Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation

HYDROCOLLOIDS AS EMULSIFIERS AND STABILIZERS IN BEVERAGE PRESERVATION

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Mara Krempel⁎, Kristen Griffin†, Hanna Khouryieh‡ ⁎

Department of Chemistry, Western Kentucky University, Bowling Green, KY, United States, †Clinical Laboratory Technology Program, Saint Louis Community College, St. Louis, MO, United States, ‡Food Processing and Technology Program, School of Engineering & Applied Sciences, Western Kentucky University, Bowling Green, KY, United States

13.1 Introduction Beverage emulsions are formed as oil-in-water emulsions, with flavor oils, antioxidants, and weighting agents as the dispersed phase, and water, stabilizers, emulsifiers, sweeteners, dyes, and preservatives as the continuous phase (Raikos et al., 2017; Zhang et al., 2015; Tan, 2004). Beverage emulsions, unlike other emulsions, are typically consumed in a diluted form rather than their natural concentrated form. It is also important that the beverage is stable in both its concentrated and diluted forms (Rezvani et al., 2012; Taherian et al., 2008; Dłużewska et al., 2006; Tan, 1998). Emulsions are naturally unstable systems because they tend to break down into their constituent oil and aqueous phases. Emulsion stability is the ability of an emulsion to resist this breakdown, as indicated by growth in the average size of oil droplets or change in their spatial distribution within the sample. The slower that these properties change, the more stable the emulsion, thus the need for additional stabilizers. The characteristics of an ideal stabilizer in beverage emulsions include the capability to lower surface tension at the oil-water interface, having a low viscosity when mixed with water, and not gelling or thickening with age (Rezvani et al., 2012). The most important group of ingredients used in stabilizing and protecting emulsions are the hydrocolloids. The term hydrocolloid has commonly been used in the food industry to describe a range of polysaccharides and proteins (Table 13.1). Hydrocolloids may act as emulsifiers, as stabilizers, or both. Emulsifiers are surface-active ingredients which adsorb at the oil-water Preservatives for the Beverage Industry. https://doi.org/10.1016/B978-0-12-816685-7.00013-6 © 2019 Elsevier Inc. All rights reserved.

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Table 13.1  Sources, Classifications, and Main Functionalities of Commonly Used Hydrocolloids in the Food Industry. Hydrocolloid

Source

Classification

Main Functionality

Gum arabic (gum acacia) Xanthan

Acacia tree Microbial fermentation

Exudate gum Microbial polysaccharide

Curdlan Gellan Locust bean gum (carob gum)

Microbial fermentation Microbial fermentation Locust bean seed

Microbial polysaccharide Microbial polysaccharide Seed galactomannan

Guar gum Carrageenan

Guar seed Red seaweed

Seed galactomannan Algal extracts

Alginate Pectin

Brown seaweed Citrus peel and apple pomace

Algal extracts Plant extract

Cellulose Carboxymethylcellulose (CMC) Starch

Wood pulp Cellulose Grain seeds and plant tubers and roots

Plant extract Modified cellulose Plant extract

Gelatin Caseinate Whey

Animal Animal Animal

Animal protein Milk protein Milk protein

Emulsifier and stabilizer Thickener, stabilizer, emulsifier, and foaming agent Gelling agent Gelling agent Low viscosity, forms rigid gels when used with xanthan or carrageenan at high concentrations Thickener and stabilizer Thickening and gelling agent; forms rigid gels when used with locust bean Gelling agent and stabilizer Gelling agent, but can also act as thickener, water binder and stabilizer Thickener Thickener and water binder Thickener, water binder, and gelling agent; modified starch acts as emulsion stabilizer Gelling agent Emulsifier and stabilizer Emulsifier and stabilizer

