009).
794
Consequently, the chemical degradation of citral in beverage emulsions can be improved by
795
ensuring that the citral molecules are located primarily in an oil phase rather than in the aqueous
796
phase. Indeed, studies have shown that citral stability can be improved by increasing the oil
797
droplet concentration (Choi, et al., 2009) or by adding surfactant micelles to the aqueous phase
798
(Choi, et al., 2010b), although these strategies may not be practical for most commercial
799
products. It was proposed that citral stability may be improved by encapsulating it within solid
800
lipid particles rather than within liquid oil droplets, however the opposite was found to be true
801
experimentally, which was attributed to the expulsion of the citral molecules into the aqueous
802
phase after droplet crystallization (Mei, et al., 2010). Addition of various kinds of natural
803
antioxidants to flavor oil emulsions has also been shown to improve the stability of citral to
804
chemical degradation (Yang, Tian, Ho, & Huang, 2011). The oil droplets in beverage emulsions
805
are surrounded by a coating of emulsifier molecules, and so it may be possible to improve the
806
stability of the citral molecules within them by engineering the properties of the interfacial layer
807
(Decker & McClements, 2001; Given Jr., 2009). Indeed, studies have shown that citral
808
degradation was faster in flavor oil droplets coated by an anionic surfactant than those coated by
809
a non-ionic or cationic surfactant, which was attributed to differences in the accumulation of
810
catalytic protons near the droplet surfaces (Choi, Decker, Henson, Popplewell, & McClements,
811
2010a). A high local concentration of protons is believed to accelerate the citral degradation
812
mechanism at the droplet surfaces. Coating flavor oil droplets with a cationic biopolymer layers
813
has also been shown to improve the stability of citral to chemical degradation (Djordjevic,
814
Cercaci, Alamed, McClements, & Decker, 2007, 2008; Yang, Tian, Ho, & Huang, 2012)
AC C
EP
TE D
M AN U
SC
788
ACCEPTED MANUSCRIPT 29
815
Polyunsaturated Lipid Degradation. There has been great interest in the beverage industry in fortifying products with ω-3 lipids (such as flax, fish and algal oils) since these lipids have
817
been claimed to have health benefits and are currently under-consumed by the general
818
population. Nevertheless, there are many technical difficulties associated will incorporating
819
these lipids into beverage products due to their high susceptibility to oxidation. Lipid oxidation
820
affects the quality of emulsion-based products, influencing their flavor, odor, and nutritive value
821
(Frankel, Satué-Gracia, Meyer, & German, 2002). The oxidation of polyunsaturated lipids is a
822
highly complex series of chemical reactions that is initiated when a lipid interacts with an
823
oxygen reactive species, and proceeds through molecular cleavage and oxygen addition reactions
824
to the formation of a wide variety of volatile compounds (McClements & Decker, 2000; Waraho,
825
McClements, & Decker, 2011). The rate at which oxidation takes place is dependent on several
826
factors: the molecular structure of the lipids; storage conditions; the presence of pro-oxidants and
827
antioxidants; and the structural organization of the system. Based on this knowledge a variety of
828
strategies have been developed to inhibit or prevent lipid oxidation in emulsified products:
829
addition of oil-soluble and water-soluble antioxidants; chelation of pro-oxidant transition metals;
830
engineering the interface to prevent pro-oxidants from coming into close proximity to lipid
831
substrates; controlling environmental conditions, such as exposure to heat, oxygen, or light.
832
Carotenoid degradation. Carotenoids are natural compounds found in many fruits and
833
vegetables that are may be used in foods an colorants or nutraceuticals because of their potential
834
health benefits (Mayne, 1996; Ryan, O'Connell, O'Sullivan, Aherne, & O'Brien, 2008). One of
835
the major factors currently limiting the incorporation of carotenoids into many food and
836
beverage products is their high susceptibility to chemical degradation. In particular, carotenoids
837
have a conjugated polyunsaturated hydrocarbon chain that makes them highly prone to
838
autoxidation (Boon, McClements, Weiss, & Decker, 2009). A number of factors have previously
839
been shown to promote the oxidation of carotenoids, including highly acidic environments
840
(Konovalov & Kispert, 1999), light (Mortensen & Skibsted, 1996), heat (Mader, 1964), singlet
841
oxygen (Krinsky, 1998), transition metals (Gao & Kispert, 2003; Williams, et al., 2001), and free
842
radicals (Liebler & McClure, 1996; Woodall, Lee, Weesie, Jackson, & Britton, 1997). Once
843
carotenoid degradation has been initiated a number of secondary reaction products may form,
844
including epoxides, endoperoxides, apocarotenals and apocarotenones (Gao & Kispert, 2003;
AC C
EP
TE D
M AN U
SC
RI PT
816
ACCEPTED MANUSCRIPT 30
845
Woodall, et al., 1997; Yamauchi, Miyake, Inoue, & Kato, 1993). The chemical degradation of
846
carotenoids leads to color fading, and may reduce their beneficial health properties.
847
Recent studies have examined the influence of interfacial properties (i.e., emulsifier type), storage conditions (i.e., pH, ionic strength, and temperature) and antioxidant addition (i.e.,
849
vitamin E, Coenzyme Q10, EDTA and ascorbic acid) on the chemical degradation of β-carotene
850
encapsulated within oil-in-water nanoemulsions (Qian, Decker, Xiao, & McClements, 2012).
851
The rate of β-carotene degradation was found to increase with decreasing pH and increasing
852
temperature, was faster for a non-ionic surfactant (Tween 20) than for a protein (β-
853
lactoglobulin), and decreased with increasing antioxidant addition to either the oil or aqueous
854
phase.
855
5.3. Defining the End of Shelf Life
SC
M AN U
856
RI PT
848
The end of the shelf life of a product can be defined as the time when it becomes unacceptable to consumers, which depends on the rate of the various physical and chemical
858
instability mechanisms occurring. A product may become unacceptable when a ring of oil
859
droplets is visible at the top of the bottle, when the flavor components decompose/oxidize and
860
create an unacceptable flavor profile, when the color changes beyond an acceptable level, or
861
when the product is microbiologically unsafe to consume. A beverage manufacture should
862
establish quantitative criteria that can be used to establish the end of the shelf life of their
863
particular product. They should then develop a systematic testing scheme that can be used to
864
predict the shelf life of products.
865
6. Beverage Emulsion Manufacture
AC C
EP
TE D
857
866
Beverage emulsions are usually prepared using a two-step process: a beverage emulsion
867
concentrate (3 – 30 wt% oil) is prepared, which is then diluted extensively to create the finished
868
product (< 0.1 wt% oil) (Tan, 2004). In this section, we briefly describe the major characteristics
869
of the concentrate and finished products, and then review some of the major manufacturing
870
approaches for forming stable beverage emulsions.
871
Beverage emulsion concentrates: When beverages are prepared using high energy
872
homogenization methods (see below), all the aqueous phase components are usually mixed
873
together and all of the oil phase components are mixed together prior to homogenization. The
ACCEPTED MANUSCRIPT 31
aqueous phase often has to be heated and mechanically agitated to facilitate dissolution and
875
dispersion of water-soluble components (such as emulsifiers, thickening agents, buffers, minerals
876
and other functional ingredients). Similarly, the oil phase may also have to be heated and
877
mechanically agitated to facilitate the melting and dispersion of any antioxidants, weighting
878
agents, ripening inhibitors, or colors. Once the oil and aqueous phases have been prepared they
879
are blended together using a high-shear mixer to form a coarse emulsion (d ≈ 1 to 10 µm), which
880
is then homogenized using a mechanical device to form a fine emulsion (d ≈ 0.1 to 1 µm). When
881
beverages are prepared using low energy homogenization methods a different approach may be
882
taken (see below). In this case, water-soluble surfactants and some other water-soluble
883
components may initially be incorporated into the oil phase, which is then mixed with the
884
aqueous phase. This process can lead to the spontaneous formation of a microemulsion,
885
nanoemulsion, or emulsion depending on system composition and preparation procedure. After
886
preparation the beverage emulsion concentrate is often pasteurized to reduce the microbial load,
887
and then stored or transported to the place where it will be used.
M AN U
SC
RI PT
874
Finished Product: The finished product is created by diluting the beverage emulsion
889
concentrate with another aqueous phase, which may contain various other ingredients, such as
890
colors, flavors, preservatives, pH regulators etc. Typically, the concentrate is diluted 500-1000
891
times to produce a final product that often has an oil concentration < 20 mg per liter for a ready-
892
to-drink product (Given, 2009). The final product may be homogenized again to ensure that any
893
non-polar colors, flavors, and preservatives are incorporated into the oil droplets. Appropriate
894
selection of ingredients and processing conditions may lead to beverage products with shelf lives
895
longer than 12 months. However, the perceived quality of a product may deteriorate after
896
extended storage due to detrimental changes in its physical or chemical properties. Beverage
897
emulsions are susceptible to various physical instability mechanisms that can lead to undesirable
898
changes in appearance, such as ringing and oiling off (see earlier section). Beverage emulsions
899
are also liable to undesirable quality changes due to chemical degradation, e.g., changes in flavor
900
profile due to degradation of flavor molecules (such as citral) or color fading due to degradation
901
of colorants (such as carotenoids). These challenges can often be overcome using appropriate
902
packaging materials and/or proper product formation. The design and formulation of successful
903
soft drink products requires careful selection of functional ingredients (oil type, emulsifiers,
904
weighting agents, ripening inhibitors, antioxidants, etc.), preparation conditions (e.g.,
AC C
EP
TE D
888
ACCEPTED MANUSCRIPT 32
905
homogenization method and operating conditions), and storage conditions (e.g., exposure to
906
elevated temperatures, light, and oxygen). A number of different methods can be used to form beverage emulsion concentrates. In
908
general, these approaches can be categorized as either high-energy or low-energy approaches
909
depending on the underlying physical principle of droplet formation (Acosta, 2009; Anton &
910
Vandamme, 2009; Leong, Wooster, Kentish, & Ashokkumar, 2009; Pouton & Porter, 2006;
911
Tadros, Izquierdo, Esquena, & Solans, 2004). High-energy approaches utilize mechanical
912
devices (“homogenizers”) that generate intense forces capable of disrupting and intermingling
913
the oil and aqueous phases leading to the formation of very fine oil droplets (Figure 9). The
914
most commonly used homogenizers utilized in the beverage industry for forming emulsions are
915
high pressure valve homogenizers, but microfluidizers and ultrasonic methods may also be used
916
(Gutierrez, et al., 2008; Leong, et al., 2009; Velikov & Pelan, 2008; Wooster, et al., 2008b).
917
High-energy approaches are probably the most common method used for preparing beverage
918
emulsions at present because they are capable of large-scale production, and they can be used to
919
prepare emulsions from a variety of different starting materials. Low energy approaches rely on
920
the spontaneous formation of fine oil droplets within mixed surfactant-oil-water systems when
921
the solution or environmental conditions are altered (Anton, Benoit, & Saulnier, 2008;
922
Bouchemal, Briancon, Perrier, & Fessi, 2004; Chu, Ichikawa, Kanafusa, & Nakajima, 2007;
923
Freitas, Merkle, & Gander, 2005; Tadros, et al., 2004; Yin, Chu, Kobayashi, & Nakajima, 2009).
924
A number of different low energy approaches have been developed, and some of these are
925
suitable for utilization within the beverage industry, e.g., phase inversion and spontaneous
926
emulsification methods (Figure 10). The minimum particle size that can be produced using
927
either approach depends on many different factors, which are highlighted in the sections below.
928
6.1. High-Energy Approaches
SC
M AN U
TE D
EP
AC C
929
RI PT
907
The size of the droplets generated by high energy approaches is determined by a balance
930
between two opposing processes occurring within the homogenizer: droplet disruption and
931
droplet coalescence (Jafari, Assadpoor, He, & Bhandari, 2008). Only those mechanical devices
932
that are capable of generating extremely intense disruptive forces are capable of producing the
933
tiny droplets required for most beverage emulsion applications (Figure 9), i.e., high-pressure
934
valve homogenizers, microfluidizers, and ultrasonic devices (Leong, et al., 2009; Tadros, et al.,
ACCEPTED MANUSCRIPT 33
2004). The smallest droplet size that can be produced by a particular high-energy device
936
depends on homogenizer design (e.g., flow and force profiles), homogenizer operating
937
conditions (e.g., energy intensity, duration), environmental conditions (e.g. temperature), sample
938
composition (e.g., oil type, emulsifier type, concentrations), and the physicochemical properties
939
of the component phases (e.g., interfacial tension, viscosity) (Kentish, et al., 2006; Wooster, et
940
al., 2008b).
941
RI PT
935
High energy homogenizers are widely used to produce beverage emulsions because they can be utilized with a wide variety of different types of oils and emulsifiers. Once the
943
homogenization conditions have been optimized, beverage emulsions can be produced using
944
triacylglycerol oils or flavor oils as the oil phase, and proteins, polysaccharides, phospholipids,
945
or surfactants as emulsifiers. Thus, high-energy methods are suitable for producing both cloud
946
emulsions and flavor emulsions. Even so, the size of the droplets produced depends strongly on
947
the characteristics of the oil and emulsifier used (see below). For example, it is usually easier to
948
produce very small droplets when the oil phase has a low viscosity and/or interfacial tension
949
(e.g., flavor oils) than when it has a high viscosity and/or interfacial tension (e.g., triacylglycerol
950
oils).
951
6.1.1. High Pressure Valve Homogenizers
M AN U
TE D
952
SC
942
High pressure valve homogenizers are currently the most common high-energy method of producing beverage emulsions. Initially, a coarse emulsion is produced using a high shear mixer
954
and then this is fed directly into the inlet of the high pressure valve homogenizer. The
955
homogenizer has a pump that pulls the coarse emulsion into a chamber on its backstroke and
956
then forces it through a narrow valve at the end of the chamber on its forward stroke (Figure 9).
957
As the coarse emulsion passes through the valve it experiences a combination of intense
958
disruptive forces that cause the larger droplets to be broken down into smaller ones. Different
959
nozzle designs are available to increase the efficiency of droplet disruption. The droplet size
960
produced using a high pressure valve homogenizer usually decreases as the number of passes
961
and/or the homogenization pressure increases. It also depends on the viscosity ratio of the two
962
phases (usually oil and water) being homogenized. Small droplets can only usually be produced
963
when the disperse-to-continuous phase viscosity ratio falls within a certain range (0.05 < ηD/ηC <
964
5) (Tadros, et al., 2004; Walstra, 1993, 2003). Finally, it is important to have sufficient
AC C
EP
953
ACCEPTED MANUSCRIPT 34
emulsifier present to cover all the new droplet surfaces formed during homogenization, and to
966
use an emulsifier that can rapidly adsorb to the droplet surfaces to prevent re-coalescence (Jafari,
967
et al., 2008). Usually, there is a linear relationship between the logarithm of the homogenization
968
pressure (P) and the logarithm of the droplet diameter (d): log d ∝ log P, with the constant of
969
proportionality depending on homogenizer type (McClements, 2005b). To reduce the droplet
970
size to the level required in beverage emulsions it is sometimes necessary to operate at extremely
971
high pressures and to use multiple passes through the homogenizer. Even then, it is only
972
possible under certain circumstances to obtain droplets less than 100 nm in radius (e.g., high
973
emulsifier levels, low interfacial tensions, and appropriate viscosity ratios).
974
6.1.2. Microfluidizers
SC
The formation of beverage emulsions using a microfluidizer also involves forcing a coarse
M AN U
975
RI PT
965
emulsion through a narrow orifice under high pressure to facilitate droplet disruption. However,
977
the design of the channels through which the emulsion is made to flow within a microfluidizer is
978
different from that of a high pressure valve homogenizer (Figure 9). The microfluidizer divides
979
an emulsion into two streams that are then made to impinge on each other in an interaction
980
chamber. Intense disruptive forces are generated within the interaction chamber when the two
981
fast moving streams of emulsion collide, leading to highly efficient droplet disruption.
TE D
976
A number of studies have examined the potential application of microfluidizers for the
983
production of model beverage emulsions (Dalgleish, West, & Hallett, 1997; Henry, Fryer, Frith,
984
& Norton, 2010; Klein, Aserin, Svitov, & Garti, 2010). These studies have shown that small
985
droplets can be produced provided that conditions are optimized to facilitate droplet disruption
986
and inhibit droplet coalescence. The droplet size tends to decrease with increasing
987
homogenization pressure, number of passes, emulsifier concentration, and decreasing disperse-
988
to-continuous phase viscosity ratio (Wooster, et al., 2008b). Again, there is usually a linear
989
relationship between the logarithm of homogenization pressure and the logarithm of the droplet
990
diameter: log d ∝ log P. Recent studies have identified some of the major factors influencing the
991
formation of fine emulsions using microfluidizers (Qian & McClements, 2011). This study
992
showed that the logarithm of the mean droplet diameter decreased linearly with the logarithm of
993
the homogenization pressure for both an ionic surfactant (SDS) and a globular protein (β-
994
lactoglobulin) (Figure 11). However, the slope of the log(d) versus log(P) relationship was
AC C
EP
982
ACCEPTED MANUSCRIPT 35
appreciably steeper for the surfactant (-0.57) than for the protein (-0.29), which was attributed to
996
the fact that the protein may adsorb more slowly to the droplet surfaces, and that it may form a
997
viscoelastic coating that inhibits further droplet breakup. The dependence of the mean droplet
998
diameter on viscosity ratio (ηD/ηC) was also examined by preparing emulsions using different oil
999
phase and aqueous phase compositions. For the ionic surfactant there was a distinct decrease in
1000
mean droplet diameter with decreasing viscosity ratio, which suggested that droplet disruption
1001
within the homogenizer became easier as the viscosity of the two phases became more similar.
1002
On the other hand, little change was found in mean droplet size with viscosity ratio when a
1003
globular protein was used as an emulsifier, which again may be due to the relatively slow
1004
adsorption of the protein and its ability to form a coating that inhibits further droplet disruption.
1005
6.1.3. Ultrasonic Homogenizers
SC
M AN U
1006
RI PT
995
Beverage emulsions can also be formed continuously using ultrasonic homogenizers. This type of homogenizer utilizes high intensity ultrasonic waves to generate intense disruptive forces
1008
(mainly generated by cavitation) that break the oil and water phases into very small droplets
1009
(Kentish, et al., 2006; Leong, et al., 2009; Lin & Chen, 2008). Batch and continuous ultrasonic
1010
homogenizers are available for producing emulsions (Leong, et al., 2009). However, continuous
1011
ultrasonic homogenizers are probably the most commonly used methods for the large scale
1012
production of fine emulsions (Figure 9). The size of the droplets produced using these devices
1013
tends to decrease as the intensity of the ultrasonic waves is increased or the residence time in the
1014
disruption zone is increased (Abismail, Canselier, Wilhelm, Delmas, & Gourdon, 1999; Maa &
1015
Hsu, 1999). The homogenization efficiency also depends on the type and amount of emulsifier
1016
present, and the viscosity of the oil and aqueous phases (Jafari, He, & Bhandari, 2006; Kentish,
1017
et al., 2006; Leong, et al., 2009; Maa & Hsu, 1999). Ultrasonic homogenizers are particularly
1018
suitable for low-viscosity fluids, but are less suitable for more viscous systems.
1019
6.2. Low-Energy Approaches
EP
AC C
1020
TE D
1007
The formation of emulsions using low-energy approaches relies on the spontaneous
1021
formation of oil droplets in surfactant-oil-water mixtures when either their composition or their
1022
environment is altered in a specific way (Anton, et al., 2008; Anton & Vandamme, 2009;
1023
Bouchemal, et al., 2004; Yin, et al., 2009). A number of methods for preparing fine emulsions
1024
are based on the low-energy approach, including spontaneous emulsification and phase inversion
ACCEPTED MANUSCRIPT 36
methods (Figure 10) (Anton, et al., 2008; Anton & Vandamme, 2009; Fernandez, Andre, Rieger,
1026
& Kuhnle, 2004; Maestro, Sole, Gonzalez, Solans, & Gutierrez, 2008). Some of these low
1027
energy methods are already used in the beverage industry for forming oil-in-water emulsions,
1028
whereas others may also be for certain applications.
1029
RI PT
1025
Low energy approaches are often more effective at producing small droplet sizes than high energy approaches, but they are often more limited in the types of oils and emulsifiers that can be
1031
used. For example, it is currently not possible to use proteins or polysaccharides as emulsifiers
1032
in most of the low energy approaches used to form very fine emulsions. Instead, relatively high
1033
concentrations of synthetic surfactants are usually required to form stable emulsions by these
1034
approaches, which limit their use for some beverage applications.
1035
6.2.1. Spontaneous Emulsification Methods
M AN U
1036
SC
1030
In this method an emulsion is spontaneously formed when an organic phase and an aqueous phase are mixed together (Anton & Vandamme, 2009; Miller, 1988; Pouton & Porter, 2006).
1038
The organic phase usually contains oil, hydrophilic surfactant, and possibly water-miscible
1039
solvent, whereas the aqueous usually contains water and possibly cosolvent and cosurfactant.
1040
The two phases can be brought together in various ways: the organic phase may be titrated into
1041
the aqueous phase (Anton & Vandamme, 2009), or the aqueous phase may be titrated into the
1042
organic phase (Sonneville-Aubrun, et al., 2009). When the two phases come into contact the
1043
hydrophilic surfactant and/or solvent move from the oil phase to the aqueous phase, which
1044
generates tiny oil droplets at the phase boundary (Horn & Rieger, 2001). The size of the droplets
1045
produced can be controlled by varying the compositions of the two initial phases, as well as the
1046
mixing conditions.
EP
The spontaneous emulsification method was recently compared with a high energy method
AC C
1047
TE D
1037
1048
(microfluidization) of producing fine emulsions (Yang, Marshall-Breton, Leser, Sher, &
1049
McClements, 2012). The surfactant-oil-water system used consisted of 15.4 wt% non-ionic
1050
surfactant, 23.1 wt% MCT, and 61.5% water, with the surfactant containing a 50:50 mixture of a
1051
hydrophilic (Tween 80) and lipophilic (Tween 85) surfactant. In the high energy method, the
1052
surfactant, oil and water were blended together, and then passed through a microfluidizer. In the
1053
low energy method, the oil and surfactant were mixed together, and then simply titrated into the
1054
water using slow stirring. The microfluidization method produced droplets with a diameter of
ACCEPTED MANUSCRIPT 37
about 110 nm, whereas the spontaneous emulsification method could produce droplets with
1056
diameters around 140 nm. This simple experiment demonstrated that nanoemulsions could be
1057
produced using the spontaneous emulsification method, provided that the system composition
1058
was optimized, i.e., surfactant, oil, and water contents. However, a much higher surfactant-to-oil
1059
ratio (SOR) is required to produce small droplets using the low energy method than with the high
1060
energy method. The influence of SOR on the mean droplet diameter of vitamin E nanoemulsions
1061
produced using the spontaneous emulsification approach is shown in Figure 12. These
1062
measurements show that there is a minimum in the mean droplet size at a particular SOR, and
1063
therefore the composition of the system has to be carefully controlled to obtain a particular
1064
droplet size.
The ability of fine emulsions to be formed using the spontaneous emulsification method
M AN U
1065
SC
RI PT
1055
depends on the system composition (surfactant, oil, and water levels), surfactant type, oil type,
1067
and preparation method (order of addition, mixing speeds). This approach is suitable for
1068
producing flavor or cloud emulsions, provided that an appropriate surfactant is used (such as a
1069
non-ionic surfactant with a particular HLB number) and the surfactant-to-oil ratio is optimized.
1070
The main advantage of this method is that it is inexpensive, simple, and requires no major
1071
equipment, whereas the disadvantage is that relatively high surfactant levels are required (SOR ≈
1072
1)
1073
6.2.2. Phase Inversion Methods
A number of methods have been developed to formulate fine emulsions that depend on
EP
1074
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1066
inducing a phase inversion from a W/O to O/W form, e.g., phase inversion temperature (PIT)
1076
and emulsion inversion point (EIP) methods (Fernandez, et al., 2004; Thakur, Villette, Aubry, &
1077
Delaplace, 2008). In the PIT method the change from one type of an emulsion to another involves a
1078
transitional phase inversion, whereas in the EIP method it involves a catastrophic phase inversion
1079
(Figure 13). A transitional phase inversion occurs when the surfactant properties are altered by
1080
adjusting a formulation variable, such as temperature, pH, or ionic strength. A catastrophic phase
1081
inversion occurs when the ratio of the oil-to-water phases is altered while the surfactant properties
1082
remain constant.
AC C
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ACCEPTED MANUSCRIPT 38
6.2.2.1. Phase Inversion Temperature Method The PIT method is based on changes in the optimum curvature and relative solubility of
1085
non-ionic surfactants with changing temperature (Anton, Gayet, Benoit, & Saulnier, 2007; Anton
1086
& Vandamme, 2009; Gutierrez, et al., 2008). Emulsions containing very fine droplets can be
1087
spontaneously formed using the PIT method by varying the temperature-time profile of
1088
appropriate mixtures of surfactant, oil, and water. This type of phase inversion usually involves
1089
the controlled transformation of an emulsion from one type to another (e.g., W/O to O/W or vice
1090
versa) through an intermediate state. This intermediate state is typically a liquid crystalline or
1091
bicontinuous microemulsion phase. The driving force for this type of phase inversion is the
1092
change in the physicochemical properties of the surfactant molecules caused by temperature
1093
changes, e.g., molecular geometry and solubility (Figure 10). The molecular geometry of a
1094
surfactant can be characterized by a packing parameter, p (Israelachvili, 2011):
1096
SC
p=
M AN U
1095
RI PT
1083 1084
aT aH
(7)
where, aT and aH are the cross-sectional areas of the lipophilic tail-group and hydrophilic headgroup respectively. Surfactants spontaneously associate in water to form monolayers that have a
1098
curvature that allows the most efficient packing of the surfactant molecules. The optimum
1099
curvature of a monolayer depends on the packing parameter of the surfactant: for p < 1, the
1100
optimum curvature is convex (favoring O/W systems); for p = 1, monolayers with zero curvature
1101
are preferred (favoring lamellar structures); and, for p > 1 the optimum curvature is concave
1102
(favoring W/O systems). For non-ionic surfactants, the head group is relatively large compared to
1103
the tail group (p < 1) at temperatures below the PIT and so O/W emulsions are favored. Upon
1104
heating, the head group becomes progressively dehydrated and so the packing parameter increases.
1105
At the phase inversion temperature (PIT), the head group and tail group have similar sizes (p = 1)
1106
and so liquid crystals or microemulsions are formed. Above the PIT, the head group is relatively
1107
small compared to the tail group (p > 1) and so W/O emulsions are favored. The relative solubility
1108
of non-surfactants in the oil and water phases also changes with temperature because of head group
1109
dehydration, which has also been used to interpret the PIT phenomenon (Anton, et al., 2007; Anton
1110
& Vandamme, 2009). At low temperatures, the head group is highly hydrated and so the surfactant
1111
tends to be more soluble in water. As the temperature is raised and the head group becomes
1112
progressively dehydrated and the solubility of the surfactant in water decreases, while its solubility
AC C
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ACCEPTED MANUSCRIPT 39
in oil increases. At a particular temperature (≈ PIT), the solubility of the surfactant in the oil and
1114
water phases is approximately equal. At higher temperatures, the surfactant becomes more soluble
1115
in the oil phase than in the water phase. Thus at high temperatures the surfactant is mainly located
1116
in the oil phase, and when the system is cooled it has a tendency to move into the aqueous phase.
1117
Thus, oil droplets are formed by a mechanism similar to spontaneous emulsification.
1118
RI PT
1113
The PIT method is relatively straightforward to implement (Figure 10). A mixture of
surfactant, oil, and water (SOW) is initially heated up to a temperature around or slightly above the
1120
PIT, which leads to the formation of a microemulsion or liquid crystalline phase. The SOW system
1121
is then quench cooled to a temperature well below the PIT with continuous stirring, which leads to
1122
the spontaneous formation of an oil-in-water emulsion or nanoemulsion (Anton & Vandamme,
1123
2009). An example of the phase behavior of a SOW mixture consisting of a non-ionic surfactant
1124
(13% Tween 80), a flavor oil (10 wt% lemon oil), and water (77%) upon heating and cooling is
1125
shown in Figure 14 (Rao & McClements, 2010). Initially, the surfactant, oil and water were
1126
blended together to form a coarse O/W emulsion that was optically opaque. Upon heating the
1127
system becomes transparent when the PIT is reached, and then becomes opaque when heated above
1128
the PIT due to formation of a W/O emulsion. Upon cooling, the system goes from turbid to
1129
transparent indicating that a nanoemulsion was formed (d = 45 nm).
M AN U
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1130
SC
1119
One of the limitations of the phase inversion temperature method is that the emulsions produced are often highly prone to droplet coalescence when they are stored at temperatures
1132
approaching the PIT. This could be a problem in food and beverage applications that require some
1133
form of thermal treatment (such as pasteurization, sterilization or cooking) or that are stored at
1134
elevated temperatures (e.g., in warm or hot climates). Recently, we developed an approach to
1135
overcome this problem by forming emulsions using a non-ionic surfactant with a relatively low PIT,
1136
and then diluting the resulting emulsions in a solution containing another surfactant with a high PIT
1137
(Rao & McClements, 2010). This second surfactant partially displaces some of the original
1138
surfactant from the droplet surfaces, thereby altering the optimum curvature and PIT of the
1139
surfactant monolayer, as well as increasing the repulsive interactions between the droplets. This
1140
approach could be used in the food and beverage industry to form very fine droplets using the PIT
1141
method, and then stabilizing them by diluting them in a different surfactant solution.
1142 1143
AC C
EP
1131
The PIT method can only be used for certain combinations of oils and surfactants. The PIT must be within a practical range that can be implemented within the food industry (e.g. somewhere
ACCEPTED MANUSCRIPT 40
between 40 and 90 ºC) – if it is too high then it will not be possible to create emulsions, and if it is
1145
too low the emulsions formed will be highly unstable to droplet growth. The PIT tends to increase
1146
with increasing molecular weight and hydrophobicity of oil molecules, which means that for many
1147
triacylglycerol oils the PIT is too high to practically reach. On the other hand, flavor oils seem to be
1148
amenable to emulsification using the PIT method. Nevertheless, it is still important to select an
1149
appropriate type and amount of surfactant to ensure the PIT is in the correct range.
1150 1151
6.2.2.2. Emulsion Inversion Point (EIP) Method The emulsion inversion point (EIP) method can also be implemented very easily (Figure 10).
1152
It simply involves titrating increasing amounts of an aqueous phase into an organic phase to induce
1153
a catastrophic phase inversion from a W/O to an O/W system. Initially, an organic phase is
1154
prepared that contains oil and a hydrophilic surfactant. The aqueous phase is then slowly titrated
1155
into this organic phase with constant stirring. As the amount of water added increases the system
1156
converts from a W/O emulsion, to an O/W/O multiple emulsion, to an O/W emulsion (Jahanzad,
1157
Crombie, Innes, & Sajjadi, 2009; Sajjadi, 2006). The formation of these multiple emulsions
1158
requires that the hydrophilic surfactant is initially located in the oil phase. At relatively low
1159
surfactant concentrations the formation of multiple emulsions is suppressed and only relatively
1160
large oil droplets are produced in the final emulsion, which are similar in size to those that would
1161
be produced if the surfactant had been dissolved in the water phase prior to homogenization
1162
(Jahanzad, et al., 2009). At relatively high surfactant concentrations, multiple emulsions are
1163
formed during the titration process, and the final oil droplet size within the O/W emulsions is
1164
determined by the size of the inner oil droplets in the intermediate O/W/O emulsions. The value
1165
of the critical concentration where phase inversion occurs, as well as the size of the oil droplets
1166
produced, depends on process variables, such as the stirring speed, the rate of water addition, and
1167
the emulsifier concentration (Thakur, et al., 2008). The emulsifiers used in catastrophic phase
1168
inversion are usually limited to small molecule surfactants that can stabilize both W/O emulsions (at
1169
least over the short term) and O/W emulsions (over the long term).
SC
M AN U
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EP
AC C
1170
RI PT
1144
We have recently used the EIP method to produce emulsions with relatively small droplet
1171
diameters (< 100 nm) (Ostertag, Weiss, & McClements, 2012). We titrated water into an organic
1172
phase containing a mixture of oil and water-miscible surfactant. The droplet size produced by this
1173
method depended on: (i) oil type: medium chain triglycerides (MCT) < flavor oils (orange and
ACCEPTED MANUSCRIPT 41
limonene) < long chain triglycerides (olive, grape, sesame, peanut and canola oils); (ii) surfactant
1175
type: Tween 80 < Tween 20 < Tween 85; (iii) surfactant concentration: smaller droplets were
1176
produced at higher surfactant-to-oil ratios (SOR); (iv) surfactant location: surfactant initially in oil <
1177
surfactant initially in water. As an example, the data for the influence of oil type on droplet size
1178
produced using the EIP method is shown in Figure 15. The EIP method was also compared to a
1179
high energy method (microfluidization). Small droplets (d < 160 nm) could be produced by both
1180
methods, but that much less surfactant was needed for the high energy method (SOR ≥ 0.1) than the
1181
low energy method (SOR ≥ 0.7). The EIP method may therefore be suitable for producing fine
1182
emulsions suitable for use in the beverage industry without the need for expensive homogenization
1183
equipment, provided that having relatively high amounts of surfactant present is not a problem. In
1184
the finished product, the levels of surfactant may be quite small (even when SOR ≈ 1), since the
1185
beverage emulsion concentrates are diluted so much.
1186
7. Beverage Emulsion Ingredients
1187
7.1. Oil Phase Components
1188
7.1.1. Flavor Oils
SC
M AN U
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1189
RI PT
1174
Flavor oils, such as orange, lemon and peppermint oils, are widely used in the food and beverage industries as flavoring agents, since they contain volatile constituents with
1191
characteristic aroma profiles. The composition of natural flavor oils depends on their biological
1192
origin, the extraction procedure used to isolate them, and any subsequent processing steps. For
1193
example, lemon oil is usually obtained from lemon peel by cold pressing, and it may then be
1194
further refined using steam distillation and various other methods (Misharina, Terenina,
1195
Krikunova, & Medvedeva, 2010). Commercially, flavor oils are available in various forms
1196
(“folds”) that differ in chemical composition due to differences in isolation and processing
1197
procedures used. Flavor oils extracted by cold pressing are usually referred to as single-fold (1×)
1198
oils, while those that have undergone further processing (such as distillation) are referred to as
1199
higher fold oils e.g., three-fold (3×), five-fold (5×), or ten-fold (10×) oils (Gamarra, Sakanaka,
1200
Tambourgi, & Cabral, 2006). Normally, higher-fold oils have different and more intense flavor
1201
profiles than lower-fold oils. Differences in the chemical composition of flavor oils also leads to
AC C
EP
1190
ACCEPTED MANUSCRIPT 42
differences in their physicochemical properties (e.g., water solubility, density, viscosity,
1203
refractive index, and optical properties), which influences their ability to form and stabilize
1204
beverage emulsions (Rao & McClements, 2012a; Rao & McClements, 2012c). In recent studies,
1205
our laboratory has examined the influence of lemon oil fold (1×, 3×, 5× and 10×) on the
1206
formation and properties of oil-in-water microemulsions, nanoemulsions and emulsions (Rao &
1207
McClements, 2012b; Rao & McClements, 2012c). The composition, molecular characteristics,
1208
and physicochemical properties of the four lemon oils were tabulated. The main constituents in
1209
1× lemon oil were monoterpenes (> 90 %), whereas the major constituents in 10× lemon oil were
1210
monoterpenes (≈ 35%), sesquiterpenes (≈ 14%) and oxygenates (≈ 33%). The concentration of
1211
both polar and non-polar components increased with increasing oil fold, while intermediate polar
1212
components decreased. The density, interfacial tension, viscosity, and refractive index of the
1213
lemon oils increased as the oil fold increased (i.e., 1× < 3× < 5× < 10×). The stability of oil-in-
1214
water emulsions produced by high pressure homogenization was strongly influenced by lemon
1215
oil type. Emulsions produced using lower fold oils (1×, 3×, 5×) were highly unstable to droplet
1216
growth during storage, with the growth rate increasing with increasing storage temperature and
1217
decreasing oil fold. Droplet growth was mainly attributed to Ostwald ripening i.e., the diffusion
1218
of water-soluble lemon oil components from small to large droplets. Emulsions prepared using
1219
the highest fold oil (10×) were stable to droplet growth, which was attributed to the presence of
1220
an appreciable fraction of low water-solubility constituents that acted as “ripening inhibitors”
1221
(see later). On the other hand, the lower fold oils were more suitable for forming
1222
microemulsions than the higher fold oils, presumably because they contained constituents that
1223
more easily fit into micelle structures.
SC
M AN U
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EP
A number of recent studies have examined the major factors influencing the formation and
AC C
1224
RI PT
1202
1225
stability of beverage emulsions containing flavor oils, including the preparation method used
1226
(Cheng & McClements, 2011; Rao & McClements, 2011, 2012c), emulsifier type (Mirhosseini,
1227
Tan, Aghlara, et al., 2008; Mirhosseini, Tan, Hamid, & Yusof, 2008; Qian & McClements,
1228
2011), droplet charge (Choi, et al., 2010a), addition of antioxidants (Yang, et al., 2011), presence
1229
of surfactant micelles (Choi, et al., 2010b), and the physical state of the oil phase (Mei, et al.,
1230
2010). Many flavor oils used in beverage emulsions are unstable to chemical degradation during
1231
storage. For example, citral decomposes rapidly during storage at acidic pH by a series of
1232
cyclization and oxidation reactions (Choi, et al., 2009). This effect is highlighted in Figure 16,
ACCEPTED MANUSCRIPT 43
which shows the amount of citral remaining in aqueous solution during storage at different pH
1234
values: the degradation rate increases rapidly as the pH is decreased. Citral degradation reduces
1235
the level of desirable aroma compounds present as well as increasing the levels of undesirable
1236
aroma compounds, thus limiting the shelf life of many beverages. The rate of citral degradation
1237
in emulsions is affected by environmental and compositional factors, such as temperature,
1238
oxygen availability, and antioxidant addition (Choi, et al., 2009; Liang, Wang, Simon, & Ho,
1239
2004a, 2004b; Ueno, Kiyohapa, Ho, & Masuda, 2006; Ueno, et al., 2003). It also depends on the
1240
location of the citral molecules within an emulsion, e.g., oil, water, or interfacial regions (Choi,
1241
et al., 2009; Choi, et al., 2010b). If a labile component is located within an environment where it
1242
is isolated from other components that promote its chemical degradation, then it is possible to
1243
reduce the degradation rate. Since citral degradation occurs predominantly in acidic aqueous
1244
solutions, it is possible to reduce its degradation rate by altering its partitioning between the oil
1245
and aqueous phases. For example, when citral is located predominantly within a non-polar
1246
environment, such as the interior of a lipid droplet or surfactant micelle, it is partially protected
1247
from degradation. This effect is highlighted in Figure 17, which shows that the rate of citral
1248
degradation in oil-in-water emulsions decreases as the total fat droplet concentration increases.
1249
This effect can be attributed to the fact that a lower fraction of the citral is present within the
1250
acidic aqueous phase as the fat content increases due to equilibrium partitioning effects (Figure
1251
18). This phenomenon may be important for beverage emulsion concentrates that have relatively
1252
high fat contents, since in this case an appreciable fraction of the citral will be present within the
1253
oil phase. Choosing an oil type and concentration that ensures that most of the citral is present
1254
within the oil droplets, rather than the aqueous phase, will then help to inhibit citral degradation
1255
during storage. On the other hand, once an emulsion concentrate is diluted to form the finished
1256
product nearly all of the flavor molecules partition into the aqueous phase and therefore any
1257
protective effect is lost.
1258
7.1.2. Cloud Oils
AC C
EP
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M AN U
SC
RI PT
1233
1259
The oil droplets in cloud emulsions are composed mainly of non-flavor oils, such as
1260
triacylglycerol or terpene oils. Triacylglycerol oils are typically derived from natural sources
1261
such as vegetable, corn, canola and sunflower oils, whereas terpene oils are usually isolated by
1262
distillation of natural flavor oils. These oils have very low water-solubilities and are therefore
ACCEPTED MANUSCRIPT 44
stable to Ostwald ripening (unlike flavor oils). The main purpose of cloud oils is to produce
1264
droplets that scatter light strongly so as to give a product a desirable turbid or cloudy appearance,
1265
and so it is important that the droplets are within the size range where efficient light scattering
1266
occurs (typically around 200 to 500 nm). For commercial applications, it is also important that
1267
the droplets remain physically and chemically stable during storage, transport, and handling.
1268
Cloud oils do not usually contribute directly to the flavor profile of beverage emulsions, but they
1269
may indirectly influence flavor due to their effects on the partitioning of flavor molecules
1270
between the oil, water, and headspace regions (McClements, 2005b). They may also adversely
1271
affect the flavor profile of a beverage if they are susceptible to chemical degradation, e.g., the
1272
oxidation of unsaturated lipids products rancid off-flavors.
SC
The physicochemical properties of cloud oils may also play an important role in the
M AN U
1273
RI PT
1263
formation and stability of cloud emulsions. The viscosity of cloud oils will influence their ability
1275
to form small droplets during the homogenization process, with higher viscosities normally
1276
leading to larger droplets. The density of cloud oils determines the creaming velocity of the
1277
droplets within an emulsion, and may therefore influence their long-term stability. The melting
1278
characteristics of cloud oils may also be important for certain applications since partially
1279
crystalline oil droplets are susceptible to partial coalescence leading to droplet aggregation and
1280
phase separation (McClements, 2005b). This may be particularly important for beverages that
1281
are stored in a refrigerator for prolonged periods. In this case, it may be necessary to use an oil
1282
phase that remains liquid at refrigerator temperatures (e.g., winterized or fractionated oil).
1283
7.1.3. Nutraceutical Lipids
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1284
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1274
There has been growing interest in the incorporation of various kinds of nutraceutical lipids into commercial food and beverage products, such as polyunsaturated fats, conjugated linoleic
1286
acid, carotenoids, phytosterols, and fat-soluble vitamins. These nutraceutical lipids are either
1287
mixed with the other oil phase components prior to homogenization or they are encapsulated
1288
separately and then introduced intro the product. The incorporation of nutraceutical lipids into
1289
commercial beverage emulsions is often challenging because they are prone to various physical
1290
or chemical degradation mechanisms. Each type of nutraceutical lipid has its own particular
1291
challenges depends on its physicochemical properties, such as solubility, oil-water partition
1292
coefficient, melting point, and chemical stability (McClements, Decker, Park, & Weiss, 2009).
AC C
1285
ACCEPTED MANUSCRIPT 45
Beverage products must therefore be carefully formulated taking into account the potential
1294
problems associated with the specific types of nutraceutical lipids present.