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interface during emulsion preparation and protect the formed d ­ roplets against coalescence. Surface-active compounds have a hydrophilic head group that is attracted to the aqueous phase, and a lipophilic tail that is attracted to the oil phase (Hasenhuettl and Hartel, 2008). Gum arabic and modified starches are the most commonly used hydrocolloids in beverage emulsions due to their ability to act as emulsifying agents. On the other hand, the stabilizing action of polysaccharide-based hydrocolloids, such as xanthan, guar, carboxymethylcellulose, carrageenan, pectin, is attributed to the structuring, thickening, and gelation of the aqueous continuous phase (Phillips and Williams, 2009). Their stabilizing effect on emulsions derives from the increased viscosity of the aqueous phase. The increase in the continuous-phase viscosity reduces the kinetic motion of the oil droplets and the rate of flocculation and coalescence, which increase emulsion stability. Because polysaccharides are mainly hydrophilic in molecular character and have little to no surface activity, they cannot act as primary emulsifiers (Phillips and Williams, 2009). Proteins are the emulsifiers of choice in the food processing industry, especially milk proteins. Proteins are surface-active emulsifiers due to the occurrence of lipophilic amino acids, such as phenylalanine, leucine, and isoleucine (Hasenhuettl and Hartel, 2008). This chapter begins by discussing the types of, main ingredients in, and production of beverage emulsions, and then discusses the mechanism of beverage emulsion stability. Finally, the effect of the most important protein and polysaccharide hydrocolloids on the stability and physicochemical properties of beverage emulsions is discussed.

13.2  Categories of Beverage Emulsions Beverage emulsions are divided into two categories, beverage flavor emulsions and beverage cloud emulsions. Beverage flavor emulsions provide the beverage with flavor and cloudiness, whereas beverage cloud emulsions provide mainly cloudiness. These two categories are dependent on the type of oil used for the final product. Beverage­ flavor emulsions provide taste and aroma through lipophilic compounds (such as citrus oils), while beverage cloud emulsions provide desirable optical properties to the specific product by utilizing a highly water-insoluble oil phase. Both cloud and flavor oils are unstable when mixed with water due to the hydrophobic nature of the oil. Since oil is less dense than the water, it rises to the top of the water, becoming susceptible to oxidation by the air in the surrounding environment. When these oils oxidize and degrade, they lose the characteristics they once possessed that created the desirable finished beverage product. Therefore, to these oil-in-water beverage emulsions, various hydrocolloids and stabilizers can be added to reduce the oil droplet i­ nstabilities

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described later in the chapter (Zhang et  al., 2015; Piorkowski and McClements, 2014; Mirhosseini et al., 2009; Tan, 1998).

13.2.1 Beverage Cloud Emulsions Beverage cloud emulsions are so named because of their cloudy appearance, which is caused by a low concentration of oil droplets in the diluted phase. The oil droplets scatter incoming light and create the “cloudy” or “turbid” optical quality that is characteristic to these beverages (Taherian et al., 2008). Beverage cloud emulsions are typically utilized in juice beverages that contain a low percentage of juice, which makes a cloudy appearance desirable. The cloudiness of these drinks hides any sedimentation that may occur (Piorkowski and McClements, 2014; McClements, 2015). For instance, consumers are advised to shake juice beverages that are made from the concentrate because of the natural sedimentation that occurs during storage; the sedimentation is rarely seen with a casual glance because the cloudiness of the beverage conceals it. The oils chosen for beverage cloud emulsions must be water-insoluble and chemically stable. Both of these qualities ensure the presence of the oil droplets in the beverage, thereby ensuring the continual cloudiness of the beverage. The particle size of the droplets must remain small in order to have the correct dimensions to perform the light scattering effect that aids in the cloudiness of these emulsions. The small particle size also ensures that no gravitational separation of the two phases occurs, retaining a consistent turbidity throughout the emulsion storage time (Piorkowski and McClements, 2014; Mirhosseini et al., 2009; Chanamai and McClements, 2001).

13.2.2 Flavor Emulsions Beverage flavor emulsions utilize oils that provide aromas and desired tastes to the final product, rather than an optically cloudy appearance as in beverage cloud emulsions. One of the most common oils for these emulsions is orange oil, which contains high amounts of limonene (Raikos et al., 2017; Tan, 2004). Flavor oils can provide different results in the final, diluted beverage, depending on where in the emulsion the oil can be found, what the physical state of the oil is, or what the environmental conditions are. Therefore, beverage manufacturers must be more careful and knowledgeable when working with beverage flavor emulsions, considering the flavor of the final beverage is a very crucial characteristic. Many flavor oils are prone to chemical degradation, which can alter the perceived flavor of the beverage by the consumer. Too much oil in the emulsion can lead to excess volatile oil in the headspace, which can alter the flavor of the beverage as well. Flavor oils are also susceptible to losing their flavor when placed in acidic aqueous phases (Piorkowski and McClements, 2014; Rao and McClements, 2012a, 2012b).