1295 1296
7.1.3.1. Polyunsaturated Lipids There has been growing interest in the incorporation of polyunsaturated fatty acids (PUFAs),
1297
and especially omega-3 (ω-3) fatty acids, into food and beverage products because of their
1298
potential health benefits (Ruxton, Reed, Simpson, & Millington, 2004, 2007; Siddiqui, et al.,
1299
2004). Omega-3 fatty acids are unsaturated fatty acids that have a double bond that is three
1300
carbon atoms from the methyl end of the molecule. The most common ω-3 fatty acids in food
1301
oils are α-linolenic acid (ALA, 18:3), eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic
1302
acid (DHA, 22:6), with EPA and DHA being the most bioactive. PUFAs are normally
1303
incorporated into foods as part of triacylglycerol molecules, but they may also be incorporated as
1304
monoacylglycerols, diacylglycerols, or phospholipids. Consumption of adequate amounts of ω-3
1305
fatty acids has been linked to decreased risks of various diseases, including cardiovascular
1306
disease, immune response disorders, mental disorders, and poor infant development (Orchard,
1307
Pan, Cheek, Ing, & Jackson, 2012; Tur, Bibiloni, Sureda, & Pons, 2012). This has prompted
1308
food manufacturers to attempt to fortify their products with ω-3 fatty acids at levels sufficiently
1309
high to have a beneficial biological effect (McClements & Decker, 2000; Waraho, et al., 2011).
1310
However, PUFAs are extremely susceptible to oxidative deterioration, which causes problems
1311
for the long-term storage of these products (Arab-Tehrany, et al., 2012).
SC
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The oxidation of ω-3 fatty acids in emulsions involves a complex series of chemical
EP
1312
RI PT
1293
reactions that results in the production of rancid off-flavors. The control of lipid oxidation often
1314
requires a combination of different approaches to achieve adequate long-term stability, including
1315
control of initial ingredient quality, removal/deactivation of pro-oxidants (such as oxygen or
1316
transition metals), addition of anti-oxidants, and interfacial engineering (McClements & Decker,
1317
2000; Waraho, et al., 2011). A more detailed discussion of the chemical stability of PUFA was
1318
given in an earlier section.
1319 1320
7.1.3.2. Fat Soluble Antioxidants Lipophilic antioxidants can be incorporated into the oil phase of oil-in-water emulsions to
1321
inhibit the oxidation of encapsulated chemically labile substances (such as ω-3 oils or
1322
carotenoids), thereby extending product shelf life (Boon, McClements, Weiss, & Decker, 2010;
AC C
1313
ACCEPTED MANUSCRIPT 46
McClements & Decker, 2000; Waraho, et al., 2011). Alternatively, they can be encapsulated as
1324
functional food components themselves so as to make an “antioxidant” claim on a product label.
1325
Some commonly used lipophilic antioxidants utilized within the food industry include alpha-
1326
tocopherols, ascorbyl palmitate, BHT, BHA, and rosemary extracts (McClements & Decker,
1327
2008). These five molecules act as free radical scavengers and they each have the ability to
1328
absorb two free radicals before becoming inactive. The addition of these antioxidants typically
1329
increases the lag phase of the oxidation reaction.
1330 1331
7.1.3.3. Fat Soluble Vitamins There is considerable interest in incorporating various types of fat-soluble vitamins (A, D, E,
1332
and K) and nutraceuticals (e.g., carotenoids, phytosterols, flavonoids, and curcumin) into food
1333
and beverage products to improve their nutritional value (Sagalowicz & Leser, 2010; Velikov &
1334
Pelan, 2008). In this section, we use recent studies on the incorporation of vitamin E into
1335
emulsions to demonstrate some of the potential benefits and challenges associated with its use in
1336
beverages. The term “vitamin E” actually refers to a group of fat-soluble vitamins that are
1337
widely used as functional ingredients in food, pharmaceutical, and cosmetic preparations, with α-
1338
tocopherol being the most biologically active form (Chiu & Yang, 1992). The major biological
1339
function of Vitamin E appears to be as an oil-soluble antioxidant, although other health benefits
1340
have been claimed, including reducing cardiovascular disease, diabetes, and cancer (Sylvester, et
1341
al., 2011; Traber, Frei, & Beckman, 2008; Weng-Yew & Brown, 2011). For these reasons, there
1342
has been interest in fortifying many foods and beverages with Vitamin E (Sagalowicz & Leser,
1343
2010). Vitamin E is unstable to oxidation and may therefore be lost during processing, storage,
1344
and utilization of commercial products (Gawrysiak-Witulska, Siger, & Nogala-Kalucka, 2009;
1345
Yoon & Choe, 2009). For this reason, vitamin E acetate (rather than vitamin E) is used in many
1346
food and beverage applications since it has a higher oxidative stability. After consumption
1347
vitamin E acetate is broken down to vitamin E in the gastrointestinal tract by the action of
1348
pancreatic esterases (Brisson, et al., 2008). The recommended daily intake (RDI) of Vitamin E
1349
is 15 mg/day (Gonnet, Lethuaut, & Boury, 2010; Institute.of.Medicine, 2000).
1350
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1323
Vitamin E is a highly lipophilic molecule that cannot be directly dispersed into aqueous
1351
solutions (Sagalowicz & Leser, 2010). Instead, it must be incorporated into an appropriate
1352
colloidal delivery system prior to dispersion (Gonnet, et al., 2010). A number of previous
ACCEPTED MANUSCRIPT 47
studies have shown that vitamin E can be successfully incorporated into emulsion-based delivery
1354
systems, such as microemulsions (Chiu & Yang, 1992; Feng, Wang, Zhang, Wang, & Liu,
1355
2009b), nanoemulsions (Hatanaka, et al., 2010; Li, et al., 2011; Shukat, Bourgaux, & Relkin,
1356
2012) and emulsions (Chen & Wagner, 2004; Gonnet, et al., 2010). Encapsulation of vitamin E
1357
has been reported to improve its physicochemical stability during storage and its biological
1358
activity after consumption (Cortesi, Esposito, Luca, & Nastruzzi, 2002; Zuccari, Carosio, Fini,
1359
Montaldo, & Orienti, 2005). Indeed, studies have suggested that the oral bioavailability of
1360
Vitamin E is increased when it is delivered in colloidal form rather than in bulk form (Feng,
1361
Wang, Zhang, Wang, & Liu, 2009a), which would make beverages particularly suitable delivery
1362
systems for this type of nutraceutical lipid.
1363 1364
7.1.3.4. Carotenoids Carotenoids are a diverse group of lipophilic compounds that contribute to the yellow,
1365
orange and red colors of many foods, and that have also been claimed to have some health
1366
benefits. They are polyenes consisting of 3 to 13 conjugated double bonds and in some cases 6
1367
carbon ring structures at one or both ends of the molecule. Carotenoids containing oxygen are
1368
known as xanthophylls (e.g. lutein and zeaxanthin) while those without oxygen are known as
1369
carotenes (e.g., lycopene and β-carotene). The carotenoids have been proposed to exhibit several
1370
potential health benefits: lutein and zeaxanthin may decrease age related macular degeneration
1371
and cataracts (Stringham & Hammond, 2005); lycopene may decrease the risk of prostate cancer
1372
(Basu & Imrhan, 2007). Carotenoids are relatively stable to chemical degradation when they are
1373
present in their natural environment. However, they become highly unstable to degradation once
1374
they are isolated, becoming susceptible to light, oxygen, pH, and temperature (Xianquan, Shi,
1375
Kakuda, & Yueming, 2005). Consequently, dispersion of carotenoids into ingredient systems
1376
can result in their rapid degradation (Heinonen, Haila, Lampi, & Piironen, 1997; Ribeiro, Ax, &
1377
Schubert, 2003). Carotenoids can be degraded by reactions that cause the loss of double bonds
1378
or scission of the molecule. In addition, the double bonds in carotenoids can undergo
1379
isomerization to the cis configuration (Xianquan, et al., 2005). Isomerization reactions might
1380
actually be beneficial since cis isomers of carotenoids such as lycopene are thought to be more
1381
bioavailable and bioactive (Schieber & Carle, 2005). An additional challenge to using
1382
carotenoids as ingredients in functional foods is their high melting point, making them crystalline
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ACCEPTED MANUSCRIPT 48
at food storage and body temperatures. A more detailed discussion of the factors governing the
1384
chemical degradation of carotenoids was given earlier.
1385 1386
7.1.3.5. Phytosterols Phytosterols and phytostanols are a group of bioactive lipids derived from plants that have
1387
been shown to be have biological activity (Chaiyasit, Elias, McClements, & Decker, 2007). The
1388
fortification of foods and beverages with these components has become popular due to their
1389
ability to decrease total and low density lipoprotein cholesterol in humans, which is mainly
1390
attributed to their ability to inhibit the absorption of dietary cholesterol (Ostlund, 2004; Wong,
1391
2001). Intake of 1.6 g phytosterols/day results in an approximately 10% reduction in LDL
1392
cholesterol (Hallikainen, Sarkkinen, & Uusitupa, 2000). The intestinal absorption of
1393
phytosterols is very low and so dietary phytosterols do not have adverse effects on health.
M AN U
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1383
Incorporation of phytosterols and phytostanols into aqueous-based beverages is particularly
1395
difficult due to their low-water solubility, low-oil solubility, high melting point, and tendency to
1396
form crystals. Some of these problems have been overcome by esterification of phytosterols
1397
with polyunsaturated fatty acids. After consumption, the phytosterol esters are hydrolyzed by
1398
enzymes in the gastrointestinal tract to produce free fatty acids and phytosterols. The initial
1399
applications of phytosterols were in high fat foods (e.g. margarine and spreads) where
1400
solublization and dispersion within the oil phase were relatively simple.
1401
introduced into aqueous-based foods, they need to be either suspended or emulsified.
1402
Phytosterols are also susceptible to chemical degradation (oxidation) during storage
1403
(Bortolomeazzi, Cordaro, Pizzale, & Conte, 2003; Cercaci, Rodriguez-Estrada, Lercker, &
1404
Decker, 2007; Dutta, 1997; Lambelet, et al., 2003; Soupas, Juntunen, Lampi, & Piironen, 2004).
1405
At present, it is unclear whether oxidized forms of phytosterols lose their bioactivity or have
1406
some toxicity as has been observed for oxidized cholesterol. Phytosterols and phytostanols must
1407
therefore be encapsulated in delivery systems that will maintain their physical and chemical
1408
stability in beverage emulsions.
1409
7.1.4. Fat Soluble Colorants
1410
For phytosterols to be
AC C
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1394
A number of natural fat-soluble compounds have intense colors and can therefore be used as
1411
pigments in foods, including paprika, lycopene, and β-carotene (Gibbs, Kermasha, Alli, &
1412
Mulligan, 1999; Kandansamy & Somasundaram, 2012; Wrolstad & Culver, 2012). Some of
ACCEPTED MANUSCRIPT 49
these components also have biological activity, and may therefore be used as nutraceutical lipids
1414
also. Many colorants from natural sources are highly unstable and may chemically degrade and
1415
fade rapidly during storage, e.g., the carotenoids discussed earlier (Qian, et al., 2012). It is
1416
therefore important to establish the mechanism of chemical degradation for each type of fat
1417
soluble colorant used in a beverage product and to determine the major factors affecting its
1418
degradation (such as pH, light, oxygen, pro-oxidants) so that an effective delivery system can be
1419
designed to ensure appropriate product shelf-life.
1420
7.2. Emulsifiers
1421
7.2.1. Mechanism of Action
SC
Emulsifiers are surface-active molecules that are used in emulsions to facilitate droplet
M AN U
1422
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1413
breakup within the homogenizer (thereby forming smaller droplets) and to prevent droplet
1424
growth after formation (thereby increasing long-term stability) (McClements, 2005b; Stauffer,
1425
1999). During homogenization, emulsifiers absorb to the oil-water interface, which leads to a
1426
reduction in the interfacial tension thereby facilitating further droplet disruption. Once they have
1427
adsorbed to the droplet surface the emulsifiers should prevent the droplets from coalescing with
1428
each other, which means they must for a protective coating around the oil droplets. An effective
1429
emulsifier system should meet a number of criteria to prevent droplet coalescence within the
1430
homogenizer (Jafari, et al., 2008): (1) the emulsifier molecules must be present at a sufficiently
1431
high concentration to cover all of the oil-water surface formed: (2) the emulsifier molecules must
1432
form a coating around the droplet surfaces faster than the droplets collide with each other; and,
1433
(3) the adsorbed emulsifier molecules must form a coating that prevents the droplets from
1434
coming into close proximity and merging together (McClements, 2005b). There are a number of
1435
food-grade emulsifiers that fulfill these requirements, but they differ considerably in their
1436
effectiveness at forming and stabilizing beverage emulsions.
1437
7.2.2. Factors Influencing Selection
1438
AC C
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1423
Emulsion Formation: Two of the most important factors to consider when selecting an
1439
appropriate emulsifier for forming a beverage emulsion is its ability to produce small droplets
1440
during homogenization, and the minimum amount of emulsifier required to form a stable system
1441
(McClements, 2005b). The performance of an emulsifier can be characterized by plotting the
ACCEPTED MANUSCRIPT 50
mean droplet diameter versus emulsifier concentration under standardized homogenization
1443
conditions, such as homogenizer type, energy input, and duration (McClements, 2007). A
1444
typical dependence of mean droplet diameter on emulsifier concentration for oil-in-water
1445
emulsions produced using a high pressure homogenizer is shown in Figure 19. These curves can
1446
be divided into two regions: an emulsifier-limited region and a homogenizer-limited region.
1447
In the emulsifier-limited region the droplet size decreases with increasing emulsifier
1448
concentration and the size of the droplets is primarily limited by the total amount of emulsifier
1449
present (rather than by homogenization conditions). The minimum size of the stable droplets
1450
that can be produced in this region is given by (McClements, 2007):
1451
SC
6 ⋅ Γ⋅ φ 6 ⋅ Γ⋅ φ = cS c'S (1 − φ )
(8)
M AN U
d min =
RI PT
1442
Here, Γ is the surface load of the emulsifier at saturation (kg m–2), φ is the disperse phase volume fraction (dimensionless), cS is the concentration of emulsifier in the emulsion (kg m–3),
1453
and c′S is the concentration of emulsifier in the aqueous phase (kg m–3). This equation shows
1454
that the droplet size should by linearly related to the inverse of the surfactant concentration in
1455
this region. In this region, the homogenizer initially produces small droplets but there is
1456
insufficient emulsifier present to completely cover them all, and so they tend to coalesce with
1457
each other and form larger droplets under they are covered and protected from coalescence.
1458
TE D
1452
In the homogenizer-limited region there is more than enough emulsifier present to cover all of the droplets formed by the homogenizer, but the disruptive energy generated by the
1460
homogenizer is insufficient to form any smaller droplets. In this region, the droplet size is
1461
therefore limited by the maximum disruptive energy that the homogenizer can generate, rather
1462
than by the emulsifier concentration. In this region, the droplet size could be reduced further by
1463
increasing the homogenization pressure to that smaller droplets are produced.
AC C
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1459
1464
Emulsifiers differ in the abilities to form small droplets and in the amounts required to form
1465
stable emulsions due to differences in their molecular and physicochemical characteristics. The
1466
most important physical characteristics are: (i) the rate at which they absorb to the oil-water
1467
interface during homogenization; (ii) the amount that they reduce the interfacial tension after
1468
absorption; and, (iii) the amount required to cover a unit area of interface (surface load).
1469
Typically, the size of the droplets produced under standardized conditions tends to decrease with
1470
increasing absorption kinetics, decreasing interfacial tension, and decreasing surface load.
ACCEPTED MANUSCRIPT 51
Emulsion Stability: The nature of the emulsifier used to form a beverage emulsion has a
1472
pronounced influence on its physical and chemical stability, and so beverage manufacturers must
1473
carefully select an emulsifier with appropriate stability characteristics. When selecting an
1474
emulsifier it is important to establish the set of conditions that it is expected to work under (e.g.
1475
pH range, temperature range, ionic composition, solvent composition, and mechanical abuse),
1476
which is likely to be highly product specific. Emulsifiers may vary considerably in their ability
1477
to form stable emulsions under different environmental conditions. For example, many globular
1478
proteins form stable emulsions at low ionic strengths and pH values far from their isoelectric
1479
point, but tend to flocculate at high ionic strengths or pH values close to their isoelectric point
1480
(McClements, 2004). In addition, they tend to flocculate when heated above their thermal
1481
denaturation temperature due to an increase in droplet surface hydrophobicity. Emulsions
1482
stabilized by some types of non-ionic surfactants are also unstable at elevated temperatures (Rao
1483
& McClements, 2010), in this case because the rate of droplet coalescence increases when the
1484
surfactant approaches its phase inversion temperature (PIT). Emulsions stabilized by
1485
amphiphilic polysaccharides, such as gum arabic and modified starch, tend to be have good
1486
stability to droplet aggregation across a wide range of pH values, ionic strengths, and
1487
temperatures (Chanamai & McClements, 2002b; Charoen, et al., 2011; Qian, Decker, Xiao, &
1488
McClements, 2011). However, these emulsifiers are often not very efficient at producing small
1489
droplets, or they have to be used at high levels. The stability characteristics of a particular
1490
emulsifier are largely determined by its influence on the colloidal interactions operating between
1491
oil droplets (McClements, 2005b). Globular proteins and ionic surfactants tend to form thin
1492
interfacial layers that mainly stabilize droplets against aggregation through electrostatic
1493
repulsion. On the other hand, polysaccharides and non-ionic surfactants tend to form thick
1494
hydrophilic interfacial layers that mainly stabilize droplets through steric repulsion. Emulsions
1495
stabilized by electrostatic interactions are particularly susceptible to environmental changes that
1496
alter the magnitude or range of the electrical charge, e.g., pH and ionic strength. An example of
1497
differences in the electrical characteristics and aggregation stability of emulsions stabilized by a
1498
protein (whey protein isolate) and two polysaccharides (gum Arabic and modified starch) are
1499
shown in Figure 20. The protein-coated droplets tend to aggregate near their isoelectric point
1500
due to their low net charge, whereas polysaccharide-coated droplets are stable across the whole
1501
pH range due to their thick adsorbed layers.
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Legal and Labeling Requirements: Traditionally, many of the emulsifiers used in food and
1503
beverage applications were synthetic small-molecule surfactants due to their low cost and high
1504
efficiency (Kralova & Sjoblom, 2009). However, there has been increasing interest within the
1505
food and beverage industry in replacing synthetic ingredients with natural alternatives so as to
1506
create product labels that are more consumer-friendly. Natural ingredients often have a higher
1507
price than synthetic ingredients, and poorer functional performances. The change from artificial
1508
to natural food components can also increase manufacturing costs and reduce product shelf-life.
1509
In the following sections, we provide a brief overview of some of the synthetic and natural
1510
emulsifiers that are currently available for use in beverage emulsions.
1511
7.2.3. Ingredient Examples
SC
In principle, many different kinds of emulsifiers can be used to stabilize the oil-in-water
M AN U
1512
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1502
emulsions used in the beverage industry. In practice, the vast majority of commercial beverage
1514
emulsions are made with a limited number of emulsifiers, mainly gum arabic and modified
1515
starch. In this section, we provide an overview of both currently used and potential emulsifiers
1516
that can be utilized in beverage products, and classify them according to the nature of the
1517
molecules involved (i.e., polysaccharides, proteins, or surfactants).
1518 1519
7.2.3.1. Polysaccharide-based emulsifiers Gum Arabic: Currently, gum arabic (also known as gum acacia) is probably the most widely
1520
used emulsifier in the beverage industry to formulate cloud and flavor emulsions (Given, 2009;
1521
Tan, 2004). Gum arabic is a natural exudate harvested from acacia trees in sub-Saharan Africa,
1522
especially Sudan (Buffo, Reineccius, & Oehlert, 2002). Exudates from Acacia seyal and senegal
1523
have both been approved for use in food, but the beverage industry generally acknowledges A.
1524
senegal as the “gold standard” (Reiner, Reineccius, & Peppard, 2010). Acacia senegal consists
1525
of at least three high molecular weight biopolymer fractions. It has been proposed that the
1526
surface-active fraction consists of branched arabinogalactan blocks attached to a polypeptide
1527
backbone (Dickinson, 2003; Jayme, Dunstan, & Gee, 1999; Phillips & Williams, 2000). The
1528
hydrophobic polypeptide chain is believed to anchor the molecules to the droplet surface, while
1529
the hydrophilic carbohydrate blocks extend into the surrounding aqueous phase. The interfacial
1530
layer formed by gum arabic is believed to provide stability against droplet aggregation mainly
1531
through steric repulsion, but with some contribution from electrostatic repulsion also. The
AC C
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1513
ACCEPTED MANUSCRIPT 53
influence of a variety of processing conditions on gum arabic functionality has been examined
1533
(Buffo & Reineccius, 2001; Buffo, Reineccius, & Oehlert, 2001; Buffo, et al., 2002; Chanamai
1534
& McClements, 2002b). For example, it has been shown that gum arabic-coated oil droplets
1535
remain stable to flocculation when exposed to a wide range of environmental conditions, e.g., pH
1536
(3 to 9), ionic strength (0 to 500 mM NaCl; 0 to 25 mM CaCl2) and thermal treatment (30 to 90
1537
ºC) (Chanamai & McClements, 2002b; Charoen, et al., 2011). Nevertheless, gum arabic has a
1538
relatively low affinity for oil-water interfaces compared to most other surface-active
1539
biopolymers, which means that it has to be used at relatively high concentrations to form stable
1540
emulsions. For example, as much as 20 % gum arabic may be required to produce a stable 12
1541
wt% oil-in-water emulsion (Tse & Reineccius, 1995a). For this reason, its application as an
1542
emulsifier is restricted to products that have relatively low droplet concentrations, e.g., beverage
1543
emulsions. In addition, frequent problems have been reported in obtaining reliable sources of
1544
consistently high quality gum arabic, which has led many food scientists to investigate
1545
alternative sources of biopolymer emulsifiers for use in beverages, such as modified starch and
1546
various proteins (Chanamai & McClements, 2002a; Charoen, et al., 2011; Garti, 1999; Kim,
1547
Morr, & Schenz, 1996; Qian, et al., 2011; Tan, 1998; Trubiano, 1995a). Gum arabic has a high
1548
water-solubility and a relatively low solution viscosity compared to other gums, which facilitates
1549
its application as an emulsifier.
SC
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1550
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1532
Modified Starch: Natural (unmodified) starches have poor surface activity due to their hydrophilic glucose backbones. Nevertheless, starches can be chemically modified to make
1552
them effective emulsifiers by attaching non-polar chains along their backbones (Trubiano,
1553
1995b). These types of modified starch are widely used as emulsifiers in the beverage industry.
1554
One of the most commonly used modified starches is an octenyl succinate derivative of waxy-
1555
maize (Stauffer, 1999; Trubiano, 1995b). It consists primarily of amylopectin that has been
1556
chemically modified to contain a side-group that is anionic and non-polar. These side-groups
1557
anchor the molecule to the oil droplet surface, while the hydrophilic starch chains protrude into
1558
the aqueous phase and protect droplets against aggregation through steric repulsion. Because the
1559
dominant stabilizing mechanism is steric repulsion, emulsions stabilized by modified starch are
1560
resistant to changes in pH (3 to 9), ionic strength (0 to 100 mM NaCl; 0 to 25 mM CaCl2) and
1561
temperature (30 to 90 ºC) (Chanamai & McClements, 2002b; Charoen, et al., 2011; Qian, et al.,
1562
2011). Like gum arabic, modified starch has a relatively low interfacial activity (compared to
AC C
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1551
ACCEPTED MANUSCRIPT 54
proteins or surfactants), and so a large excess must be added to ensure that all the droplet
1564
surfaces are adequately coated. For example, it is had been recommended that a 1:1 mass ratio
1565
of modified starch to oil was required to produce a stable oil-in-water emulsion (Tse &
1566
Reineccius, 1995a). Nevertheless, there have recently been technical advances in the
1567
manufacture of modified starches that have led to emulsifiers that can be used at much lower
1568
levels (1:5 mass ratio) to form stable emulsions with small droplet sizes (Charoen, et al., 2011;
1569
Qian, et al., 2011). Studies have shown that if the concentration of non-adsorbed modified starch
1570
in the aqueous phase surrounding the oil droplets is too high, it may promote depletion
1571
flocculation (Chanamai & McClements, 2001). Modified starches usually come in powdered or
1572
granular forms that are easily dispersible in cold water.
SC
Other polysaccharides: A number of other surface-active polysaccharides have been studied
M AN U
1573
RI PT
1563
1574
for their potential use as emulsifiers in oil-in-water emulsions. Although these polysaccharides
1575
are not widely used in the beverage industry at present they may find some applications in the
1576
future. There has been debate about the molecular origin of the surface activity of these
1577
polysaccharides, and about whether their ability to stabilize emulsions is primarily due to their
1578
surface activity or their ability to thicken the aqueous phase (Dickinson, 2003). Cellulose is not a good emulsifier in its natural state because it forms strong intermolecular
TE D
1579
hydrogen bonds, which make it insoluble in water. Nevertheless, cellulose can be physically,
1581
chemically, or enzymatically modified to produce food-grade ingredients that have interfacial
1582
activity and can be used as emulsifiers (Garti, et al., 1993), such as methyl cellulose (MC),
1583
hydroxypropyl cellulose (HPC), and methyl hydroxypropyl cellulose (MHPC). These
1584
ingredients are all non-ionic polymers that are soluble in cold water, but tend to become
1585
insoluble when the solution is heated above a critical temperature (around 50 – 90 ºC). They
1586
have good stability to pH (2 – 11), salt and freeze-thaw cycling, which may be beneficial in a
1587
number of beverage emulsion applications.
AC C
EP
1580
1588
Certain types of pectin have been shown to be surface active molecules that can able to
1589
stabilize oil-in-water emulsions, e.g., beet pectin (Jung & Wicker, 2012; Nakauma, et al., 2008).
1590
The origin of this effect is believed to be due to the pectin having some protein molecules
1591
associated with it. The amphiphilic protein portion anchors the pectin molecules to the oil phase,
1592
while the hydrophilic polysaccharide chains protrude into the aqueous phase thereby providing
1593
some steric repulsion. In addition, pectin molecules usually have a relatively high negative
ACCEPTED MANUSCRIPT 55
1594
charge because of the presence of carboxylic groups (pKa = 3.5), and therefore they can also
1595
generate an electrostatic repulsion (Nakauma, et al., 2008).
1596
Soy soluble polysaccharide (SSPS) is a soluble, acidic polysaccharide extracted from okara that has a molecular structure that closely resembles pectin (Nakamura, Takahashi, Yoshida,
1598
Maeda, & Corredig, 2004; Nakamura, Yoshida, Maeda, Furuta, & Corredig, 2004). Similar to
1599
gum Arabic and beet pectin, SSPS is believed to have a protein backbone that anchors the
1600
molecule to the oil phase and hydrophilic anionic polysaccharide chains that protrude into the
1601
aqueous phase and provide stabilization through steric and electrostatic repulsion (Nakauma, et
1602
al., 2008).
SC
1603
RI PT
1597
Corn fiber gum (CFG) is a by-product of the milling of corn (Yadav, Johnston, Hotchkiss, & Hicks, 2007). Preliminary studies have shown that CFG can facilitate the formation and stability
1605
of oil-in-water emulsions (Yadav, et al., 2007). The surface activity of this polysaccharide may
1606
also be due to the presence of small amounts of protein or lipids that are associated with it.
1607
M AN U
1604
Research is continuing in this area, and it is likely that a variety of other polysaccharidebased emulsifiers will be identified that may be suitable for stabilization of beverage emulsions.
1609 1610
7.2.3.2. Protein-based emulsifiers Proteins are widely used in the food industry to stabilize various commercial food and
1611
beverage products, but they are not commonly used to stabilize soft drinks. Surface-active
1612
proteins can be isolated from various sources, including milk, meat, fish, and plants, but the most
1613
widely used in the food industry are those derived from bovine milk: casein and whey proteins.
1614
The interfacial coatings formed by proteins are usually relatively thin and electrically charged,
1615
and hence the major mechanism preventing droplet flocculation in protein-stabilized emulsions
1616
is electrostatic repulsion (rather than steric repulsion). Consequently, protein-stabilized
1617
emulsions are particularly sensitive to pH and ionic strength effects (Figures 20 and 21), and
1618
will tend to flocculate at pH values close to their isoelectric points or when the ionic strength
1619
exceeds a certain level (Demetriades, et al., 1997a, 1997b; Dickinson, 2010; McClements, 2004).
1620
Proteins can typically be used at much lower levels that amphiphilic polysaccharides to
1621
stabilize oil-in-water emulsions. For example, as much as 20 % gum arabic may be required to
1622
produce a stable 12.5 wt% oil-in-water emulsion, whereas less than 1% whey protein can be used
1623
(Chanamai & McClements, 2002a; Tse & Reineccius, 1995b). On the other hand, globular
AC C
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1608
ACCEPTED MANUSCRIPT 56
protein stabilized emulsions are much more susceptible to pH, ionic strength, and thermal
1625
processing than polysaccharide stabilized emulsions as shown in Figure 20 (Chanamai &
1626
McClements, 2002a). The stability of protein-stabilized emulsions to pH, salts, and thermal
1627
processing can often be improved by mixing them with ionic polysaccharides either before or
1628
after homogenization, such as gum arabic, pectin, alginate, and carrageenan (Guzey &
1629
McClements, 2006, 2007; Harnsilawat, Pongsawatmanit, & McClements, 2006; Klein, et al.,
1630
2010). These ionic polysaccharides form an interfacial complex with the adsorbed protein
1631
molecules, which increases the steric and electrostatic repulsion between the droplets
1632
(Dickinson, 2011).
SC
1633
RI PT
1624
A number of other methods have also been developed to improve the emulsifying properties of protein ingredients, including limited hydrolysis to form peptides, modification of protein
1635
structure by chemical, physical, enzymatic or genetic means, and blending of the proteins with
1636
other ingredients, although not all of these methods are legally permitted at present.
1637
M AN U
1634
Globular Proteins: Native globular proteins tend to have fairly compact spheroid structures with the non-polar amino acids contained within their hydrophobic interiors (away from water)
1639
and the polar amino acids located at the exterior (in contact with water). There are many sources
1640
of globular proteins in foods that are surface-active and capable of forming and stabilizing
1641
emulsions, such as those from whey, soy, egg, pea, and various other plants (McClements,
1642
2005b). Globular proteins tend to form relatively thin interfacial coatings around fat droplets,
1643
and therefore primarily stabilize emulsions against flocculation through electrostatic repulsion
1644
(McClements, 2004). They are therefore particularly sensitive to droplet flocculation at pH
1645
values near the isoelectric point (where the droplet charge is low) or at high salt concentrations
1646
(where the droplet charge is screened). Emulsions stabilized by globular proteins are also
1647
particularly sensitive to heat treatments, because these proteins unfold when the temperature
1648
exceeds the thermal denaturation temperature thereby exposing reactive non-polar and sulfhydryl
1649
groups (Kim, Decker, & McClements, 2002). These reactive groups increase the attractive
1650
interactions between droplets, which may lead to droplet flocculation through increased
1651
hydrophobic attraction and disulfide bond formation.
AC C
EP
TE D
1638
1652
The most common source of globular proteins in the food industry is the whey protein
1653
derived from bovine milk. A number of different whey protein ingredients are available for use
1654
in the food industry as emulsifiers, including whey protein concentrate (WPC), whey protein
ACCEPTED MANUSCRIPT 57
isolate (WPI), and highly purified protein fractions such as β-lactoglobulin and α-lactalbumin.
1656
These ingredients normally come as white powders that are dissolved in water prior to
1657
utilization. WPC and WPI are commonly used as emulsifiers in commercial food and beverage
1658
products, whereas β-lactoglobulin and α-lactalbumin are more frequently used in fundamental
1659
studies of protein functionality in research laboratories. Whey protein actually contains a
1660
mixture of globular proteins: ≈ 55% β-lactoglobulin; ≈ 24% α-lactalbumin; ≈ 15%
1661
immunoglobulins; and ≈ 5% serum albumin (McClements, 2005b). The isoelectric point of the
1662
major whey proteins is around pH 5, and therefore droplets stabilized by whey proteins tend to
1663
flocculate around this pH, and especially between about pH 4 to 6 (Figure 20). The thermal
1664
denaturation temperature of whey proteins is around 70 to 80 ºC, and so they are prone to droplet
1665
aggregation when heated above this temperature, especially in the presence of salts that screen
1666
electrostatic interactions (Kim, et al., 2002).
SC
M AN U
1667
RI PT
1655
Flexible Proteins: Another group of protein-based emulsifiers commonly used in foods can be classified as flexible proteins (Dickinson, 1992a). These molecules tend to have a more
1669
flexible random coil type structure (although they often retain some secondary structure). The
1670
main sources of flexible proteins are gelatin from fish and animals, and caseins from bovine
1671
milk. Gelatin is a relatively high molecular weight protein derived from fish, porcine, or bovine
1672
collagen (McClements, 2005b). It is typically prepared by disrupting the native structure of
1673
collagen by boiling it in the presence of either acid (Type A gelatin) or base (Type B gelatin).
1674
The isoelectric point of Type A gelatin (∼7-9) tends be higher than that of Type B gelatin (∼5).
1675
Gelatin exists as a random coil molecule at relatively high temperatures, but undergoes a coil-to-
1676
helix transition upon cooling below a critical temperature, which is about 10 to 25 ºC for animal
1677
gelatin and about 0 to 5 ºC for fish gelatin. Gelatin has been shown to be surface-active and
1678
capable of acting as an emulsifier in oil-in-water emulsions. Nevertheless, it often produces
1679
relatively large droplet sizes when used in isolation, and therefore it may be utilized with other
1680
ingredients to improve its effectiveness at forming and stabilizing emulsions.
AC C
EP
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1668
1681
There are various casein-based emulsifiers that can be purchased commercially with
1682
different compositions and functional properties, such as sodium caseinate, calcium caseinate,
1683
and purified protein fractions, such as a β-casein. There are four main protein fractions in
1684
casein: αS1 (∼ 44%), αS2 (∼ 11%), β (∼ 32%) and κ (∼ 11%). Casein-coated droplets have been
1685
shown to be unstable to droplet flocculation at pH values (3.5 – 5.3) close to the protein’s
ACCEPTED MANUSCRIPT 58
isoelectric point. Caseinate-stabilized emulsions tend to be more stable to heating than whey
1687
protein-stabilized emulsions, presumably because the relatively flexible casein molecules do not
1688
undergo appreciable heat-induced conformational changes like globular proteins do.
1689 1690
7.2.3.3. Small-molecule Surfactants In general, small-molecule surfactants consist of a polar head-group and a non-polar tail
1691
group (Kralova & Sjoblom, 2009; McClements, 2005b; Stauffer, 1999). The head group may be
1692
non-ionic, anionic or cationic, while the tail group may vary in the number, length, and degree of
1693
unsaturation of the chains. Many surfactants can be used to fabricate beverage emulsions using
1694
either low or high energy approaches, which is different from amphiphilic biopolymers that can
1695
only be used to fabricate them using high energy methods. In addition, many surfactants are able
1696
to form smaller oil droplets than amphiphilic biopolymers using high energy methods because
1697
they can adsorb more quickly to the oil-water interfaces during homogenization and lower the
1698
interfacial tension more. Surfactants are also able to stabilize emulsions at lower concentrations
1699
than amphiphilic biopolymers. Despite these advantages, the food industry has been trying to
1700
replace synthetic small molecule surfactants with emulsifiers that are natural and more label
1701
friendly due to consumer demands. Recently, some natural small molecule surfactants have
1702
become commercially available, such as quillaja saponin. A summary of some small-molecule
1703
surfactants that could be utilized in beverage emulsions is given in Table 4.
SC
M AN U
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1704
RI PT
1686
Tweens: Tweens (also sold under the name polysorbates) are synthetic non-ionic surfactants that consist of a non-polar fatty acid group esterified to a polar polyoxyethylene sorbitan group.
1706
The nature of the fatty acid and polyoxyethylene chains determines the nomenclature and
1707
functional properties of the surfactant. Tween 20 (monolaurate), Tween 40 (monopalmitate),
1708
Tween 60 (monostearate), and Tween 80 (monooleate) are hydrophilic molecules with relatively
1709
high HLB values (14.9 and 16). They are therefore predominantly water-soluble, form micelles
1710
in aqueous solutions, and stabilize oil-in-water emulsions. Since Tweens are nonionic they tend
1711
to produce droplets that are stable to aggregation over a wide range of pH and ionic strength
1712
values. However, Tween-coated droplets may become unstable to coalescence at elevated
1713
temperatures close to the phase inversion temperature.
1714 1715
AC C
EP
1705
Sugar esters: Sugar esters are synthetic nonionic surfactants that consist of a polar sugar esterified to a fatty acid chain. There has been growing interest in the utilization of sugar esters
ACCEPTED MANUSCRIPT 59
as emulsifiers within the food and beverage industry (Szuts, Budai-Szucs, Eros, Otomo, &
1717
Szabó-Révész, 2010), which can be attributed to their desirable sensory attributes, low toxicity,
1718
and high biodegradability (Sadtler, Guely, Marchal, & Choplin, 2004). In addition, sugar esters
1719
are produced from natural products, such as sucrose and vegetable oil, and are therefore often
1720
perceived as being more label-friendly than petrochemical-based surfactants (Huck-Iriart,
1721
Candal, & Herrera, 2009). Sucrose fatty acid esters are non-ionic surfactants that have a sucrose
1722
moiety as a hydrophilic head group and one or more fatty acids as lipophilic tail groups. Sucrose
1723
monoesters have a single fatty acid attached to the sucrose molecule, which means they are
1724
relatively hydrophilic, water-soluble, have high HLB numbers, and can stabilize O/W emulsions
1725
(Garti, Clement, Leser, Aserin, & Fanun, 1999; Glatter, et al., 2001). A number of researchers
1726
have previously examined the structure, properties, and functionality of sucrose monoesters
1727
(Fanun, 2010; Garti, Aserin, & Fanun, 2000; Garti, Clement, Fanun, & Leser, 2000; Garti, et al.,
1728
1999). These studies have shown that sucrose monoesters can form a range of different colloidal
1729
structures depending on system composition and temperature, e.g., microemulsions,
1730
nanoemulsions or emulsions. Emulsions formed using sucrose monopalmitate (SMP) as the only
1731
emulsifier are highly unstable to aggregation when stored at acidic pH values (pH 3.5), which
1732
has been attributed to a reduction of droplet charge (Rao & McClements, 2011). However,
1733
emulsions that are stable across a wide range of pH can be produced by mixing SMP with a co-
1734
surfactant (such as lecithin) that increases the electrostatic repulsion between droplets at low pH
1735
(Choi, et al., 2011).
TE D
M AN U
SC
RI PT
1716
Quillaja Saponin: Quillaja saponin is a natural surfactant extracted from the bark of the
1737
Quillaja saponaria Molina tree. It has been found to contain surface active components that are
1738
capable of forming surfactant micelles and stabilizing oil-in-water emulsions (Mitra & Dungan,
1739
1997; Waller & Yamasaki, 1996a, 1996b; Yang, Leser, Sher, & McClements, 2013; Yang &
1740
McClements, 2013). The major components within this extract have been reported to be saponins
1741
(van Setten, ten Hove, Wiertz, Kamerling, & van de Werken, 1998; van Setten, van de Werken,
1742
Zomer, & Kersten, 1995) , which are high molecular weight glycosides consisting of a sugar
1743
moiety attached to a triterpene or a steroid aglycone (Hostettmann & Marston, 1995). The
1744
saponins are surface active substances because they contain both hydrophilic regions (such as
1745
rhamnose, xylose, arabinose, galactose, fucose, and glucuronic acid) and hydrophobic regions
1746
(such as quillaic acid and gypsogenic acid) on the same molecule (Mitra & Dungan, 1997; Sidhu
AC C
EP
1736
ACCEPTED MANUSCRIPT 60
1747
& Oakenfull, 1986). Recent experiments have shown that it can be used to form emulsions
1748
containing small droplets that are stable to changes in pH, ionic strength, and temperature (Yang,
1749
et al., 2013; Yang & McClements, 2013).
1750
A food ingredient based on the quillaja saponin extract has recently been produced by the National Starch Company (Bridgewater, NJ) under the trade name Q-Naturale®. This ingredient
1752
was released in 2008 and has been marketed as a replacement for gum acacia in beverage
1753
emulsions. According to the suppliers, Q-Naturale® can be used at a low level, has high oil
1754
loading, and is stable in some forms of alcoholic beverages, unlike most forms of gum acacia or
1755
modified food starch. It is also organic certified and is stable in products stored at ambient or
1756
cold temperatures for up to a year. The ingredient also comes in a liquid form, so that dissolution
1757
and hydration steps are unnecessary.
SC
M AN U
1758
RI PT
1751
Aside from its use as a beverage emulsion emulsifier, quillaja extract has also been used as a foam-stabilizing agent in beverages. The extract is added to the aqueous phase of a beverage to
1760
stabilize foam that forms at the neck of the beverage. This foam is considered a positive attribute
1761
in root beer and some citrus juices in various parts of the world.
1762 1763
7.2.3.4. Mixed Emulsifier Systems There has been increasing interest in using mixed emulsifier systems with the aim of
1764
producing more label-friendly food ingredients. Proteins tend to be better at producing small
1765
emulsion droplets when used at low concentrations than polysaccharides, whereas
1766
polysaccharides tend to be better at producing emulsions that are more stable to environmental
1767
conditions than proteins, e.g., pH, ionic strength, temperature, freeze-thaw cycling (McClements
1768
2004). The beneficial attributes of these two kinds of biopolymer can be combined to produce
1769
small emulsion droplets with good environmental stability. A number of researchers have shown
1770
that protein-polysaccharide complexes may have better emulsifying properties than either of the
1771
biopolymers used in isolation (Dickinson, 2003, 2011; Guzey & McClements, 2006). These
1772
complexes may be held together either by physical or covalent interactions, and may be formed
1773
either before or after homogenization (McClements 2004). Ingredients based on protein-
1774
polysaccharide interactions will have to be legally acceptable, economically viable and show
1775
benefits over existing ingredients before they find widespread utilization in the food industry. It
AC C
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1759
ACCEPTED MANUSCRIPT 61
1776
should be noted that gum arabic is actually a naturally occurring protein-polysaccharide complex
1777
that is already widely used in the food industry as an emulsifier. As mentioned earlier, sucrose monoesters produce emulsions that are not very stable under
1779
acidic conditions when they are used in isolation, but their stability can be greatly improved by
1780
combining them with co-surfactants, such as lysolecithin.
1781
7.3. Weighting Agents
1782
7.3.1. Mechanism of Action
Weighting agents are additives incorporated into the oil phase of certain types of beverage
SC
1783
RI PT
1778
emulsions to inhibit gravitational separation of the oil droplets (McClements, 2005b). As
1785
discussed earlier, the creaming rate of a droplet within an emulsion is given by Stokes’ law
1786
(McClements, 2005a). This equation predicts that the rate of creaming is proportional to the
1787
droplet radius squared (r2), the density contrast between the two phases (∆ρ), and the reciprocal
1788
of the aqueous phase viscosity (η0) (Equation 3). If ∆ρ is positive the particles move downwards
1789
(sediment), but if ∆ρ is negative they move upwards (creaming). The stability of a beverage
1790
emulsion to gravitational separation can therefore be improved by ensuring that the density of the
1791
oil droplets is similar to that of the surrounding aqueous phase. The densities of some of the
1792
major components found in beverage emulsions are listed in Table 3. The densities of flavor
1793
oils and vegetable oils are considerably lower than those of water and aqueous sugar solutions.
1794
Consequently, droplets containing these oils tend to move upward during storage leading to
1795
“ringing”, i.e., the formation of a visible ring of fat droplets at the surface of the product. The
1796
creaming rate can be reduced by decreasing the density contrast between the oil droplets and the
1797
surrounding aqueous phase (Equation 3). The most common means of increasing the density of
1798
the oil phase so that it matches that of the aqueous phase is to add weighting agents, which are
1799
hydrophobic components that have a density that is considerably greater than water (Table 3).
1800
7.3.2. Factors Influencing Selection
AC C
EP
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M AN U
1784
1801
A number of different weighting agents are available for utilization within commercial
1802
beverage products, such as brominated vegetable oil (BVO), SAIB (Sucrose acetate isobutyrate),
1803
ester gum, or dammar gum (Table 3). There are a number of different factors that will
ACCEPTED MANUSCRIPT 62
1804
determine which of these different components is suitable for application within a specific
1805
beverage product. Physicochemical factors: The bulk physicochemical properties of commercial weighting
1807
agents vary considerably, which alters their ease of use and functional properties. The amount of
1808
a weighting agent that is required to match the density of the oil and aqueous phases depends on
1809
its density: the higher the density, the smaller the amount needed. Weighting agents also vary in
1810
their rheological and solubility properties, which will influence the preparation of the oil phase
1811
and the formation of stable emulsions by homogenization. Many beverage emulsions are
1812
sweetened by adding sugars to the aqueous phase. Sugars increase the aqueous phase density
1813
and would therefore be expected to increase the density contrast between the oil and aqueous
1814
phases, which should accelerate creaming. However, sugars also increase the viscosity of the
1815
continuous phase. These two effects largely cancel each other out, so that the creaming rate in
1816
sugar solutions is not much greater than in pure water (Chanamai & McClements, 2000).
SC
M AN U
1817
RI PT
1806
As mentioned previously, the overall density of an emulsion droplet actually depends on the density and volume of the oil phase and the interfacial layer. The interfacial layer is usually
1819
made up of emulsifiers (such as surfactants, phospholipids, proteins, or polysaccharides) that
1820
have a higher density than both the oil and the water phases. When the thickness of the
1821
interfacial layer is appreciable compared to the dimensions of the oil core, then the overall
1822
density of the emulsion droplet (oil core + interfacial layer) may become similar to that of the
1823
aqueous phase, and so the adsorbed emulsifier acts as a weighting agent itself (McClements,
1824
2011).
EP
1825
TE D
1818
Legal and Labeling Factors: There are legal limits on the amount of each type of weighting agent that can be used in beverages, which must be adhered to when formulating a project.
1827
These legal limits vary from country to country and therefore a manufacturer must be aware of
1828
the appropriate legislation in each country the product will be marketed in. There are also
1829
differences in the “label-friendliness” of different weighting agents. Dammar and ester gum can
1830
be considered to be natural ingredients, whereas BVO and SAIB are synthetic ingredients.
AC C
1826
1831
Brominated vegetable oil (BVO) and ester gum are the two most commonly used weighting
1832
agents in beverage emulsions. However, many countries have restricted the amount of weighting
1833
agents that can be found in the final product. For example, the United States restricts BVO and
1834
ester gum’s use to 15 and 100 ppm per serving, respectively. This low concentration of allowable
ACCEPTED MANUSCRIPT 63
weighting agents restricts the type of products that BVO and ester gum can be used in. Beverage
1836
emulsions are one of the few products where these weighting agents can be used to reduce the
1837
creaming rate, because the oil phase content is so low (<0.1%). It is also possible to blend two or
1838
more types of weighting agents together to achieve density matching but remain below their
1839
individual restriction levels.
1840
7.3.3. Ingredient Examples
1841 1842
7.3.3.1. Brominated Vegetable Oil Brominated vegetable oil (BVO) is made when bromine is added to the double bonds of the
1843
triacylglycerol molecules in corn, soybean, cotton seed, or olive oil. It typically has a density
1844
between 1240 and 1330 kg m-3, and a permitted usage level of 15 ppm per serving in the United
1845
States. Nevertheless, there has recently been consumer concern about the presence of BVO in
1846
soft drinks, which has led some major beverage manufacturers to replace it with more natural
1847
alternatives. In 2013, a leading beverage manufacturer removed BVO from a major product line
1848
after receiving consumer complaints and petitions against its use. Even though BVO is permitted
1849
for use in the United States, it has been banned from use in beverages in the European Union and
1850
Asia.
1851 1852
7.3.3.2. Ester Gum Ester gum is a hydrophobic polymer made when glycerol is esterified to gum rosin
1853
harvested from trees (such as pine trees). It is normally supplied as a crystalline solid that can be
1854
incorporated into the oil phase. Ester gum may not be considered to be a natural food ingredient
1855
due to the esterification step used in its preparation,. Nevertheless, it is derived from natural
1856
components (gum rosin) and non-animal glycerol, and the name “ester gum” may appear to be
1857
more label friendly to consumers than BVO or SAIB. Ester gum has a density of around 1080
1858
kg m-3 and a permitted usage level of 100 ppm per serving in the United States (FDA, USA). It
1859
performs similarly to BVO but a greater concentration of ester gum has to be added to the oil
1860
phase in order to raise the density (Chanamai & McClements, 2000). Aside from its traditional
1861
use as a weighting agent, recent studies have shown that ester gum also retards droplet growth in
1862
model flavor emulsions through Ostwald ripening, i.e., it acts like a ripening inhibitor (Lim, et
1863
al., 2011). This behavior would probably be demonstrated by all weighting agents because they
1864
are highly hydrophobic materials with low water-solubilities.
AC C
EP
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M AN U
SC
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1835
ACCEPTED MANUSCRIPT 64
7.3.3.3. Sucrose Acetate Isobutyrate Sucrose acetate isobutyrate (SAIB) is a synthetic weighting agent produced by the
1867
esterification of sucrose with acetic and isobutyric anhydrides. SAIB is usually supplied in the
1868
form of a high viscosity transparent liquid that can be mixed with the oil phase prior to
1869
homogenization. It has a molecular weight of ≈ 845 g mol-1, a refractive index of ≈ 1.454, and a
1870
density of ≈ 1146 kg m-3. SAIB also has good stability to chemical degradation due to lipid
1871
oxidation. The level of SAIB in beverages is currently restricted to 300 ppm per serving in the
1872
United States (FDA, USA).
1873 1874
7.3.3.4. Dammar Gum Dammar gum is a natural weighting agent that is isolated from an exudate of
1875
Caesalpinaceae and Dipterocarpaceae shrubs. Of the four weighting agents mentioned in this
1876
review, dammar gum has the lowest density at 1060 kg m-3 (Table 3). Dammar gum is approved
1877
for use in several countries, but it is not currently approved for use in the USA and it so does not
1878
have GRAS status.
1879
7.4. Ripening Inhibitors
SC
M AN U
As discussed previously, a major form of physical instability in many beverage emulsions is
TE D
1880
RI PT
1865 1866
Ostwald ripening, which is the process whereby the mean droplet size grows during storage due
1882
to diffusion of oil molecules from small droplets to large droplets (Kabalnov, 2001; Kabalnov &
1883
Shchukin, 1992). Once steady state conditions have been achieved, the increase in mean droplet
1884
diameter over time can be described by Equation 6 given earlier (Kabalnov & Shchukin, 1992).
1885
This equation predicts that the Ostwald ripening rate increases as the solubility of the oil phase in
1886
the aqueous phase increases. Many flavor oils have a relatively high water-solubility and are
1887
therefore particularly prone to droplet growth through this mechanism (Rao & McClements,
1888
2012b). On the other hand, triacylglycerol oils (such as corn, sunflower, or canola oils) contain
1889
large non-polar triacylglycerol molecules with relatively low water-solubilities, and therefore
1890
cloud emulsions prepared from these types of oil are not prone to Ostwald ripening (Coupland,
1891
Weiss, Lovy, & McClements, 1996).
1892
AC C
EP
1881
An effective means of inhibiting or preventing Ostwald ripening in an emulsion is to add a
1893
hydrophobic component with a very low water-solubility to the oil phase prior to
1894
homogenization (Kabalnov, Pertzov, & Shchukin, 1987). This hydrophobic compound is
ACCEPTED MANUSCRIPT 65
referred to as a “ripening inhibitor” due to its ability to slow droplet growth through this
1896
mechanism (Lim, et al., 2011; McClements, et al., 2012). The ability of low water-solubility
1897
compounds to retard Ostwald ripening in emulsions can be attributed to an entropy of mixing
1898
effect (Kabalnov, et al., 1987), which is discussed in more detail in Section 5.1.3.
1899
RI PT
1895
Recent studies have shown that Ostwald ripening may be inhibited in flavor oil emulsions by adding long chain triglycerides (such as corn oil) (McClements, et al., 2012) or by adding
1901
weighting agents (such as ester gum) (Lim, et al., 2011). It has also been proposed that some of
1902
the water-insoluble components within flavor oils themselves may act as ripening inhibitors,
1903
which may account for the better stability of high-fold lemon oils (e.g., 10×) to Ostwald ripening
1904
than low-fold lemon oils (e.g., 1× and 3×) (Rao & McClements, 2012c).
1905
7.5. Thickening Agents
1906
7.5.1. Mechanism of Action
M AN U
1907
SC
1900
Thickening agents are ingredients that are added to beverage emulsions to increase the viscosity of the aqueous phase, and therefore retard droplet creaming and alter product
1909
mouthfeel. Thickening agents are usually hydrophilic biopolymers that dissolve in the aqueous
1910
phase and occupy a volume that is much larger than the volume of the polymer chain itself
1911
(McClements, 2005b). The effectiveness of a thickening agent at increasing the viscosity of an
1912
aqueous solution is largely determined by its molecular characteristics (e.g., molecular weight,
1913
conformation, and interactions). To a first approximation the viscosity of a dilute aqueous
1914
solution containing a hydrophilic biopolymer can be described by Einstein’s equation:
1917 1918
EP
1916
η = η0 (1 + 2.5 Rvφ)
(9)
AC C
1915
TE D
1908
where φ is the volume fraction of the biopolymer chain, and Rv is the volume ratio (Rv =
1919
Vsphere / Vpolymer), where Vsphere is the effective volume that the biopolymer occupies in solution
1920
(which includes the polymer chain and any trapped solvent), and Vpolymer is the volume occupied
1921
by the polymer chain itself. The higher the value of the volume ratio, the more effective is the
1922
polymer at increasing the viscosity of a solution. Typically, Rv increases as the molar mass,
1923
extension, and self-association of biopolymer molecules increases. Thus, xanthan gum is a
1924
particularly effective thickening agent because it has a high molecular weight and is highly
ACCEPTED MANUSCRIPT 66
extended in aqueous solutions and therefore has a large Rv value. Most thickening agents used in
1926
beverages are long-chain hydrophilic polysaccharides, such as xanthan, locust bean gum, pectin,
1927
or carrageenan. It should be noted that thickening agents can actually promote emulsion
1928
instability under certain conditions, due to their ability to induce bridging or depletion
1929
flocculation. Bridging flocculation can occur when polymer molecules are attracted to the
1930
surfaces of oil droplets, whereas depletion flocculation occurs when they polymer molecules are
1931
dispersed in the continuous phase and their concentration exceeds a particular value.
1932
7.5.2. Factors Influencing Selection
SC
RI PT
1925
There are a number of factors that must be considered when selecting a suitable thickening
1934
agent for use in beverage emulsions. It is important to establish the solution and environmental
1935
conditions under which the thickening agent must function. This usually requires knowledge of
1936
the physicochemical properties of the biopolymer molecules involved, such as helix-coil
1937
transition temperatures (for carrageenan, alginate, pectin), electrical properties (charge groups
1938
and pKa values), hydrophobicity, ion sensitivity, and chemical/enzymatic stability (Whistler &
1939
BeMiller, 1997).
1940
of the biopolymer molecules used (ζ-potential versus pH profile), since electrostatic interactions
1941
influence ingredient interactions, emulsion stability, and mouthfeel. The electrical charge on
1942
biopolymers depends on the nature of any ionic groups along the chain background, as well as
1943
solution conditions (pH and ionic composition). Some biopolymers are neutral (e.g., starch,
1944
cellulose, LBG), some are anionic (e.g., alginate, carrageenan, xanthan), some are cationic (e.g.,
1945
chitosan and polylysine), and some are amphoteric (e.g., gelatin). The magnitude of the
1946
electrical charge on ionic biopolymers depends on the pH relative to the pKa value of the ionic
1947
groups. Anionic polysaccharides tend to be neutral at pH values sufficiently below their pKa
1948
value but negative above, whereas cationic polysaccharides tend to be neutral at pH values
1949
sufficiently above their pKa value but positive below. Proteins are positive below their
1950
isoelectric point, and negative above it (Figure 20). The most common charged groups on
1951
polysaccharide thickening agents are sulfate groups (e.g., carrageenan), carboxyl groups (e.g.,
1952
pectin, alginate, xanthan, carboxymethylcellulose) and amino groups (e.g., chitosan): -SO4H ↔ -
1953
SO4- (pKa ≈ 2); -CO2H ↔ -CO2- (pKa ≈ 3.5); -NH3+ ↔ - NH2 (pKa ≈ 6.5). The most common
1954
charged groups on proteins are carboxyl and amino groups.
M AN U
1933
AC C
EP
TE D
For some applications, it is important to establish the electrical characteristics
ACCEPTED MANUSCRIPT 67
The electrical characteristics of thickening agents in aqueous solutions may also be altered
1956
by interactions with other ionic species in their environment, such as mineral ions, surfactants,
1957
and other biopolymers. Monovalent or multivalent ions such as sodium, potassium, or calcium
1958
may bind to oppositely charged groups on biopolymer chains, altering their overall charge
1959
characteristics. In addition, knowledge of the type of environmental and solution conditions
1960
present within a particular food is often important for selecting the most appropriate thickening
1961
agent. For example, pectin can start to depolymerize when exposed to neutral or alkaline
1962
conditions as a result of a base-catalyzed β-elimination reaction that breaks down its sugar chain.
1963
Over time, this breakdown can result in a decline in viscosity and loss of texture (Sila et al.,
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2009).
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7.5.3. Ingredient Examples
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There are many different types of thickening agents that can be used in commercial
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beverage products. These thickening agents differ in their chemical compositions, molecular
1968
weight distributions, degrees of branching, conformations, and transition temperatures, which
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causes variations in their functional performances. A summary of thickening agents that could
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be utilized in beverage emulsions is given in Table 5. There are a number of excellent books that
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provide detailed information about the molecular, physicochemical, and functional properties of
1972
thickening agents in foods and beverages (Cui, 2005; Imeson, 2010; Phillips & Williams, 2009).
1973
7.6. Sweeteners
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7.6.1 Mechanism of Action
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Beverage emulsions are sweetened by adding artificial or natural substances that interact with specific taste receptors on the tongue to provide the perception of sweetness (Fernstrom, et
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al., 2012; McCaughey, 2008; Meyers & Brewer, 2008). It has been proposed that mammals
1978
evolved to perceive a pleasant sensation after ingestion of sugar-rich foods because these foods
1979
are a ready source of energy in nature (e.g., fruits and berries). A wide variety of substances
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have molecular structures that can induce a sweet sensation, although they vary greatly in the
1981
concentrations required to produce sweetness. Sugar molecules have different sweetness
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intensities due to differences in their molecular structures, which alters how they react with the
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sweetness receptors in the mouth. Conventionally, the sweetness of substances is compared to
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that of sucrose (table sugar) which is taken to have a Sweetness of 100. The relative sweetness
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of different types of sweetener is established using carefully controlled sensory tests. Fructose is
1986
the sweetest of all common sugars because it has just the right dimensions and bond orientations
1987
to strongly interact with the sweetness receptors in the mouth. However, many natural and
1988
artificial compounds have much strong sweetness intensities than sucrose, and therefore can be
1989
used at much lower levels, which is the basis of many high-intensity sweeteners used in low-
1990
calorie products.
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7.6.2 Factors Influencing Selection
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A number of factors must be considered when selecting a suitable sweetening system for a commercial beverage product. The sweeteners used must provide a flavor profile that consumers
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find acceptable and desirable. One of the main limitations of many high-intensity sweeteners is
1995
that they do not produce a flavor profile similar to that normally provided by sugars (Nahon,
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Roozen, & DeGraaf, 1996), or that they produce undesirable off-flavors (such as bitterness or
1997
metallic) or mouthfeel (such as astringency) (DuBois & Prakash, 2012). In addition, the
1998
sweeteners must remain physically and chemically stable within the product throughout
1999
processing, transport, and storage. Some sweeteners may breakdown when exposed to elevated
2000
temperatures and are therefore unsuitable for use in beverages that undergo thermal processing.
2001
Concern about overweight and obesity has driven consumer demand for reduced calorie
2002
products, which has led the beverage industry to produce low- and mid-calorie versions of their
2003
conventional high-calorie products. However, many of the high-intensity sweeteners used to
2004
replace sugars have undesirable flavor profiles or are perceived as being “unnatural”, and so the
2005
beverage industry is trying to introduce natural low-intensity sweeteners with desirable flavor
2006
profiles (such as Stevia).
2007
7.6.3. Ingredient Examples
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There are many different types of sweeteners that can be used in commercial beverage
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products, which vary according to their chemical structure, origins (natural versus artificial), and
2010
intensity (low- versus high-intensity). The most commonly used low-intensity sweeteners are
2011
sugars (such as sucrose, glucose, and fructose) and sugar alcohols (such as sorbitol, mannitol,
2012
xylitol and erythritol). These substances may be added as individual ingredients, or they may be
2013
part of more complex ingredients, such as high fructose corn syrup, honey, and sugar cane. The
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human body does not have the appropriate enzymes in its digestive tract to break down sugar
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alcohols in the same manner that sugars are broken down. Consequently, the body breaks sugar
2016
alcohols down more slowly and less efficiently that sugars, which means they have less calories
2017
per gram (about 50-75%), which may be useful for the development of reduced calorie products.
2018
However, high-intensity sweeteners are more commonly used to produce low-calorie or medium-
2019
calorie beverages, which may be artificial (such as Cyclamate, Aspartame, Acesulfame K,
2020
Saccharin, Sucrolose, and Neotame) or natural (such as Stevia).
2021
8. Commercial Aspects of Beverage Emulsion Formulation
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Beverage emulsions are developed internally and externally by commercial manufacturers of soft drinks products. External development is usually contracted to flavor houses that often
2024
compete with each another for a flavor contract with a beverage manufacturer. This contract can
2025
be worth thousands to millions of dollars depending on the product and length of the contract.
2026
Factors considered for selecting the most appropriate flavor delivery system include stability,
2027
cost, volume capacity, safety audits, kosher/halal certification, and the ingredient line of the
2028
beverage emulsion. Because of these various factors, the best-tasting, most stable flavor may not
2029
always be chosen as the final flavor used in a commercial application.
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Beverage manufacturers normally supply flavor houses with a base of the intended product into which the flavor is intended to be incorporated. A base consists of all ingredient components
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except the flavor. By doing this, the manufacturers do not have to disclose confidential
2033
formulations and the flavor houses can develop a flavor specific to the product. Shelf-life studies
2034
(accelerated and non-accelerated) are performed to determine how well the prototype flavors
2035
hold up in the formulation. If chemical or physical instabilities are observed, the flavor can be
2036
reformulated to increase its stability. Depending on the project’s timeline this can take as little as
2037
a few days or up to a year. If the product is being rushed to market and only a few days are
2038
allotted for formulation, the flavor house can rely on past knowledge of how flavors behave in
2039
similar products. This can enable the fast development of a flavor that will have a high-
2040
probability of success in a given formulation. If development is done internally, the same steps
2041
are followed: prototype formulation; shelf-life studies; reformulation (if necessary); and then
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commercialization.
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9. Recent Developments in Beverage Emulsions There have been a number of recent developments and innovations in the beverage industry
2045
over the past few years. In this section, we highlight a number of the most important ones. The
2046
compositions and calorie content of a number of common commercial beverages is summarized
2047
in Table 6, whereas some of the claims that can be made on beverages in given in Table 7.
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9.1. “All-natural” Products and Cleaner Ingredient Lines
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Consumers’ preference for natural products over artificial have caused manufacturers to
2050
claim their products as “Natural” whenever possible. As attractive as a “Natural” claim is, it
2051
comes at a product functionality and financial cost. In most cases, natural ingredients are more
2052
expensive than synthetic and their functional properties are often not as robust. The use of
2053
synthetic ingredients creates a cheaper, more stable product, but also one less attractive to many
2054
consumers. A 2007 study by the University of Southampton (UK) linked synthetic colors and
2055
additives to heightened hyperactivity in children (McCann, et al., 2007). This report led to a
2056
consumer outcry for the removal of synthetic ingredients from food and beverage products aimed
2057
at children, promoting many large companies to begin removing synthetic additives and colors.
2058
One problem with replacing artificial additives with natural ones is that they are often much less
2059
stable within commercial products. For example, natural colors may rapidly degrade in the
2060
presence of acid, light, and heat. Consequently, many commercial products have had to be
2061
reformulated to include effective strategies to extend their shelf-life, e.g., using antioxidants and
2062
other preservatives to slow color fading.
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The use of synthetic ingredients not only impacts consumer acceptance, but also limits retail channels a product can compete within. Certain local and national grocery chains prohibit
2065
artificial sweeteners, preservatives, and flavors in the foods they sell. The incorporation of
2066
brominated vegetable oil (BVO), sucrose-acetate isobutyrate (SAIB), Tweens (polysorbate) or
2067
modified-food starch into a product can restrict product sales in these marketplaces. Thus
2068
beverage suppliers have to use alternative weighting agents (such as dammar gum or ester gum)
2069
or emulsifiers (such as gum Arabic) in these products.
2070
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The removal of all synthetic components of a product may be unachievable for functional
2071
and financial reasons. However, manufacturers can remove as many synthetic components as
2072
possible to make “No, No, No” claims. For instance, an unnatural product can still claim “No
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artificial colors, preservatives, flavors, and sweeteners”. A “No, No, No” claim is not as strong
2074
as a “Natural” claim, but it may be seen favorably by consumers. Claims on product packaging
2075
are regulated by the US Food and Drug Administration within the Code of Federal Regulations
2076
(CFR), specifically Title 21.
2077
9.2. Low-Calorie and Mid-Calorie Soft Drinks
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Consumer perceptions about the potential link between overconsumption of sugary drinks and obesity promoted beverage manufacturers to create a variety of low or zero-calorie products
2080
(Slavin, 2012). These products have been on the market for numerous years, but many
2081
consumers do not like their taste, and so the beverage industry has responded by developing a
2082
number of mid-calorie soft drinks. These drinks contain blends of caloric (e.g., Sucrose or High-
2083
Fructose Corn Syrup) and high-potency sweeteners (e.g., Acesulfame-Potassium, Aspartame,
2084
Rebiana, and Sucralose) to reduce the total calories in sweetened beverage products (typically by
2085
over a half compared to regular products), while still maintaining a desirable flavor profile.
2086
Historically, these kinds of products have not sold well in the marketplace (with a number of
2087
products from different companies failing), but there have been renewed attempts to introduce
2088
them again recently in response to consumer desire for reduced calorie intake.
2089
9.3. Energy Drinks
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Energy drinks are marketed as products that can enhance physical or mental activity by
2091
providing a rapid source of energy, nutrients, and stimulants. These drinks may be taken before,
2092
during, or after some proposed activity. They usually contain a blend of ingredients, such as
2093
water, sugars, minerals, proteins, vitamins and/or stimulants (such as caffeine) that are proposed
2094
to replenish the bodies’ reserves and enhance performance. Typically, energy drinks come as a
2095
Ready-to-Drink beverage (serving ~ 8 fluid ounces), Ready-to-Drink shot (serving ~ 2 fluid
2096
ounces), or Liquid Concentrate (dilute beverage serving ~ 8 fluid ounces). All the major
2097
commercial beverage manufacturers have products in this rapidly growing product category.
2098
Emulsion technology may be used in these products to deliver oil-soluble flavors, clouding
2099
agents, preservatives, or nutrients. Further scientific research is required to demonstrate the
2100
claimed biological activity of many of these products. Recently, there has been concern about
2101
the potential risks to certain individuals associated with consuming the large amounts of caffeine
2102
in some of these products.
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9.4. Beverage Concentrates Beverage concentrates are products that are sold in a concentrated form that consumers then
2105
dilute with water prior to consumption. Concentrates may be in either a fluid or powdered form
2106
and contain ingredients such as sugars, flavors, colors, and nutrients that form a drinkable
2107
beverage upon dilution. Beverage concentrates have been used since the nineteenth century
2108
when soda syrups were diluted by pharmacists prior to consumption by consumers. This process
2109
still occurs within automated soda fountains found in many fast-food restaurants, and within
2110
some home soda beverage makers. More recently, beverages have been developed that contain
2111
high-intensity sweeteners rather than sugars, which enable higher concentration factors to be
2112
attained. One recent successful product based on this principle involves squirting a small
2113
amount of the concentrate from a plastic bottle into a glass of water to form a zero-calorie
2114
beverage. A number of beverage manufacturers have launched products within this category in
2115
recent years.
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Beverage concentrates have some unique developmental issues due to the fact that the
2117
ingredients are present in a highly concentrated form. For example, high acid, sweetener, flavor,
2118
buffer, nutraceuticals and/or color concentrations can lead to accelerated chemical degradation
2119
reactions and shorter shelf lives.
2120
9.5. Fortification with Vitamins, Minerals, and Nutraceuticals
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As discussed earlier, there is a push to fortify many beverage products with ingredients that
2122
are perceived as giving added health benefits, such as vitamins, minerals, antioxidants, peptides,
2123
proteins, dietary fibers, and ω-3 fatty acids. These nutraceutical additives vary greatly in their
2124
molecular, physicochemical, and biological properties, such as physical state, solubility, charge,
2125
polarity, interactions, and flavor profiles. For example, carotenoids and polyunsaturated lipids
2126
may oxidize during storage and produce undesirable flavors or changes in color. Multivalent
2127
mineral ions (such as calcium) may precipitate from solution, promote precipitation of other
2128
components (such as proteins), or cause undesirable mouthfeel. Dietary fibers may increase the
2129
viscosity of a product, promote depletion flocculation, or give a slimy mouthfeel. Consequently,
2130
each beverage product must be carefully formulated to take into account the specific types of
2131
nutraceutical components it contains. This segment of the beverage industry is growing rapidly
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with consumer demand for products that are perceived to be more beneficial to human health and
2133
wellness.
2134
9.6 Bottled Water There has been an appreciable increase in the global sales of bottled waters in recent years,
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including enhanced, flavored, and fruit-flavored waters. Enhanced water is fortified with
2137
electrolytes, vitamins and other nutraceuticals, whereas flavored and fruit-flavored waters are
2138
flavored with flavorants and fruit extracts, respectively. Utilization of conventional emulsions in
2139
bottled waters to deliver oil-soluble flavors and nutraceuticals is limited because of the increase
2140
in turbidity caused by light scattering from the oil droplets. However, microemulsions and
2141
nanoemulsions that contain very fine droplets that do not scatter light strongly may be used for
2142
this purpose.
2143
9.7. New Product Innovation
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New product innovation drives business growth and enables manufacturers to meet current consumer needs. Predicting future trends and having the first product to market can give
2146
manufacturers a huge advantage against competitors. However, being first to market is also a risk
2147
due to the possibility of a new product being rejected by consumers. Reasons for rejection may
2148
include product unfamiliarity or a lack of consumer need for the product.
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One objective of new product innovation is to develop a product that is so novel that it can be patented. The federal government then grants the manufacturer a 20-year monopoly on that
2151
product type. A patented product can be the main driver of strong revenue growth for years, and
2152
a manufacturer can gain additional revenue through lawsuits due to patent infringement. On the
2153
other hand, patent expiration can also drive significant revenue loss once competitors have open
2154
access to the once-patented concept.
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New product innovation does not necessarily mean creating the first prototype of a new
2156
product. Studying trends in other parts of the world and then modifying them to fit the intended
2157
marketplace can also be used to drive innovation. For instance, Acai Berry has been consumed in
2158
Brazil for decades, but it was only in the 2000’s that it became a desired component of food
2159
products in the United States. Suspending grains in beverage products has been carried out in
2160
South America and Europe for many years, but has only recently found its way into North
2161
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Determining a secondary use of manufacturing byproducts can also generate revenue
2163
through the development of functional food ingredients from material once destined for waste.
2164
For example, whey protein was once a byproduct of cheese production, but is not a widely used
2165
functional ingredient in foods and beverages. Fusing elements from two different product
2166
categories is another way to innovate.
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New product development is an integral part of the food industry and changing out
2168
components of a beverage to meet current demands is a struggle that manufacturers have to
2169
consider every day. Such demands include the removal of ingredients from current products that
2170
consumers perceive as undesirable (such as artificial dyes or High-Fructose Corn Syrup), the
2171
addition of desirable components (such as antioxidants, minerals, or nutraceutical lipids), or the
2172
replacement of expensive ingredients with inexpensive alternatives.
2173
10. Conclusions
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In principle, beverage emulsions are one of the simplest forms of oil-in-water emulsions used within the food industry, consisting of small oil droplets dispersed within an aqueous
2176
medium. Nevertheless, there are still many challenges associated with developing successful
2177
commercial products. Beverage emulsions are prepared from various ingredients and processing
2178
operations that may vary from run-to-run, which can cause changes in the properties and stability
2179
of finished products. To minimize any undesirable changes in product quality it is important that
2180
beverage manufacturers develop a robust initial formulation and manufacturing operation, and
2181
that they also ensure that ingredient quality and manufacturing conditions are standardized and
2182
monitored. The beverage industry must also reformulate products to meet changing market and
2183
consumer demands: to increase profits by replacing expensive ingredients with less expensive
2184
ones; to remove ingredients that consumers perceive as being undesirable, such as sugar, salt,
2185
and artificial colors, flavors, or preservatives; to fortify beverages with ingredients that
2186
consumers perceive as desirable, such as vitamins, minerals, proteins, nutraceuticals, and fibers.
2187
A thorough understanding of the science and technology underlying the formation, properties,
2188
and stability of beverage emulsions facilitates the development of innovative products, and
2189
allows any production problems to be solved more rapidly. This review article has attempted to
2190
provide a comprehensive overview of the scientific and technological principles underlying
2191
beverage emulsion development.
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11. Acknowledgments DJM would like to thank Prof. Eric Dickinson for introducing him to the area of food biopolymers and colloids – it has led to many years of interesting and stimulating work, for
2195
which I am very grateful. We would also like to thank a number of colleagues in industry and
2196
academia who have provided valuable comments and suggestions on the manuscript, including
2197
Ratjika Chanamai, Eric Decker, Yuan Fang, Anil Gaonkar, and Peter Given. We would also like
2198
to acknowledge that this material is partly based upon work supported by grants from the United
2199
States Department of Agriculture (CREES, NRI Grant and NSF-EPA-USDA Grant) and the
2200
Massachusetts Department of Agricultural Resources.
2201
12. References
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Bortolomeazzi, R., Cordaro, F., Pizzale, L., & Conte, L. S. (2003). Presence of phytosterol oxides in crude vegetable oils and their fate during refining. Journal of Agricultural and Food Chemistry, 51(8), 2394-2401. Bouchemal, K., Briancon, S., Perrier, E., & Fessi, H. (2004). Nano-emulsion formulation using spontaneous emulsification: solvent, oil and surfactant optimisation. International Journal of Pharmaceutics, 280(1-2), 241-251. Brisson, L., Castan, S., Fontbonne, H., Nicoletti, C., Puigserver, A., & Ajandouz, E. H. (2008). Alpha-tocopheryl acetate is absorbed and hydrolyzed by Caco-2 cells - Comparative studies with alpha-tocopherol. Chemistry and Physics of Lipids, 154(1), 33-37. Buffo, R. A., & Reineccius, G. A. (2001). Shelf-life and mechanisms of destabilization in dilute beverage emulsions. Flavour and Fragrance Journal, 16(1), 7-12. Buffo, R. A., Reineccius, G. A., & Oehlert, G. W. (2001). Factors affecting the emulsifying and rheological properties of gum acacia in beverage emulsions. Food Hydrocolloids, 15(1), 53-66. Buffo, R. A., Reineccius, G. A., & Oehlert, G. W. (2002). Influence of time-temperature treatments on the emulsifying properties of gum acacia in beverage emulsions. Journal of Food Engineering, 51(4), 341-345. Cao, Y., Dickinson, E., & Wedlock, D. J. (1990). Creaming and flocculation in emulsions containing polysaccharide. Food Hydrocolloids, 4(3), 185-195. Cercaci, L., Rodriguez-Estrada, M. T., Lercker, G., & Decker, E. A. (2007). Phytosterol oxidation in oil-in-water emulsions and bulk oil. Food Chemistry, 102(1), 161-167. Chaiyasit, W., Elias, R. J., McClements, D. J., & Decker, E. A. (2007). Role of physical structures in bulk oils on lipid oxidation. . Crit. Rev. Food Sci. Nutr., 47, 299-317. Chanamai, R., Horn, G., & McClements, D. J. (2002). Influence of oil polarity on droplet growth in oil-in-water emulsions stabilized by a weakly adsorbing biopolymer or a nonionic surfactant. Journal of Colloid and Interface Science, 247(1), 167-176. Chanamai, R., & McClements, D. J. (2000). Impact of weighting agents and sucrose on gravitational separation of beverage emulsions. Journal of Agricultural and Food Chemistry, 48(11), 5561-5565. Chanamai, R., & McClements, D. J. (2001). Depletion flocculation of beverage emulsions by gum arabic and modified starch. Journal of Food Science, 66(3), 457-463. Chanamai, R., & McClements, D. J. (2002a). Comparison of gum arabic, modified starch, and whey protein isolate as emulsifiers: Influence of pH, CaCl2 and temperature. J. Food Sci., 67(1), 120-125. Chanamai, R., & McClements, D. J. (2002b). Comparison of gum arabic, modified starch, and whey protein isolate as emulsifiers: Influence of pH, CaCl2 and temperature. Journal of Food Science, 67(1), 120-125. Charoen, R., Jangchud, A., Jangchud, K., Harnsilawat, T., Naivikul, O., & McClements, D. J. (2011). Influence of Biopolymer Emulsifier Type on Formation and Stability of Rice Bran Oil-in-Water Emulsions: Whey Protein, Gum Arabic, and Modified Starch. Journal of Food Science, 76(1), E165-E172. Chen, C. C., & Wagner, G. (2004). Vitamin E nanoparticle for beverage applications. Chemical Engineering Research & Design, 82(A11), 1432-1437. Cheng, Q., & McClements, D. J. (2011). Formation of nanoemulsions stabilized by model foodgrade emulsifiers using high-pressure homogenization: Factors affecting particle size. Food Hydrocolloids, In Press.
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Nakamura, A., Takahashi, T., Yoshida, R., Maeda, H., & Corredig, M. (2004). Emulsifying properties of soybean soluble polysaccharide. Food Hydrocolloids, 18(5), 795-803. Nakamura, A., Yoshida, R., Maeda, H., Furuta, H., & Corredig, M. (2004). Study of the role of the carbohydrate and protein moieties of soy soluble polysaccharides in their emulsifying properties. Journal of Agricultural and Food Chemistry, 52(17), 5506-5512. Nakauma, M., Funami, T., Noda, S., Ishihara, S., Al-Assaf, S., Nishinari, K., & Phillips, G. O. (2008). Comparison of sugar beet pectin, soybean soluble polysaccharide, and gum arabic as food emulsifiers. 1. Effect of concentration, pH, and salts on the emulsifying properties. Food Hydrocolloids, 22(7), 1254-1267. Ogawa, S., Decker, E. A., & McClements, D. J. (2003). Production and characterization of O/W emulsions containing cationic droplets stabilized by lecithin-chitosan membranes. Journal of Agricultural and Food Chemistry, 51(9), 2806-2812. Orchard, T. S., Pan, X. L., Cheek, F., Ing, S. W., & Jackson, R. D. (2012). A systematic review of omega-3 fatty acids and osteoporosis. British Journal of Nutrition, 107, S253-S260. Ostertag, F., Weiss, J., & McClements, D. J. (2012). Low-energy formation of edible nanoemulsions: Factors influencing droplet size produced by emulsion phase inversion. Journal of Colloid and Interface Science, 388, 95-102. Ostlund, R. E. (2004). Phytosterols and cholesterol metabolism. Current Opinion in Lipidology, 15(1), 37-41. Phillips, G. O., & Williams, P. A. (2000). Gum arabic. In Handbook of Hydrocolloids (pp. 165): CRC. Phillips, G. O., & Williams, P. A. (2009). Handbook of Hydrocolloids (Second ed.). Boca Raton, FL: CRC Press. Pouton, C. W., & Porter, C. J. H. (2006). Formulation of lipid-based delivery systems for oral administration: Materials, methods and strategies. In Annual Meeting of the AmericanAssociation-of-Pharmaceutical-Scientists (pp. 625-637). San Antonio, TX. Qian, C., Decker, E. A., Xiao, H., & McClements, D. J. (2011). Comparison of Biopolymer Emulsifier Performance in Formation and Stabilization of Orange Oil-in-Water Emulsions. Journal of the American Oil Chemists Society, 88(1), 47-55. Qian, C., Decker, E. A., Xiao, H., & McClements, D. J. (2012). Physical and chemical stability of beta-carotene-enriched nanoemulsions: Influence of pH, ionic strength, temperature, and emulsifier type. Food Chemistry, 132(3), 1221-1229. Qian, C., & McClements, D. J. (2011). Formation of nanoemulsions stabilized by model foodgrade emulsifiers using high-pressure homogenization: Factors affecting particle size. Food Hydrocolloids, 25(5), 1000-1008. Quemada, D., & Berli, C. (2002). Energy of interaction in colloids and its implications in rheological modeling. Advances in Colloid and Interface Science, 98(1), 51-85. Rao, J., & McClements, D. J. (2012a). Food-grade microemulsions and nanoemulsions: Role of oil phase composition on formation and stability. Food Hydrocolloids, 29(2), 326-334. Rao, J., & McClements, D. J. (2012b). Impact of lemon oil composition on formation and stability of model food and beverage emulsions. Food Chemistry, In press. Rao, J. J., & McClements, D. J. (2010). Stabilization of Phase Inversion Temperature Nanoemulsions by Surfactant Displacement. Journal of Agricultural and Food Chemistry, 58(11), 7059-7066.
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2595 2596 2597 2598 2599 2600 2601 2602 2603 2604 2605 2606 2607 2608 2609 2610 2611 2612 2613 2614 2615 2616 2617 2618 2619 2620 2621 2622 2623 2624 2625 2626 2627 2628 2629 2630 2631 2632 2633 2634 2635 2636 2637 2638
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Rao, J. J., & McClements, D. J. (2011). Food-Grade Microemulsions, Nanoemulsions and Emulsions: Fabrication from Sucrose Monopalmitate & Lemon Oil Food Hydrocolloids, 25(6), 1413-1423. Rao, J. J., & McClements, D. J. (2012c). Lemon oil solubilization in mixed surfactant solutions: Rationalizing microemulsion & nanoemulsion formation. Food Hydrocolloids, 26(1), 268-276. Reiner, S. J., Reineccius, G. A., & Peppard, T. L. (2010). A Comparison of the Stability of Beverage Cloud Emulsions Formulated with Different Gum Acacia- and Starch-Based Emulsifiers. Journal of Food Science, 75(5), E236-E246. Ribeiro, H. S., Ax, K., & Schubert, H. (2003). Stability of lycopene emulsions in food systems. Journal of Food Science, 68(9), 2730-2734. Ruxton, C. H. S., Reed, S. C., Simpson, M. J. A., & Millington, K. J. (2004). The health benefits of omega-3 polyunsaturated fatty acids: a review of the evidence. Journal of Human Nutrition and Dietetics, 17(5), 449-459. Ruxton, C. H. S., Reed, S. C., Simpson, M. J. A., & Millington, K. J. (2007). The health benefits of omega-3 polyunsaturated fatty acids: a review of the evidence. Journal of Human Nutrition and Dietetics, 20(3), 275-285. Ryan, L., O'Connell, O., O'Sullivan, L., Aherne, S. A., & O'Brien, N. M. (2008). Micellarisation of carotenoids from raw and cooked vegetables. Plant Foods for Human Nutrition (Formerly Qualitas Plantarum), 63(3), 127-133. Sadtler, V., Guely, M., Marchal, P., & Choplin, L. (2004). Shear-induced phase transitions in sucrose ester surfactant. Journal of Colloid and Interface Science, 270(2), 270-275. Sagalowicz, L., & Leser, M. E. (2010). Delivery systems for liquid food products. Current Opinion in Colloid & Interface Science, 15(1-2), 61-72. Sajjadi, S. (2006). Nanoemulsion formation by phase inversion emulsification: On the nature of inversion. Langmuir, 22(13), 5597-5603. Schieber, A., & Carle, R. (2005). Occurrence of carotenoid cis-isomers in food: Technological, analytical, and nutritional implications. Trends in Food Science & Technology, 16(9), 416-422. Shukat, R., Bourgaux, C., & Relkin, P. (2012). Crystallisation behaviour of palm oil nanoemulsions carrying vitamin E. Journal of Thermal Analysis and Calorimetry, 108(1), 153-161. Siddiqui, R. A., Shaikh, Sr., Sech, L. A., Yount, H. R., Stillwell, W., & Zaloga, G. P. (2004). Omega 3-fatty acids: Health benefits and cellular mechanisms of action. Mini-Reviews in Medicinal Chemistry, 4(8), 859-871. Sidhu, G., & Oakenfull, D. (1986). A mechanism for the hypocholesterolaemic activity of saponins. British Journal of Nutrition, 55(3), 643-649. Slavin, J. (2012). Beverages and body weight: challenges in the evidence-based review process of the Carbohydrate Subcommittee from the 2010 Dietary Guidelines Advisory Committee. Nutrition Reviews, 70, S111-S120. Sonneville-Aubrun, O., Babayan, D., Bordeaux, D., Lindner, P., Rata, G., & Cabane, B. (2009). Phase transition pathways for the production of 100 nm oil-in-water emulsions. Physical Chemistry Chemical Physics, 11(1), 101-110. Soupas, L., Juntunen, L., Lampi, A. M., & Piironen, V. (2004). Effects of sterol structure, temperature, and lipid medium on phytosterol oxidation. Journal of Agricultural and Food Chemistry, 52(21), 6485-6491.