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13.3  Beverage Emulsion Ingredients Beverage emulsions contain a greater variety of ingredients than do normal oil-in-water emulsions. Typical oil-in-water emulsions contain water, protein, oil, and a hydrocolloid for added stabilization. A beverage emulsion is a more complex system due to the various types of beverages produced and the different qualities desired for the different beverages, and therefore, they contain a greater variety of ingredients. Beverage emulsions contain two phases, the continuous and dispersed phases, as do regular emulsions. The dispersed phase consists of hydrophobic ingredients, such as oils, weighting agents, lipid-soluble antioxidants, and sweeteners. In the continuous phase, water, emulsifiers, and stabilizers are added. Dyes, water-soluble antioxidants, and preservatives can be added as well (Raikos et al., 2017; Zhang et al., 2015).

13.3.1 Dispersed-Phase Ingredients 13.3.1.1 Oils Oils are responsible for the flavor and optical appearance of the f­ inal beverage product. Flavor oils are typically citrus oils that are composed of various types of citrus flavors (e.g., orange and limonene) to obtain a balanced final flavor. These oils can also be supplemented with other organic compounds, such as alcohols, esters, and ketones which provide flavors and aromas characteristic to their respective molecules. The differing chemical compositions of the mixtures of flavor oils with various organic additives affect the physicochemical properties of the final beverage product (Piorkowski and McClements, 2014; Rao and McClements, 2012a; Tan, 2004). Cloud oils are added to beverage emulsions purely for the cloudy appearance provided to the final beverage product. These flavorless oils are terpenes or triacylglycerols that are derived, respectively, from flavor oils and natural sources, such as canola and corn (Piorkowski and McClements, 2014; Mirhosseini et al., 2008a; Tan, 2004). Although these oils are flavorless, they can inadvertently alter the flavor of the beverage due to potential chemical degradation and partitioning of various species within the emulsion (McClements, 2015).

13.3.1.2  Weighting Agents Weighting agents are oil-soluble materials that do not alter the flavor of but are heavier than, the flavor oil and are added to lengthen the shelf life of the beverages (Tan, 1998, 2004). Common weighting (­ density-adjusting) agents used in the beverage industry are glyceryl abietate, ester gum, sucrose acetate isobutyrate, damar gum, and

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­ rominated vegetable oil (Table 13.2). An efficient weighting agent must b exhibit the ­following properties: oil soluble, specific gravity higher than the oil, flavorless, and food regulatory agency approved (McClements, 2015; Tan, 2004). These ingredients lower the density difference between the continuous and oil phases, decreasing the likelihood of phase separation. Rather than decrease the density of the continuous phase, these weighting agents are added to the oil phase to increase the density of the dispersed phase. Weighting agents are important for the stability of beverage emulsions due to their ability to slow the rate of gravitational separation (Linke and Drusch, 2016; Chanamai and McClements, 2000). Restricted concentrations have been placed on weighting agents, however, no attempts have been made at modifying the water phase in contrast to the oil phase (Meybodi et al., 2014; Tan, 2004).

13.3.2 Continuous-Phase Ingredients 13.3.2.1 Water Water is the main ingredient of the continuous phase in beverage emulsions. The continuous phase is the liquid substance in which the oil droplets are suspended in. It may seem contradictory to place a hydrophobic ingredient, such as oil into an aqueous continuous phase,

Table 13.2  Common Weighting Agents Used in Beverage Emulsions and Their Effects Regarding the Oil-Phase Density. Weighting Agent

Effect on Beverage Emulsion

Brominated vegetable oil (BVO) Sucrose acetate isobutyrate

• • • • • • • • •

Damar gum

Ester gum

Increase in oil-phase density when oil concentration is low (<0.006 wt%) Small-phase density difference when oil phase is 75% soybean oil and 25% BVO Increases viscosity at room temperature Excellent oxidative stability Used at <300 ppm Aids in color and cloudiness of beverage cloud emulsions Can be utilized in unmodified form Typically added in 1:1 ratio with oil Aids in cloudiness of beverage cloud emulsions when combined with emulsifying agents • Used in carbonated beverages at <100 ppm • Typically added in 1:1 ratio with oil

Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation   433

but it is important that this phase is a hydrophilic environment because the ingredients that are added for stabilization are hydrophilic and/or amphiphilic. Water is a universal solvent, which contributes to its suitability as the basis for the continuous phase considering ingredients of various types are added into a beverage emulsion (McClements, 2015). In general, the water content in beverage emulsions formulation is about 60%–70%, and sometimes, it can be as high as 80%. The quality of the water in the continuous phase is important because any charged species can affect the stability of the product depending on the emulsifiers and stabilizers added (Tan, 2004).