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2639 2640 2641 2642 2643 2644 2645 2646 2647 2648 2649 2650 2651 2652 2653 2654 2655 2656 2657 2658 2659 2660 2661 2662 2663 2664 2665 2666 2667 2668 2669 2670 2671 2672 2673 2674 2675 2676 2677 2678 2679 2680 2681 2682 2683 2684
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Stauffer, S. (1999). Emulsifiers. St Paul, MN: Eagen Press. Stringham, J. M., & Hammond, B. R. (2005). Dietary lutein and zeaxanthin: Possible effects on visual function. Nutrition Reviews, 63(2), 59-64. Sylvester, P. W., Wali, V. B., Bachawal, S. V., Shirode, A. B., Ayoub, N. M., & Akl, M. R. (2011). Tocotrienol combination therapy results in synergistic anticancer response. Frontiers in Bioscience-Landmark, 16, 3183-3195. Szuts, A., Budai-Szucs, M., Eros, I., Otomo, N., & Szabó-Révész, P. (2010). Study of gelforming properties of sucrose esters for thermosensitive drug delivery systems. International Journal of Pharmaceutics, 383(1-2), 132-137. Tadros, T., Izquierdo, R., Esquena, J., & Solans, C. (2004). Formation and stability of nanoemulsions. Advances in Colloid and Interface Science, 108-09, 303-318. Tan, C. (2004). Beverage emulsions. In F. S, L. K & S. J (Eds.), Food Emulsions (4th ed.). New York: Marcel Decker. Tan, C. T. (1998). Beverage flavor emulsion-a form of emulsion liquid membrane encapsulation. In E. T. Contis (Ed.), Food Flavors: Formation, Analysis and Packaging Influences (pp. 29). New York: Elsevier. Thakur, R. K., Villette, C., Aubry, J. M., & Delaplace, G. (2008). Dynamic emulsification and catastrophic phase inversion of lecithin-based emulsions. Colloids and Surfaces aPhysicochemical and Engineering Aspects, 315(1-3), 285-293. Traber, M. G., Frei, B., & Beckman, J. S. (2008). Vitamin E revisited: do new data validate benefits for chronic disease prevention? Current Opinion in Lipidology, 19(1), 30-38. Trubiano, P. C. (1995a). The role of specialty food starches in flavor emulsions. Flavor Technology, 610, 199-209. Trubiano, P. C. (1995b). The role of specialty food starches in flavor encapsulation. Flavor Technology, 610, 244-253. Tse, K. Y., & Reineccius, G. A. (1995a). Methods to predict the physical stability of flavor Cloud emulsion. In Flavor Technology (Vol. 610, pp. 172-182). Tse, K. Y., & Reineccius, G. A. (1995b). Methods to predict the physical stability of flavor Cloud emulsion. Flavor Technology, 610, 172-182. Tur, J. A., Bibiloni, M. M., Sureda, A., & Pons, A. (2012). Dietary sources of omega 3 fatty acids: public health risks and benefits. British Journal of Nutrition, 107, S23-S52. Ueno, T., Kiyohapa, S., Ho, C. T., & Masuda, H. (2006). Potent inhibitory effects of black tea theaflavins on off-odor formation from citral. Journal of Agricultural and Food Chemistry, 54(8), 3055-3061. Ueno, T., Masuda, H., & Ho, C.-T. (2004). Formation mechanism of p-methylacetophenone from citral via a tert-alkoxy radical intermediate. Journal of Agricultural and Food Chemistry, 52, 5677-5684. Ueno, T., Masuda, H., Muranishi, S., Kiyohara, S., Sekiguchi, Y., & Ho, C. T. (2003). Inhibition of the formation of off-odour compounds from citral in an acidic aqueous solution. In J. L. LeQuere & P. X. Etievant (Eds.), Flavour Research at the Dawn of the Twenty-First Century (pp. 128-131). van der Graaf, S., Schroen, C. G. P. H., & Boom, R. M. (2005). Preparation of double emulsions by membrane emulsification - a review. Journal of Membrane Science, 251(1-2), 7-15. van Setten, D. C., ten Hove, G. J., Wiertz, E. J. H. J., Kamerling, J. P., & van de Werken, G. (1998). Multiple-stage tandem mass spectrometry for structural characterization of saponins. Analytical chemistry, 70(20), 4401-4409.
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2731 2732 2733 2734 2735 2736 2737 2738 2739 2740 2741 2742 2743 2744 2745 2746 2747 2748 2749 2750 2751 2752 2753 2754 2755 2756 2757 2758 2759 2760 2761 2762 2763 2764 2765 2766 2767 2768 2769 2770 2771 2772 2773 2774 2775
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Yang, X. Q., Tian, H. X., Ho, C. T., & Huang, Q. R. (2012). Stability of Citral in Emulsions Coated with Cationic Biopolymer Layers. Journal of Agricultural and Food Chemistry, 60(1), 402-409. Yang, Y., Leser, M. E., Sher, A. A., & McClements, D. J. (2013). Formation and stability of emulsions using a natural small molecule surfactant: Quillaja saponin (Q-Naturale (R)). Food Hydrocolloids, 30(2), 589-596. Yang, Y., Marshall-Breton, C., Leser, M. E., Sher, A. A., & McClements, D. J. (2012). Fabrication of ultrafine edible emulsions: Comparison of high-energy and low-energy homogenization methods. Food Hydrocolloids, 29(2), 398-406. Yang, Y., & McClements, D. J. (2013). Encapsulation of vitamin E in edible emulsions fabricated using a natural surfactant. Food Hydrocolloids, 30(2), 712-720. Yin, L. J., Chu, B. S., Kobayashi, I., & Nakajima, M. (2009). Performance of selected emulsifiers and their combinations in the preparation of beta-carotene nanodispersions. Food Hydrocolloids, 23, 1617-1622. Yoon, Y., & Choe, E. (2009). Lipid Oxidation and Stability of Tocopherols and Phospholipids in Soy-added Fried Products During Storage in the Dark. Food Science and Biotechnology, 18(2), 356-361. Zuccari, G., Carosio, R., Fini, A., Montaldo, P., & Orienti, I. (2005). Modified polyvinylalcohol for encapsulation of all-trans-retinoic acid in polymeric micelles. Journal of Controlled Release, 103(2), 369-380.
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2776 2777 2778 2779 2780 2781 2782 2783 2784 2785 2786 2787 2788 2789 2790 2791 2792 2793 2794 2795 2796 2797
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Figure 1: Schematic representation of different kinds of colloidal dispersions that can be used in the beverage industry: emulsions, nanoemulsions, and microemulsions.
SC
• Thermodynamically unstable • d > 100 nm • Optically opaque
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• Thermodynamically unstable • d < 100 nm • Transparent or slightly turbid
Microemulsion
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Nanoemulsion
• Thermodynamically stable • d < 100 nm • Transparent or slightly turbid
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Kinetically Stable Nanoemulsion
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Figure 2: Schematic diagram of most common instability mechanisms that occur in food emulsions: creaming, sedimentation, flocculation, coalescence, Ostwald ripening and phase inversion.
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Gravitational Separation
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Sedimentation Creaming
Flocculation
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Oiling Off
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Figure 3. The particles in beverage emulsions can be designed to have different functional performances by varying their physicochemical and structural properties, such as size, composition, charge and interfacial properties.
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Charge -
+ +
+ -
-
-
+ -
-
+
-
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Interfacial properties
Negative
Neutral
+
+
Particle Size
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w(h)
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Energy Barrier
h Attraction
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Repulsion
Coalesced
1º Min
2º Min
Figure 4. Schematic representation of the interaction potential between two emulsion droplets. The droplets may be stable, weakly flocculated, strongly flocculated, or coalesced depending on the attractive and repulsive forces between them.
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Figure 5. Lipid droplets have a major impact on the texture, appearance and stability of emulsions
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Flavor Polarity
Figure 6. Initial flavor intensity of emulsions with fixed flavor content is strongly dependent on oil droplet content.
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Figure 7. Droplet growth due to Ostwald ripening of orange oil emulsions can be inhibited by adding a corn oil, which acts as a ripening inhibitor (McClements et al 2012)
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Figure 8. Influence of pH on the chemical degradation rate of citral in aqueous solutions: the degradation rate increases with decreasing pH (Choi et al 2009).
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Figure 9: Schematic representation of mechanical devices that can be used to produce beverages emulsions using the high-energy approach: high pressure valve homogenizer, microfluidizer and continuous ultrasonic homogenizer
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Droplet disruption
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High Pressure Valve Homogenizer
Continuous Ultrasonic Homogenizer
Microfluidizer
Droplet disruption
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Figure 10: Schematic representation of various methods that can be used to produce beverages emulsions using a low-energy approach: emulsion inversion point method, spontaneous emulsification and phase inversion temperature.
T > PIT
W/O
O/W/O
O/W
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Emulsion inversion point method
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Bicontinuous Microemulsion
Heat T < PIT
O/W Emulsion
Spontaneous emulsification method PIT method
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Figure 11: Influence of homogenization pressure in a microfluidizer on the mean droplet diameter of oil-in-water emulsions stabilized by either β-lactoglobulin (BLG) or sodium dodecyl sulfate (SDS). The oil phase density was increased by adding increasing amounts of weighting agent (Qian, McClements 2011).
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Figure 12: Influence of surfactant-to-oil ratio on the formation of 20 wt% oil-in-water emulsions stabilized by a non-ionic surfactant (Tween 80) prepared using the spontaneous emulsification method (Sabeti, Fang, and McClements 2012). The oil phase consisted of 80% vitamin E acetate and 20% MCT.
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Figure 13: Schematic diagram of formulation-composition map for a typical surfactant-oil-water system. This map can be used to understand the formation of emulsions and nanoemulsions suing various phase inversion methods.
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W/O/W
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W/O
Transitional
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O/W
O/W/O
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Catastrophic
• HLD – Hydrophilic-Lipophilic Deviation • Relative affinity of surfactant for water and oil phases (surfactant & system dependent) • WOR – Water-to-Oil Ratio • Depends on system composition
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Figure 14: Change in turbidity when a coarse lemon oil emulsion (10% lemon oil, 13% Tween 80, 77% water) is heated and cooled.. A very fine transparent nanoemulsion is formed when the system is heated, and then cooled below its phase inversion temperature. (Rao and McClements 2010)
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Figure 15: Influence of oil type on mean droplet diameter in emulsions produced using the emulsion inversion point (EIP) method by titrating water into an oil-surfactant mixture (Ostertag, Weiss and McClements, 2012).
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Figure 16: Influence of pH and storage time on the chemical degradation of 0.05% citral dispersed in citrate buffer solutions (Choi et al 2009). The citral chemically degrades rapidly at acidic pH values.
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Figure 17: Influence of oil droplet concentration on the chemical degradation of citral dispersed within MCT oil-in-water emulsions (Choi et al 2009). The citral chemically degrades more rapidly in an aqueous than an oil environment, and therefore the less oil present the faster the degradation rate.
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Figure 18: Influence of oil droplet concentration on the partitioning of citral within MCT oil-in-water emulsions (Choi et al 2009). The mass fraction of citral in the aqueous phase decreases as the oil concentration increases.
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Figure 19: Influence of emulsifier concentration on the mean particle diameter produced using a microfluidizer using either whey protein isolated (WPI), modified starch (MS) or gum arabic (GA) are emulsifiers under standardized conditions.
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Figure 20: Influence of pH on the particle size and charge produced using a microfluidizer using either whey protein isolated (WPI), modified starch (MS) or gum arabic (GA).
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Figure 21: Influence of ionic strength on the particle size and charge produced using a microfluidizer using either whey protein isolated (WPI), modified starch (MS) or gum arabic (GA).
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Table 1: Global revenue from non-alcoholic beverage sales by major beverage manufacturing companies in 2010 (Beverage
35.1
PepsiCo
21.4
Kraft Foods
8.8
Dr. Pepper Snapple Group
5.6
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Coca-Cola Co.
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Annual Sales (Billions of Dollars)
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Executive. October 2011, “Worldwide 100”, Pages 44-48).
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Table 2. Summary of major colloidal interactions operating between oil droplets in beverage emulsions. It is assumed that the
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droplets are coated by the same type of emulsifier. Sign
Magnitude
Range
Factors Affecting
Van der Waals
Attractive
Intermediate
Intermediate
Always present
Steric
Repulsive
Strong
Short
Thickness and chemistry of interface
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Interactions
Repulsive
Strong-to-Weak
Long-to-Short
pH and ionic strength
Depletion
Attractive
Weak-to-Medium
Short
Amount & type of non-
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Electrostatic
adsorbed polymer
Short
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Attractive
Strong
Strong
Long
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Hydrophobic
Attractive
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Bridging
Amount & type of adsorbing polymer Surface hydrophobicity
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Table 3. Physicochemical properties of oil phase, water phase, and weighting agents commonly used in the formulations of
reported.
Density (kg m-3)
Restriction Level (ppm)
-
≈1050
-
-
≈1000
-
Yes Yes
840-850 900-920
-
No
1240-1330
15
Yes
1060
Defined by manufacturer
1080
Not approved for use in the United States 100
No
1146
300
Structure
Natural
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Components
Oil Phases Flavor Oils Vegetable Oils
Mainly Terpenes Mainly Triglycerides
Weighting Agents
Ester Gum Sucrose Acetate Isobutyrate
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Dammar Gum
Brominated Hydrocarbon Chain of Triacylglycerols Exudate from Caesalpinaceae and Dipterocarpaceae shrubs Hydrophobic Polymer
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Brominated Vegetable Oil
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Low calorie
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Aqueous Phases Sugared
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beverage emulsions (Chanamai & McClements, 2000; McClements, 2005). The restriction levels based on USA regulations are also
Esterification of Sucrose with Acetic and Isobutyric Anhydrides
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HLB Value 16 15 6 2 16 15.6 14.9 15 >9 8.6 6.7 4.7 4.3
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Surfactant Sucrose monopalmitate Sucrose monostearate Sucrose distearate Sucrose polystearate Tween 20 (Polyoxyethylene Sorbitan Monolaurate) Tween 40 (Polyoxyethylene Sorbitan Monopalmitate) Tween 60 (Polyoxyethylene Sorbitan Monostearate) Tween 80 (Polyoxyethylene Sorbitan Monoooleate) Q-Naturale (Quillaja saponin) Span 20 (Sorbitan monolaurate) Span 40 (Sorbitane monopalmitate) Span 60 (Sorbitan monostearate) Span 80 (Sorbitan monooleate)
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Table 4. HLB numbers of selected small molecule surfactants and co-surfactants that could be used to produce beverage emulsions. Information from various sources: McClements (2005), National Starch, and Rahn-Group.com.
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Source
Main Structure Type
Major Monomer(s)
Algal
Linear
Carrageenan
Algal
Linear/Helical
Sulfated Galactose & Anhydrogalactose
Chitosan
Crustaceans, Invertebrates
Linear
Acacia Sap
Branched Coil Domains on Protein Scaffold
Carob seeds
Linear
Linear
Modified Starch
Waxy Maize
Linear
Glucose with some nonpolar branches
Plant Cell Walls Xanthomonas campestris exudate
Highly Branched Coil
Glucuronate (backbone)
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Wood Pulp
Xanthan Gum
Negative
Neutral
Galactose and mannose
Modified Celluloses
Pectin
Positive
Galactose
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Locust bean gum
Negative
Glucosamine
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Gum Arabic
Negative
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Alginate
Mannuronic & Guluronic Acids
Charge
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Name
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Table 5. Summary of molecular characteristics of some common food-grade polysaccharides. Adapted from Matalanis et al (2011).
Linear/Helical (High MW)
Glucose and modified Glucoses
Neutral or Negative Negative Negative Negative
Glucose (backbone)
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Table 6. Examples of the composition and calorie content of some commercial beverage products currently on the market. Emulsifier
Weighting Natural Agent Claim
50
GA
None Listed
No
0
GA
None Listed
No
Cane Sugar
70
GA
EG
Yes
Cane Sugar
70
GA
EG
Yes
HFCS
80
GA
EG
No
HFCS
80
GA
EG
No
RTD
HFCS
80
GA
EG
No
RTD
HFCS
80
M-FS
MCT, SAIB
No
RTD
HFCS
120
M-FS
EG, BVO
No
APM, Ace-K
0
GA
EG, BVO
No
RTD
Fructose, Cane Sugar
120
GA
EG
No
Coca-Cola
RTD
Fructose, Cane Sugar
120
GA
EG
No
Coca-Cola
RTD
Fructose, Cane Sugar, Sorbitol
120
GA, M-FS
EG
No
Coca-Cola
RTD
Erythritol, Rebiana
0
GA
EG
No
Beverage Sweetener Type System
Half and Half Iced Tea Lemonade
Arnold Palmer
AriZona
RTD
HFCS, Sucralose, Ace-K
Half and Half Iced Tea Lemonade
Arnold Palmer Zero
AriZona
RTD
Sucralose
Cherry Citrus
BodyArmor
Body Armor Nutrition
RTD
Tropical Mandarin
BodyArmor
Body Armor Nutrition
RTD
Lemon Lime
Xion4 (Powerade)
Coca-Cola
RTD
Orange
Xion4 (Powerade)
Coca-Cola
RTD
Strawberry Lemonade
Xion4 (Powerade)
Coca-Cola
Sour Melon
Xion4 (Powerade)
Coca-Cola
Orange Soda
Fanta
Coca-Cola
Original Citrus
Fresca
Coca-Cola
RTD
Energy - Tropical Citrus
Vitamin Water
Coca-Cola
Multi-V - Lemonade
Vitamin Water
Essential Orange-Orange
Vitamin Water
Squeezed Lemonade
Vitamin Water Zero
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EP
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SC
Manufacturer
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Brand
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Calories (per 8 oz. Ready to Drink)
Product
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Sunkist
Dr. Pepper Snapple
RTD
HFCS
110
M-FS
EG
No
Orange
Diet Crush
Dr. Pepper Snapple
RTD
APM, Ace-K
0
GA
EG
No
Citrus Soda
Sun Drop
Dr. Pepper Snapple
RTD
HFCS
120
GA
EG, BVO
No
Pineapple Orange Guava
Nantucket Nectar's
Dr. Pepper Snapple
RTD
Sucrose
120
GA
None Listed
Yes
Orange Mango
Nantucket Nectar's
Dr. Pepper Snapple
RTD
Sucrose
120
GA
None Listed
Yes
Strawberry Kiwi
Snapple
Dr. Pepper Snapple
RTD
Sucrose
100
GA
EG
Yes
Lemonade
Farmer's Daughter
The Farmer's Cow
RTD
Cane Sugar
120
GA
EG
Yes
Strawberry Lemonade
Farmer's Daughter
The Farmer's Cow
RTD
Cane Sugar
110
GA
EG
Yes
Lemonade
MiO
Kraft Foods
LC
Sucralose, Ace-K
0
GA
SAIB
No
Orange Tangerine
MiO
Kraft Foods
LC
Sucralose, Ace-K
0
GA
SAIB
No
Sparkling Lemonade
N/A
Lorina
RTD
Sucrose
90
GA
EG
No
Pink Citrus Lemonade
N/A
Lorina
RTD
Sucrose
120
GA
EG
No
Lemonade
G2 of Gatorade G-Series
PepsiCo
RTD
Sucrose
80
GA
EG, SAIB
No
Lemon-Lime
G2 of Gatorade G-Series
PepsiCo
RTD
Sucrose
80
GA
EG
No
Orange
G2 of Gatorade G-Series
PepsiCo
RTD
Sucrose
80
GA
EG, BVO
No
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Orange Soda
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RTD
HFCS
110
GA
BVO
No
N/A
PepsiCo
RTD
APM, Ace-K
0
GA
BVO
No
Nature's Blends
Poland Spring
RTD
Cane Sugar
50
GA
EG
Yes
Orange Cranberry Tangerine
Skinny Water
Skinny Nutritional
RTD
Sucralose, Ace-K
0
GA
EG
No
Old-Fashioned Lemonade
N/A
Zeigler's Beverages
RTD
Sucrose
100
GA
EG
No
SC
Key:
Ace-K
Acesulfame Potassium
APM
Aspartame
BVO
Brominated Vegetable Oil
EG
Ester Gum
GA
Gum Acacia
HFCS
High Fructose Corn Syrup
Liquid Concentrate
LC
Medium Chain Triglycerides
MCT
M-FS RTD SAIB
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Lemon
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Diet Mtn Dew
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PepsiCo
EP
N/A
ModifiedFood Starch
AC C
Mtn Dew
Ready to Drink Sucrose Acetate Isobutyrate
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Claim
Justification
“0 Calories”
Less than 5 cal per RACC.
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Table 7. Claims and justifications that can be made on beverage products.
Source 21 CFR 101.60(b)
“All Natural”
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components.
SC
Undefined, but regarded as a product lacking any synthetic
N/A
Any substance that imparts flavor that is not derived from its natural “Artificially Flavored”
21 CFR 101.22 (a) (1)
source.
For juice made from concentrate, calculate use-percentage from the
21 CFR 101.30(j),
Brix table in 21 CFR 101.30(h)(1) as the basis for 100% juice.
21 CFR 101.30(h)
Contains 20% or more of the DV per RACC.
21 CFR 101.54(b)
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“Contains % Juice”
“Excellent Source of
Vitamin” “Low Calorie Soda” “Low Sodium”
Contains 10%-19% of the DV per RACC.
21 CFR 101.54(e)
40 cal or less per RACC.
21 CFR 101.60(b)
140 mg or less per RACC.
21 CFR 101.61
AC C
“Good Source of
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Vitamin”
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21 CFR 101.22 (a) (1), “Natural and Artificial Contains Natural and Artficial Flavors.
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Flavor”
21 CFR 101.22 (a) (3), 21 CFR (g) 101.22 (3)
The flavor constituents function in the food as flavor, not solely as a “Naturally Flavored”
21 CFR 101.22 (a) (3)
No Sugar or Sugar-Containing Ingredients are added during processing.
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“No Added Sugar”
SC
source of nutrition are derived from its natural source in nature.
21 CFR 101.60(c) (2)
Contains no colarant source outside the principle flavor components of
“No Artificial Colors”
21 CFR 101.22(k)(1) (2),
beverage. Natural color is considered artficial color when used for 21 CFR 74
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colorant purposes.
“No Pulp”
“Reduced Sugar”
EP
21 CFR 101.65 (b)(2) 21 CFR 101.65(b)(1)
Cannot Contain Preservatives.
AC C
“No Preservatives”
21 CFR 101.65(b)(1)
Cannot Contain HFCS.
“No HFCS”
21 CFR 101.65 (b)(2) 21 CFR 101.65(b)(1)
Cannot Contain Pulp.
At least 25% less sugars per RACC than an appropriate reference food.
21 CFR 101.65 (b)(2) 21 CFR 101.60 (c)
S0268-005X(13)00211-7
DOI:
10.1016/j.foodhyd.2013.07.009
Reference:
FOOHYD 2311
To appear in:
Food Hydrocolloids
Received Date: 13 May 2013 Revised Date:
8 July 2013
Accepted Date: 9 July 2013
Please cite this article as: Piorkowski, D.T., McClements, D.J., Beverage Emulsions: Recent Developments in Formulation, Production, and applications, Food Hydrocolloids (2013), doi: 10.1016/ j.foodhyd.2013.07.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract “BEVERAGE EMULSIONS: RECENT DEVELOPMENTS IN FORMULATION, PRODUCTION, AND APPLICATIONS” by D.T. Piorkowski and D.J. McClements
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Food Hydrocolloids
The article provides an overview of recent research on the formation, stability, and
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properties of beverage emulsions.
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BEVERAGE EMULSIONS: RECENT DEVELOPMENTS IN
2
FORMULATION, PRODUCTION, AND APPLICATIONS
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Daniel T. Piorkowski and David Julian McClements1
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Department of Food Science, University of Massachusetts, Amherst, MA 01003
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Journal: Food Hydrocolloids
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Submitted: March 2013
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Revised: July 2013
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1
Department of Food Science, University of Massachusetts, Amherst, MA 01003, [email protected], 413 545 1019
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Abstract Soft drinks are one of the most widely consumed and profitable beverages in the world.
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This review article focuses on the utilization of emulsion science and technology for the
29
fabrication of soft drinks by the beverage industry. A brief overview of the various high and low
30
energy methods available for preparing this type of beverage emulsions is given, as well as a
31
discussion of the functional ingredients used to formulate these systems, including oil phases,
32
emulsifiers, weighting agents, ripening inhibitors, and thickening agents. The influence of
33
droplet characteristics on the physicochemical and sensory properties of beverage emulsions is
34
reviewed, with special focus on their influence on product stability. Finally, we discuss recent
35
developments in the soft drinks area, including fortification with vitamins, reduced calorie
36
beverages, and “all-natural” products.
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Keywords: beverages; soft drinks; nutraceuticals; flavors; emulsions; nanoemulsions
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1. Introduction Globally, soft drinks are one of the most widely consumed and profitable beverages (Table
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1). Cola is the top soft drink flavor currently consumed in the United States, with lemon-lime
43
and orange being the second and third. All three of these soft drink flavors contain hydrophobic
44
citrus compounds extracted from fruit peels. Soft drinks may also contain a variety of other
45
hydrophobic components, such as clouding agents, weighting agents, nutraceuticals, oil-soluble
46
vitamins, and oil-soluble antimicrobials. The non-polar character of flavor oils and other
47
hydrophobic ingredients means that these ingredients cannot simply be dispersed directly into an
48
aqueous phase – they would rapidly coalesce and separate through gravitational forces leading to
49
a layer of oil on top of the product (Given, 2009). Instead they first have to be converted into a
50
colloidal dispersion consisting of flavor molecules encapsulated within small particles suspended
51
within an aqueous medium, e.g., a microemulsion, nanoemulsion, or emulsion (McClements,
52
2011; McClements & Li, 2010). These colloidal delivery systems must be carefully designed to
53
provide desirable physicochemical, sensory, and biological attributes to the final product. A
54
number of desirable attributes of colloidal delivery systems suitable for application in beverage
55
products are highlighted below (McClements, Decker, & Weiss, 2007; McClements & Li, 2010):
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•
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Fabrication: Ideally, the delivery systems should be fabricated using robust, reliable and inexpensive manufacturing methods that are easily implemented.
•
Stability: The delivery systems should be designed to withstand all of the stresses
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Composition: Ideally, the delivery systems should be fabricated entirely from “label friendly” food-grade ingredients that are economic and easy to handle.
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that a product may experience during its production, storage, transport and utilization, such as temperature fluctuations, exposure to light and oxygen, exposure to mechanical forces (such as stirring, flow through a pipe, and vibrations), variations in aqueous phase composition (such as pH, ionic strength, buffer type,
65
ingredient interactions), and exposure to microorganisms (such as yeasts, molds or
66
bacteria).
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Physicochemical and sensory properties: The delivery system should not adversely
68
affect the optical properties, rheology, or flavor profile (aroma, taste, and mouthfeel)
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of the beverage product into which it is incorporated. Biological activity: The delivery system should not adversely affect the biological
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•
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activity of any encapsulated bioactive components, such as antimicrobials, vitamins,
72
or nutraceuticals.
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This review article provides an overview of the current status of the design, formulation, and production of emulsion-based delivery systems suitable for utilization within the beverage
75
industry.
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2. Emulsion Science and Technology in the Beverage Industry
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Hydrophobic components (such as flavor oils, clouding agents, oil-soluble vitamins, and nutraceuticals) can be incorporated into a variety of different colloidal delivery systems suitable
79
for application within beverage products (McClements, 2012; McClements & Rao, 2011), with
80
the most common being microemulsions, nanoemulsions, and emulsions (Figure 1). Each of
81
these colloidal dispersions has particular benefits and limitations for the encapsulation of
82
hydrophobic compounds. Microemulsions are thermodynamically stable systems under a
83
specific set of environmental conditions (e.g., composition and temperature), and are therefore
84
easy to fabricate (often by simple mixing) and tend to have good long-term stability.
85
Microemulsions typically contain very small particles (r < 25 nm) and therefore tend to be
86
optically transparent, which is desirable for soft drinks that should be clear. On the other hand,
87
the formation of microemulsions usually requires relatively high levels of synthetic surfactants
88
and sometimes the use of cosurfactants/cosolvents, which can be undesirable for cost, taste, and
89
labeling reasons. Microemulsions may also become thermodynamically unstable if
90
environmental conditions are altered (such as temperature or composition).
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Conventional emulsions (r > 100 nm) and nanoemulsions (r < 100 nm) are both
92
thermodynamically unstable systems, and therefore tend to breakdown during storage through a
93
variety of instability mechanisms (Figure 2), such as gravitational separation, flocculation,
94
coalescence and Ostwald ripening (McClements, et al., 2007; McClements & Rao, 2011).
95
Emulsion systems must therefore be carefully designed to inhibit these instability mechanisms
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and provide sufficient kinetic stability throughout the lifetime of the product. Emulsions usually
97
contain larger droplets than microemulsions and therefore they scatter light more strongly and
98
appear more turbid or cloudy. This is an advantage for soft drinks that are required to have a
99
cloudy appearance, but a disadvantage for products where optical clarity is required.
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Nevertheless, recently it has been shown that emulsions with ultrafine droplets, often referred to
101
as “nanoemulsions”, can be prepared that are optically transparent (McClements, 2012;
102
McClements & Rao, 2011). A major advantage of emulsions and nanoemulsions is that the
103
emulsifier-to-oil ratio required to formulate them is often much less than that required for
104
microemulsions, and they can be formulated from all natural ingredients (such as proteins and
105
polysaccharides) rather than synthetic surfactants (such as Tweens). In this article, we focus
106
primarily on the utilization of emulsion systems (conventional emulsions and nanoemulsions) in
107
the preparation of soft drinks but much of the material is also relevant to the formulation of
108
microemulsions.
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It should be noted that the emulsions used in the beverage industry are typically divided into two groups: flavor emulsions and cloud emulsions. Flavor emulsions contain lipophilic
111
compounds that are primarily present to provide taste and aroma to a beverage product (such as
112
lemon, lime, or orange oils). On the other hand, cloud emulsions are used to provide specific
113
optical properties to certain beverage products, i.e., to increase their turbidity (“cloudiness”).
114
Cloud emulsions are typically prepared using an oil phase that is highly water-insoluble and that
115
is not prone to chemical degradation, such as flavorless vegetable oils. In addition, the size of the
116
droplets within cloud emulsions is designed so that they have dimensions where strong light
117
scattering occurs, but are not too large to undergo gravitational separation (e.g., r = 100-200
118
nm). Cloud emulsions are often added to beverages that only contain a relatively low percentage
119
of juice and provide a desirable cloudy appearance that hides sedimentation and ringing.
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In this article, we will use the term “emulsion” to refer to both nanoemulsions and
121
conventional emulsions because they have similar structures and properties. Generally, an
122
emulsion consists of at least two immiscible liquids (usually oil and water), with one of the
123
liquids being dispersed as small spherical droplets in the other (Dickinson, 1992a; Dickinson &
124
Stainsby, 1982; Friberg, Larsson, & Sjoblom, 2004; McClements, 2005b). In general, emulsions
125
are classified according to the relative spatial organization of the oil and water phases. A system
126
that contains oil droplets dispersed within water is called an oil-in-water (O/W) emulsion,
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whereas a system that contains water droplets dispersed in oil is called a water-in-oil (W/O)
128
emulsion. It is possible to prepare more complex emulsion structures, e.g., oil-in-water-in-oil
129
(O/W/O), water-in-oil-in-water (W/O/W) or oil-in-water-in-water (O/W/W) emulsions
130
(Benichou, Aserin, & Garti, 2004; Garti & Bisperink, 1998; van der Graaf, Schroen, & Boom,
131
2005). Currently, almost all of the emulsions used in the beverage industry are of the O/W type,
132
although there may be certain advantages to using other emulsion types for certain applications.
133
For example, in principle it is possible to trap a hydrophilic bioactive component within the inner
134
water phase of a W/O/W emulsion to protect it from chemical degradation or for taste masking.
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In practice, it is often difficult to formulate W/O/W emulsions that have sufficient stability for
136
commercial applications, although this is still an active area of research.
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Emulsions are thermodynamically unfavorable systems that tend to break down over time
138
though a variety of physicochemical mechanisms, including gravitational separation (creaming
139
and sedimentation), droplet aggregation (flocculation and coalescence) and droplet growth
140
(Ostwald ripening) (Dickinson, 1992a; Friberg, et al., 2004; McClements, 2005b). It is possible
141
to form emulsions that are kinetically stable for a reasonable period of time by including
142
substances known as stabilizers, e.g., emulsifiers, weighting agents, ripening inhibitors, or
143
texture modifiers. It is important to clearly distinguish the different physicochemical
144
mechanisms involved in promoting emulsion stability for these different categories of stabilizers.
145
Emulsifiers are surface-active molecules that adsorb to the surface of freshly formed droplets
146
during homogenization, forming a protective layer that prevents the droplets from aggregating.
147
Weighting agents are dense hydrophobic components added to low-density oils to prevent
148
gravitational separation. Ripening inhibitors are water-insoluble components added to polar oils
149
to prevent Ostwald ripening. Texture modifiers are substances used to increase the viscosity or
150
gel aqueous solutions, thereby retarding or preventing droplet movement. A more detailed
151
description of different types of stabilizers that can be used in beverage emulsions is given in a
152
later section. Selecting the most appropriate stabilizer(s) for a particular application is one of the
153
most important factors determining the shelf-life and physicochemical properties of beverage
154
emulsions.
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3. Controlling Droplet Characteristics The bulk physicochemical properties of beverage emulsions (such as optical properties,
157
stability, rheology, molecular partitioning, and release characteristics) are largely determined by
158
the properties of the droplets they contain (McClements, 2005b), such as composition,
159
concentration, size, and charge (Figure 3). In this section, we discuss some of the most
160
important droplet characteristics that can be controlled by beverage manufacturers in order to
161
create products with specific desirable functional properties.
162
3.1. Droplet composition
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The composition of the oil phase has a major influence on the formation and stability of beverage emulsions, which has often been overlooked in academic research. Beverage
165
emulsions may contain a variety of different hydrophobic components, including flavor oils,
166
essential oils, triacylglycerol oils, oil-soluble vitamins, nutraceuticals, weighting agents, and
167
ripening inhibitors. These components vary in their molecular characteristics (such as molecular
168
weight, molecular conformation, and functional groups), which leads to changes in their
169
physicochemical properties (such as polarity, water-solubility, density, viscosity, refractive
170
index, physical state, and melting point). Many of these molecular and physicochemical
171
properties have a major influence on the formation, stability, and functionality of emulsions. For
172
example, oil viscosity influences the efficiency of droplet disruption during high energy
173
homogenization – the closer the ratio of dispersed phase viscosity to continuous phase viscosity
174
(ηD/ ηC) is to unity, the more efficient is droplet disruption and the smaller is the particle size
175
produced (Walstra, 1993, 2003). Oil density determines the rate of particle creaming or
176
sedimentation within emulsions – the greater the density contrast between the droplets and
177
surrounding fluid, the faster the rate of gravitational separation (McClements, 2005c). Oil
178
refractive index determines the efficiency of light scattering by droplets in emulsions – the
179
greater the refractive index contrast between the droplets and surrounding fluid, the stronger the
180
degree of light scattering and the more turbid the appearance (Chanamai & McClements, 2002b).
181
The water-solubility of an oil phase determines the stability of an emulsion to Ostwald ripening
182
due to diffusion of oil molecules through the aqueous phase (Kabalnov, 2001; McClements,
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Henson, Popplewell, Decker, & Choi, 2012). Oil interfacial tension plays a number of important
184
roles in determining emulsion formation and stability. First, the ease of droplet disruption during
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high energy homogenization decreases as the interfacial tension decreases (Walstra, 1993).
186
Second, the rate of droplet coalescence increases as the interfacial tension decreases (Kabalnov
187
& Wennerstrom, 1996). Third, the ability of emulsifiers to adhere to droplet surfaces decreases
188
as the bare oil-water interfacial tension decreases (Chanamai, Horn, & McClements, 2002).
189
Finally, the rate of droplet growth due to Ostwald ripening depends on the interfacial tension at
190
the oil-water interface (Kabalnov, 2001).
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For flavor emulsions, it is important to control the type and concentration of the flavor
192
molecules initially present in the oil phase. It is also important to be aware that the location of
193
the flavor molecules within an emulsion is governed by their oil-water partition, which depends
194
on carrier oil type (Choi, Decker, Henson, Popplewell, & McClements, 2009; Choi, Decker,
195
Henson, Popplewell, & McClements, 2010b). The flavor profile of an emulsion may therefore
196
change if the carrier oil type is altered, if the physical state of the carrier oil changes, or if an
197
emulsion is diluted, since this will change the distribution of the flavor molecules in the oil,
198
water and air (Choi, et al., 2009; Choi, et al., 2010b; Mei, et al., 2010).
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It is important for beverage manufacturers to understand the composition of the oil phases
200
used to formulate commercial products, and to understand how specific lipophilic components
201
influence the formation, stability, and properties of final products.
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3.2. Droplet concentration
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In general, the concentration of droplets in an emulsion influences its texture, stability, appearance, sensory attributes, and nutritional quality (McClements, 2005b; McClements & Rao,
205
2011). Droplet concentration is usually characterized in terms of the dispersed phase volume
206
fraction (φ), which is the volume of emulsion droplets (VD) divided by the total volume of
207
emulsion (VE): φ = VD/VE. Practically, it is often more convenient to express the droplet
208
concentration in terms of the dispersed phase mass fraction (φm), which is the mass of emulsion
209
droplets (mD) divided by the total mass of emulsion (mE): φm = mD/mE. When the densities of the
210
two phases are equal, the mass fraction is equivalent to the volume fraction. It is particularly
211
important to convert the droplet concentration to the appropriate units when comparing
212
experimental work with theoretical predictions.
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In beverage emulsions, controlling the droplet concentration is important for a number of
214
reasons. Beverage emulsions are often prepared in a concentrated form (> 10% oil) because this
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facilitates handling and transport, but they are highly diluted when they are introduced into the
216
final product (< 0.1% oil). The amount to which an emulsion concentrate is diluted influences
217
the appearance of a final product, since emulsion turbidity or cloudiness increases with oil
218
droplet concentration. Dilution also influences the total amount of flavor molecules present in
219
final products, as well as their partitioning between the oil and water phases (Choi, et al., 2009).
220
In the concentrate, droplet concentration has a major impact on the rheological properties of the
221
system. From a practical point of view, it may be important to have a high oil loading in the
222
concentrate emulsion so as to reduce transport and storage costs, but not have the oil content so
223
high that the product is unstable or cannot easily be dispersed into the final product.
224
3.3. Droplet size distribution
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The size distribution of the droplets in a beverage emulsion has a strong impact on its
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physical stability (e.g., to gravitational separation, flocculation, coalescence and Ostwald
227
ripening) and its optical properties (e.g., lightness and color) (McClements, 2005b). Beverage
228
manufacturers must therefore specify the optimum droplet size distribution required for their
229
particular product based on the properties required, e.g., optical clarity and shelf-life. They must
230
then develop a formulation and manufacturing process that can reliably produce a beverage with
231
this droplet size distribution. Immediately after the product has been manufactured it is usually
232
important to measure the droplet size distribution to ensure that it has met the specified quality
233
criteria, e.g., using light scattering instruments. It may also be important to measure changes in
234
the droplet size distribution of the product during storage or after an accelerated storage test to
235
predict its long-term stability (McClements, 2007).
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The particle size distribution (PSD) of an emulsion specifies the concentration of droplets within different size classes, and can be conveniently measured using various commercially
238
available instruments (McClements, 2005b). When presenting or interpreting PSD data on a
239
beverage emulsion it is important to pay particular attention to the manner in which the particle
240
concentration and particle size are presented. The concentration of particles within a particular
241
size class is usually presented as either the volume or number percent, whereas the size of the
242
particles in a particular size class is usually presented as either the mid-point particle radius or
243
diameter. The same PSD may look very different if it is plotted as volume versus particle size or
244
as number versus particle size. Typically, particle volume versus particle diameter is the most
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widespread and informative way of presenting particle size data. Commercial beverage
246
emulsions are always polydisperse systems that can be characterized as being "monomodal",
247
"bimodal" or "multimodal" depending on whether there are one, two, or more peaks in the
248
particle size distribution. Typically, beverage manufacturers would like to produce a final
249
product that has a narrow monomodal distribution, as this usually provides the best long-term
250
stability.
251
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In many practical situations it is important to have knowledge of the full PSD of a beverage emulsion since this contains information about the size characteristics of all of the particles
253
present, as well as providing insights into the possible origin and nature of any instability
254
mechanisms. For example, it may be possible to detect a small population of large particles that
255
may cause problems with creaming during long-term storage (i.e., ringing). In addition, by
256
measuring changes in the PSD overtime it is sometimes possible to distinguish between different
257
instability mechanisms (e.g., coalescence versus Ostwald ripening). Nevertheless, in some
258
situations it is more convenient to represent the full particle size distribution by a measure of its
259
central tendency and spread. The mean, median, or modal particle sizes are often used as
260
measures of the central tendency, whereas the relative standard deviation is often used as a
261
measure of spread (Walstra, 2003). The mean particle size is the most widely used method of
262
representing the central tendency of emulsion particle size distributions in the beverage industry.
263
It is important to realize that a number of different mean particles sizes can be derived from
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a full PSD and each mean size can have a different magnitude and physical meaning
265
(McClements, 2007). The three most commonly used mean particle sizes are the number-
266
weighted mean diameter (dN or d10 = Σnidi/ Σni), the surface-weighted mean diameter (dS or d32 =
267
Σnidi3/ Σnidi2) and the volume-weighted mean diameter (dV or d43 = Σnidi4/ Σnidi3). Generally, the
268
volume-weighted mean diameter is more sensitive to the presence of large particles than the
269
number-weighted mean diameter, and so it often provides the most rigorous test of the physical
270
stability of a beverage emulsion, i.e., if d43 is small then the emulsion is more likely to remain
271
stable. Appreciable differences between the values of d10, d32 and d43 generally indicate that the
272
particle size distribution is broad or multimodal. One must therefore be very careful when
273
interpreting or reporting particle size data to identify which mean particle size value (and
274
corresponding relative standard deviation) is being used. It should also be noted that mean
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275
values should be treated with caution when used to represent highly polydisperse emulsions (e.g.,
276
aggregated systems), and it is always useful to examine the full particle size distribution.
277
Commercial beverage manufacturers usually develop a set of standardized particle size criteria that they use to determine whether a particular batch of product has the desired
279
physicochemical characteristics, e.g., long-term stability and optical properties. For example, a
280
manufacturer might specify that that mean droplet diameter (d43) of a particular class of products
281
should be < 500 nm, and that > 90% of the droplets should be smaller than 800 nm. The precise
282
criteria used will depend on the product being manufactured (especially whether it should be
283
clear or opaque).
284
3.4. Droplet charge
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The droplets in most beverage emulsions have an electrical charge because of adsorption of
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ionic species to their surfaces, e.g., proteins, ionic polysaccharides, ionic surfactants,
287
phospholipids, fatty acids, and some small ions (McClements, 2005b). The electrical
288
characteristics of a droplet surface depend on the type, concentration and organization of the
289
ionized species present, as well as the ionic composition and physical properties of the
290
surrounding aqueous phase. The electrical charge on the oil droplets in a beverage may be
291
important for a number of reasons: it determines the stability of the droplets to aggregation due
292
to its influence of the magnitude, range and sign of electrostatic interactions; it determines the
293
interactions of droplets with other charged species in an emulsion e.g., ions (such as calcium or
294
iron), or polyelectrolytes (such as proteins or polysaccharides); it influences how the droplets
295
interact with electrically charged surfaces, such as storage vessels, bottles, cups, and the mouth;
296
it influences the behavior of the droplets in an electrical field, which is important for measuring
297
their charge using electrophoresis.
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The electrical characteristics of a droplet in an emulsion are usually characterized in terms
299
of its surface charge density (σ), electrical potential (Ψ0), and/or ζ−potential (ζ) (Hunter, 1986).
300
The surface charge density is the amount of electrical charge per unit surface area, which
301
depends on the net number of ionized groups per unit interfacial area. The electrical potential is
302
the amount of energy required to increase the surface charge density from zero to σ. The
303
electrical potential depends on the surface charge density, but also on the ionic composition of
304
the surrounding medium due to electrostatic screening effects. At a fixed surface charge density,
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the electrical potential decreases with increasing ionic strength due to these effects. The zeta-
306
potential (ζ) is the electrical potential at the "shear plane", which is defined as the distance away
307
from the droplet surface below which the counter-ions remain strongly attached to the droplet
308
when it moves in an electrical field. Practically, the ζ-potential is a better representation of the
309
electrical characteristics of an oil droplet because it inherently accounts for the adsorption of any
310
counter ions or ionic species to the droplet surface. In addition, the ζ-potential is more
311
convenient to measure than the surface charge density or electrical potential (Hunter, 1986).
312
Typically, the electrical characteristics of the droplets in an emulsion are determined by
313
measuring the ζ-potential versus pH under appropriate measurement conditions (such as ionic
314
composition).
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Droplet aggregation is inhibited in many beverage emulsions by using ionic emulsifiers that
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adsorb to the droplet surfaces and prevent them from coming close together because of
317
electrostatic repulsion (Dickinson, 1992b; Friberg, et al., 2004; McClements, 2005b).
318
Electrostatic repulsion plays a major role in determining the aggregation stability of fat droplets
319
coated by charged emulsifiers that only form thin layers that generate short range steric
320
repulsion, such as globular proteins and ionic surfactants. On the other hand, electrostatic
321
repulsion is less important in systems where the fat droplets are coated by emulsifiers that form
322
thick interfacial layers that generate long range steric repulsion, such as polysaccharides (gum
323
arabic and modified starch). For electrostatically-stabilized emulsions, the magnitude of the ζ-
324
potential should be greater than about 20 mV to produce systems that are stable during long-term
325
storage. For sterically-stabilized emulsions, the droplet charge may not be important in terms of
326
their physical stability, but it may still be important in systems where chemical reactions occur
327
within the oil droplets that are induced by water-soluble ionic species, such as oxidation of ω-3
328
fatty acids by transition metals.
329
3.5. Interfacial properties
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The boundary between the oil and water phases in an emulsion consists of a narrow region
331
(≈ 1 to 50 nm thick) that surrounds each oil droplet, and contains a mixture of oil, water, and
332
emulsifier molecules, as well as possibly other molecular species, such as mineral ions,
333
polyelectrolytes, and polar lipids. The interfacial region makes up a significant fraction of the
334
volume of a droplet when the droplet diameter is less than about 1 µm (McClements & Rao,
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2011), and is therefore particularly important in beverage emulsions since they usually contain
336
droplets considerably smaller than this size . The interfacial region can influence many
337
important physicochemical and sensory properties of beverages emulsions, including their
338
stability, rheology, mouthfeel, and flavor. For this reason, it is often important to have
339
knowledge about the interfacial properties of the droplets in a beverage emulsion, and to
340
establish the major factors that influence them. Some of the most important properties of the
341
interfacial region are: composition; structural organization; thickness; rheology; interfacial
342
tension; and charge. These properties are determined by the type, concentration and interactions
343
of any surface-active species present, as well as by the events that occur before, during, and after
344
emulsion formation, e.g., complexation, competitive adsorption, layer-by-layer formation
345
(Dickinson, 2003). As mentioned earlier, the electrical charge on the droplet interface influences
346
its interaction with other charged molecules, as well as its stability to aggregation. The thickness
347
and rheology of the interfacial region influences the stability of emulsions to gravitational
348
separation, coalescence and flocculation, and determines the rate at which molecules leave or
349
enter the droplets (Dickinson, 2003; McClements, 2005b). For example, the ability of interfacial
350
coatings to prevent droplet flocculation is strongly influenced by their thickness.
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Beverage manufacturers should therefore be aware of the nature of the interfacial region
352
surrounding the oil droplets in their products, and the fact that they may be able to manipulate its
353
properties to improve product performance.
354
3.6. Colloidal interactions
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The attractive and repulsive colloidal interactions that operate between the oil droplets in beverage emulsions determine their stability to flocculation and coalescence, which in turn
357
influences their creaming stability and rheology (Friberg, et al., 2004; McClements, 2005b). The
358
colloidal interactions between two oil droplets can be described in terms of an interaction
359
potential (w(h)), which is the energy required to bring two droplets from an infinite distance
360
apart to a surface-to-surface separation of h (Figure 4). The overall interaction potential is made
361
up from contributions from various types of interactions, with the most important being van der
362
Waals, steric, electrostatic, depletion, and hydrophobic interactions (Israelachvili, 2011;
363
McClements, 2005b). These individual interactions vary in their sign (attractive or repulsive),
364
magnitude (weak to strong) and range (short to long). Each of the individual interactions usually
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has a simple dependence on surface-to-surface separation, but the sum of the interactions can
366
exhibit a more complex dependence. For example, the interaction potential between two oil
367
droplets coated by a layer of charged polymer molecules would have a number of maximum and
368
minimum values at certain separations, such as short- and long-range energy barriers, and
369
primary and secondary minima (Figure 4). Generally, droplets tend to aggregate when attractive
370
interactions dominate, but remain as individual entities when repulsive interactions dominate
371
(McClements, 2005b).
372
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It is particularly important for scientists working in the beverage industry to identify and understand the major colloidal interactions operating between the droplets in their particular
374
product. This knowledge can then be used to establish the optimum approach for maintaining
375
product stability during production, transport and storage. For example, if a beverage emulsion
376
is stabilized by a protein-based emulsifier, then electrostatic repulsive interactions will play an
377
important role in preventing droplet aggregation. In this situation, the system will be sensitive to
378
environmental changes that reduce the magnitude and range of the electrostatic repulsion acting
379
between droplets, such as altering the pH or adding salts (particularly multivalent counter-ions).
380
On the other hand, if the beverage emulsion is stabilized by a polysaccharide-based emulsifier,
381
then steric repulsive interactions will be most important for preventing droplet aggregation. In
382
this case, the product will be much less sensitive to droplet aggregation when the pH or ionic
383
strength is changed. In this latter case, emulsion stability depends on the thickness and
384
hydrophilicity of the interfacial layer, which will depend on the molecular characteristics of the
385
polysaccharide molecules. A summary of the major colloidal interactions in beverage emulsions
386
is given in Table 2.
387
4. Physicochemical Properties
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The physicochemical properties of beverage emulsions play an important role during the
389
manufacturing process, as well as in determining the perceived quality attributes of the final
390
product. The most important physicochemical attributes of these systems are briefly discussed in
391
this section. Some of these properties are more important in the concentrated form of the
392
beverage, whereas others are more important in the diluted form.
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393 394
4.1. Optical properties The first cue that a consumer uses to judge the quality or desirability of a finished beverage product is its visual appearance (provided it is packaged or poured into a transparent container,
396
such as a bottle or cup). Each type of beverage product is expected to have a particular
397
appearance depending on its nature, e.g., a dark brown cola, a cloudy orange juice, or a clear
398
green lime juice. From a scientific viewpoint, emulsion appearance is categorized in terms of
399
their opacity and color, which can be quantitatively described using tristimulus color coordinates,
400
such as the L*a*b* system (McClements, 2005b). In this color system, L* represents the
401
lightness, and a* and b* are color coordinates: where +a* is the red direction, -a* is the green
402
direction; +b* is the yellow direction, -b* is the blue direction; low L* is dark and high L* is
403
light. The opacity of an emulsion can therefore by characterized by the lightness (L*), while the
404
color intensity can be characterized by the chroma: C = (a*2 + b*2)1/2. The color intensity is
405
usually inversely related to the lightness, so that the chroma decreases (fades) when the lightness
406
increases. The optical properties of emulsions are mainly determined by the relative refractive
407
index, concentration, and size distribution of the droplets they contain (Chanamai &
408
McClements, 2002b; Danviriyakul, McClements, Decker, Nawar, & Chinachoti, 2002;
409
McClements, 2005b). The lightness of an emulsion tends to increase with increasing refractive
410
index contrast and increasing droplet concentration, and has a maximum value at a particular
411
droplet size. This has important implications for the development of beverage products that
412
should be either clear or opaque. In general, the lightness of emulsions increases steeply as the
413
oil droplet concentration increases from about 0 to 5 wt%, but then increases more gradually at
414
higher droplet concentrations (Figure 5).
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As mentioned earlier, some beverages are expected to have optical clarity, whereas others
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416
are expected to be cloudy. Optimizing the initial particle size distribution of a beverage
417
emulsion, as well as inhibiting any changes in the particle size during storage, is therefore a
418
particularly important part of designing a commercial product with the desired optical properties.
419
For clear products, the majority of droplets should be less than about 50 nm in diameter so that
420
the light scattering is very weak (Wooster, Golding, & Sanguansri, 2008a). The scattering
421
efficiency of the individual oil droplets determines the maximum amount of oil phase that can be
422
incorporated into a clear beverage before it becomes noticeably cloudy. As a rule of thumb, a
423
turbidity of 0.05 cm-1 (at 600 nm) can be considered to be a rough cut-off point between
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transparent and cloudy products. For cloudy products, the majority of droplets should be
425
between about 200 and 400 nm in diameter so that the light scattering is very strong
426
(McClements, 2002). In this case, the scattering efficiency of the individual oil droplets will
427
determine the minimum amount of a clouding emulsion required to reach a particular turbidity in
428
the final product.
429
4.2. Rheology
430
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The rheological properties of beverage emulsions are also an important factor determining their manufacture and utilization. Most beverage emulsions are initially manufactured in a
432
concentrated form, which is diluted appreciably during the production of the final beverage
433
product. The droplet concentration in the beverage concentrate typically ranges from 3 to 30%,
434
while that in the final product is typically < 0.1%. Industrially, the rheology of the beverage
435
concentrate is important since it influences the ease of mixing, flow through a pipe, and
436
packaging. A manufacturer typically wants to have as high an oil loading as possible, without
437
the product becoming too viscous or gel-like to handle easily. This requires careful control of
438
the total droplet concentration in the system. The droplet concentration in the final beverage
439
concentration is usually so low that the rheology is dominated by the properties of the aqueous
440
continuous phase (see discussion below).
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431
The rheology of dilute colloidal dispersions is normally characterized by the shear viscosity
442
(Genovese, Lozano, & Rao, 2007; McClements, 2005b). When the droplet concentration is less
443
than about 5% (φ < 0.05), the shear viscosity can be described by Einstein’s equation:
444
446 447
η = η 0 (1 + 2.5φ )
(1)
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441
Here, η is the viscosity of the overall system, η0 is the viscosity of the continuous phase, and
448
φ is the disperse phase volume fraction. This equation under-predicts the viscosity of colloidal
449
dispersions with higher droplet concentrations due to the effects of droplet-droplet interactions.
450
Einstein’s equation was derived assuming that the colloidal particles were rigid isolated spheres
451
surrounded by a Newtonian fluid. Nevertheless, it still provides a good approximation of the
452
rheological properties of dilute beverage emulsions because the flow of the liquid within the oil
453
phase is inhibited by the emulsifier coating. The above equation shows that the viscosity of a
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dilute emulsion increases linearly with droplet concentration, but that the most important factor
455
affecting the overall rheology is the viscosity of the continuous phase. Thus the most effective
456
means of controlling the viscosity of a dilute beverage emulsion is to change the viscosity of the
457
continuous phase, e.g., by adding sugars or polymer thickening agents.
458
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The viscosity of concentrated emulsions can be described by a semi-empirical equation that
459
takes into account droplet-droplet interactions (Berli, Deiber, & Quemada, 2005; McClements,
460
2005b; Quemada & Berli, 2002): −2
461
φ η = η 0 1 − φc
462
Here, φ is the disperse phase volume fraction, and φc (≈ 0.63) is a critical disperse phase
(2)
SC
volume fraction above which the droplets are so closely packed together that they cannot easily
464
flow past each other. This equation shows that the viscosity of an emulsion increases with
465
increasing droplet concentration, gradually initially and then steeply as the droplets become more
466
closely packed (Figure 5). Around and above the droplet concentration where close packing
467
occurs, the emulsion becomes highly viscosity and may exhibit solid-like characteristics, such as
468
visco-elasticity and plasticity (Berli, et al., 2005; McClements, 2005b; Quemada & Berli, 2002).
469
In flocculated systems the critical concentration where the system becomes highly viscous or
470
solid-like may be much lower than in a non-flocculated system. It is therefore important for
471
beverage manufactures to consider the influence of droplet concentration and interactions on the
472
rheological properties of emulsion concentrates (Genovese, et al., 2007; McClements, 2005b;
473
Walstra, 2003).
474
4.3. Molecular distribution and release characteristics
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A beverage emulsion may contain a number of constituents that partition into different
476
phases within the product, e.g., oil, aqueous, interfacial, or gas phases (McClements, 2005b).
477
The physical location of some of these constituents may have a major impact on the quality
478
attributes of the final product. For example, many flavor oils contain constituents that are
479
chemically unstable and therefore prone to degradation during storage (e.g., citral). The
480
chemical stability of these constituents is often dependent on their molecular environment. For
481
example, the rate of acid-catalyzed degradation of citral has been shown to occur considerably
482
faster when it is located within an aqueous phase than when it is present within an oil phase
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(Choi, et al., 2009; Choi, et al., 2010b). This would suggest that it is better to keep citral under
484
neutral conditions or within the emulsion concentrate as long as possible before the final dilution
485
into the acid phase is carried out. The perceived flavor profile of beverage emulsions depends on
486
the distribution of volatile molecules between the liquid and gas phases. Increasing the oil
487
content of an emulsion decreases the concentration of hydrophobic (KOW > 1) volatiles in the
488
headspace and therefore reduces the perceived flavor profile (Figure 6). This phenomenon is
489
important to take into account when reformulating a beverage product so that it contains a
490
different fat concentration, e.g., fortification with a bioactive lipid such as ω-3 oils.
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The location of a constituent within a beverage emulsion is governed by its equilibrium
492
partition coefficients (e.g., oil-water, oil-air, oil-interface) and its mass transport kinetics through
493
the system (McClements, 2005b). When a beverage emulsion is placed in the mouth there is a
494
redistribution of flavor molecules, with some of the aroma compounds leaving the product and
495
entering the nasal cavity. The rate at which flavor molecules leave the droplets in beverage
496
emulsions is usually extremely quick (< 0.1 s for KOW < 1000), and therefore droplet dimensions
497
tend to have little impact on the flavor release profile (McClements, 2005b). Nevertheless, it
498
may be possible to encapsulate oil droplets within hydrogel matrices to slow down the release of
499
flavor molecules within the mouth.
500
5. Beverage Emulsion Shelf-Life
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One of the most important factors determining the commercial viability of beverage
502
emulsions is their ability to resist changes in their physical and chemical properties after their
503
production. Beverage emulsions experience a range of environmental stresses during their
504
manufacture, transport, storage, and utilization that may reduce their shelf lives: mechanical
505
forces (e.g., stirring, flow through a pipe, centrifugation, vibrations, and pouring); temperature
506
variations (e.g., freezing, chilling, warming, pasteurization, and sterilization); exposure to light
507
(e.g., natural or artificial visible or ultraviolet waves); exposure to oxygen; variations in solution
508
properties (e.g., pH and mineral composition of water). Exposure to these environmental
509
stresses may promote emulsion instability through a variety of physicochemical mechanisms:
510
loss of ingredient functionality (e.g., changes in solubility, surface activity, or stabilization
511
capacity); acceleration of chemical degradation reactions (e.g., oxidation, polymerization, or
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hydrolysis); acceleration of physical instability mechanisms, (e.g., flocculation, coalescence or
513
Ostwald ripening). In this section, a brief overview of some of the major instability mechanisms
514
in beverage emulsions is given, and some suggestions for preventing them from occurring are
515
provided.
516
5.1. Physical Stability
517
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Emulsions are thermodynamically unfavorable systems that tend to break down over time due to a variety of physicochemical mechanisms (Figure 2), including gravitational separation,
519
flocculation, coalescence and Ostwald ripening (Dickinson, 1992a; Friberg, et al., 2004;
520
McClements, 2005b). All of these instability mechanisms lead to a change in the structural
521
organization of the various components within the system, rather than in the type of molecules
522
present. Nevertheless, changes in the chemical structure of active components can lead to
523
changes in physical stability, and vice versa.
524
5.1.1. Gravitational Separation
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Gravitational separation is one of the most common forms of physical instability in commercial beverage emulsions, and it may take the form of either creaming or sedimentation
527
depending on the relative densities of the oil droplets and the surrounding aqueous phase.
528
Creaming is the upward movement of droplets when they have a lower density than the aqueous
529
phase, whereas sedimentation is the downwards movement of droplets when they have a higher
530
density than the aqueous phase. The oil phases used in beverage emulsions consist primarily of
531
triacylglycerol and/or flavor oils, which have lower densities than water and so creaming is more
532
prevalent (Table 3). However, if a beverage emulsion contained an excess of weighting agent
533
within the oil phase then it may be prone to sedimentation. A beverage emulsion is also prone to
534
sedimentation if it contains very small oil droplets covered by relatively thick and dense
535
interfacial layers (see below) (McClements, 2011).
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536
One of the most common problems reported in beverage emulsions is “ringing”, which is the
537
accumulation of a visible ring of oil droplets on the top of a product. This effect is due to droplet
538
creaming, which may have occurred because a population of droplets in the initial emulsion was
539
too large, or because some droplet growth occurred during storage, e.g., due to flocculation,
540
coalescence, or Ostwald ripening (see later sections). To a first approximation, the velocity that
541
an oil droplet moves upwards in a dilute emulsion due to gravity is given by Stokes’ law:
ACCEPTED MANUSCRIPT 20 2 2 grparticle ( ρ particle − ρ 0 )
542
v=−
543
where, v is the creaming velocity, rparticle is the particle radius, ρparticle is the particle
544
density, ρ0 is the aqueous phase density, η0 is the aqueous phase viscosity, and g is the
545
acceleration due to gravity. This equation shows that the rate of droplet creaming should
546
decrease as the droplet size decreases, the density contrast decreases, or the aqueous phase
547
viscosity increases. Gravitational forces cause droplets to move either upwards or downwards
548
depending on their density relative to the surrounding aqueous phase. Hence, if only
549
gravitational forces operated, then the droplets would accumulate at either the top or the bottom
550
of an emulsion. In practice, droplets may also move because of Brownian motion associated
551
with the thermal energy of the system. Brownian motion favors the random distribution of the
552
droplets throughout the entire volume of the emulsion, rather than their accumulation at either
553
the top or bottom.
554
containing relatively large droplets (r > 100 nm), whereas Brownian motion forces tend to
555
dominate droplet movement in emulsions containing smaller droplets (McClements, 2011).
556
Consequently, emulsions become more stable to creaming or sedimentation as the particle size
557
decreases because the creaming velocity decreases (v ∝ r2) and because Brownian motion effects
558
increase.
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Gravitational forces tend to dominate droplet movement in emulsions
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(3)
9η 0
The above calculations assume that the particles in beverage emulsions are homogeneous spheres consisting entirely of oil phase. In practice, the particles in beverage emulsions actually
561
have a core-shell structure, consisting of an oil core and an interfacial shell. In this case, the
562
overall particle radius is given by rparticle = rcore + δ, and the overall particle density (ρparticle)
563
depends on the densities of the core (ρC) and shell (ρS) materials and the volume fraction of the
564
shell (Φ S):
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565
ρ particle = Φ S ρ S + (1 − Φ S ) ρ C
566
The shell layer usually has a higher density than the oil or aqueous phases, so that an
(4)
567
increase in the volume fraction of the shell layer will tend to increase the overall particle density.
568
This has important implications for preventing gravitational separation in beverage emulsions
569
with small droplets sizes since it reduces the density contrast between the particles and aqueous
570
phase. In addition, very small particles may actually sediment rather than cream if they contain
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571
sufficiently thick and dense emulsifier layers. Thus, it should be possible to produce density
572
matched particles in beverage emulsions by controlling the oil core size and the thickness of the
573
adsorbed emulsifier layer. The above discussion has highlighted a number of approaches that can be used to inhibit or
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prevent gravitational separation in beverage emulsions. First, gravitational separation can be
576
prevented by matching the density of the dispersed (oil) and continuous (aqueous) phases. The
577
density of the aqueous phase typically varies from about 1000 to 1050 kg m-3, depending on the
578
amount of sugars and other solutes present (Table 3). The density of most oil phases is less than
579
this value, and therefore oil droplets will tend to move upwards. As already mentioned, the
580
density of the core-shell particles within a beverage emulsion can be matched to the surrounding
581
aqueous phase by adding a weighting agent to the oil phase, or by controlling the thickness and
582
density of the emulsifier layer. Second, gravitational separation can be inhibited by reducing the
583
size of the droplets in the emulsion, since the creaming velocity is proportional to the droplet size
584
squared (Stoke’s Law). If the droplets are sufficiently small, then Brownian motion effects will
585
dominate and the system will remain stable to creaming or sedimentation. Third, gravitational
586
separation can be inhibited by increasing the viscosity of the aqueous phase, e.g., by adding
587
thickening or gelling agents. This approach may not always be viable since it will also influence
588
the texture and mouthfeel of the final product.
M AN U
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589
SC
575
Another approach some beverage manufacturers have used to mask the undesirable effects of creaming (“ringing”) on the appearance of a product is to design the packaging so as to
591
obscure the effect, e.g., with appropriate placement of the labels or cap.
592
5.1.2. Droplet Aggregation
The aggregation state of the droplets in a beverage emulsion is important because it
AC C
593
EP
590
594
influences the stability of the product to gravitational separation. Changes in particle size during
595
storage may also influence other important quality attributes of beverage products, such as their
596
appearance (cloudiness or homogeneity). The tendency for droplet aggregation to occur in a
597
beverage emulsion depends on the balance of attractive and repulsive forces operating between
598
the droplets (see earlier). The nature of the colloidal interactions operating in a particular
599
beverage emulsion depends on the physicochemical properties of the oil, water and interfacial
600
phases (e.g., dielectric constant and refractive index), oil core characteristics (such as radius),
ACCEPTED MANUSCRIPT 22
interfacial shell characteristics (such as thickness, charge, packing, rheology and
602
hydrophobicity), and the properties of the intervening fluid (such as pH, ionic strength, osmotic
603
pressure, and temperature). To a first approximation the overall colloidal interactions between a
604
pair of droplets in a beverage emulsion can be described by the sum of the van der Waals (wVDV),
605
electrostatic (wE), and steric (wS) interactions (McClements, 2005b):
606 607
w(h) = wVDV(h) + wE(h) + wS(h)
(5)
608
The van der Waals interactions are attractive, whereas the steric and electrostatic
SC
609
RI PT
601
interactions are usually repulsive (Table 2). The van der Waals attraction operates between all
611
kinds of droplets and would always cause aggregation if there were no opposing repulsive forces.
612
The magnitude and range of the steric repulsion depend on the thickness and chemistry of the
613
interfacial layer, whereas the magnitude and range of the electrostatic repulsion depend on the
614
droplet charge (ζ-potential) and the ionic composition of the aqueous phase. To design a product
615
that is stable to droplet aggregation one must assure that the repulsive interactions dominate the
616
attractive interactions. This is usually achieved by using an emulsifier that generates repulsive
617
interactions between the droplets. The emulsifiers used in the beverage industry typically
618
stabilize the droplets against aggregation by generating steric and/or electrostatic repulsive
619
interactions. Emulsifiers that form relatively thick open interfaces (such as polysaccharides and
620
non-ionic surfactants with large hydrophilic head-groups) can generate a steric repulsion that is
621
sufficient strong and long range to overcome the attractive van der Waals interactions, and
622
thereby stabilize the system against aggregation. Emulsifiers that form highly charged interfaces
623
(such as proteins and ionic surfactants) can generate a strong electrostatic repulsion between
624
droplets that prevent aggregation. However, emulsifiers that can only stabilize emulsions due to
625
electrostatic interactions may be prone to instability when the pH or ionic strength is changed.
626
Some emulsifiers use a combination of electrostatic and steric repulsion to stabilize the system,
627
e.g., such as casein and whey proteins.
AC C
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610
628
The droplets in emulsions are in continual motion because of the effects of thermal energy,
629
gravity, or applied mechanical forces, and as they move about they frequently collide with their
630
neighbors. After a collision, emulsion droplets may either move apart or remain aggregated,
631
depending on the relative magnitude of the attractive and repulsive interactions between them.
ACCEPTED MANUSCRIPT 23
Droplets aggregate when there is a primary or secondary minimum in the interaction potential
633
that is sufficiently deep and accessible to the droplets (Figure 4). The two major types of
634
aggregation in beverage emulsions are flocculation and coalescence.
635 636
5.1.2.1. Flocculation Droplet flocculation is the process whereby two or more droplets come together to form an
637
aggregate in which the droplets retain their individual integrity (Figure 2). Droplet flocculation
638
is usually detrimental to beverage emulsion quality because it accelerates the rate of gravitational
639
separation thereby reducing their shelf-life. Flocculation can also cause an appreciable increase
640
in the viscosity of beverage emulsion concentrates, and may even lead to the formation of a gel.
641
This may be undesirable since it would influence the transport, handling and dispersibility of the
642
product. Flocculation may occur in beverage emulsions through a variety of different processes
643
that either increase the attractive forces or decrease the repulsive forces operating between the
644
droplets. The mechanism that is important in a particular emulsion depends largely on the nature
645
of the emulsifier used and the solution conditions (e.g., pH, ion type and concentration, and
646
functional ingredients).
SC
M AN U
647
RI PT
632
Reduced electrostatic repulsion: Electrostatically stabilized emulsions may flocculate when the electrostatic repulsion between the droplets is reduced. A number of physicochemical
649
changes may cause this reduction in electrostatic repulsion (Israelachvili, 2011): (i) the pH is
650
altered so that the net charge on the droplets is reduced; (ii) counter-ions bind to the surface of
651
the droplets and reduce their charge (“charge neutralization”); (iii) the ionic strength of the
652
aqueous phase is increased to screen the electrostatic interactions (“electrostatic screening”).
653
Protein-coated oil droplets are particularly sensitive to flocculation due to reduction in the
654
electrostatic repulsion between them when the pH or ionic composition is altered (Demetriades,
655
Coupland, & McClements, 1997a; McClements, 2004).
EP
AC C
656
TE D
648
Increased depletion attraction: The presence of non-adsorbing colloidal entities in the
657
continuous phase of an emulsion, such as biopolymers or surfactant micelles, generates an
658
increase in the attractive force between the droplets due to an osmotic effect associated with the
659
exclusion of the colloidal entities from a narrow region surrounding each droplet (Israelachvili,
660
2011). This attractive force increases as the concentration of colloidal entities increases, until
661
eventually it becomes large enough to overcome the repulsive interactions between the droplets and
ACCEPTED MANUSCRIPT 24
causes them to flocculate. This type of droplet aggregation is usually referred to as depletion
663
flocculation. The presence of relatively high concentrations of non-adsorbed biopolymer
664
emulsifiers (gum arabic and modified starch) have been shown to induce depletion flocculation in
665
model beverage emulsions (Chanamai & McClements, 2001). Depletion flocculation may also be
666
promoted by other kinds of biopolymers that might be used in beverages, such as maltodextrin,
667
pectin, xanthan gum, and carrageenan (Cao, Dickinson, & Wedlock, 1990; Cho & McClements,
668
2009; Gu, Decker, & McClements, 2004; Gunning, Hibberd, Howe, & Robins, 1988).
669
RI PT
662
Increased hydrophobic interactions: This type of interaction is important in emulsions that contain droplets that have some non-polar regions exposed to the aqueous phase. A good
671
example of this type of interaction is the effect of thermal processing on the flocculation stability
672
of oil-in-water emulsions stabilized by globular proteins (Demetriades, Coupland, &
673
McClements, 1997b). At room temperature, whey protein stabilized emulsions (pH 7) are stable
674
to flocculation because of the large electrostatic repulsion between the droplets, but when they
675
are heated above 70 oC they become unstable. The globular proteins adsorbed to the surface of
676
the droplets unfold above this temperature and expose non-polar amino acids that were originally
677
located in their interior. Exposure of these non-polar amino acids increases the hydrophobic
678
character of the droplet surface and therefore leads to flocculation because of the increased
679
hydrophobic attraction between the droplets.
TE D
M AN U
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670
Formation of biopolymer bridges: Many types of biopolymer promote flocculation by
681
forming bridges between two or more droplets. Biopolymers may adsorb either directly to the
682
bare oil surfaces of the droplets or to the adsorbed emulsifier molecules that form the interfacial
683
layer. To be able to bind to the droplets there must be a sufficiently strong attractive interaction
684
between segments of the biopolymer and the droplet surface. The most common types of
685
interaction that operate in food emulsions are hydrophobic and electrostatic (Dickinson, 2003).
686
For example, a positively charged biopolymer (such as chitosan) might adsorb to the surface of
687
two negatively charged emulsion droplets causing them to flocculate (Ogawa, Decker, &
688
McClements, 2003) or a negatively charged biopolymer (such as pectin, carrageenan or xanthan)
689
might adsorb to the surface of two positively charged droplets causing them to flocculate
690
(Dickinson, 2003; Guzey & McClements, 2006).
691 692
AC C
EP
680
The development of a suitable strategy to prevent droplet flocculation in a particular beverage emulsion therefore depends on identification of the physicochemical origin of
ACCEPTED MANUSCRIPT 25
flocculation in that system. In general, flocculation can be prevented by ensuring that the
694
repulsive forces dominate the attractive forces, and that there are no additives that can promote
695
bridging.
696 697
5.1.2.2. Coalescence Coalescence is the process whereby two or more liquid droplets merge together to form a
698
single larger droplet (Figure 2). Coalescence causes emulsion droplets to cream or sediment
699
more rapidly because of the increase in their particle size. In beverage emulsions, coalescence
700
eventually leads to the formation of a layer of oil on top of the material, which is referred to as
701
oiling off. This process is one of the main reasons for the shiny oily layers often seen on top of
702
unstable beverage emulsions.
SC
The susceptibility of a beverage emulsion to droplet coalescence is highly dependent on the
M AN U
703
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693
nature of the emulsifier used to stabilize the system, since this instability mechanism involves
705
two or more droplets fusing together. In general, the susceptibility of oil droplets to coalescence
706
is determined by the nature of the forces that act between the droplets (i.e. gravitational,
707
colloidal, hydrodynamic and mechanical forces) and the resistance of the interfacial layer to
708
rupture. The stability of emulsions to coalescence can be improved by preventing the droplets
709
from coming into close proximity for extended periods, e.g., by preventing droplet flocculation,
710
preventing the formation of a creamed layer, or having too high droplet concentrations
711
(McClements, 2005b). Alternatively, one can control the properties of the interfacial layer
712
surrounding the oil droplets to make it more resistant to rupture, e.g., by selecting an appropriate
713
emulsifier or other additives that alter surface properties.
714
5.1.3. Ostwald Ripening
EP
AC C
715
TE D
704
This susceptibility of a beverage emulsion to Ostwald ripening (OR) is mainly determined
716
by the solubility of the oil phase in the aqueous phase: the higher the solubility, the more
717
unstable the emulsion. Oil phases with very low water-solubilities (such as the vegetable oils
718
used in clouding emulsions) do not exhibit OR, but oil phases with relatively high water-
719
solubilities (such as flavor or essential oils) may be highly unstable. Mechanistically, OR is the
720
process whereby the size of the oil droplets in an oil-in-water emulsion increases over time due
721
to diffusion of oil molecules from small to large droplets through the intervening aqueous phase
722
(Kabalnov, 2001; Kabalnov & Shchukin, 1992). The driving force for this effect is the fact that
ACCEPTED MANUSCRIPT 26
the water-solubility of an oil contained within a spherical droplet increases as the radius of the
724
droplet decreases, which means that there is a higher concentration of solubilized oil molecules
725
in the aqueous phase surrounding a small droplet than surrounding a larger one (Kabalnov &
726
Shchukin, 1992; McClements, 2005b). The presence of this concentration gradient means that
727
solubilized oil molecules tend to move from the immediate vicinity of smaller droplets to that of
728
larger droplets. This leads to an increase in mean droplet size over time, which can be described
729
by the following equation once steady state conditions have been achieved (Kabalnov &
730
Shchukin, 1992):
RI PT
723
732
d (t ) 3 − d ( 0 ) 3 = ω t =
32 9
αS ∞ Dt
(6)
M AN U
733 734
SC
731
Here, d(t) is the number-weighted mean droplet diameter at time t, d0 is the initial number-
735
weighted mean droplet diameter, ω is the Ostwald ripening rate, α =2γVm/RT, S∞ is the water-
736
solubility of the oil phase in the aqueous phase, D is the translational diffusion coefficient of the
737
oil molecules through the aqueous phase, Vm is the molar volume of the oil, γ is the oil-water
738
interfacial tension, R is the gas constant, and T is the absolute temperature. The most important factor determining the stability of a beverage emulsion to OR is the
740
water-solubility of the oil phase (S∞) (Weiss, Herrmann, & McClements, 1999). For this reason
741
OR is not usually a problem for emulsions prepared using oils with a very low water-solubility,
742
such as long chain triglycerides (e.g., corn, soy, sunflower, or fish oils). On the other hand, OR
743
may occur rapidly for emulsions prepared using oils with an appreciable water-solubility, such as
744
flavor oils and essential oils (Li, Le Maux, Xiao, & McClements, 2009; McClements, et al.,
745
2012; Wooster, Golding, & Sanguansri, 2008b). OR can be retarded in these systems by adding
746
a substance known as a ripening inhibitor. A ripening inhibitor is a non-polar molecule that is
747
soluble in the oil phase but insoluble in the water phase, e.g., a long chain triacylglycerol (such
748
as corn oil). This type of molecule can inhibit OR by generating an entropy of mixing effect that
749
counter-balances the curvature effects.
AC C
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739
750
Consider an oil-in-water emulsion that contains droplets comprised of two different lipid
751
components: a water-insoluble component and a water-soluble component. The water-soluble
752
molecules will diffuse from the small to the large droplets due to OR. Consequently, there will
753
be a greater concentration of water-insoluble molecules in the smaller droplets than in the larger
ACCEPTED MANUSCRIPT 27
droplets after OR occurs. Differences in the composition of emulsion droplets are
755
thermodynamically unfavorable because of the entropy associated with mixing: it is more
756
favorable to have the two lipids distributed evenly throughout all of the droplets, rather than to
757
be located in particular droplets. Consequently, there is a thermodynamic driving force that
758
operates in the opposite direction to the OR effect. The change in droplet size distribution with
759
time then depends on the concentration and solubility of the two components within the oil
760
droplets. This approach has previously been used to improve the stability of food-grade
761
nanoemulsions, such as those containing short chain triglycerides, essential oils, and flavor oils
762
(Li, et al., 2009; McClements, et al., 2012; Wooster, et al., 2008b). An example of this effect is
763
shown in Figure 7 which shows that droplet growth in orange oil-in-water emulsions during
764
storage can be inhibited by adding a sufficiently high concentration of corn oil (the ripening
765
inhibitor) (McClements, et al., 2012),. Orange oil (4-fold) has a relatively high solubility in
766
water, and therefore is highly prone to OR, which leads to an appreciable increase in mean
767
droplet size during storage. On the other hand, corn oil has a very low solubility in water, and
768
therefore it can retard OR if it is incorporated into the oil phase prior to homogenization. These
769
results show that incorporating ≥ 10% corn oil into the oil phase was sufficient to inhibit OR in
770
these systems (Figure 7). OR may also be retarded by adding certain kinds of weighting agents
771
(such as ester gums) since these substances also have a very low water solubility and therefore
772
act as ripening inhibitors (Lim, et al., 2011).
773
5.2. Chemical Stability
SC
M AN U
TE D
EP
774
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754
A number of lipophilic compounds that may be present in beverage emulsions can undergo chemical degradation during storage, which leads to a loss of color, flavor and/or nutrients. A
776
few representative examples of chemical degradation of lipophilic components in oil-in-water
777
emulsions are given below.
778
AC C
775
Citrus Degradation. Several mechanisms lead to the chemical decomposition of citrus flavor
779
components (such as citral, d-limonene, and citronellal), including oxidation, hydrolytic
780
reactions, the formation of terpene alcohols, and the cyclization of terpene aldehydes (Clark,
781
Powell, & Radford, 1977; Kimura, Iwata, & Nishimura, 1982; Kimura, Nishimura, Iwata, &
782
Mizutani, 1983a, 1983b). Acid-catalyzed decomposition and oxidation reactions change the
783
desirable flavor profile of citrus oils by reducing the concentration of desirable flavor
ACCEPTED MANUSCRIPT 28
784
components and increasing the concentration of undesirable flavor components (Tan, 2004;
785
Ueno, Masuda, & Ho, 2004). The beverage industry would therefore like to identify effective
786
strategies for preventing these undesirable chemical degradation reactions. There has been a great deal of research on establishing the major factors that influence the
RI PT
787
chemical degradation of citral because this is one of the most important flavor compounds found
789
in commercial beverages. The degradation rate of citral in aqueous solutions has been shown to
790
increase with decreasing pH (Choi, et al., 2009) (Figure 8). Most commercial beverages have
791
acidic aqueous phases and are therefore highly susceptible to flavor loss during storage due to
792
this acid-catalyzed mechanism. The chemical stability of citral has been shown to be much
793
higher when it is located within an oil phase than in an aqueous phase (Choi, et al., 2009).
794
Consequently, the chemical degradation of citral in beverage emulsions can be improved by
795
ensuring that the citral molecules are located primarily in an oil phase rather than in the aqueous
796
phase. Indeed, studies have shown that citral stability can be improved by increasing the oil
797
droplet concentration (Choi, et al., 2009) or by adding surfactant micelles to the aqueous phase
798
(Choi, et al., 2010b), although these strategies may not be practical for most commercial
799
products. It was proposed that citral stability may be improved by encapsulating it within solid
800
lipid particles rather than within liquid oil droplets, however the opposite was found to be true
801
experimentally, which was attributed to the expulsion of the citral molecules into the aqueous
802
phase after droplet crystallization (Mei, et al., 2010). Addition of various kinds of natural
803
antioxidants to flavor oil emulsions has also been shown to improve the stability of citral to
804
chemical degradation (Yang, Tian, Ho, & Huang, 2011). The oil droplets in beverage emulsions
805
are surrounded by a coating of emulsifier molecules, and so it may be possible to improve the
806
stability of the citral molecules within them by engineering the properties of the interfacial layer
807
(Decker & McClements, 2001; Given Jr., 2009). Indeed, studies have shown that citral
808
degradation was faster in flavor oil droplets coated by an anionic surfactant than those coated by
809
a non-ionic or cationic surfactant, which was attributed to differences in the accumulation of
810
catalytic protons near the droplet surfaces (Choi, Decker, Henson, Popplewell, & McClements,
811
2010a). A high local concentration of protons is believed to accelerate the citral degradation
812
mechanism at the droplet surfaces. Coating flavor oil droplets with a cationic biopolymer layers
813
has also been shown to improve the stability of citral to chemical degradation (Djordjevic,
814
Cercaci, Alamed, McClements, & Decker, 2007, 2008; Yang, Tian, Ho, & Huang, 2012)
AC C
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788
ACCEPTED MANUSCRIPT 29
815
Polyunsaturated Lipid Degradation. There has been great interest in the beverage industry in fortifying products with ω-3 lipids (such as flax, fish and algal oils) since these lipids have
817
been claimed to have health benefits and are currently under-consumed by the general
818
population. Nevertheless, there are many technical difficulties associated will incorporating
819
these lipids into beverage products due to their high susceptibility to oxidation. Lipid oxidation
820
affects the quality of emulsion-based products, influencing their flavor, odor, and nutritive value
821
(Frankel, Satué-Gracia, Meyer, & German, 2002). The oxidation of polyunsaturated lipids is a
822
highly complex series of chemical reactions that is initiated when a lipid interacts with an
823
oxygen reactive species, and proceeds through molecular cleavage and oxygen addition reactions
824
to the formation of a wide variety of volatile compounds (McClements & Decker, 2000; Waraho,
825
McClements, & Decker, 2011). The rate at which oxidation takes place is dependent on several
826
factors: the molecular structure of the lipids; storage conditions; the presence of pro-oxidants and
827
antioxidants; and the structural organization of the system. Based on this knowledge a variety of
828
strategies have been developed to inhibit or prevent lipid oxidation in emulsified products:
829
addition of oil-soluble and water-soluble antioxidants; chelation of pro-oxidant transition metals;
830
engineering the interface to prevent pro-oxidants from coming into close proximity to lipid
831
substrates; controlling environmental conditions, such as exposure to heat, oxygen, or light.
832
Carotenoid degradation. Carotenoids are natural compounds found in many fruits and
833
vegetables that are may be used in foods an colorants or nutraceuticals because of their potential
834
health benefits (Mayne, 1996; Ryan, O'Connell, O'Sullivan, Aherne, & O'Brien, 2008). One of
835
the major factors currently limiting the incorporation of carotenoids into many food and
836
beverage products is their high susceptibility to chemical degradation. In particular, carotenoids
837
have a conjugated polyunsaturated hydrocarbon chain that makes them highly prone to
838
autoxidation (Boon, McClements, Weiss, & Decker, 2009). A number of factors have previously
839
been shown to promote the oxidation of carotenoids, including highly acidic environments
840
(Konovalov & Kispert, 1999), light (Mortensen & Skibsted, 1996), heat (Mader, 1964), singlet
841
oxygen (Krinsky, 1998), transition metals (Gao & Kispert, 2003; Williams, et al., 2001), and free
842
radicals (Liebler & McClure, 1996; Woodall, Lee, Weesie, Jackson, & Britton, 1997). Once
843
carotenoid degradation has been initiated a number of secondary reaction products may form,
844
including epoxides, endoperoxides, apocarotenals and apocarotenones (Gao & Kispert, 2003;
AC C
EP
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816
ACCEPTED MANUSCRIPT 30
845
Woodall, et al., 1997; Yamauchi, Miyake, Inoue, & Kato, 1993). The chemical degradation of
846
carotenoids leads to color fading, and may reduce their beneficial health properties.