13.3.2.2 Hydrocolloids Many hydrocolloids are utilized in beverage emulsions as stabilizers due to their ability to stabilize the oil droplets and keep the aqueous and oil phases homogenized throughout a defined storage time. Hydrocolloids can act as surface-active emulsifiers and thickeners or nonsurface-active stabilizers. Emulsifiers, surface-active molecules that are primary stabilizers, need to be able to adsorb onto the surface of the droplet while interacting with the surrounding aqueous phase; in other words, the primary emulsifier must be an amphiphilic biopolymer, which consists of both polar and nonpolar regions. As the first additive incorporated into the emulsion, primary emulsifiers increase the kinetic stability, thus elongating the length of time the emulsion will remain homogenized (McClements, 2015; Lam and Nickerson, 2013; Tan, 2004; Stauffer, 1999). Some hydrocolloids are utilized more frequently than others because they display certain characteristics that are ideal for beverages. As stated earlier, the characteristics for ideal stabilizer in beverage emulsions include keeping the viscosity low while still stabilizing the oil droplet, lowering the surface tension at the oil-water interface, and not increasing viscosity or gelling with increased storage time (Rezvani et  al., 2012; Mirhosseini and Tan, 2010; Tse and Reineccius, 1995). A low viscosity is wanted because beverages containing stabilizers that display low surface tensions and viscosities are easily poured and have a pleasant texture when consumed. However, there is no single hydrocolloid that is ideal for use in every type of food emulsion because a hydrocolloid’s ability to prevent instabilities varies under different environmental stresses. The effect of hydrocolloids on the stability and physicochemical properties of beverage emulsions is discussed in detail later in the chapter.

13.3.2.3 Acids Acids are a minor but significant ingredient in beverage e­ mulsions because the overall stability of the final product can be dependent on the presence of acid, depending on the characteristics of the other species used. Acids control the pH value, which is an ­important

434  Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation

f­actor when considering interactions with proteins as well as microbial growth. Different proteins have different isoelectric points, which are pH values at which the protein chain is neutrally charged. The pH value of the final emulsion can affect the stability of various protein-polysaccharide complexes, and therefore, affect the quality of the beverage (Owens et al., 2018; Lam and Nickerson, 2013; Tan, 2004). Some acids are used purely to control the pH, such as hydrochloric acid, but others are used to enhance the flavor of the finished beverage product. Such acids are most commonly found in fruit and cola emulsions that are noncarbonated because the acids complement the natural flavor of the emulsion. Examples of flavored acids that have been used in beverage emulsions are citric acid, phosphoric acid, tartaric acid, and acetic acid (McClements, 2015; Tan, 2004).

13.3.2.4 Preservatives Preservatives are added to beverage emulsions to aid in the lengthening of shelf life for the final product by inhibiting microbial growth (Javanmardi et al., 2015). A common preservative is sodium benzoate, the water-soluble salt of benzoic acid. The effectiveness of this preservative largely depends on the pH value of the beverage because the preservative is present in its salt form. Sodium benzoate, specifically, is most effective at preserving when not dissociated, meaning it requires acidic environments (Tan, 2004).

13.3.2.5 Colorings Colorings help with the optical quality of the final beverage products. Appropriate colorings and dyes are chosen to correlate with the proposed flavor of the final product. For example, FD&C Red 40 and FD&C 6 Yellow are chosen to create an optimal orange color for orange flavor emulsions (Tan, 2004). An appropriate hue of color aids in the flavor perception of the beverage. Safety concerns have arisen regarding synthetic colorings, so manufacturers have begun investigating more natural sources of color. Examples of natural colorings are beetroot and cranberry for red, saffron for orange, and chamomile flowers for yellow. Roots, leaves, and barks can also be used (Lakshmi, 2014).