847
Recent studies have examined the influence of interfacial properties (i.e., emulsifier type), storage conditions (i.e., pH, ionic strength, and temperature) and antioxidant addition (i.e.,
849
vitamin E, Coenzyme Q10, EDTA and ascorbic acid) on the chemical degradation of β-carotene
850
encapsulated within oil-in-water nanoemulsions (Qian, Decker, Xiao, & McClements, 2012).
851
The rate of β-carotene degradation was found to increase with decreasing pH and increasing
852
temperature, was faster for a non-ionic surfactant (Tween 20) than for a protein (β-
853
lactoglobulin), and decreased with increasing antioxidant addition to either the oil or aqueous
854
phase.
855
5.3. Defining the End of Shelf Life
SC
M AN U
856
RI PT
848
The end of the shelf life of a product can be defined as the time when it becomes unacceptable to consumers, which depends on the rate of the various physical and chemical
858
instability mechanisms occurring. A product may become unacceptable when a ring of oil
859
droplets is visible at the top of the bottle, when the flavor components decompose/oxidize and
860
create an unacceptable flavor profile, when the color changes beyond an acceptable level, or
861
when the product is microbiologically unsafe to consume. A beverage manufacture should
862
establish quantitative criteria that can be used to establish the end of the shelf life of their
863
particular product. They should then develop a systematic testing scheme that can be used to
864
predict the shelf life of products.
865
6. Beverage Emulsion Manufacture
AC C
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857
866
Beverage emulsions are usually prepared using a two-step process: a beverage emulsion
867
concentrate (3 – 30 wt% oil) is prepared, which is then diluted extensively to create the finished
868
product (< 0.1 wt% oil) (Tan, 2004). In this section, we briefly describe the major characteristics
869
of the concentrate and finished products, and then review some of the major manufacturing
870
approaches for forming stable beverage emulsions.
871
Beverage emulsion concentrates: When beverages are prepared using high energy
872
homogenization methods (see below), all the aqueous phase components are usually mixed
873
together and all of the oil phase components are mixed together prior to homogenization. The
ACCEPTED MANUSCRIPT 31
aqueous phase often has to be heated and mechanically agitated to facilitate dissolution and
875
dispersion of water-soluble components (such as emulsifiers, thickening agents, buffers, minerals
876
and other functional ingredients). Similarly, the oil phase may also have to be heated and
877
mechanically agitated to facilitate the melting and dispersion of any antioxidants, weighting
878
agents, ripening inhibitors, or colors. Once the oil and aqueous phases have been prepared they
879
are blended together using a high-shear mixer to form a coarse emulsion (d ≈ 1 to 10 µm), which
880
is then homogenized using a mechanical device to form a fine emulsion (d ≈ 0.1 to 1 µm). When
881
beverages are prepared using low energy homogenization methods a different approach may be
882
taken (see below). In this case, water-soluble surfactants and some other water-soluble
883
components may initially be incorporated into the oil phase, which is then mixed with the
884
aqueous phase. This process can lead to the spontaneous formation of a microemulsion,
885
nanoemulsion, or emulsion depending on system composition and preparation procedure. After
886
preparation the beverage emulsion concentrate is often pasteurized to reduce the microbial load,
887
and then stored or transported to the place where it will be used.
M AN U
SC
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874
Finished Product: The finished product is created by diluting the beverage emulsion
889
concentrate with another aqueous phase, which may contain various other ingredients, such as
890
colors, flavors, preservatives, pH regulators etc. Typically, the concentrate is diluted 500-1000
891
times to produce a final product that often has an oil concentration < 20 mg per liter for a ready-
892
to-drink product (Given, 2009). The final product may be homogenized again to ensure that any
893
non-polar colors, flavors, and preservatives are incorporated into the oil droplets. Appropriate
894
selection of ingredients and processing conditions may lead to beverage products with shelf lives
895
longer than 12 months. However, the perceived quality of a product may deteriorate after
896
extended storage due to detrimental changes in its physical or chemical properties. Beverage
897
emulsions are susceptible to various physical instability mechanisms that can lead to undesirable
898
changes in appearance, such as ringing and oiling off (see earlier section). Beverage emulsions
899
are also liable to undesirable quality changes due to chemical degradation, e.g., changes in flavor
900
profile due to degradation of flavor molecules (such as citral) or color fading due to degradation
901
of colorants (such as carotenoids). These challenges can often be overcome using appropriate
902
packaging materials and/or proper product formation. The design and formulation of successful
903
soft drink products requires careful selection of functional ingredients (oil type, emulsifiers,
904
weighting agents, ripening inhibitors, antioxidants, etc.), preparation conditions (e.g.,
AC C
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888
ACCEPTED MANUSCRIPT 32
905
homogenization method and operating conditions), and storage conditions (e.g., exposure to
906
elevated temperatures, light, and oxygen). A number of different methods can be used to form beverage emulsion concentrates. In
908
general, these approaches can be categorized as either high-energy or low-energy approaches
909
depending on the underlying physical principle of droplet formation (Acosta, 2009; Anton &
910
Vandamme, 2009; Leong, Wooster, Kentish, & Ashokkumar, 2009; Pouton & Porter, 2006;
911
Tadros, Izquierdo, Esquena, & Solans, 2004). High-energy approaches utilize mechanical
912
devices (“homogenizers”) that generate intense forces capable of disrupting and intermingling
913
the oil and aqueous phases leading to the formation of very fine oil droplets (Figure 9). The
914
most commonly used homogenizers utilized in the beverage industry for forming emulsions are
915
high pressure valve homogenizers, but microfluidizers and ultrasonic methods may also be used
916
(Gutierrez, et al., 2008; Leong, et al., 2009; Velikov & Pelan, 2008; Wooster, et al., 2008b).
917
High-energy approaches are probably the most common method used for preparing beverage
918
emulsions at present because they are capable of large-scale production, and they can be used to
919
prepare emulsions from a variety of different starting materials. Low energy approaches rely on
920
the spontaneous formation of fine oil droplets within mixed surfactant-oil-water systems when
921
the solution or environmental conditions are altered (Anton, Benoit, & Saulnier, 2008;
922
Bouchemal, Briancon, Perrier, & Fessi, 2004; Chu, Ichikawa, Kanafusa, & Nakajima, 2007;
923
Freitas, Merkle, & Gander, 2005; Tadros, et al., 2004; Yin, Chu, Kobayashi, & Nakajima, 2009).
924
A number of different low energy approaches have been developed, and some of these are
925
suitable for utilization within the beverage industry, e.g., phase inversion and spontaneous
926
emulsification methods (Figure 10). The minimum particle size that can be produced using
927
either approach depends on many different factors, which are highlighted in the sections below.
928
6.1. High-Energy Approaches
SC
M AN U
TE D
EP
AC C
929
RI PT
907
The size of the droplets generated by high energy approaches is determined by a balance
930
between two opposing processes occurring within the homogenizer: droplet disruption and
931
droplet coalescence (Jafari, Assadpoor, He, & Bhandari, 2008). Only those mechanical devices
932
that are capable of generating extremely intense disruptive forces are capable of producing the
933
tiny droplets required for most beverage emulsion applications (Figure 9), i.e., high-pressure
934
valve homogenizers, microfluidizers, and ultrasonic devices (Leong, et al., 2009; Tadros, et al.,
ACCEPTED MANUSCRIPT 33
2004). The smallest droplet size that can be produced by a particular high-energy device
936
depends on homogenizer design (e.g., flow and force profiles), homogenizer operating
937
conditions (e.g., energy intensity, duration), environmental conditions (e.g. temperature), sample
938
composition (e.g., oil type, emulsifier type, concentrations), and the physicochemical properties
939
of the component phases (e.g., interfacial tension, viscosity) (Kentish, et al., 2006; Wooster, et
940
al., 2008b).
941
RI PT
935
High energy homogenizers are widely used to produce beverage emulsions because they can be utilized with a wide variety of different types of oils and emulsifiers. Once the
943
homogenization conditions have been optimized, beverage emulsions can be produced using
944
triacylglycerol oils or flavor oils as the oil phase, and proteins, polysaccharides, phospholipids,
945
or surfactants as emulsifiers. Thus, high-energy methods are suitable for producing both cloud
946
emulsions and flavor emulsions. Even so, the size of the droplets produced depends strongly on
947
the characteristics of the oil and emulsifier used (see below). For example, it is usually easier to
948
produce very small droplets when the oil phase has a low viscosity and/or interfacial tension
949
(e.g., flavor oils) than when it has a high viscosity and/or interfacial tension (e.g., triacylglycerol
950
oils).
951
6.1.1. High Pressure Valve Homogenizers
M AN U
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952
SC
942
High pressure valve homogenizers are currently the most common high-energy method of producing beverage emulsions. Initially, a coarse emulsion is produced using a high shear mixer
954
and then this is fed directly into the inlet of the high pressure valve homogenizer. The
955
homogenizer has a pump that pulls the coarse emulsion into a chamber on its backstroke and
956
then forces it through a narrow valve at the end of the chamber on its forward stroke (Figure 9).
957
As the coarse emulsion passes through the valve it experiences a combination of intense
958
disruptive forces that cause the larger droplets to be broken down into smaller ones. Different
959
nozzle designs are available to increase the efficiency of droplet disruption. The droplet size
960
produced using a high pressure valve homogenizer usually decreases as the number of passes
961
and/or the homogenization pressure increases. It also depends on the viscosity ratio of the two
962
phases (usually oil and water) being homogenized. Small droplets can only usually be produced
963
when the disperse-to-continuous phase viscosity ratio falls within a certain range (0.05 < ηD/ηC <
964
5) (Tadros, et al., 2004; Walstra, 1993, 2003). Finally, it is important to have sufficient
AC C
EP
953
ACCEPTED MANUSCRIPT 34
emulsifier present to cover all the new droplet surfaces formed during homogenization, and to
966
use an emulsifier that can rapidly adsorb to the droplet surfaces to prevent re-coalescence (Jafari,
967
et al., 2008). Usually, there is a linear relationship between the logarithm of the homogenization
968
pressure (P) and the logarithm of the droplet diameter (d): log d ∝ log P, with the constant of
969
proportionality depending on homogenizer type (McClements, 2005b). To reduce the droplet
970
size to the level required in beverage emulsions it is sometimes necessary to operate at extremely
971
high pressures and to use multiple passes through the homogenizer. Even then, it is only
972
possible under certain circumstances to obtain droplets less than 100 nm in radius (e.g., high
973
emulsifier levels, low interfacial tensions, and appropriate viscosity ratios).
974
6.1.2. Microfluidizers
SC
The formation of beverage emulsions using a microfluidizer also involves forcing a coarse
M AN U
975
RI PT
965
emulsion through a narrow orifice under high pressure to facilitate droplet disruption. However,
977
the design of the channels through which the emulsion is made to flow within a microfluidizer is
978
different from that of a high pressure valve homogenizer (Figure 9). The microfluidizer divides
979
an emulsion into two streams that are then made to impinge on each other in an interaction
980
chamber. Intense disruptive forces are generated within the interaction chamber when the two
981
fast moving streams of emulsion collide, leading to highly efficient droplet disruption.
TE D
976
A number of studies have examined the potential application of microfluidizers for the
983
production of model beverage emulsions (Dalgleish, West, & Hallett, 1997; Henry, Fryer, Frith,
984
& Norton, 2010; Klein, Aserin, Svitov, & Garti, 2010). These studies have shown that small
985
droplets can be produced provided that conditions are optimized to facilitate droplet disruption
986
and inhibit droplet coalescence. The droplet size tends to decrease with increasing
987
homogenization pressure, number of passes, emulsifier concentration, and decreasing disperse-
988
to-continuous phase viscosity ratio (Wooster, et al., 2008b). Again, there is usually a linear
989
relationship between the logarithm of homogenization pressure and the logarithm of the droplet
990
diameter: log d ∝ log P. Recent studies have identified some of the major factors influencing the
991
formation of fine emulsions using microfluidizers (Qian & McClements, 2011). This study
992
showed that the logarithm of the mean droplet diameter decreased linearly with the logarithm of
993
the homogenization pressure for both an ionic surfactant (SDS) and a globular protein (β-
994
lactoglobulin) (Figure 11). However, the slope of the log(d) versus log(P) relationship was
AC C
EP
982
ACCEPTED MANUSCRIPT 35
appreciably steeper for the surfactant (-0.57) than for the protein (-0.29), which was attributed to
996
the fact that the protein may adsorb more slowly to the droplet surfaces, and that it may form a
997
viscoelastic coating that inhibits further droplet breakup. The dependence of the mean droplet
998
diameter on viscosity ratio (ηD/ηC) was also examined by preparing emulsions using different oil
999
phase and aqueous phase compositions. For the ionic surfactant there was a distinct decrease in
1000
mean droplet diameter with decreasing viscosity ratio, which suggested that droplet disruption
1001
within the homogenizer became easier as the viscosity of the two phases became more similar.
1002
On the other hand, little change was found in mean droplet size with viscosity ratio when a
1003
globular protein was used as an emulsifier, which again may be due to the relatively slow
1004
adsorption of the protein and its ability to form a coating that inhibits further droplet disruption.
1005
6.1.3. Ultrasonic Homogenizers
SC
M AN U
1006
RI PT
995
Beverage emulsions can also be formed continuously using ultrasonic homogenizers. This type of homogenizer utilizes high intensity ultrasonic waves to generate intense disruptive forces
1008
(mainly generated by cavitation) that break the oil and water phases into very small droplets
1009
(Kentish, et al., 2006; Leong, et al., 2009; Lin & Chen, 2008). Batch and continuous ultrasonic
1010
homogenizers are available for producing emulsions (Leong, et al., 2009). However, continuous
1011
ultrasonic homogenizers are probably the most commonly used methods for the large scale
1012
production of fine emulsions (Figure 9). The size of the droplets produced using these devices
1013
tends to decrease as the intensity of the ultrasonic waves is increased or the residence time in the
1014
disruption zone is increased (Abismail, Canselier, Wilhelm, Delmas, & Gourdon, 1999; Maa &
1015
Hsu, 1999). The homogenization efficiency also depends on the type and amount of emulsifier
1016
present, and the viscosity of the oil and aqueous phases (Jafari, He, & Bhandari, 2006; Kentish,
1017
et al., 2006; Leong, et al., 2009; Maa & Hsu, 1999). Ultrasonic homogenizers are particularly
1018
suitable for low-viscosity fluids, but are less suitable for more viscous systems.
1019
6.2. Low-Energy Approaches
EP
AC C
1020
TE D
1007
The formation of emulsions using low-energy approaches relies on the spontaneous
1021
formation of oil droplets in surfactant-oil-water mixtures when either their composition or their
1022
environment is altered in a specific way (Anton, et al., 2008; Anton & Vandamme, 2009;
1023
Bouchemal, et al., 2004; Yin, et al., 2009). A number of methods for preparing fine emulsions
1024
are based on the low-energy approach, including spontaneous emulsification and phase inversion
ACCEPTED MANUSCRIPT 36
methods (Figure 10) (Anton, et al., 2008; Anton & Vandamme, 2009; Fernandez, Andre, Rieger,
1026
& Kuhnle, 2004; Maestro, Sole, Gonzalez, Solans, & Gutierrez, 2008). Some of these low
1027
energy methods are already used in the beverage industry for forming oil-in-water emulsions,
1028
whereas others may also be for certain applications.
1029
RI PT
1025
Low energy approaches are often more effective at producing small droplet sizes than high energy approaches, but they are often more limited in the types of oils and emulsifiers that can be
1031
used. For example, it is currently not possible to use proteins or polysaccharides as emulsifiers
1032
in most of the low energy approaches used to form very fine emulsions. Instead, relatively high
1033
concentrations of synthetic surfactants are usually required to form stable emulsions by these
1034
approaches, which limit their use for some beverage applications.
1035
6.2.1. Spontaneous Emulsification Methods
M AN U
1036
SC
1030
In this method an emulsion is spontaneously formed when an organic phase and an aqueous phase are mixed together (Anton & Vandamme, 2009; Miller, 1988; Pouton & Porter, 2006).
1038
The organic phase usually contains oil, hydrophilic surfactant, and possibly water-miscible
1039
solvent, whereas the aqueous usually contains water and possibly cosolvent and cosurfactant.
1040
The two phases can be brought together in various ways: the organic phase may be titrated into
1041
the aqueous phase (Anton & Vandamme, 2009), or the aqueous phase may be titrated into the
1042
organic phase (Sonneville-Aubrun, et al., 2009). When the two phases come into contact the
1043
hydrophilic surfactant and/or solvent move from the oil phase to the aqueous phase, which
1044
generates tiny oil droplets at the phase boundary (Horn & Rieger, 2001). The size of the droplets
1045
produced can be controlled by varying the compositions of the two initial phases, as well as the
1046
mixing conditions.
EP
The spontaneous emulsification method was recently compared with a high energy method
AC C
1047
TE D
1037
1048
(microfluidization) of producing fine emulsions (Yang, Marshall-Breton, Leser, Sher, &
1049
McClements, 2012). The surfactant-oil-water system used consisted of 15.4 wt% non-ionic
1050
surfactant, 23.1 wt% MCT, and 61.5% water, with the surfactant containing a 50:50 mixture of a
1051
hydrophilic (Tween 80) and lipophilic (Tween 85) surfactant. In the high energy method, the
1052
surfactant, oil and water were blended together, and then passed through a microfluidizer. In the
1053
low energy method, the oil and surfactant were mixed together, and then simply titrated into the
1054
water using slow stirring. The microfluidization method produced droplets with a diameter of
ACCEPTED MANUSCRIPT 37
about 110 nm, whereas the spontaneous emulsification method could produce droplets with
1056
diameters around 140 nm. This simple experiment demonstrated that nanoemulsions could be
1057
produced using the spontaneous emulsification method, provided that the system composition
1058
was optimized, i.e., surfactant, oil, and water contents. However, a much higher surfactant-to-oil
1059
ratio (SOR) is required to produce small droplets using the low energy method than with the high
1060
energy method. The influence of SOR on the mean droplet diameter of vitamin E nanoemulsions
1061
produced using the spontaneous emulsification approach is shown in Figure 12. These
1062
measurements show that there is a minimum in the mean droplet size at a particular SOR, and
1063
therefore the composition of the system has to be carefully controlled to obtain a particular
1064
droplet size.
The ability of fine emulsions to be formed using the spontaneous emulsification method
M AN U
1065
SC
RI PT
1055
depends on the system composition (surfactant, oil, and water levels), surfactant type, oil type,
1067
and preparation method (order of addition, mixing speeds). This approach is suitable for
1068
producing flavor or cloud emulsions, provided that an appropriate surfactant is used (such as a
1069
non-ionic surfactant with a particular HLB number) and the surfactant-to-oil ratio is optimized.
1070
The main advantage of this method is that it is inexpensive, simple, and requires no major
1071
equipment, whereas the disadvantage is that relatively high surfactant levels are required (SOR ≈
1072
1)
1073
6.2.2. Phase Inversion Methods
A number of methods have been developed to formulate fine emulsions that depend on
EP
1074
TE D
1066
inducing a phase inversion from a W/O to O/W form, e.g., phase inversion temperature (PIT)
1076
and emulsion inversion point (EIP) methods (Fernandez, et al., 2004; Thakur, Villette, Aubry, &
1077
Delaplace, 2008). In the PIT method the change from one type of an emulsion to another involves a
1078
transitional phase inversion, whereas in the EIP method it involves a catastrophic phase inversion
1079
(Figure 13). A transitional phase inversion occurs when the surfactant properties are altered by
1080
adjusting a formulation variable, such as temperature, pH, or ionic strength. A catastrophic phase
1081
inversion occurs when the ratio of the oil-to-water phases is altered while the surfactant properties
1082
remain constant.
AC C
1075
ACCEPTED MANUSCRIPT 38
6.2.2.1. Phase Inversion Temperature Method The PIT method is based on changes in the optimum curvature and relative solubility of
1085
non-ionic surfactants with changing temperature (Anton, Gayet, Benoit, & Saulnier, 2007; Anton
1086
& Vandamme, 2009; Gutierrez, et al., 2008). Emulsions containing very fine droplets can be
1087
spontaneously formed using the PIT method by varying the temperature-time profile of
1088
appropriate mixtures of surfactant, oil, and water. This type of phase inversion usually involves
1089
the controlled transformation of an emulsion from one type to another (e.g., W/O to O/W or vice
1090
versa) through an intermediate state. This intermediate state is typically a liquid crystalline or
1091
bicontinuous microemulsion phase. The driving force for this type of phase inversion is the
1092
change in the physicochemical properties of the surfactant molecules caused by temperature
1093
changes, e.g., molecular geometry and solubility (Figure 10). The molecular geometry of a
1094
surfactant can be characterized by a packing parameter, p (Israelachvili, 2011):
1096
SC
p=
M AN U
1095
RI PT
1083 1084
aT aH
(7)
where, aT and aH are the cross-sectional areas of the lipophilic tail-group and hydrophilic headgroup respectively. Surfactants spontaneously associate in water to form monolayers that have a
1098
curvature that allows the most efficient packing of the surfactant molecules. The optimum
1099
curvature of a monolayer depends on the packing parameter of the surfactant: for p < 1, the
1100
optimum curvature is convex (favoring O/W systems); for p = 1, monolayers with zero curvature
1101
are preferred (favoring lamellar structures); and, for p > 1 the optimum curvature is concave
1102
(favoring W/O systems). For non-ionic surfactants, the head group is relatively large compared to
1103
the tail group (p < 1) at temperatures below the PIT and so O/W emulsions are favored. Upon
1104
heating, the head group becomes progressively dehydrated and so the packing parameter increases.
1105
At the phase inversion temperature (PIT), the head group and tail group have similar sizes (p = 1)
1106
and so liquid crystals or microemulsions are formed. Above the PIT, the head group is relatively
1107
small compared to the tail group (p > 1) and so W/O emulsions are favored. The relative solubility
1108
of non-surfactants in the oil and water phases also changes with temperature because of head group
1109
dehydration, which has also been used to interpret the PIT phenomenon (Anton, et al., 2007; Anton
1110
& Vandamme, 2009). At low temperatures, the head group is highly hydrated and so the surfactant
1111
tends to be more soluble in water. As the temperature is raised and the head group becomes
1112
progressively dehydrated and the solubility of the surfactant in water decreases, while its solubility
AC C
EP
TE D
1097
ACCEPTED MANUSCRIPT 39
in oil increases. At a particular temperature (≈ PIT), the solubility of the surfactant in the oil and
1114
water phases is approximately equal. At higher temperatures, the surfactant becomes more soluble
1115
in the oil phase than in the water phase. Thus at high temperatures the surfactant is mainly located
1116
in the oil phase, and when the system is cooled it has a tendency to move into the aqueous phase.
1117
Thus, oil droplets are formed by a mechanism similar to spontaneous emulsification.
1118
RI PT
1113
The PIT method is relatively straightforward to implement (Figure 10). A mixture of
surfactant, oil, and water (SOW) is initially heated up to a temperature around or slightly above the
1120
PIT, which leads to the formation of a microemulsion or liquid crystalline phase. The SOW system
1121
is then quench cooled to a temperature well below the PIT with continuous stirring, which leads to
1122
the spontaneous formation of an oil-in-water emulsion or nanoemulsion (Anton & Vandamme,
1123
2009). An example of the phase behavior of a SOW mixture consisting of a non-ionic surfactant
1124
(13% Tween 80), a flavor oil (10 wt% lemon oil), and water (77%) upon heating and cooling is
1125
shown in Figure 14 (Rao & McClements, 2010). Initially, the surfactant, oil and water were
1126
blended together to form a coarse O/W emulsion that was optically opaque. Upon heating the
1127
system becomes transparent when the PIT is reached, and then becomes opaque when heated above
1128
the PIT due to formation of a W/O emulsion. Upon cooling, the system goes from turbid to
1129
transparent indicating that a nanoemulsion was formed (d = 45 nm).
M AN U
TE D
1130
SC
1119
One of the limitations of the phase inversion temperature method is that the emulsions produced are often highly prone to droplet coalescence when they are stored at temperatures
1132
approaching the PIT. This could be a problem in food and beverage applications that require some
1133
form of thermal treatment (such as pasteurization, sterilization or cooking) or that are stored at
1134
elevated temperatures (e.g., in warm or hot climates). Recently, we developed an approach to
1135
overcome this problem by forming emulsions using a non-ionic surfactant with a relatively low PIT,
1136
and then diluting the resulting emulsions in a solution containing another surfactant with a high PIT
1137
(Rao & McClements, 2010). This second surfactant partially displaces some of the original
1138
surfactant from the droplet surfaces, thereby altering the optimum curvature and PIT of the
1139
surfactant monolayer, as well as increasing the repulsive interactions between the droplets. This
1140
approach could be used in the food and beverage industry to form very fine droplets using the PIT
1141
method, and then stabilizing them by diluting them in a different surfactant solution.
1142 1143
AC C
EP
1131
The PIT method can only be used for certain combinations of oils and surfactants. The PIT must be within a practical range that can be implemented within the food industry (e.g. somewhere
ACCEPTED MANUSCRIPT 40
between 40 and 90 ºC) – if it is too high then it will not be possible to create emulsions, and if it is
1145
too low the emulsions formed will be highly unstable to droplet growth. The PIT tends to increase
1146
with increasing molecular weight and hydrophobicity of oil molecules, which means that for many
1147
triacylglycerol oils the PIT is too high to practically reach. On the other hand, flavor oils seem to be
1148
amenable to emulsification using the PIT method. Nevertheless, it is still important to select an
1149
appropriate type and amount of surfactant to ensure the PIT is in the correct range.
1150 1151
6.2.2.2. Emulsion Inversion Point (EIP) Method The emulsion inversion point (EIP) method can also be implemented very easily (Figure 10).
1152
It simply involves titrating increasing amounts of an aqueous phase into an organic phase to induce
1153
a catastrophic phase inversion from a W/O to an O/W system. Initially, an organic phase is
1154
prepared that contains oil and a hydrophilic surfactant. The aqueous phase is then slowly titrated
1155
into this organic phase with constant stirring. As the amount of water added increases the system
1156
converts from a W/O emulsion, to an O/W/O multiple emulsion, to an O/W emulsion (Jahanzad,
1157
Crombie, Innes, & Sajjadi, 2009; Sajjadi, 2006). The formation of these multiple emulsions
1158
requires that the hydrophilic surfactant is initially located in the oil phase. At relatively low
1159
surfactant concentrations the formation of multiple emulsions is suppressed and only relatively
1160
large oil droplets are produced in the final emulsion, which are similar in size to those that would
1161
be produced if the surfactant had been dissolved in the water phase prior to homogenization
1162
(Jahanzad, et al., 2009). At relatively high surfactant concentrations, multiple emulsions are
1163
formed during the titration process, and the final oil droplet size within the O/W emulsions is
1164
determined by the size of the inner oil droplets in the intermediate O/W/O emulsions. The value
1165
of the critical concentration where phase inversion occurs, as well as the size of the oil droplets
1166
produced, depends on process variables, such as the stirring speed, the rate of water addition, and
1167
the emulsifier concentration (Thakur, et al., 2008). The emulsifiers used in catastrophic phase
1168
inversion are usually limited to small molecule surfactants that can stabilize both W/O emulsions (at
1169
least over the short term) and O/W emulsions (over the long term).
SC
M AN U
TE D
EP
AC C
1170
RI PT
1144
We have recently used the EIP method to produce emulsions with relatively small droplet
1171
diameters (< 100 nm) (Ostertag, Weiss, & McClements, 2012). We titrated water into an organic
1172
phase containing a mixture of oil and water-miscible surfactant. The droplet size produced by this
1173
method depended on: (i) oil type: medium chain triglycerides (MCT) < flavor oils (orange and
ACCEPTED MANUSCRIPT 41
limonene) < long chain triglycerides (olive, grape, sesame, peanut and canola oils); (ii) surfactant
1175
type: Tween 80 < Tween 20 < Tween 85; (iii) surfactant concentration: smaller droplets were
1176
produced at higher surfactant-to-oil ratios (SOR); (iv) surfactant location: surfactant initially in oil <
1177
surfactant initially in water. As an example, the data for the influence of oil type on droplet size
1178
produced using the EIP method is shown in Figure 15. The EIP method was also compared to a
1179
high energy method (microfluidization). Small droplets (d < 160 nm) could be produced by both
1180
methods, but that much less surfactant was needed for the high energy method (SOR ≥ 0.1) than the
1181
low energy method (SOR ≥ 0.7). The EIP method may therefore be suitable for producing fine
1182
emulsions suitable for use in the beverage industry without the need for expensive homogenization
1183
equipment, provided that having relatively high amounts of surfactant present is not a problem. In
1184
the finished product, the levels of surfactant may be quite small (even when SOR ≈ 1), since the
1185
beverage emulsion concentrates are diluted so much.
1186
7. Beverage Emulsion Ingredients
1187
7.1. Oil Phase Components
1188
7.1.1. Flavor Oils
SC
M AN U
TE D
1189
RI PT
1174
Flavor oils, such as orange, lemon and peppermint oils, are widely used in the food and beverage industries as flavoring agents, since they contain volatile constituents with
1191
characteristic aroma profiles. The composition of natural flavor oils depends on their biological
1192
origin, the extraction procedure used to isolate them, and any subsequent processing steps. For
1193
example, lemon oil is usually obtained from lemon peel by cold pressing, and it may then be
1194
further refined using steam distillation and various other methods (Misharina, Terenina,
1195
Krikunova, & Medvedeva, 2010). Commercially, flavor oils are available in various forms
1196
(“folds”) that differ in chemical composition due to differences in isolation and processing
1197
procedures used. Flavor oils extracted by cold pressing are usually referred to as single-fold (1×)
1198
oils, while those that have undergone further processing (such as distillation) are referred to as
1199
higher fold oils e.g., three-fold (3×), five-fold (5×), or ten-fold (10×) oils (Gamarra, Sakanaka,
1200
Tambourgi, & Cabral, 2006). Normally, higher-fold oils have different and more intense flavor
1201
profiles than lower-fold oils. Differences in the chemical composition of flavor oils also leads to
AC C
EP
1190
ACCEPTED MANUSCRIPT 42
differences in their physicochemical properties (e.g., water solubility, density, viscosity,
1203
refractive index, and optical properties), which influences their ability to form and stabilize
1204
beverage emulsions (Rao & McClements, 2012a; Rao & McClements, 2012c). In recent studies,
1205
our laboratory has examined the influence of lemon oil fold (1×, 3×, 5× and 10×) on the
1206
formation and properties of oil-in-water microemulsions, nanoemulsions and emulsions (Rao &
1207
McClements, 2012b; Rao & McClements, 2012c). The composition, molecular characteristics,
1208
and physicochemical properties of the four lemon oils were tabulated. The main constituents in
1209
1× lemon oil were monoterpenes (> 90 %), whereas the major constituents in 10× lemon oil were
1210
monoterpenes (≈ 35%), sesquiterpenes (≈ 14%) and oxygenates (≈ 33%). The concentration of
1211
both polar and non-polar components increased with increasing oil fold, while intermediate polar
1212
components decreased. The density, interfacial tension, viscosity, and refractive index of the
1213
lemon oils increased as the oil fold increased (i.e., 1× < 3× < 5× < 10×). The stability of oil-in-
1214
water emulsions produced by high pressure homogenization was strongly influenced by lemon
1215
oil type. Emulsions produced using lower fold oils (1×, 3×, 5×) were highly unstable to droplet
1216
growth during storage, with the growth rate increasing with increasing storage temperature and
1217
decreasing oil fold. Droplet growth was mainly attributed to Ostwald ripening i.e., the diffusion
1218
of water-soluble lemon oil components from small to large droplets. Emulsions prepared using
1219
the highest fold oil (10×) were stable to droplet growth, which was attributed to the presence of
1220
an appreciable fraction of low water-solubility constituents that acted as “ripening inhibitors”
1221
(see later). On the other hand, the lower fold oils were more suitable for forming
1222
microemulsions than the higher fold oils, presumably because they contained constituents that
1223
more easily fit into micelle structures.
SC
M AN U
TE D
EP
A number of recent studies have examined the major factors influencing the formation and
AC C
1224
RI PT
1202
1225
stability of beverage emulsions containing flavor oils, including the preparation method used
1226
(Cheng & McClements, 2011; Rao & McClements, 2011, 2012c), emulsifier type (Mirhosseini,
1227
Tan, Aghlara, et al., 2008; Mirhosseini, Tan, Hamid, & Yusof, 2008; Qian & McClements,
1228
2011), droplet charge (Choi, et al., 2010a), addition of antioxidants (Yang, et al., 2011), presence
1229
of surfactant micelles (Choi, et al., 2010b), and the physical state of the oil phase (Mei, et al.,
1230
2010). Many flavor oils used in beverage emulsions are unstable to chemical degradation during
1231
storage. For example, citral decomposes rapidly during storage at acidic pH by a series of
1232
cyclization and oxidation reactions (Choi, et al., 2009). This effect is highlighted in Figure 16,
ACCEPTED MANUSCRIPT 43
which shows the amount of citral remaining in aqueous solution during storage at different pH
1234
values: the degradation rate increases rapidly as the pH is decreased. Citral degradation reduces
1235
the level of desirable aroma compounds present as well as increasing the levels of undesirable
1236
aroma compounds, thus limiting the shelf life of many beverages. The rate of citral degradation
1237
in emulsions is affected by environmental and compositional factors, such as temperature,
1238
oxygen availability, and antioxidant addition (Choi, et al., 2009; Liang, Wang, Simon, & Ho,
1239
2004a, 2004b; Ueno, Kiyohapa, Ho, & Masuda, 2006; Ueno, et al., 2003). It also depends on the
1240
location of the citral molecules within an emulsion, e.g., oil, water, or interfacial regions (Choi,
1241
et al., 2009; Choi, et al., 2010b). If a labile component is located within an environment where it
1242
is isolated from other components that promote its chemical degradation, then it is possible to
1243
reduce the degradation rate. Since citral degradation occurs predominantly in acidic aqueous
1244
solutions, it is possible to reduce its degradation rate by altering its partitioning between the oil
1245
and aqueous phases. For example, when citral is located predominantly within a non-polar
1246
environment, such as the interior of a lipid droplet or surfactant micelle, it is partially protected
1247
from degradation. This effect is highlighted in Figure 17, which shows that the rate of citral
1248
degradation in oil-in-water emulsions decreases as the total fat droplet concentration increases.
1249
This effect can be attributed to the fact that a lower fraction of the citral is present within the
1250
acidic aqueous phase as the fat content increases due to equilibrium partitioning effects (Figure
1251
18). This phenomenon may be important for beverage emulsion concentrates that have relatively
1252
high fat contents, since in this case an appreciable fraction of the citral will be present within the
1253
oil phase. Choosing an oil type and concentration that ensures that most of the citral is present
1254
within the oil droplets, rather than the aqueous phase, will then help to inhibit citral degradation
1255
during storage. On the other hand, once an emulsion concentrate is diluted to form the finished
1256
product nearly all of the flavor molecules partition into the aqueous phase and therefore any
1257
protective effect is lost.
1258
7.1.2. Cloud Oils
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1233
1259
The oil droplets in cloud emulsions are composed mainly of non-flavor oils, such as
1260
triacylglycerol or terpene oils. Triacylglycerol oils are typically derived from natural sources
1261
such as vegetable, corn, canola and sunflower oils, whereas terpene oils are usually isolated by
1262
distillation of natural flavor oils. These oils have very low water-solubilities and are therefore
ACCEPTED MANUSCRIPT 44
stable to Ostwald ripening (unlike flavor oils). The main purpose of cloud oils is to produce
1264
droplets that scatter light strongly so as to give a product a desirable turbid or cloudy appearance,
1265
and so it is important that the droplets are within the size range where efficient light scattering
1266
occurs (typically around 200 to 500 nm). For commercial applications, it is also important that
1267
the droplets remain physically and chemically stable during storage, transport, and handling.
1268
Cloud oils do not usually contribute directly to the flavor profile of beverage emulsions, but they
1269
may indirectly influence flavor due to their effects on the partitioning of flavor molecules
1270
between the oil, water, and headspace regions (McClements, 2005b). They may also adversely
1271
affect the flavor profile of a beverage if they are susceptible to chemical degradation, e.g., the
1272
oxidation of unsaturated lipids products rancid off-flavors.
SC
The physicochemical properties of cloud oils may also play an important role in the
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1273
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1263
formation and stability of cloud emulsions. The viscosity of cloud oils will influence their ability
1275
to form small droplets during the homogenization process, with higher viscosities normally
1276
leading to larger droplets. The density of cloud oils determines the creaming velocity of the
1277
droplets within an emulsion, and may therefore influence their long-term stability. The melting
1278
characteristics of cloud oils may also be important for certain applications since partially
1279
crystalline oil droplets are susceptible to partial coalescence leading to droplet aggregation and
1280
phase separation (McClements, 2005b). This may be particularly important for beverages that
1281
are stored in a refrigerator for prolonged periods. In this case, it may be necessary to use an oil
1282
phase that remains liquid at refrigerator temperatures (e.g., winterized or fractionated oil).
1283
7.1.3. Nutraceutical Lipids
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There has been growing interest in the incorporation of various kinds of nutraceutical lipids into commercial food and beverage products, such as polyunsaturated fats, conjugated linoleic
1286
acid, carotenoids, phytosterols, and fat-soluble vitamins. These nutraceutical lipids are either
1287
mixed with the other oil phase components prior to homogenization or they are encapsulated
1288
separately and then introduced intro the product. The incorporation of nutraceutical lipids into
1289
commercial beverage emulsions is often challenging because they are prone to various physical
1290
or chemical degradation mechanisms. Each type of nutraceutical lipid has its own particular
1291
challenges depends on its physicochemical properties, such as solubility, oil-water partition
1292
coefficient, melting point, and chemical stability (McClements, Decker, Park, & Weiss, 2009).
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1285
ACCEPTED MANUSCRIPT 45
Beverage products must therefore be carefully formulated taking into account the potential
1294
problems associated with the specific types of nutraceutical lipids present.
1295 1296
7.1.3.1. Polyunsaturated Lipids There has been growing interest in the incorporation of polyunsaturated fatty acids (PUFAs),
1297
and especially omega-3 (ω-3) fatty acids, into food and beverage products because of their
1298
potential health benefits (Ruxton, Reed, Simpson, & Millington, 2004, 2007; Siddiqui, et al.,
1299
2004). Omega-3 fatty acids are unsaturated fatty acids that have a double bond that is three
1300
carbon atoms from the methyl end of the molecule. The most common ω-3 fatty acids in food
1301
oils are α-linolenic acid (ALA, 18:3), eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic
1302
acid (DHA, 22:6), with EPA and DHA being the most bioactive. PUFAs are normally
1303
incorporated into foods as part of triacylglycerol molecules, but they may also be incorporated as
1304
monoacylglycerols, diacylglycerols, or phospholipids. Consumption of adequate amounts of ω-3
1305
fatty acids has been linked to decreased risks of various diseases, including cardiovascular
1306
disease, immune response disorders, mental disorders, and poor infant development (Orchard,
1307
Pan, Cheek, Ing, & Jackson, 2012; Tur, Bibiloni, Sureda, & Pons, 2012). This has prompted
1308
food manufacturers to attempt to fortify their products with ω-3 fatty acids at levels sufficiently
1309
high to have a beneficial biological effect (McClements & Decker, 2000; Waraho, et al., 2011).
1310
However, PUFAs are extremely susceptible to oxidative deterioration, which causes problems
1311
for the long-term storage of these products (Arab-Tehrany, et al., 2012).
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The oxidation of ω-3 fatty acids in emulsions involves a complex series of chemical
EP
1312
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1293
reactions that results in the production of rancid off-flavors. The control of lipid oxidation often
1314
requires a combination of different approaches to achieve adequate long-term stability, including
1315
control of initial ingredient quality, removal/deactivation of pro-oxidants (such as oxygen or
1316
transition metals), addition of anti-oxidants, and interfacial engineering (McClements & Decker,
1317
2000; Waraho, et al., 2011). A more detailed discussion of the chemical stability of PUFA was
1318
given in an earlier section.
1319 1320
7.1.3.2. Fat Soluble Antioxidants Lipophilic antioxidants can be incorporated into the oil phase of oil-in-water emulsions to
1321
inhibit the oxidation of encapsulated chemically labile substances (such as ω-3 oils or
1322
carotenoids), thereby extending product shelf life (Boon, McClements, Weiss, & Decker, 2010;
AC C
1313
ACCEPTED MANUSCRIPT 46
McClements & Decker, 2000; Waraho, et al., 2011). Alternatively, they can be encapsulated as
1324
functional food components themselves so as to make an “antioxidant” claim on a product label.
1325
Some commonly used lipophilic antioxidants utilized within the food industry include alpha-
1326
tocopherols, ascorbyl palmitate, BHT, BHA, and rosemary extracts (McClements & Decker,
1327
2008). These five molecules act as free radical scavengers and they each have the ability to
1328
absorb two free radicals before becoming inactive. The addition of these antioxidants typically
1329
increases the lag phase of the oxidation reaction.
1330 1331
7.1.3.3. Fat Soluble Vitamins There is considerable interest in incorporating various types of fat-soluble vitamins (A, D, E,
1332
and K) and nutraceuticals (e.g., carotenoids, phytosterols, flavonoids, and curcumin) into food
1333
and beverage products to improve their nutritional value (Sagalowicz & Leser, 2010; Velikov &
1334
Pelan, 2008). In this section, we use recent studies on the incorporation of vitamin E into
1335
emulsions to demonstrate some of the potential benefits and challenges associated with its use in
1336
beverages. The term “vitamin E” actually refers to a group of fat-soluble vitamins that are
1337
widely used as functional ingredients in food, pharmaceutical, and cosmetic preparations, with α-
1338
tocopherol being the most biologically active form (Chiu & Yang, 1992). The major biological
1339
function of Vitamin E appears to be as an oil-soluble antioxidant, although other health benefits
1340
have been claimed, including reducing cardiovascular disease, diabetes, and cancer (Sylvester, et
1341
al., 2011; Traber, Frei, & Beckman, 2008; Weng-Yew & Brown, 2011). For these reasons, there
1342
has been interest in fortifying many foods and beverages with Vitamin E (Sagalowicz & Leser,
1343
2010). Vitamin E is unstable to oxidation and may therefore be lost during processing, storage,
1344
and utilization of commercial products (Gawrysiak-Witulska, Siger, & Nogala-Kalucka, 2009;
1345
Yoon & Choe, 2009). For this reason, vitamin E acetate (rather than vitamin E) is used in many
1346
food and beverage applications since it has a higher oxidative stability. After consumption
1347
vitamin E acetate is broken down to vitamin E in the gastrointestinal tract by the action of
1348
pancreatic esterases (Brisson, et al., 2008). The recommended daily intake (RDI) of Vitamin E
1349
is 15 mg/day (Gonnet, Lethuaut, & Boury, 2010; Institute.of.Medicine, 2000).
1350
AC C
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1323
Vitamin E is a highly lipophilic molecule that cannot be directly dispersed into aqueous
1351
solutions (Sagalowicz & Leser, 2010). Instead, it must be incorporated into an appropriate
1352
colloidal delivery system prior to dispersion (Gonnet, et al., 2010). A number of previous
ACCEPTED MANUSCRIPT 47
studies have shown that vitamin E can be successfully incorporated into emulsion-based delivery
1354
systems, such as microemulsions (Chiu & Yang, 1992; Feng, Wang, Zhang, Wang, & Liu,
1355
2009b), nanoemulsions (Hatanaka, et al., 2010; Li, et al., 2011; Shukat, Bourgaux, & Relkin,
1356
2012) and emulsions (Chen & Wagner, 2004; Gonnet, et al., 2010). Encapsulation of vitamin E
1357
has been reported to improve its physicochemical stability during storage and its biological
1358
activity after consumption (Cortesi, Esposito, Luca, & Nastruzzi, 2002; Zuccari, Carosio, Fini,
1359
Montaldo, & Orienti, 2005). Indeed, studies have suggested that the oral bioavailability of
1360
Vitamin E is increased when it is delivered in colloidal form rather than in bulk form (Feng,
1361
Wang, Zhang, Wang, & Liu, 2009a), which would make beverages particularly suitable delivery
1362
systems for this type of nutraceutical lipid.
1363 1364
7.1.3.4. Carotenoids Carotenoids are a diverse group of lipophilic compounds that contribute to the yellow,
1365
orange and red colors of many foods, and that have also been claimed to have some health
1366
benefits. They are polyenes consisting of 3 to 13 conjugated double bonds and in some cases 6
1367
carbon ring structures at one or both ends of the molecule. Carotenoids containing oxygen are
1368
known as xanthophylls (e.g. lutein and zeaxanthin) while those without oxygen are known as
1369
carotenes (e.g., lycopene and β-carotene). The carotenoids have been proposed to exhibit several
1370
potential health benefits: lutein and zeaxanthin may decrease age related macular degeneration
1371
and cataracts (Stringham & Hammond, 2005); lycopene may decrease the risk of prostate cancer
1372
(Basu & Imrhan, 2007). Carotenoids are relatively stable to chemical degradation when they are
1373
present in their natural environment. However, they become highly unstable to degradation once
1374
they are isolated, becoming susceptible to light, oxygen, pH, and temperature (Xianquan, Shi,
1375
Kakuda, & Yueming, 2005). Consequently, dispersion of carotenoids into ingredient systems
1376
can result in their rapid degradation (Heinonen, Haila, Lampi, & Piironen, 1997; Ribeiro, Ax, &
1377
Schubert, 2003). Carotenoids can be degraded by reactions that cause the loss of double bonds
1378
or scission of the molecule. In addition, the double bonds in carotenoids can undergo
1379
isomerization to the cis configuration (Xianquan, et al., 2005). Isomerization reactions might
1380
actually be beneficial since cis isomers of carotenoids such as lycopene are thought to be more
1381
bioavailable and bioactive (Schieber & Carle, 2005). An additional challenge to using
1382
carotenoids as ingredients in functional foods is their high melting point, making them crystalline
AC C
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ACCEPTED MANUSCRIPT 48
at food storage and body temperatures. A more detailed discussion of the factors governing the
1384
chemical degradation of carotenoids was given earlier.
1385 1386
7.1.3.5. Phytosterols Phytosterols and phytostanols are a group of bioactive lipids derived from plants that have
1387
been shown to be have biological activity (Chaiyasit, Elias, McClements, & Decker, 2007). The
1388
fortification of foods and beverages with these components has become popular due to their
1389
ability to decrease total and low density lipoprotein cholesterol in humans, which is mainly
1390
attributed to their ability to inhibit the absorption of dietary cholesterol (Ostlund, 2004; Wong,
1391
2001). Intake of 1.6 g phytosterols/day results in an approximately 10% reduction in LDL
1392
cholesterol (Hallikainen, Sarkkinen, & Uusitupa, 2000). The intestinal absorption of
1393
phytosterols is very low and so dietary phytosterols do not have adverse effects on health.
M AN U
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1383
Incorporation of phytosterols and phytostanols into aqueous-based beverages is particularly
1395
difficult due to their low-water solubility, low-oil solubility, high melting point, and tendency to
1396
form crystals. Some of these problems have been overcome by esterification of phytosterols
1397
with polyunsaturated fatty acids. After consumption, the phytosterol esters are hydrolyzed by
1398
enzymes in the gastrointestinal tract to produce free fatty acids and phytosterols. The initial
1399
applications of phytosterols were in high fat foods (e.g. margarine and spreads) where
1400
solublization and dispersion within the oil phase were relatively simple.
1401
introduced into aqueous-based foods, they need to be either suspended or emulsified.
1402
Phytosterols are also susceptible to chemical degradation (oxidation) during storage
1403
(Bortolomeazzi, Cordaro, Pizzale, & Conte, 2003; Cercaci, Rodriguez-Estrada, Lercker, &
1404
Decker, 2007; Dutta, 1997; Lambelet, et al., 2003; Soupas, Juntunen, Lampi, & Piironen, 2004).
1405
At present, it is unclear whether oxidized forms of phytosterols lose their bioactivity or have
1406
some toxicity as has been observed for oxidized cholesterol. Phytosterols and phytostanols must
1407
therefore be encapsulated in delivery systems that will maintain their physical and chemical
1408
stability in beverage emulsions.
1409
7.1.4. Fat Soluble Colorants
1410
For phytosterols to be
AC C
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1394
A number of natural fat-soluble compounds have intense colors and can therefore be used as
1411
pigments in foods, including paprika, lycopene, and β-carotene (Gibbs, Kermasha, Alli, &
1412
Mulligan, 1999; Kandansamy & Somasundaram, 2012; Wrolstad & Culver, 2012). Some of
ACCEPTED MANUSCRIPT 49
these components also have biological activity, and may therefore be used as nutraceutical lipids
1414
also. Many colorants from natural sources are highly unstable and may chemically degrade and
1415
fade rapidly during storage, e.g., the carotenoids discussed earlier (Qian, et al., 2012). It is
1416
therefore important to establish the mechanism of chemical degradation for each type of fat
1417
soluble colorant used in a beverage product and to determine the major factors affecting its
1418
degradation (such as pH, light, oxygen, pro-oxidants) so that an effective delivery system can be
1419
designed to ensure appropriate product shelf-life.
1420
7.2. Emulsifiers
1421
7.2.1. Mechanism of Action
SC
Emulsifiers are surface-active molecules that are used in emulsions to facilitate droplet
M AN U
1422
RI PT
1413
breakup within the homogenizer (thereby forming smaller droplets) and to prevent droplet
1424
growth after formation (thereby increasing long-term stability) (McClements, 2005b; Stauffer,
1425
1999). During homogenization, emulsifiers absorb to the oil-water interface, which leads to a
1426
reduction in the interfacial tension thereby facilitating further droplet disruption. Once they have
1427
adsorbed to the droplet surface the emulsifiers should prevent the droplets from coalescing with
1428
each other, which means they must for a protective coating around the oil droplets. An effective
1429
emulsifier system should meet a number of criteria to prevent droplet coalescence within the
1430
homogenizer (Jafari, et al., 2008): (1) the emulsifier molecules must be present at a sufficiently
1431
high concentration to cover all of the oil-water surface formed: (2) the emulsifier molecules must
1432
form a coating around the droplet surfaces faster than the droplets collide with each other; and,
1433
(3) the adsorbed emulsifier molecules must form a coating that prevents the droplets from
1434
coming into close proximity and merging together (McClements, 2005b). There are a number of
1435
food-grade emulsifiers that fulfill these requirements, but they differ considerably in their
1436
effectiveness at forming and stabilizing beverage emulsions.
1437
7.2.2. Factors Influencing Selection
1438
AC C
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1423
Emulsion Formation: Two of the most important factors to consider when selecting an
1439
appropriate emulsifier for forming a beverage emulsion is its ability to produce small droplets
1440
during homogenization, and the minimum amount of emulsifier required to form a stable system
1441
(McClements, 2005b). The performance of an emulsifier can be characterized by plotting the
ACCEPTED MANUSCRIPT 50
mean droplet diameter versus emulsifier concentration under standardized homogenization
1443
conditions, such as homogenizer type, energy input, and duration (McClements, 2007). A
1444
typical dependence of mean droplet diameter on emulsifier concentration for oil-in-water
1445
emulsions produced using a high pressure homogenizer is shown in Figure 19. These curves can
1446
be divided into two regions: an emulsifier-limited region and a homogenizer-limited region.
1447
In the emulsifier-limited region the droplet size decreases with increasing emulsifier
1448
concentration and the size of the droplets is primarily limited by the total amount of emulsifier
1449
present (rather than by homogenization conditions). The minimum size of the stable droplets
1450
that can be produced in this region is given by (McClements, 2007):
1451
SC
6 ⋅ Γ⋅ φ 6 ⋅ Γ⋅ φ = cS c'S (1 − φ )
(8)
M AN U
d min =
RI PT
1442
Here, Γ is the surface load of the emulsifier at saturation (kg m–2), φ is the disperse phase volume fraction (dimensionless), cS is the concentration of emulsifier in the emulsion (kg m–3),
1453
and c′S is the concentration of emulsifier in the aqueous phase (kg m–3). This equation shows
1454
that the droplet size should by linearly related to the inverse of the surfactant concentration in
1455
this region. In this region, the homogenizer initially produces small droplets but there is
1456
insufficient emulsifier present to completely cover them all, and so they tend to coalesce with
1457
each other and form larger droplets under they are covered and protected from coalescence.
1458
TE D
1452
In the homogenizer-limited region there is more than enough emulsifier present to cover all of the droplets formed by the homogenizer, but the disruptive energy generated by the
1460
homogenizer is insufficient to form any smaller droplets. In this region, the droplet size is
1461
therefore limited by the maximum disruptive energy that the homogenizer can generate, rather
1462
than by the emulsifier concentration. In this region, the droplet size could be reduced further by
1463
increasing the homogenization pressure to that smaller droplets are produced.
AC C
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1459
1464
Emulsifiers differ in the abilities to form small droplets and in the amounts required to form
1465
stable emulsions due to differences in their molecular and physicochemical characteristics. The
1466
most important physical characteristics are: (i) the rate at which they absorb to the oil-water
1467
interface during homogenization; (ii) the amount that they reduce the interfacial tension after
1468
absorption; and, (iii) the amount required to cover a unit area of interface (surface load).
1469
Typically, the size of the droplets produced under standardized conditions tends to decrease with
1470
increasing absorption kinetics, decreasing interfacial tension, and decreasing surface load.
ACCEPTED MANUSCRIPT 51
Emulsion Stability: The nature of the emulsifier used to form a beverage emulsion has a
1472
pronounced influence on its physical and chemical stability, and so beverage manufacturers must
1473
carefully select an emulsifier with appropriate stability characteristics. When selecting an
1474
emulsifier it is important to establish the set of conditions that it is expected to work under (e.g.
1475
pH range, temperature range, ionic composition, solvent composition, and mechanical abuse),
1476
which is likely to be highly product specific. Emulsifiers may vary considerably in their ability
1477
to form stable emulsions under different environmental conditions. For example, many globular
1478
proteins form stable emulsions at low ionic strengths and pH values far from their isoelectric
1479
point, but tend to flocculate at high ionic strengths or pH values close to their isoelectric point
1480
(McClements, 2004). In addition, they tend to flocculate when heated above their thermal
1481
denaturation temperature due to an increase in droplet surface hydrophobicity. Emulsions
1482
stabilized by some types of non-ionic surfactants are also unstable at elevated temperatures (Rao
1483
& McClements, 2010), in this case because the rate of droplet coalescence increases when the
1484
surfactant approaches its phase inversion temperature (PIT). Emulsions stabilized by
1485
amphiphilic polysaccharides, such as gum arabic and modified starch, tend to be have good
1486
stability to droplet aggregation across a wide range of pH values, ionic strengths, and
1487
temperatures (Chanamai & McClements, 2002b; Charoen, et al., 2011; Qian, Decker, Xiao, &
1488
McClements, 2011). However, these emulsifiers are often not very efficient at producing small
1489
droplets, or they have to be used at high levels. The stability characteristics of a particular
1490
emulsifier are largely determined by its influence on the colloidal interactions operating between
1491
oil droplets (McClements, 2005b). Globular proteins and ionic surfactants tend to form thin
1492
interfacial layers that mainly stabilize droplets against aggregation through electrostatic
1493
repulsion. On the other hand, polysaccharides and non-ionic surfactants tend to form thick
1494
hydrophilic interfacial layers that mainly stabilize droplets through steric repulsion. Emulsions
1495
stabilized by electrostatic interactions are particularly susceptible to environmental changes that
1496
alter the magnitude or range of the electrical charge, e.g., pH and ionic strength. An example of
1497
differences in the electrical characteristics and aggregation stability of emulsions stabilized by a
1498
protein (whey protein isolate) and two polysaccharides (gum Arabic and modified starch) are
1499
shown in Figure 20. The protein-coated droplets tend to aggregate near their isoelectric point
1500
due to their low net charge, whereas polysaccharide-coated droplets are stable across the whole
1501
pH range due to their thick adsorbed layers.
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Legal and Labeling Requirements: Traditionally, many of the emulsifiers used in food and
1503
beverage applications were synthetic small-molecule surfactants due to their low cost and high
1504
efficiency (Kralova & Sjoblom, 2009). However, there has been increasing interest within the
1505
food and beverage industry in replacing synthetic ingredients with natural alternatives so as to
1506
create product labels that are more consumer-friendly. Natural ingredients often have a higher
1507
price than synthetic ingredients, and poorer functional performances. The change from artificial
1508
to natural food components can also increase manufacturing costs and reduce product shelf-life.
1509
In the following sections, we provide a brief overview of some of the synthetic and natural
1510
emulsifiers that are currently available for use in beverage emulsions.
1511
7.2.3. Ingredient Examples
SC
In principle, many different kinds of emulsifiers can be used to stabilize the oil-in-water
M AN U
1512
RI PT
1502
emulsions used in the beverage industry. In practice, the vast majority of commercial beverage
1514
emulsions are made with a limited number of emulsifiers, mainly gum arabic and modified
1515
starch. In this section, we provide an overview of both currently used and potential emulsifiers
1516
that can be utilized in beverage products, and classify them according to the nature of the
1517
molecules involved (i.e., polysaccharides, proteins, or surfactants).
1518 1519
7.2.3.1. Polysaccharide-based emulsifiers Gum Arabic: Currently, gum arabic (also known as gum acacia) is probably the most widely
1520
used emulsifier in the beverage industry to formulate cloud and flavor emulsions (Given, 2009;
1521
Tan, 2004). Gum arabic is a natural exudate harvested from acacia trees in sub-Saharan Africa,
1522
especially Sudan (Buffo, Reineccius, & Oehlert, 2002). Exudates from Acacia seyal and senegal
1523
have both been approved for use in food, but the beverage industry generally acknowledges A.
1524
senegal as the “gold standard” (Reiner, Reineccius, & Peppard, 2010). Acacia senegal consists
1525
of at least three high molecular weight biopolymer fractions. It has been proposed that the
1526
surface-active fraction consists of branched arabinogalactan blocks attached to a polypeptide
1527
backbone (Dickinson, 2003; Jayme, Dunstan, & Gee, 1999; Phillips & Williams, 2000). The
1528
hydrophobic polypeptide chain is believed to anchor the molecules to the droplet surface, while
1529
the hydrophilic carbohydrate blocks extend into the surrounding aqueous phase. The interfacial
1530
layer formed by gum arabic is believed to provide stability against droplet aggregation mainly
1531
through steric repulsion, but with some contribution from electrostatic repulsion also. The
AC C
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1513
ACCEPTED MANUSCRIPT 53
influence of a variety of processing conditions on gum arabic functionality has been examined
1533
(Buffo & Reineccius, 2001; Buffo, Reineccius, & Oehlert, 2001; Buffo, et al., 2002; Chanamai
1534
& McClements, 2002b). For example, it has been shown that gum arabic-coated oil droplets
1535
remain stable to flocculation when exposed to a wide range of environmental conditions, e.g., pH
1536
(3 to 9), ionic strength (0 to 500 mM NaCl; 0 to 25 mM CaCl2) and thermal treatment (30 to 90
1537
ºC) (Chanamai & McClements, 2002b; Charoen, et al., 2011). Nevertheless, gum arabic has a
1538
relatively low affinity for oil-water interfaces compared to most other surface-active
1539
biopolymers, which means that it has to be used at relatively high concentrations to form stable
1540
emulsions. For example, as much as 20 % gum arabic may be required to produce a stable 12
1541
wt% oil-in-water emulsion (Tse & Reineccius, 1995a). For this reason, its application as an
1542
emulsifier is restricted to products that have relatively low droplet concentrations, e.g., beverage
1543
emulsions. In addition, frequent problems have been reported in obtaining reliable sources of
1544
consistently high quality gum arabic, which has led many food scientists to investigate
1545
alternative sources of biopolymer emulsifiers for use in beverages, such as modified starch and
1546
various proteins (Chanamai & McClements, 2002a; Charoen, et al., 2011; Garti, 1999; Kim,
1547
Morr, & Schenz, 1996; Qian, et al., 2011; Tan, 1998; Trubiano, 1995a). Gum arabic has a high
1548
water-solubility and a relatively low solution viscosity compared to other gums, which facilitates
1549
its application as an emulsifier.
SC
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1550
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1532
Modified Starch: Natural (unmodified) starches have poor surface activity due to their hydrophilic glucose backbones. Nevertheless, starches can be chemically modified to make
1552
them effective emulsifiers by attaching non-polar chains along their backbones (Trubiano,
1553
1995b). These types of modified starch are widely used as emulsifiers in the beverage industry.
1554
One of the most commonly used modified starches is an octenyl succinate derivative of waxy-
1555
maize (Stauffer, 1999; Trubiano, 1995b). It consists primarily of amylopectin that has been
1556
chemically modified to contain a side-group that is anionic and non-polar. These side-groups
1557
anchor the molecule to the oil droplet surface, while the hydrophilic starch chains protrude into
1558
the aqueous phase and protect droplets against aggregation through steric repulsion. Because the
1559
dominant stabilizing mechanism is steric repulsion, emulsions stabilized by modified starch are
1560
resistant to changes in pH (3 to 9), ionic strength (0 to 100 mM NaCl; 0 to 25 mM CaCl2) and
1561
temperature (30 to 90 ºC) (Chanamai & McClements, 2002b; Charoen, et al., 2011; Qian, et al.,
1562
2011). Like gum arabic, modified starch has a relatively low interfacial activity (compared to
AC C
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1551
ACCEPTED MANUSCRIPT 54
proteins or surfactants), and so a large excess must be added to ensure that all the droplet
1564
surfaces are adequately coated. For example, it is had been recommended that a 1:1 mass ratio
1565
of modified starch to oil was required to produce a stable oil-in-water emulsion (Tse &
1566
Reineccius, 1995a). Nevertheless, there have recently been technical advances in the
1567
manufacture of modified starches that have led to emulsifiers that can be used at much lower
1568
levels (1:5 mass ratio) to form stable emulsions with small droplet sizes (Charoen, et al., 2011;
1569
Qian, et al., 2011). Studies have shown that if the concentration of non-adsorbed modified starch
1570
in the aqueous phase surrounding the oil droplets is too high, it may promote depletion
1571
flocculation (Chanamai & McClements, 2001). Modified starches usually come in powdered or
1572
granular forms that are easily dispersible in cold water.
SC
Other polysaccharides: A number of other surface-active polysaccharides have been studied
M AN U
1573
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1563
1574
for their potential use as emulsifiers in oil-in-water emulsions. Although these polysaccharides
1575
are not widely used in the beverage industry at present they may find some applications in the
1576
future. There has been debate about the molecular origin of the surface activity of these
1577
polysaccharides, and about whether their ability to stabilize emulsions is primarily due to their
1578
surface activity or their ability to thicken the aqueous phase (Dickinson, 2003). Cellulose is not a good emulsifier in its natural state because it forms strong intermolecular
TE D
1579
hydrogen bonds, which make it insoluble in water. Nevertheless, cellulose can be physically,
1581
chemically, or enzymatically modified to produce food-grade ingredients that have interfacial
1582
activity and can be used as emulsifiers (Garti, et al., 1993), such as methyl cellulose (MC),
1583
hydroxypropyl cellulose (HPC), and methyl hydroxypropyl cellulose (MHPC). These
1584
ingredients are all non-ionic polymers that are soluble in cold water, but tend to become
1585
insoluble when the solution is heated above a critical temperature (around 50 – 90 ºC). They
1586
have good stability to pH (2 – 11), salt and freeze-thaw cycling, which may be beneficial in a
1587
number of beverage emulsion applications.
AC C
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1580
1588
Certain types of pectin have been shown to be surface active molecules that can able to
1589
stabilize oil-in-water emulsions, e.g., beet pectin (Jung & Wicker, 2012; Nakauma, et al., 2008).
1590
The origin of this effect is believed to be due to the pectin having some protein molecules
1591
associated with it. The amphiphilic protein portion anchors the pectin molecules to the oil phase,
1592
while the hydrophilic polysaccharide chains protrude into the aqueous phase thereby providing
1593
some steric repulsion. In addition, pectin molecules usually have a relatively high negative
ACCEPTED MANUSCRIPT 55
1594
charge because of the presence of carboxylic groups (pKa = 3.5), and therefore they can also
1595
generate an electrostatic repulsion (Nakauma, et al., 2008).
1596
Soy soluble polysaccharide (SSPS) is a soluble, acidic polysaccharide extracted from okara that has a molecular structure that closely resembles pectin (Nakamura, Takahashi, Yoshida,
1598
Maeda, & Corredig, 2004; Nakamura, Yoshida, Maeda, Furuta, & Corredig, 2004). Similar to
1599
gum Arabic and beet pectin, SSPS is believed to have a protein backbone that anchors the
1600
molecule to the oil phase and hydrophilic anionic polysaccharide chains that protrude into the
1601
aqueous phase and provide stabilization through steric and electrostatic repulsion (Nakauma, et
1602
al., 2008).
SC
1603
RI PT
1597
Corn fiber gum (CFG) is a by-product of the milling of corn (Yadav, Johnston, Hotchkiss, & Hicks, 2007). Preliminary studies have shown that CFG can facilitate the formation and stability
1605
of oil-in-water emulsions (Yadav, et al., 2007). The surface activity of this polysaccharide may
1606
also be due to the presence of small amounts of protein or lipids that are associated with it.
1607
M AN U
1604
Research is continuing in this area, and it is likely that a variety of other polysaccharidebased emulsifiers will be identified that may be suitable for stabilization of beverage emulsions.
1609 1610
7.2.3.2. Protein-based emulsifiers Proteins are widely used in the food industry to stabilize various commercial food and
1611
beverage products, but they are not commonly used to stabilize soft drinks. Surface-active
1612
proteins can be isolated from various sources, including milk, meat, fish, and plants, but the most
1613
widely used in the food industry are those derived from bovine milk: casein and whey proteins.
1614
The interfacial coatings formed by proteins are usually relatively thin and electrically charged,
1615
and hence the major mechanism preventing droplet flocculation in protein-stabilized emulsions
1616
is electrostatic repulsion (rather than steric repulsion). Consequently, protein-stabilized
1617
emulsions are particularly sensitive to pH and ionic strength effects (Figures 20 and 21), and
1618
will tend to flocculate at pH values close to their isoelectric points or when the ionic strength
1619
exceeds a certain level (Demetriades, et al., 1997a, 1997b; Dickinson, 2010; McClements, 2004).
1620
Proteins can typically be used at much lower levels that amphiphilic polysaccharides to
1621
stabilize oil-in-water emulsions. For example, as much as 20 % gum arabic may be required to
1622
produce a stable 12.5 wt% oil-in-water emulsion, whereas less than 1% whey protein can be used
1623
(Chanamai & McClements, 2002a; Tse & Reineccius, 1995b). On the other hand, globular
AC C
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1608
ACCEPTED MANUSCRIPT 56
protein stabilized emulsions are much more susceptible to pH, ionic strength, and thermal
1625
processing than polysaccharide stabilized emulsions as shown in Figure 20 (Chanamai &
1626
McClements, 2002a). The stability of protein-stabilized emulsions to pH, salts, and thermal
1627
processing can often be improved by mixing them with ionic polysaccharides either before or
1628
after homogenization, such as gum arabic, pectin, alginate, and carrageenan (Guzey &
1629
McClements, 2006, 2007; Harnsilawat, Pongsawatmanit, & McClements, 2006; Klein, et al.,
1630
2010). These ionic polysaccharides form an interfacial complex with the adsorbed protein
1631
molecules, which increases the steric and electrostatic repulsion between the droplets
1632
(Dickinson, 2011).
SC
1633
RI PT
1624
A number of other methods have also been developed to improve the emulsifying properties of protein ingredients, including limited hydrolysis to form peptides, modification of protein
1635
structure by chemical, physical, enzymatic or genetic means, and blending of the proteins with
1636
other ingredients, although not all of these methods are legally permitted at present.
1637
M AN U
1634
Globular Proteins: Native globular proteins tend to have fairly compact spheroid structures with the non-polar amino acids contained within their hydrophobic interiors (away from water)
1639
and the polar amino acids located at the exterior (in contact with water). There are many sources
1640
of globular proteins in foods that are surface-active and capable of forming and stabilizing
1641
emulsions, such as those from whey, soy, egg, pea, and various other plants (McClements,
1642
2005b). Globular proteins tend to form relatively thin interfacial coatings around fat droplets,
1643
and therefore primarily stabilize emulsions against flocculation through electrostatic repulsion
1644
(McClements, 2004). They are therefore particularly sensitive to droplet flocculation at pH
1645
values near the isoelectric point (where the droplet charge is low) or at high salt concentrations
1646
(where the droplet charge is screened). Emulsions stabilized by globular proteins are also
1647
particularly sensitive to heat treatments, because these proteins unfold when the temperature
1648
exceeds the thermal denaturation temperature thereby exposing reactive non-polar and sulfhydryl
1649
groups (Kim, Decker, & McClements, 2002). These reactive groups increase the attractive
1650
interactions between droplets, which may lead to droplet flocculation through increased
1651
hydrophobic attraction and disulfide bond formation.
AC C
EP
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1638
1652
The most common source of globular proteins in the food industry is the whey protein
1653
derived from bovine milk. A number of different whey protein ingredients are available for use
1654
in the food industry as emulsifiers, including whey protein concentrate (WPC), whey protein
ACCEPTED MANUSCRIPT 57
isolate (WPI), and highly purified protein fractions such as β-lactoglobulin and α-lactalbumin.
1656
These ingredients normally come as white powders that are dissolved in water prior to
1657
utilization. WPC and WPI are commonly used as emulsifiers in commercial food and beverage
1658
products, whereas β-lactoglobulin and α-lactalbumin are more frequently used in fundamental
1659
studies of protein functionality in research laboratories. Whey protein actually contains a
1660
mixture of globular proteins: ≈ 55% β-lactoglobulin; ≈ 24% α-lactalbumin; ≈ 15%
1661
immunoglobulins; and ≈ 5% serum albumin (McClements, 2005b). The isoelectric point of the
1662
major whey proteins is around pH 5, and therefore droplets stabilized by whey proteins tend to
1663
flocculate around this pH, and especially between about pH 4 to 6 (Figure 20). The thermal
1664
denaturation temperature of whey proteins is around 70 to 80 ºC, and so they are prone to droplet
1665
aggregation when heated above this temperature, especially in the presence of salts that screen
1666
electrostatic interactions (Kim, et al., 2002).
SC
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1667
RI PT
1655
Flexible Proteins: Another group of protein-based emulsifiers commonly used in foods can be classified as flexible proteins (Dickinson, 1992a). These molecules tend to have a more
1669
flexible random coil type structure (although they often retain some secondary structure). The
1670
main sources of flexible proteins are gelatin from fish and animals, and caseins from bovine
1671
milk. Gelatin is a relatively high molecular weight protein derived from fish, porcine, or bovine
1672
collagen (McClements, 2005b). It is typically prepared by disrupting the native structure of
1673
collagen by boiling it in the presence of either acid (Type A gelatin) or base (Type B gelatin).
1674
The isoelectric point of Type A gelatin (∼7-9) tends be higher than that of Type B gelatin (∼5).
1675
Gelatin exists as a random coil molecule at relatively high temperatures, but undergoes a coil-to-
1676
helix transition upon cooling below a critical temperature, which is about 10 to 25 ºC for animal
1677
gelatin and about 0 to 5 ºC for fish gelatin. Gelatin has been shown to be surface-active and
1678
capable of acting as an emulsifier in oil-in-water emulsions. Nevertheless, it often produces
1679
relatively large droplet sizes when used in isolation, and therefore it may be utilized with other
1680
ingredients to improve its effectiveness at forming and stabilizing emulsions.
AC C
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1668
1681
There are various casein-based emulsifiers that can be purchased commercially with
1682
different compositions and functional properties, such as sodium caseinate, calcium caseinate,
1683
and purified protein fractions, such as a β-casein. There are four main protein fractions in
1684
casein: αS1 (∼ 44%), αS2 (∼ 11%), β (∼ 32%) and κ (∼ 11%). Casein-coated droplets have been
1685
shown to be unstable to droplet flocculation at pH values (3.5 – 5.3) close to the protein’s
ACCEPTED MANUSCRIPT 58
isoelectric point. Caseinate-stabilized emulsions tend to be more stable to heating than whey
1687
protein-stabilized emulsions, presumably because the relatively flexible casein molecules do not
1688
undergo appreciable heat-induced conformational changes like globular proteins do.
1689 1690
7.2.3.3. Small-molecule Surfactants In general, small-molecule surfactants consist of a polar head-group and a non-polar tail
1691
group (Kralova & Sjoblom, 2009; McClements, 2005b; Stauffer, 1999). The head group may be
1692
non-ionic, anionic or cationic, while the tail group may vary in the number, length, and degree of
1693
unsaturation of the chains. Many surfactants can be used to fabricate beverage emulsions using
1694
either low or high energy approaches, which is different from amphiphilic biopolymers that can
1695
only be used to fabricate them using high energy methods. In addition, many surfactants are able
1696
to form smaller oil droplets than amphiphilic biopolymers using high energy methods because
1697
they can adsorb more quickly to the oil-water interfaces during homogenization and lower the
1698
interfacial tension more. Surfactants are also able to stabilize emulsions at lower concentrations
1699
than amphiphilic biopolymers. Despite these advantages, the food industry has been trying to
1700
replace synthetic small molecule surfactants with emulsifiers that are natural and more label
1701
friendly due to consumer demands. Recently, some natural small molecule surfactants have
1702
become commercially available, such as quillaja saponin. A summary of some small-molecule
1703
surfactants that could be utilized in beverage emulsions is given in Table 4.
SC
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1704
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1686
Tweens: Tweens (also sold under the name polysorbates) are synthetic non-ionic surfactants that consist of a non-polar fatty acid group esterified to a polar polyoxyethylene sorbitan group.
1706
The nature of the fatty acid and polyoxyethylene chains determines the nomenclature and
1707
functional properties of the surfactant. Tween 20 (monolaurate), Tween 40 (monopalmitate),
1708
Tween 60 (monostearate), and Tween 80 (monooleate) are hydrophilic molecules with relatively
1709
high HLB values (14.9 and 16). They are therefore predominantly water-soluble, form micelles
1710
in aqueous solutions, and stabilize oil-in-water emulsions. Since Tweens are nonionic they tend
1711
to produce droplets that are stable to aggregation over a wide range of pH and ionic strength
1712
values. However, Tween-coated droplets may become unstable to coalescence at elevated
1713
temperatures close to the phase inversion temperature.
1714 1715
AC C
EP
1705
Sugar esters: Sugar esters are synthetic nonionic surfactants that consist of a polar sugar esterified to a fatty acid chain. There has been growing interest in the utilization of sugar esters
ACCEPTED MANUSCRIPT 59
as emulsifiers within the food and beverage industry (Szuts, Budai-Szucs, Eros, Otomo, &
1717
Szabó-Révész, 2010), which can be attributed to their desirable sensory attributes, low toxicity,
1718
and high biodegradability (Sadtler, Guely, Marchal, & Choplin, 2004). In addition, sugar esters
1719
are produced from natural products, such as sucrose and vegetable oil, and are therefore often
1720
perceived as being more label-friendly than petrochemical-based surfactants (Huck-Iriart,
1721
Candal, & Herrera, 2009). Sucrose fatty acid esters are non-ionic surfactants that have a sucrose
1722
moiety as a hydrophilic head group and one or more fatty acids as lipophilic tail groups. Sucrose
1723
monoesters have a single fatty acid attached to the sucrose molecule, which means they are
1724
relatively hydrophilic, water-soluble, have high HLB numbers, and can stabilize O/W emulsions
1725
(Garti, Clement, Leser, Aserin, & Fanun, 1999; Glatter, et al., 2001). A number of researchers
1726
have previously examined the structure, properties, and functionality of sucrose monoesters
1727
(Fanun, 2010; Garti, Aserin, & Fanun, 2000; Garti, Clement, Fanun, & Leser, 2000; Garti, et al.,
1728
1999). These studies have shown that sucrose monoesters can form a range of different colloidal
1729
structures depending on system composition and temperature, e.g., microemulsions,
1730
nanoemulsions or emulsions. Emulsions formed using sucrose monopalmitate (SMP) as the only
1731
emulsifier are highly unstable to aggregation when stored at acidic pH values (pH 3.5), which
1732
has been attributed to a reduction of droplet charge (Rao & McClements, 2011). However,
1733
emulsions that are stable across a wide range of pH can be produced by mixing SMP with a co-
1734
surfactant (such as lecithin) that increases the electrostatic repulsion between droplets at low pH
1735
(Choi, et al., 2011).
TE D
M AN U
SC
RI PT
1716
Quillaja Saponin: Quillaja saponin is a natural surfactant extracted from the bark of the
1737
Quillaja saponaria Molina tree. It has been found to contain surface active components that are
1738
capable of forming surfactant micelles and stabilizing oil-in-water emulsions (Mitra & Dungan,
1739
1997; Waller & Yamasaki, 1996a, 1996b; Yang, Leser, Sher, & McClements, 2013; Yang &
1740
McClements, 2013). The major components within this extract have been reported to be saponins
1741
(van Setten, ten Hove, Wiertz, Kamerling, & van de Werken, 1998; van Setten, van de Werken,
1742
Zomer, & Kersten, 1995) , which are high molecular weight glycosides consisting of a sugar
1743
moiety attached to a triterpene or a steroid aglycone (Hostettmann & Marston, 1995). The
1744
saponins are surface active substances because they contain both hydrophilic regions (such as
1745
rhamnose, xylose, arabinose, galactose, fucose, and glucuronic acid) and hydrophobic regions
1746
(such as quillaic acid and gypsogenic acid) on the same molecule (Mitra & Dungan, 1997; Sidhu
AC C
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1736
ACCEPTED MANUSCRIPT 60
1747
& Oakenfull, 1986). Recent experiments have shown that it can be used to form emulsions
1748
containing small droplets that are stable to changes in pH, ionic strength, and temperature (Yang,
1749
et al., 2013; Yang & McClements, 2013).
1750
A food ingredient based on the quillaja saponin extract has recently been produced by the National Starch Company (Bridgewater, NJ) under the trade name Q-Naturale®. This ingredient
1752
was released in 2008 and has been marketed as a replacement for gum acacia in beverage
1753
emulsions. According to the suppliers, Q-Naturale® can be used at a low level, has high oil
1754
loading, and is stable in some forms of alcoholic beverages, unlike most forms of gum acacia or
1755
modified food starch. It is also organic certified and is stable in products stored at ambient or
1756
cold temperatures for up to a year. The ingredient also comes in a liquid form, so that dissolution
1757
and hydration steps are unnecessary.
SC
M AN U
1758
RI PT
1751
Aside from its use as a beverage emulsion emulsifier, quillaja extract has also been used as a foam-stabilizing agent in beverages. The extract is added to the aqueous phase of a beverage to
1760
stabilize foam that forms at the neck of the beverage. This foam is considered a positive attribute
1761
in root beer and some citrus juices in various parts of the world.
1762 1763
7.2.3.4. Mixed Emulsifier Systems There has been increasing interest in using mixed emulsifier systems with the aim of
1764
producing more label-friendly food ingredients. Proteins tend to be better at producing small
1765
emulsion droplets when used at low concentrations than polysaccharides, whereas
1766
polysaccharides tend to be better at producing emulsions that are more stable to environmental
1767
conditions than proteins, e.g., pH, ionic strength, temperature, freeze-thaw cycling (McClements
1768
2004). The beneficial attributes of these two kinds of biopolymer can be combined to produce
1769
small emulsion droplets with good environmental stability. A number of researchers have shown
1770
that protein-polysaccharide complexes may have better emulsifying properties than either of the
1771
biopolymers used in isolation (Dickinson, 2003, 2011; Guzey & McClements, 2006). These
1772
complexes may be held together either by physical or covalent interactions, and may be formed
1773
either before or after homogenization (McClements 2004). Ingredients based on protein-
1774
polysaccharide interactions will have to be legally acceptable, economically viable and show
1775
benefits over existing ingredients before they find widespread utilization in the food industry. It
AC C
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1759
ACCEPTED MANUSCRIPT 61
1776
should be noted that gum arabic is actually a naturally occurring protein-polysaccharide complex
1777
that is already widely used in the food industry as an emulsifier. As mentioned earlier, sucrose monoesters produce emulsions that are not very stable under
1779
acidic conditions when they are used in isolation, but their stability can be greatly improved by
1780
combining them with co-surfactants, such as lysolecithin.
1781
7.3. Weighting Agents
1782
7.3.1. Mechanism of Action
Weighting agents are additives incorporated into the oil phase of certain types of beverage
SC
1783
RI PT
1778
emulsions to inhibit gravitational separation of the oil droplets (McClements, 2005b). As
1785
discussed earlier, the creaming rate of a droplet within an emulsion is given by Stokes’ law
1786
(McClements, 2005a). This equation predicts that the rate of creaming is proportional to the
1787
droplet radius squared (r2), the density contrast between the two phases (∆ρ), and the reciprocal
1788
of the aqueous phase viscosity (η0) (Equation 3). If ∆ρ is positive the particles move downwards
1789
(sediment), but if ∆ρ is negative they move upwards (creaming). The stability of a beverage
1790
emulsion to gravitational separation can therefore be improved by ensuring that the density of the
1791
oil droplets is similar to that of the surrounding aqueous phase. The densities of some of the
1792
major components found in beverage emulsions are listed in Table 3. The densities of flavor
1793
oils and vegetable oils are considerably lower than those of water and aqueous sugar solutions.
1794
Consequently, droplets containing these oils tend to move upward during storage leading to
1795
“ringing”, i.e., the formation of a visible ring of fat droplets at the surface of the product. The
1796
creaming rate can be reduced by decreasing the density contrast between the oil droplets and the
1797
surrounding aqueous phase (Equation 3). The most common means of increasing the density of
1798
the oil phase so that it matches that of the aqueous phase is to add weighting agents, which are
1799
hydrophobic components that have a density that is considerably greater than water (Table 3).
1800
7.3.2. Factors Influencing Selection
AC C
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M AN U
1784
1801
A number of different weighting agents are available for utilization within commercial
1802
beverage products, such as brominated vegetable oil (BVO), SAIB (Sucrose acetate isobutyrate),
1803
ester gum, or dammar gum (Table 3). There are a number of different factors that will
ACCEPTED MANUSCRIPT 62
1804
determine which of these different components is suitable for application within a specific
1805
beverage product. Physicochemical factors: The bulk physicochemical properties of commercial weighting
1807
agents vary considerably, which alters their ease of use and functional properties. The amount of
1808
a weighting agent that is required to match the density of the oil and aqueous phases depends on
1809
its density: the higher the density, the smaller the amount needed. Weighting agents also vary in
1810
their rheological and solubility properties, which will influence the preparation of the oil phase
1811
and the formation of stable emulsions by homogenization. Many beverage emulsions are
1812
sweetened by adding sugars to the aqueous phase. Sugars increase the aqueous phase density
1813
and would therefore be expected to increase the density contrast between the oil and aqueous
1814
phases, which should accelerate creaming. However, sugars also increase the viscosity of the
1815
continuous phase. These two effects largely cancel each other out, so that the creaming rate in
1816
sugar solutions is not much greater than in pure water (Chanamai & McClements, 2000).
SC
M AN U
1817
RI PT
1806
As mentioned previously, the overall density of an emulsion droplet actually depends on the density and volume of the oil phase and the interfacial layer. The interfacial layer is usually
1819
made up of emulsifiers (such as surfactants, phospholipids, proteins, or polysaccharides) that
1820
have a higher density than both the oil and the water phases. When the thickness of the
1821
interfacial layer is appreciable compared to the dimensions of the oil core, then the overall
1822
density of the emulsion droplet (oil core + interfacial layer) may become similar to that of the
1823
aqueous phase, and so the adsorbed emulsifier acts as a weighting agent itself (McClements,
1824
2011).
EP
1825
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1818
Legal and Labeling Factors: There are legal limits on the amount of each type of weighting agent that can be used in beverages, which must be adhered to when formulating a project.
1827
These legal limits vary from country to country and therefore a manufacturer must be aware of
1828
the appropriate legislation in each country the product will be marketed in. There are also
1829
differences in the “label-friendliness” of different weighting agents. Dammar and ester gum can
1830
be considered to be natural ingredients, whereas BVO and SAIB are synthetic ingredients.
AC C
1826
1831
Brominated vegetable oil (BVO) and ester gum are the two most commonly used weighting
1832
agents in beverage emulsions. However, many countries have restricted the amount of weighting
1833
agents that can be found in the final product. For example, the United States restricts BVO and
1834
ester gum’s use to 15 and 100 ppm per serving, respectively. This low concentration of allowable
ACCEPTED MANUSCRIPT 63
weighting agents restricts the type of products that BVO and ester gum can be used in. Beverage
1836
emulsions are one of the few products where these weighting agents can be used to reduce the
1837
creaming rate, because the oil phase content is so low (<0.1%). It is also possible to blend two or
1838
more types of weighting agents together to achieve density matching but remain below their
1839
individual restriction levels.
1840
7.3.3. Ingredient Examples
1841 1842
7.3.3.1. Brominated Vegetable Oil Brominated vegetable oil (BVO) is made when bromine is added to the double bonds of the
1843
triacylglycerol molecules in corn, soybean, cotton seed, or olive oil. It typically has a density
1844
between 1240 and 1330 kg m-3, and a permitted usage level of 15 ppm per serving in the United
1845
States. Nevertheless, there has recently been consumer concern about the presence of BVO in
1846
soft drinks, which has led some major beverage manufacturers to replace it with more natural
1847
alternatives. In 2013, a leading beverage manufacturer removed BVO from a major product line
1848
after receiving consumer complaints and petitions against its use. Even though BVO is permitted
1849
for use in the United States, it has been banned from use in beverages in the European Union and
1850
Asia.