13.4  Beverage Emulsion Production Beverage emulsion production is a two-step process: beverage emulsion concentrate and finished product. Beverage emulsions are first produced in concentrated forms (10–30 wt% oil) and then diluted several hundred times to create the finished product (<0.1 wt% oil) (McClements, 2015; Tan, 2004). Dilution of concentrated beverage emulsion influences the appearance of final products (Piorkowski and

Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation   435

McClements, 2014) and the total amount of flavor molecules present in final products (Choi et al., 2009). The methods for production must create a beverage emulsion that remains physically stable for a very long time, usually between 4 and 12 months (Given, 2009). A typical beverage emulsion composition is described in Table 13.3. The first step in formulating a beverage emulsion is to blend together the continuous-phase ingredients (water, preservatives, and hydrocolloids). Next, the dispersed-phase ingredients (oil and weighting agents) are blended and then added to the continuous-phase mixture; this phase is called the primary emulsion because any emulsifying hydrocolloids can adsorb onto the oil droplet and form the first layer of protection. After homogenization of the continuous and dispersed phases, any thickeners being added for increased stability will be incorporated; these will form a stabilizing film around the primary emulsion (Rezvani et al., 2012; Zhang et al., 2015). The concentrated form of beverage emulsions is usually prepared using either high-energy or low-energy approaches (Piorkowski and McClements, 2014). High-energy approaches include high-pressure valve homogenizers, microfluidizers, and ultrasonic methods that are capable of producing relatively high shear forces that disrupt the oil and aqueous phases to form very small oil droplets (0.1–1 μm). The emulsions are typically homogenized at a total pressure of 2000–5000 psi using a two-stage high-pressure homogenization (Given, 2009). The low energy approaches include phase inversion and spontaneous emulsification methods that rely on the spontaneous formation of fine oil droplets within mixed surfactant-oil-water systems when the solution or environmental conditions are altered (Piorkowski and McClements, 2014).

Table 13.3  Typical Formulations for Beverage Emulsions. Ingredient

Using Gum Arabic (%)

Using Modified Starch (%)

Flavor oil Gum arabic Modified starch Citric acid Sodium benzoate Water

10 10–20 – 0.3 0.1 79.6–69.6

11 – 11 0.3 0.1 77.6

436  Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation

Once the concentrated beverage emulsions are complete, dilution can occur at any desired time. Dilution is carried out with aqueous-phase ingredients and will result in a dilution factor between 300 and 2000 times the original concentration (Chanamai and McClements, 2001). Typical diluting solutions comprise of water and additional sweeteners, colorings, and preservatives (McClements, 2015; Rezvani et al., 2012; Taherian et al., 2008; Dłużewska et al., 2006; Tan, 2004). The diluted emulsion may be homogenized briefly to ensure all newly introduced ingredients are fully incorporated. The final, ready-to-drink beverage product consists of less than 0.1 wt% or 20 mg L−1 oil and is ready for package appropriately (Given, 2009). Postponing the dilution of the beverage emulsion elongates the period of time the beverage product remains stable. The concentrated form of the final emulsion can remain stabilized longer than the diluted form, due to the increased viscosity exhibited by the higher concentration of hydrocolloids (10%–20% adsorbing hydrocolloid) in the concentrated form. This concentrated form aids in the storage and transport of the beverage emulsions because of the increased stability. Typically, the concentrated beverage emulsions are manufactured in one factory, from which they are shipped to another factory to be diluted (McClements, 2015; Tan, 2004).

13.5  Stability of Beverage Emulsions Beverage emulsions must utilize a delivery system that is able to combine hydrophilic ingredients with the necessary hydrophobic ingredients (e.g., flavor oils, nutraceuticals, and clouding agents). The delivery systems that are most frequently used to mix these normally immiscible liquids are emulsions, microemulsions, and nanoemulsions, as shown in Fig. 13.1. Of these three types of emulsions, conventional emulsions and Emulsion

Thermodynamically unstable Droplet size: 0.1–100 µm

Nanoemulsion

Microemulsion

Thermodynamically unstable Droplet size: <100 nm

Thermodynamically stable Droplet size: 5–100 nm

Fig. 13.1  Stability and droplet size of emulsions, nanoemulsions, and microemulsions.

Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation   437

nanoemulsions are preferred, as microemulsions generally require synthetic surfactants and the inclusion of cosurfactants/cosolvents which can have a negative impact on the cost, taste, and labeling of the product (Piorkowski and McClements, 2014). Although more preferable and more frequently used, the remaining two types of emulsions are inherently thermodynamically unstable as the hydrophobic material will eventually attempt to separate from the hydrophilic during storage of the beverages. Emulsions and nanoemulsions are both thermodynamically unstable due to the larger droplet sizes observed in the system. Emulsions have droplet diameters of greater than 100 nm, while nanoemulsions have droplet sizes smaller than 100 nm, but still large enough to be considered unstable. Microemulsions are thermodynamically stable due to their small droplet sizes (5–50 nm) that are even smaller than those found in nanoemulsions (Tadros, 2013). The stability of the smaller oil droplets is retained via synthetic surfactants that comprise of a nonpolar tail and polar head, which is opposite of commonly used proteins. Since synthetic surfactants are undesirable for food products, microemulsions are used less than nanoemulsions and emulsions. Nanoemulsions (transparent) and emulsions (opaque) can also have an impact on the visual quality of the beverage in which they are utilized. The classes of beverage emulsions in which opacity and transparency are wanted are described later (McClements, 2012).

13.5.1 Mechanism of Droplet Stability Given that emulsions are thermodynamically unstable systems, the kinetic stabilization of these mixtures is crucial in beverage emulsions. Emulsion stability is simply referred to as the resistance of phase separation over time. The slower the phase separation, the more stable the emulsion. Physical instabilities include creaming and sedimentation, flocculation, coalescence, and Ostwald ripening. As oil settles on the top of the emulsion, lipid oxidation may also introduce additional physical and chemical instabilities due to the aggregation of oil droplets through the free radical mechanism (Meybodi et al., 2014; Mosca et al., 2013). Instabilities often occur as the system breaks down over time; therefore, accomplishing droplet stability is an integral part of maintaining physiochemical stability so as to maintain a fresh and palatable final product (Zhang et al., 2015; Tan, 1998). Phase separation results in an instability called creaming, and commonly referred to as ringing or oiling-off. This occurs when a white ring forms at the neck of the container in which the beverage is being kept, which is indicative of oil droplets rising to the top of the emulsion due to differences in densities between the aqueous and oil phases. The separation of phases is ultimately caused by flocculation, coalescence, and gravitational separation, as illustrated in Fig. 13.2 (Rezvani et al., 2012).

438  Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation

g

in

Homogenized emulsion

m ea

r

C

nta

me edi

tion

S

Floc c

ulat

Co

ale

sce

ion

nc

e

Fig. 13.2  Schematic representation of physical instability in oil-in-water emulsions.

Gravitational separation covers two-phase separation processes: creaming and sedimentation. Creaming is the separation of oil droplets as a layer at the top of emulsions, while sedimentation is the separation of oil droplets at the bottom of the emulsion. This gravitational separation is due to the tendency for emulsion components to separate according to varying densities, with least dense at the top and most dense at the bottom. Coalescence and flocculation are the results of droplet aggregation. Flocculation occurs when oil droplets come into contact with one another without losing their individual integrities. The more flocculation that occurs, the more prone the oil droplets are to rise to the top of the emulsion due to a greater density difference. Coalescence is seen when two oil droplets merge together to form a larger oil droplet, which, yet again, allows the oil droplets to separate from the aqueous phase more easily. Coalescence occurs because either the emulsifier concentration is too low whereby oil droplets become larger, or the emulsifier used does not have the required characteristics to stabilize the emulsion (McClements, 2015). Stoke’s Law can be utilized to predict the creaming velocity of the oil droplets in the emulsions. U Stokes =

2 gr 2 ( ρd − ρc ) 9ηc

(13.1)

Chapter 13  Hydrocolloids as Emulsifiers and Stabilizers in Beverage Preservation   439

In Eq. (13.1), “UStokes” is the rate of creaming or sedimentation, “g” is the acceleration of gravity, “r” is the oil droplet radius, “ρd” is the density of dispersed phase, “ρc” is the density of the continuous phase, and “ηc” is the viscosity of the continuous phase. The equation relates shear viscosity, particle radius, and the densities of the continuous and dispersed phases. By looking at this equation, different ways of delaying gravitational separation can be determined, such as minimizing the difference in density between the two phases, reducing the droplet size, or increasing the aqueous-phase viscosity. Stoke’s Law illustrates that differences in densities between the dispersed and continuous phases in the emulsion are a large factor in droplet instability, which explains why incorporating weighting agents into the dispersed phase can be so useful (Rezvani et al., 2012; Dłużewska et al., 2006; Taherian et al., 2006; Chanamai and McClements, 2000). However, Stoke’s Law only predicts creaming stability and does not take into account other mechanisms of instability (Tse and Reineccius, 1995).