1851 1852
7.3.3.2. Ester Gum Ester gum is a hydrophobic polymer made when glycerol is esterified to gum rosin
1853
harvested from trees (such as pine trees). It is normally supplied as a crystalline solid that can be
1854
incorporated into the oil phase. Ester gum may not be considered to be a natural food ingredient
1855
due to the esterification step used in its preparation,. Nevertheless, it is derived from natural
1856
components (gum rosin) and non-animal glycerol, and the name “ester gum” may appear to be
1857
more label friendly to consumers than BVO or SAIB. Ester gum has a density of around 1080
1858
kg m-3 and a permitted usage level of 100 ppm per serving in the United States (FDA, USA). It
1859
performs similarly to BVO but a greater concentration of ester gum has to be added to the oil
1860
phase in order to raise the density (Chanamai & McClements, 2000). Aside from its traditional
1861
use as a weighting agent, recent studies have shown that ester gum also retards droplet growth in
1862
model flavor emulsions through Ostwald ripening, i.e., it acts like a ripening inhibitor (Lim, et
1863
al., 2011). This behavior would probably be demonstrated by all weighting agents because they
1864
are highly hydrophobic materials with low water-solubilities.
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7.3.3.3. Sucrose Acetate Isobutyrate Sucrose acetate isobutyrate (SAIB) is a synthetic weighting agent produced by the
1867
esterification of sucrose with acetic and isobutyric anhydrides. SAIB is usually supplied in the
1868
form of a high viscosity transparent liquid that can be mixed with the oil phase prior to
1869
homogenization. It has a molecular weight of ≈ 845 g mol-1, a refractive index of ≈ 1.454, and a
1870
density of ≈ 1146 kg m-3. SAIB also has good stability to chemical degradation due to lipid
1871
oxidation. The level of SAIB in beverages is currently restricted to 300 ppm per serving in the
1872
United States (FDA, USA).
1873 1874
7.3.3.4. Dammar Gum Dammar gum is a natural weighting agent that is isolated from an exudate of
1875
Caesalpinaceae and Dipterocarpaceae shrubs. Of the four weighting agents mentioned in this
1876
review, dammar gum has the lowest density at 1060 kg m-3 (Table 3). Dammar gum is approved
1877
for use in several countries, but it is not currently approved for use in the USA and it so does not
1878
have GRAS status.
1879
7.4. Ripening Inhibitors
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As discussed previously, a major form of physical instability in many beverage emulsions is
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1865 1866
Ostwald ripening, which is the process whereby the mean droplet size grows during storage due
1882
to diffusion of oil molecules from small droplets to large droplets (Kabalnov, 2001; Kabalnov &
1883
Shchukin, 1992). Once steady state conditions have been achieved, the increase in mean droplet
1884
diameter over time can be described by Equation 6 given earlier (Kabalnov & Shchukin, 1992).
1885
This equation predicts that the Ostwald ripening rate increases as the solubility of the oil phase in
1886
the aqueous phase increases. Many flavor oils have a relatively high water-solubility and are
1887
therefore particularly prone to droplet growth through this mechanism (Rao & McClements,
1888
2012b). On the other hand, triacylglycerol oils (such as corn, sunflower, or canola oils) contain
1889
large non-polar triacylglycerol molecules with relatively low water-solubilities, and therefore
1890
cloud emulsions prepared from these types of oil are not prone to Ostwald ripening (Coupland,
1891
Weiss, Lovy, & McClements, 1996).
1892
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An effective means of inhibiting or preventing Ostwald ripening in an emulsion is to add a
1893
hydrophobic component with a very low water-solubility to the oil phase prior to
1894
homogenization (Kabalnov, Pertzov, & Shchukin, 1987). This hydrophobic compound is
ACCEPTED MANUSCRIPT 65
referred to as a “ripening inhibitor” due to its ability to slow droplet growth through this
1896
mechanism (Lim, et al., 2011; McClements, et al., 2012). The ability of low water-solubility
1897
compounds to retard Ostwald ripening in emulsions can be attributed to an entropy of mixing
1898
effect (Kabalnov, et al., 1987), which is discussed in more detail in Section 5.1.3.
1899
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Recent studies have shown that Ostwald ripening may be inhibited in flavor oil emulsions by adding long chain triglycerides (such as corn oil) (McClements, et al., 2012) or by adding
1901
weighting agents (such as ester gum) (Lim, et al., 2011). It has also been proposed that some of
1902
the water-insoluble components within flavor oils themselves may act as ripening inhibitors,
1903
which may account for the better stability of high-fold lemon oils (e.g., 10×) to Ostwald ripening
1904
than low-fold lemon oils (e.g., 1× and 3×) (Rao & McClements, 2012c).
1905
7.5. Thickening Agents
1906
7.5.1. Mechanism of Action
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Thickening agents are ingredients that are added to beverage emulsions to increase the viscosity of the aqueous phase, and therefore retard droplet creaming and alter product
1909
mouthfeel. Thickening agents are usually hydrophilic biopolymers that dissolve in the aqueous
1910
phase and occupy a volume that is much larger than the volume of the polymer chain itself
1911
(McClements, 2005b). The effectiveness of a thickening agent at increasing the viscosity of an
1912
aqueous solution is largely determined by its molecular characteristics (e.g., molecular weight,
1913
conformation, and interactions). To a first approximation the viscosity of a dilute aqueous
1914
solution containing a hydrophilic biopolymer can be described by Einstein’s equation:
1917 1918
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η = η0 (1 + 2.5 Rvφ)
(9)
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1908
where φ is the volume fraction of the biopolymer chain, and Rv is the volume ratio (Rv =
1919
Vsphere / Vpolymer), where Vsphere is the effective volume that the biopolymer occupies in solution
1920
(which includes the polymer chain and any trapped solvent), and Vpolymer is the volume occupied
1921
by the polymer chain itself. The higher the value of the volume ratio, the more effective is the
1922
polymer at increasing the viscosity of a solution. Typically, Rv increases as the molar mass,
1923
extension, and self-association of biopolymer molecules increases. Thus, xanthan gum is a
1924
particularly effective thickening agent because it has a high molecular weight and is highly
ACCEPTED MANUSCRIPT 66
extended in aqueous solutions and therefore has a large Rv value. Most thickening agents used in
1926
beverages are long-chain hydrophilic polysaccharides, such as xanthan, locust bean gum, pectin,
1927
or carrageenan. It should be noted that thickening agents can actually promote emulsion
1928
instability under certain conditions, due to their ability to induce bridging or depletion
1929
flocculation. Bridging flocculation can occur when polymer molecules are attracted to the
1930
surfaces of oil droplets, whereas depletion flocculation occurs when they polymer molecules are
1931
dispersed in the continuous phase and their concentration exceeds a particular value.
1932
7.5.2. Factors Influencing Selection
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1925
There are a number of factors that must be considered when selecting a suitable thickening
1934
agent for use in beverage emulsions. It is important to establish the solution and environmental
1935
conditions under which the thickening agent must function. This usually requires knowledge of
1936
the physicochemical properties of the biopolymer molecules involved, such as helix-coil
1937
transition temperatures (for carrageenan, alginate, pectin), electrical properties (charge groups
1938
and pKa values), hydrophobicity, ion sensitivity, and chemical/enzymatic stability (Whistler &
1939
BeMiller, 1997).
1940
of the biopolymer molecules used (ζ-potential versus pH profile), since electrostatic interactions
1941
influence ingredient interactions, emulsion stability, and mouthfeel. The electrical charge on
1942
biopolymers depends on the nature of any ionic groups along the chain background, as well as
1943
solution conditions (pH and ionic composition). Some biopolymers are neutral (e.g., starch,
1944
cellulose, LBG), some are anionic (e.g., alginate, carrageenan, xanthan), some are cationic (e.g.,
1945
chitosan and polylysine), and some are amphoteric (e.g., gelatin). The magnitude of the
1946
electrical charge on ionic biopolymers depends on the pH relative to the pKa value of the ionic
1947
groups. Anionic polysaccharides tend to be neutral at pH values sufficiently below their pKa
1948
value but negative above, whereas cationic polysaccharides tend to be neutral at pH values
1949
sufficiently above their pKa value but positive below. Proteins are positive below their
1950
isoelectric point, and negative above it (Figure 20). The most common charged groups on
1951
polysaccharide thickening agents are sulfate groups (e.g., carrageenan), carboxyl groups (e.g.,
1952
pectin, alginate, xanthan, carboxymethylcellulose) and amino groups (e.g., chitosan): -SO4H ↔ -
1953
SO4- (pKa ≈ 2); -CO2H ↔ -CO2- (pKa ≈ 3.5); -NH3+ ↔ - NH2 (pKa ≈ 6.5). The most common
1954
charged groups on proteins are carboxyl and amino groups.
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For some applications, it is important to establish the electrical characteristics
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The electrical characteristics of thickening agents in aqueous solutions may also be altered
1956
by interactions with other ionic species in their environment, such as mineral ions, surfactants,
1957
and other biopolymers. Monovalent or multivalent ions such as sodium, potassium, or calcium
1958
may bind to oppositely charged groups on biopolymer chains, altering their overall charge
1959
characteristics. In addition, knowledge of the type of environmental and solution conditions
1960
present within a particular food is often important for selecting the most appropriate thickening
1961
agent. For example, pectin can start to depolymerize when exposed to neutral or alkaline
1962
conditions as a result of a base-catalyzed β-elimination reaction that breaks down its sugar chain.
1963
Over time, this breakdown can result in a decline in viscosity and loss of texture (Sila et al.,
1964
2009).
1965
7.5.3. Ingredient Examples
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There are many different types of thickening agents that can be used in commercial
1967
beverage products. These thickening agents differ in their chemical compositions, molecular
1968
weight distributions, degrees of branching, conformations, and transition temperatures, which
1969
causes variations in their functional performances. A summary of thickening agents that could
1970
be utilized in beverage emulsions is given in Table 5. There are a number of excellent books that
1971
provide detailed information about the molecular, physicochemical, and functional properties of
1972
thickening agents in foods and beverages (Cui, 2005; Imeson, 2010; Phillips & Williams, 2009).
1973
7.6. Sweeteners
1974
7.6.1 Mechanism of Action
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Beverage emulsions are sweetened by adding artificial or natural substances that interact with specific taste receptors on the tongue to provide the perception of sweetness (Fernstrom, et
1977
al., 2012; McCaughey, 2008; Meyers & Brewer, 2008). It has been proposed that mammals
1978
evolved to perceive a pleasant sensation after ingestion of sugar-rich foods because these foods
1979
are a ready source of energy in nature (e.g., fruits and berries). A wide variety of substances
1980
have molecular structures that can induce a sweet sensation, although they vary greatly in the
1981
concentrations required to produce sweetness. Sugar molecules have different sweetness
1982
intensities due to differences in their molecular structures, which alters how they react with the
1983
sweetness receptors in the mouth. Conventionally, the sweetness of substances is compared to
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that of sucrose (table sugar) which is taken to have a Sweetness of 100. The relative sweetness
1985
of different types of sweetener is established using carefully controlled sensory tests. Fructose is
1986
the sweetest of all common sugars because it has just the right dimensions and bond orientations
1987
to strongly interact with the sweetness receptors in the mouth. However, many natural and
1988
artificial compounds have much strong sweetness intensities than sucrose, and therefore can be
1989
used at much lower levels, which is the basis of many high-intensity sweeteners used in low-
1990
calorie products.
1991
7.6.2 Factors Influencing Selection
SC
1992
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1984
A number of factors must be considered when selecting a suitable sweetening system for a commercial beverage product. The sweeteners used must provide a flavor profile that consumers
1994
find acceptable and desirable. One of the main limitations of many high-intensity sweeteners is
1995
that they do not produce a flavor profile similar to that normally provided by sugars (Nahon,
1996
Roozen, & DeGraaf, 1996), or that they produce undesirable off-flavors (such as bitterness or
1997
metallic) or mouthfeel (such as astringency) (DuBois & Prakash, 2012). In addition, the
1998
sweeteners must remain physically and chemically stable within the product throughout
1999
processing, transport, and storage. Some sweeteners may breakdown when exposed to elevated
2000
temperatures and are therefore unsuitable for use in beverages that undergo thermal processing.
2001
Concern about overweight and obesity has driven consumer demand for reduced calorie
2002
products, which has led the beverage industry to produce low- and mid-calorie versions of their
2003
conventional high-calorie products. However, many of the high-intensity sweeteners used to
2004
replace sugars have undesirable flavor profiles or are perceived as being “unnatural”, and so the
2005
beverage industry is trying to introduce natural low-intensity sweeteners with desirable flavor
2006
profiles (such as Stevia).
2007
7.6.3. Ingredient Examples
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2008
There are many different types of sweeteners that can be used in commercial beverage
2009
products, which vary according to their chemical structure, origins (natural versus artificial), and
2010
intensity (low- versus high-intensity). The most commonly used low-intensity sweeteners are
2011
sugars (such as sucrose, glucose, and fructose) and sugar alcohols (such as sorbitol, mannitol,
2012
xylitol and erythritol). These substances may be added as individual ingredients, or they may be
2013
part of more complex ingredients, such as high fructose corn syrup, honey, and sugar cane. The
ACCEPTED MANUSCRIPT 69
human body does not have the appropriate enzymes in its digestive tract to break down sugar
2015
alcohols in the same manner that sugars are broken down. Consequently, the body breaks sugar
2016
alcohols down more slowly and less efficiently that sugars, which means they have less calories
2017
per gram (about 50-75%), which may be useful for the development of reduced calorie products.
2018
However, high-intensity sweeteners are more commonly used to produce low-calorie or medium-
2019
calorie beverages, which may be artificial (such as Cyclamate, Aspartame, Acesulfame K,
2020
Saccharin, Sucrolose, and Neotame) or natural (such as Stevia).
2021
8. Commercial Aspects of Beverage Emulsion Formulation
SC
2022
RI PT
2014
Beverage emulsions are developed internally and externally by commercial manufacturers of soft drinks products. External development is usually contracted to flavor houses that often
2024
compete with each another for a flavor contract with a beverage manufacturer. This contract can
2025
be worth thousands to millions of dollars depending on the product and length of the contract.
2026
Factors considered for selecting the most appropriate flavor delivery system include stability,
2027
cost, volume capacity, safety audits, kosher/halal certification, and the ingredient line of the
2028
beverage emulsion. Because of these various factors, the best-tasting, most stable flavor may not
2029
always be chosen as the final flavor used in a commercial application.
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2030
M AN U
2023
Beverage manufacturers normally supply flavor houses with a base of the intended product into which the flavor is intended to be incorporated. A base consists of all ingredient components
2032
except the flavor. By doing this, the manufacturers do not have to disclose confidential
2033
formulations and the flavor houses can develop a flavor specific to the product. Shelf-life studies
2034
(accelerated and non-accelerated) are performed to determine how well the prototype flavors
2035
hold up in the formulation. If chemical or physical instabilities are observed, the flavor can be
2036
reformulated to increase its stability. Depending on the project’s timeline this can take as little as
2037
a few days or up to a year. If the product is being rushed to market and only a few days are
2038
allotted for formulation, the flavor house can rely on past knowledge of how flavors behave in
2039
similar products. This can enable the fast development of a flavor that will have a high-
2040
probability of success in a given formulation. If development is done internally, the same steps
2041
are followed: prototype formulation; shelf-life studies; reformulation (if necessary); and then
2042
commercialization.
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2043
9. Recent Developments in Beverage Emulsions There have been a number of recent developments and innovations in the beverage industry
2045
over the past few years. In this section, we highlight a number of the most important ones. The
2046
compositions and calorie content of a number of common commercial beverages is summarized
2047
in Table 6, whereas some of the claims that can be made on beverages in given in Table 7.
2048
9.1. “All-natural” Products and Cleaner Ingredient Lines
RI PT
2044
Consumers’ preference for natural products over artificial have caused manufacturers to
2050
claim their products as “Natural” whenever possible. As attractive as a “Natural” claim is, it
2051
comes at a product functionality and financial cost. In most cases, natural ingredients are more
2052
expensive than synthetic and their functional properties are often not as robust. The use of
2053
synthetic ingredients creates a cheaper, more stable product, but also one less attractive to many
2054
consumers. A 2007 study by the University of Southampton (UK) linked synthetic colors and
2055
additives to heightened hyperactivity in children (McCann, et al., 2007). This report led to a
2056
consumer outcry for the removal of synthetic ingredients from food and beverage products aimed
2057
at children, promoting many large companies to begin removing synthetic additives and colors.
2058
One problem with replacing artificial additives with natural ones is that they are often much less
2059
stable within commercial products. For example, natural colors may rapidly degrade in the
2060
presence of acid, light, and heat. Consequently, many commercial products have had to be
2061
reformulated to include effective strategies to extend their shelf-life, e.g., using antioxidants and
2062
other preservatives to slow color fading.
M AN U
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2063
SC
2049
The use of synthetic ingredients not only impacts consumer acceptance, but also limits retail channels a product can compete within. Certain local and national grocery chains prohibit
2065
artificial sweeteners, preservatives, and flavors in the foods they sell. The incorporation of
2066
brominated vegetable oil (BVO), sucrose-acetate isobutyrate (SAIB), Tweens (polysorbate) or
2067
modified-food starch into a product can restrict product sales in these marketplaces. Thus
2068
beverage suppliers have to use alternative weighting agents (such as dammar gum or ester gum)
2069
or emulsifiers (such as gum Arabic) in these products.
2070
AC C
2064
The removal of all synthetic components of a product may be unachievable for functional
2071
and financial reasons. However, manufacturers can remove as many synthetic components as
2072
possible to make “No, No, No” claims. For instance, an unnatural product can still claim “No
ACCEPTED MANUSCRIPT 71
artificial colors, preservatives, flavors, and sweeteners”. A “No, No, No” claim is not as strong
2074
as a “Natural” claim, but it may be seen favorably by consumers. Claims on product packaging
2075
are regulated by the US Food and Drug Administration within the Code of Federal Regulations
2076
(CFR), specifically Title 21.
2077
9.2. Low-Calorie and Mid-Calorie Soft Drinks
2078
RI PT
2073
Consumer perceptions about the potential link between overconsumption of sugary drinks and obesity promoted beverage manufacturers to create a variety of low or zero-calorie products
2080
(Slavin, 2012). These products have been on the market for numerous years, but many
2081
consumers do not like their taste, and so the beverage industry has responded by developing a
2082
number of mid-calorie soft drinks. These drinks contain blends of caloric (e.g., Sucrose or High-
2083
Fructose Corn Syrup) and high-potency sweeteners (e.g., Acesulfame-Potassium, Aspartame,
2084
Rebiana, and Sucralose) to reduce the total calories in sweetened beverage products (typically by
2085
over a half compared to regular products), while still maintaining a desirable flavor profile.
2086
Historically, these kinds of products have not sold well in the marketplace (with a number of
2087
products from different companies failing), but there have been renewed attempts to introduce
2088
them again recently in response to consumer desire for reduced calorie intake.
2089
9.3. Energy Drinks
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M AN U
SC
2079
Energy drinks are marketed as products that can enhance physical or mental activity by
2091
providing a rapid source of energy, nutrients, and stimulants. These drinks may be taken before,
2092
during, or after some proposed activity. They usually contain a blend of ingredients, such as
2093
water, sugars, minerals, proteins, vitamins and/or stimulants (such as caffeine) that are proposed
2094
to replenish the bodies’ reserves and enhance performance. Typically, energy drinks come as a
2095
Ready-to-Drink beverage (serving ~ 8 fluid ounces), Ready-to-Drink shot (serving ~ 2 fluid
2096
ounces), or Liquid Concentrate (dilute beverage serving ~ 8 fluid ounces). All the major
2097
commercial beverage manufacturers have products in this rapidly growing product category.
2098
Emulsion technology may be used in these products to deliver oil-soluble flavors, clouding
2099
agents, preservatives, or nutrients. Further scientific research is required to demonstrate the
2100
claimed biological activity of many of these products. Recently, there has been concern about
2101
the potential risks to certain individuals associated with consuming the large amounts of caffeine
2102
in some of these products.
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2103
9.4. Beverage Concentrates Beverage concentrates are products that are sold in a concentrated form that consumers then
2105
dilute with water prior to consumption. Concentrates may be in either a fluid or powdered form
2106
and contain ingredients such as sugars, flavors, colors, and nutrients that form a drinkable
2107
beverage upon dilution. Beverage concentrates have been used since the nineteenth century
2108
when soda syrups were diluted by pharmacists prior to consumption by consumers. This process
2109
still occurs within automated soda fountains found in many fast-food restaurants, and within
2110
some home soda beverage makers. More recently, beverages have been developed that contain
2111
high-intensity sweeteners rather than sugars, which enable higher concentration factors to be
2112
attained. One recent successful product based on this principle involves squirting a small
2113
amount of the concentrate from a plastic bottle into a glass of water to form a zero-calorie
2114
beverage. A number of beverage manufacturers have launched products within this category in
2115
recent years.
M AN U
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2104
Beverage concentrates have some unique developmental issues due to the fact that the
2117
ingredients are present in a highly concentrated form. For example, high acid, sweetener, flavor,
2118
buffer, nutraceuticals and/or color concentrations can lead to accelerated chemical degradation
2119
reactions and shorter shelf lives.
2120
9.5. Fortification with Vitamins, Minerals, and Nutraceuticals
TE D
2116
As discussed earlier, there is a push to fortify many beverage products with ingredients that
2122
are perceived as giving added health benefits, such as vitamins, minerals, antioxidants, peptides,
2123
proteins, dietary fibers, and ω-3 fatty acids. These nutraceutical additives vary greatly in their
2124
molecular, physicochemical, and biological properties, such as physical state, solubility, charge,
2125
polarity, interactions, and flavor profiles. For example, carotenoids and polyunsaturated lipids
2126
may oxidize during storage and produce undesirable flavors or changes in color. Multivalent
2127
mineral ions (such as calcium) may precipitate from solution, promote precipitation of other
2128
components (such as proteins), or cause undesirable mouthfeel. Dietary fibers may increase the
2129
viscosity of a product, promote depletion flocculation, or give a slimy mouthfeel. Consequently,
2130
each beverage product must be carefully formulated to take into account the specific types of
2131
nutraceutical components it contains. This segment of the beverage industry is growing rapidly
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2132
with consumer demand for products that are perceived to be more beneficial to human health and
2133
wellness.
2134
9.6 Bottled Water There has been an appreciable increase in the global sales of bottled waters in recent years,
RI PT
2135
including enhanced, flavored, and fruit-flavored waters. Enhanced water is fortified with
2137
electrolytes, vitamins and other nutraceuticals, whereas flavored and fruit-flavored waters are
2138
flavored with flavorants and fruit extracts, respectively. Utilization of conventional emulsions in
2139
bottled waters to deliver oil-soluble flavors and nutraceuticals is limited because of the increase
2140
in turbidity caused by light scattering from the oil droplets. However, microemulsions and
2141
nanoemulsions that contain very fine droplets that do not scatter light strongly may be used for
2142
this purpose.
2143
9.7. New Product Innovation
M AN U
2144
SC
2136
New product innovation drives business growth and enables manufacturers to meet current consumer needs. Predicting future trends and having the first product to market can give
2146
manufacturers a huge advantage against competitors. However, being first to market is also a risk
2147
due to the possibility of a new product being rejected by consumers. Reasons for rejection may
2148
include product unfamiliarity or a lack of consumer need for the product.
2149
TE D
2145
One objective of new product innovation is to develop a product that is so novel that it can be patented. The federal government then grants the manufacturer a 20-year monopoly on that
2151
product type. A patented product can be the main driver of strong revenue growth for years, and
2152
a manufacturer can gain additional revenue through lawsuits due to patent infringement. On the
2153
other hand, patent expiration can also drive significant revenue loss once competitors have open
2154
access to the once-patented concept.
AC C
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2155
New product innovation does not necessarily mean creating the first prototype of a new
2156
product. Studying trends in other parts of the world and then modifying them to fit the intended
2157
marketplace can also be used to drive innovation. For instance, Acai Berry has been consumed in
2158
Brazil for decades, but it was only in the 2000’s that it became a desired component of food
2159
products in the United States. Suspending grains in beverage products has been carried out in
2160
South America and Europe for many years, but has only recently found its way into North
2161
America.
ACCEPTED MANUSCRIPT 74
Determining a secondary use of manufacturing byproducts can also generate revenue
2163
through the development of functional food ingredients from material once destined for waste.
2164
For example, whey protein was once a byproduct of cheese production, but is not a widely used
2165
functional ingredient in foods and beverages. Fusing elements from two different product
2166
categories is another way to innovate.
RI PT
2162
New product development is an integral part of the food industry and changing out
2168
components of a beverage to meet current demands is a struggle that manufacturers have to
2169
consider every day. Such demands include the removal of ingredients from current products that
2170
consumers perceive as undesirable (such as artificial dyes or High-Fructose Corn Syrup), the
2171
addition of desirable components (such as antioxidants, minerals, or nutraceutical lipids), or the
2172
replacement of expensive ingredients with inexpensive alternatives.
2173
10. Conclusions
M AN U
2174
SC
2167
In principle, beverage emulsions are one of the simplest forms of oil-in-water emulsions used within the food industry, consisting of small oil droplets dispersed within an aqueous
2176
medium. Nevertheless, there are still many challenges associated with developing successful
2177
commercial products. Beverage emulsions are prepared from various ingredients and processing
2178
operations that may vary from run-to-run, which can cause changes in the properties and stability
2179
of finished products. To minimize any undesirable changes in product quality it is important that
2180
beverage manufacturers develop a robust initial formulation and manufacturing operation, and
2181
that they also ensure that ingredient quality and manufacturing conditions are standardized and
2182
monitored. The beverage industry must also reformulate products to meet changing market and
2183
consumer demands: to increase profits by replacing expensive ingredients with less expensive
2184
ones; to remove ingredients that consumers perceive as being undesirable, such as sugar, salt,
2185
and artificial colors, flavors, or preservatives; to fortify beverages with ingredients that
2186
consumers perceive as desirable, such as vitamins, minerals, proteins, nutraceuticals, and fibers.
2187
A thorough understanding of the science and technology underlying the formation, properties,
2188
and stability of beverage emulsions facilitates the development of innovative products, and
2189
allows any production problems to be solved more rapidly. This review article has attempted to
2190
provide a comprehensive overview of the scientific and technological principles underlying
2191
beverage emulsion development.
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2192 2193
11. Acknowledgments DJM would like to thank Prof. Eric Dickinson for introducing him to the area of food biopolymers and colloids – it has led to many years of interesting and stimulating work, for
2195
which I am very grateful. We would also like to thank a number of colleagues in industry and
2196
academia who have provided valuable comments and suggestions on the manuscript, including
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Ratjika Chanamai, Eric Decker, Yuan Fang, Anil Gaonkar, and Peter Given. We would also like
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to acknowledge that this material is partly based upon work supported by grants from the United
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States Department of Agriculture (CREES, NRI Grant and NSF-EPA-USDA Grant) and the
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Massachusetts Department of Agricultural Resources.
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Figure 1: Schematic representation of different kinds of colloidal dispersions that can be used in the beverage industry: emulsions, nanoemulsions, and microemulsions.
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Figure 2: Schematic diagram of most common instability mechanisms that occur in food emulsions: creaming, sedimentation, flocculation, coalescence, Ostwald ripening and phase inversion.
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Figure 3. The particles in beverage emulsions can be designed to have different functional performances by varying their physicochemical and structural properties, such as size, composition, charge and interfacial properties.
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Figure 4. Schematic representation of the interaction potential between two emulsion droplets. The droplets may be stable, weakly flocculated, strongly flocculated, or coalesced depending on the attractive and repulsive forces between them.
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Figure 5. Lipid droplets have a major impact on the texture, appearance and stability of emulsions
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Figure 6. Initial flavor intensity of emulsions with fixed flavor content is strongly dependent on oil droplet content.
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Figure 7. Droplet growth due to Ostwald ripening of orange oil emulsions can be inhibited by adding a corn oil, which acts as a ripening inhibitor (McClements et al 2012)
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Figure 8. Influence of pH on the chemical degradation rate of citral in aqueous solutions: the degradation rate increases with decreasing pH (Choi et al 2009).
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Figure 9: Schematic representation of mechanical devices that can be used to produce beverages emulsions using the high-energy approach: high pressure valve homogenizer, microfluidizer and continuous ultrasonic homogenizer
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Figure 10: Schematic representation of various methods that can be used to produce beverages emulsions using a low-energy approach: emulsion inversion point method, spontaneous emulsification and phase inversion temperature.
T > PIT
W/O
O/W/O
O/W
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Emulsion inversion point method
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W/O Emulsion
Cool
T = PIT
Bicontinuous Microemulsion
Heat T < PIT
O/W Emulsion
Spontaneous emulsification method PIT method
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Figure 11: Influence of homogenization pressure in a microfluidizer on the mean droplet diameter of oil-in-water emulsions stabilized by either β-lactoglobulin (BLG) or sodium dodecyl sulfate (SDS). The oil phase density was increased by adding increasing amounts of weighting agent (Qian, McClements 2011).
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Figure 12: Influence of surfactant-to-oil ratio on the formation of 20 wt% oil-in-water emulsions stabilized by a non-ionic surfactant (Tween 80) prepared using the spontaneous emulsification method (Sabeti, Fang, and McClements 2012). The oil phase consisted of 80% vitamin E acetate and 20% MCT.
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Figure 13: Schematic diagram of formulation-composition map for a typical surfactant-oil-water system. This map can be used to understand the formation of emulsions and nanoemulsions suing various phase inversion methods.
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W/O/W
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SC
W/O
Transitional
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O/W
O/W/O
AC C
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Catastrophic
• HLD – Hydrophilic-Lipophilic Deviation • Relative affinity of surfactant for water and oil phases (surfactant & system dependent) • WOR – Water-to-Oil Ratio • Depends on system composition
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Figure 14: Change in turbidity when a coarse lemon oil emulsion (10% lemon oil, 13% Tween 80, 77% water) is heated and cooled.. A very fine transparent nanoemulsion is formed when the system is heated, and then cooled below its phase inversion temperature. (Rao and McClements 2010)
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Figure 15: Influence of oil type on mean droplet diameter in emulsions produced using the emulsion inversion point (EIP) method by titrating water into an oil-surfactant mixture (Ostertag, Weiss and McClements, 2012).
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Figure 16: Influence of pH and storage time on the chemical degradation of 0.05% citral dispersed in citrate buffer solutions (Choi et al 2009). The citral chemically degrades rapidly at acidic pH values.
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Figure 17: Influence of oil droplet concentration on the chemical degradation of citral dispersed within MCT oil-in-water emulsions (Choi et al 2009). The citral chemically degrades more rapidly in an aqueous than an oil environment, and therefore the less oil present the faster the degradation rate.
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Figure 18: Influence of oil droplet concentration on the partitioning of citral within MCT oil-in-water emulsions (Choi et al 2009). The mass fraction of citral in the aqueous phase decreases as the oil concentration increases.
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Figure 19: Influence of emulsifier concentration on the mean particle diameter produced using a microfluidizer using either whey protein isolated (WPI), modified starch (MS) or gum arabic (GA) are emulsifiers under standardized conditions.
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Figure 20: Influence of pH on the particle size and charge produced using a microfluidizer using either whey protein isolated (WPI), modified starch (MS) or gum arabic (GA).
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Figure 21: Influence of ionic strength on the particle size and charge produced using a microfluidizer using either whey protein isolated (WPI), modified starch (MS) or gum arabic (GA).
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Table 1: Global revenue from non-alcoholic beverage sales by major beverage manufacturing companies in 2010 (Beverage
35.1
PepsiCo
21.4
Kraft Foods
8.8
Dr. Pepper Snapple Group
5.6
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Coca-Cola Co.
SC
Annual Sales (Billions of Dollars)
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Company
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Executive. October 2011, “Worldwide 100”, Pages 44-48).
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Table 2. Summary of major colloidal interactions operating between oil droplets in beverage emulsions. It is assumed that the
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droplets are coated by the same type of emulsifier. Sign
Magnitude
Range
Factors Affecting
Van der Waals
Attractive
Intermediate
Intermediate
Always present
Steric
Repulsive
Strong
Short
Thickness and chemistry of interface
SC
Interactions
Repulsive
Strong-to-Weak
Long-to-Short
pH and ionic strength
Depletion
Attractive
Weak-to-Medium
Short
Amount & type of non-
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Electrostatic
adsorbed polymer
Short
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Attractive
Strong
Strong
Long
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Hydrophobic
Attractive
AC C
Bridging
Amount & type of adsorbing polymer Surface hydrophobicity
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Table 3. Physicochemical properties of oil phase, water phase, and weighting agents commonly used in the formulations of
reported.
Density (kg m-3)
Restriction Level (ppm)
-
≈1050
-
-
≈1000
-
Yes Yes
840-850 900-920
-
No
1240-1330
15
Yes
1060
Defined by manufacturer
1080
Not approved for use in the United States 100
No
1146
300
Structure
Natural
SC
Components
Oil Phases Flavor Oils Vegetable Oils
Mainly Terpenes Mainly Triglycerides
Weighting Agents
Ester Gum Sucrose Acetate Isobutyrate
EP
Dammar Gum
Brominated Hydrocarbon Chain of Triacylglycerols Exudate from Caesalpinaceae and Dipterocarpaceae shrubs Hydrophobic Polymer
AC C
Brominated Vegetable Oil
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Low calorie
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Aqueous Phases Sugared
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beverage emulsions (Chanamai & McClements, 2000; McClements, 2005). The restriction levels based on USA regulations are also
Esterification of Sucrose with Acetic and Isobutyric Anhydrides
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HLB Value 16 15 6 2 16 15.6 14.9 15 >9 8.6 6.7 4.7 4.3
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Surfactant Sucrose monopalmitate Sucrose monostearate Sucrose distearate Sucrose polystearate Tween 20 (Polyoxyethylene Sorbitan Monolaurate) Tween 40 (Polyoxyethylene Sorbitan Monopalmitate) Tween 60 (Polyoxyethylene Sorbitan Monostearate) Tween 80 (Polyoxyethylene Sorbitan Monoooleate) Q-Naturale (Quillaja saponin) Span 20 (Sorbitan monolaurate) Span 40 (Sorbitane monopalmitate) Span 60 (Sorbitan monostearate) Span 80 (Sorbitan monooleate)
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Table 4. HLB numbers of selected small molecule surfactants and co-surfactants that could be used to produce beverage emulsions. Information from various sources: McClements (2005), National Starch, and Rahn-Group.com.
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Source
Main Structure Type
Major Monomer(s)
Algal
Linear
Carrageenan
Algal
Linear/Helical
Sulfated Galactose & Anhydrogalactose
Chitosan
Crustaceans, Invertebrates
Linear
Acacia Sap
Branched Coil Domains on Protein Scaffold
Carob seeds
Linear
Linear
Modified Starch
Waxy Maize
Linear
Glucose with some nonpolar branches
Plant Cell Walls Xanthomonas campestris exudate
Highly Branched Coil
Glucuronate (backbone)
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Wood Pulp
Xanthan Gum
Negative
Neutral
Galactose and mannose
Modified Celluloses
Pectin
Positive
Galactose
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Locust bean gum
Negative
Glucosamine
EP
Gum Arabic
Negative
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Alginate
Mannuronic & Guluronic Acids
Charge
SC
Name
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Table 5. Summary of molecular characteristics of some common food-grade polysaccharides. Adapted from Matalanis et al (2011).
Linear/Helical (High MW)
Glucose and modified Glucoses
Neutral or Negative Negative Negative Negative
Glucose (backbone)
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Table 6. Examples of the composition and calorie content of some commercial beverage products currently on the market. Emulsifier
Weighting Natural Agent Claim
50
GA
None Listed
No
0
GA
None Listed
No
Cane Sugar
70
GA
EG
Yes
Cane Sugar
70
GA
EG
Yes
HFCS
80
GA
EG
No
HFCS
80
GA
EG
No
RTD
HFCS
80
GA
EG
No
RTD
HFCS
80
M-FS
MCT, SAIB
No
RTD
HFCS
120
M-FS
EG, BVO
No
APM, Ace-K
0
GA
EG, BVO
No
RTD
Fructose, Cane Sugar
120
GA
EG
No
Coca-Cola
RTD
Fructose, Cane Sugar
120
GA
EG
No
Coca-Cola
RTD
Fructose, Cane Sugar, Sorbitol
120
GA, M-FS
EG
No
Coca-Cola
RTD
Erythritol, Rebiana
0
GA
EG
No
Beverage Sweetener Type System
Half and Half Iced Tea Lemonade
Arnold Palmer
AriZona
RTD
HFCS, Sucralose, Ace-K
Half and Half Iced Tea Lemonade
Arnold Palmer Zero
AriZona
RTD
Sucralose
Cherry Citrus
BodyArmor
Body Armor Nutrition
RTD
Tropical Mandarin
BodyArmor
Body Armor Nutrition
RTD
Lemon Lime
Xion4 (Powerade)
Coca-Cola
RTD
Orange
Xion4 (Powerade)
Coca-Cola
RTD
Strawberry Lemonade
Xion4 (Powerade)
Coca-Cola
Sour Melon
Xion4 (Powerade)
Coca-Cola
Orange Soda
Fanta
Coca-Cola
Original Citrus
Fresca
Coca-Cola
RTD
Energy - Tropical Citrus
Vitamin Water
Coca-Cola
Multi-V - Lemonade
Vitamin Water
Essential Orange-Orange
Vitamin Water
Squeezed Lemonade
Vitamin Water Zero
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EP
AC C
SC
Manufacturer
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Brand
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Calories (per 8 oz. Ready to Drink)
Product
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Sunkist
Dr. Pepper Snapple
RTD
HFCS
110
M-FS
EG
No
Orange
Diet Crush
Dr. Pepper Snapple
RTD
APM, Ace-K
0
GA
EG
No
Citrus Soda
Sun Drop
Dr. Pepper Snapple
RTD
HFCS
120
GA
EG, BVO
No
Pineapple Orange Guava
Nantucket Nectar's
Dr. Pepper Snapple
RTD
Sucrose
120
GA
None Listed
Yes
Orange Mango
Nantucket Nectar's
Dr. Pepper Snapple
RTD
Sucrose
120
GA
None Listed
Yes
Strawberry Kiwi
Snapple
Dr. Pepper Snapple
RTD
Sucrose
100
GA
EG
Yes
Lemonade
Farmer's Daughter
The Farmer's Cow
RTD
Cane Sugar
120
GA
EG
Yes
Strawberry Lemonade
Farmer's Daughter
The Farmer's Cow
RTD
Cane Sugar
110
GA
EG
Yes
Lemonade
MiO
Kraft Foods
LC
Sucralose, Ace-K
0
GA
SAIB
No
Orange Tangerine
MiO
Kraft Foods
LC
Sucralose, Ace-K
0
GA
SAIB
No
Sparkling Lemonade
N/A
Lorina
RTD
Sucrose
90
GA
EG
No
Pink Citrus Lemonade
N/A
Lorina
RTD
Sucrose
120
GA
EG
No
Lemonade
G2 of Gatorade G-Series
PepsiCo
RTD
Sucrose
80
GA
EG, SAIB
No
Lemon-Lime
G2 of Gatorade G-Series
PepsiCo
RTD
Sucrose
80
GA
EG
No
Orange
G2 of Gatorade G-Series
PepsiCo
RTD
Sucrose
80
GA
EG, BVO
No
SC
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Orange Soda
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RTD
HFCS
110
GA
BVO
No
N/A
PepsiCo
RTD
APM, Ace-K
0
GA
BVO
No
Nature's Blends
Poland Spring
RTD
Cane Sugar
50
GA
EG
Yes
Orange Cranberry Tangerine
Skinny Water
Skinny Nutritional
RTD
Sucralose, Ace-K
0
GA
EG
No
Old-Fashioned Lemonade
N/A
Zeigler's Beverages
RTD
Sucrose
100
GA
EG
No
SC
Key:
Ace-K
Acesulfame Potassium
APM
Aspartame
BVO
Brominated Vegetable Oil
EG
Ester Gum
GA
Gum Acacia
HFCS
High Fructose Corn Syrup
Liquid Concentrate
LC
Medium Chain Triglycerides
MCT
M-FS RTD SAIB
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Lemon
TE D
Diet Mtn Dew
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PepsiCo
EP
N/A
ModifiedFood Starch
AC C
Mtn Dew
Ready to Drink Sucrose Acetate Isobutyrate
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Claim
Justification
“0 Calories”
Less than 5 cal per RACC.
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Table 7. Claims and justifications that can be made on beverage products.
Source 21 CFR 101.60(b)
“All Natural”
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components.
SC
Undefined, but regarded as a product lacking any synthetic
N/A
Any substance that imparts flavor that is not derived from its natural “Artificially Flavored”
21 CFR 101.22 (a) (1)
source.
For juice made from concentrate, calculate use-percentage from the
21 CFR 101.30(j),
Brix table in 21 CFR 101.30(h)(1) as the basis for 100% juice.
21 CFR 101.30(h)
Contains 20% or more of the DV per RACC.
21 CFR 101.54(b)
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“Contains % Juice”
“Excellent Source of
Vitamin” “Low Calorie Soda” “Low Sodium”
Contains 10%-19% of the DV per RACC.
21 CFR 101.54(e)
40 cal or less per RACC.
21 CFR 101.60(b)
140 mg or less per RACC.
21 CFR 101.61
AC C
“Good Source of
EP
Vitamin”
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21 CFR 101.22 (a) (1), “Natural and Artificial Contains Natural and Artficial Flavors.
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Flavor”
21 CFR 101.22 (a) (3), 21 CFR (g) 101.22 (3)
The flavor constituents function in the food as flavor, not solely as a “Naturally Flavored”
21 CFR 101.22 (a) (3)
No Sugar or Sugar-Containing Ingredients are added during processing.
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“No Added Sugar”
SC
source of nutrition are derived from its natural source in nature.
21 CFR 101.60(c) (2)
Contains no colarant source outside the principle flavor components of
“No Artificial Colors”
21 CFR 101.22(k)(1) (2),
beverage. Natural color is considered artficial color when used for 21 CFR 74
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colorant purposes.
“No Pulp”
“Reduced Sugar”
EP
21 CFR 101.65 (b)(2) 21 CFR 101.65(b)(1)
Cannot Contain Preservatives.
AC C
“No Preservatives”
21 CFR 101.65(b)(1)
Cannot Contain HFCS.
“No HFCS”
21 CFR 101.65 (b)(2) 21 CFR 101.65(b)(1)
Cannot Contain Pulp.
At least 25% less sugars per RACC than an appropriate reference food.
21 CFR 101.65 (b)(2) 21 CFR 101.60 (c)