13.5.2  Properties of Oil Droplets The stability of beverage emulsions is dependent on the properties of the particles they contain. Composition, concentration, size, and charge are some properties of droplets that contribute to overall stability (Piorkowski and McClements, 2014). The two main types of forces that determine the propensity of emulsion droplets to coalesce or flocculate are attractive interactions and repulsive factors. Attractive interactions include Van der Waals and osmotic forces, and repulsive factors are comprised of electrostatic, steric, and hydration forces. Van der Waals interactions become more prominent as the oil droplets become closer in proximity and as the droplet size increases; therefore, the droplets need to be prevented from interacting in order to prevent flocculation, coalescence, and further instabilities. Emulsion system optical characteristics, rheological behavior, and physical stability can all be affected by the attractive and repulsive forces between droplets; thus, maximizing the repulsive forces between droplets is important in ensuring a more stable emulsion and preventing the droplets from aggregating (McClements, 2015; Tse and Reineccius, 1995).

13.5.2.1  Droplet Composition The usage of hydrophobic components, such as flavor oils, triacylglycerol oils, oil-soluble vitamins, nutraceuticals, weighting agents, and ripening agents, in the composition of the oil phase in beverage emulsions greatly contributes to the physical and chemical characteristics of the system. The beverage industry utilizes these various additives because they all have unique properties, including but not limited to molecular weight, conformation, polarity, and melting point that

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contribute to the overall characteristics of the emulsion (Piorkowski and McClements, 2014). If the beverage emulsions lose stability over time, the unique and desirable qualities introduced to the product by these additives will be lost. The presence of flavor agents in many beverage emulsions complicates the particle distribution aspect as many of these agents contain small chain fatty acids that can pass through the continuous phase and merge into a larger droplet, thereby making the emulsion less stable. It should be noted that while emulsions with smaller droplet sizes are generally more stable, this opens up the opportunity for more oxidation to occur at the outside of the droplet as there is a greater surface area to volume ratio, meaning more surface area is vulnerable to oxidation. Continuous-phase characteristics can also be determinant of the stability of the emulsion to creaming, as the rheological properties of this phase affect the stability of droplets (Meybodi et al., 2014). Ostwald ripening presents another challenge to emulsion stabilization as the solubility of the oil increases as the size of the droplet increases, creating a concentration gradient that causes smaller droplets to move toward larger droplets which results in an increase in the overall size of the droplet (Evans et al., 2013).

13.5.2.2  Droplet Concentration It is imperative that droplet concentration is controlled in beverage emulsions because it can affect the texture, stability, appearance, sensory attributes, and nutritional quality of the final beverage product. Shelf life is a critical aspect to consider when formulating beverage emulsions in which the creaming of oil droplets plays a large role (Buffo and Reineccius, 2002). The nature in which beverage emulsions are typically made complicates the factor of droplet concentration. Beverage emulsions are typically produced in concentrated forms (>10% oil) to ease transport and handling and are then diluted (0.1% oil) when they are ready to be used (Piorkowski and McClements, 2014). Altering the droplet concentration of the emulsion can have drastic effects on the stability of the emulsion, especially if the oil content in the concentrated form is made to be too high. Droplet aggregation can occur through a free radical mechanism when lipid oxidation takes place during a prolonged storage time, creating difficulties when diluting the concentrated beverage emulsion (Mosca et al., 2013).

13.5.2.3  Droplet Particle Size Droplet diameter is directly correlated with emulsion stability. Oilin-water emulsion stability can be largely determined by Stoke’s Law (Eq. 13.1) in which droplet diameter is a critical variable (Meybodi et al., 2014). Droplet size distribution is yet another factor that plays a strong role in determining the physical stability of beverage emulsions.

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Improper droplet size distribution can lead to problems, such as gravitational separation, flocculation, coalescence, and Ostwald ripening. A consistent and reliable droplet size distribution helps manufacturers and producers anticipate the stability, shelf life, and characteristics of the beverage emulsion. Aesthetic characteristics, such as lightness and color can also be affected. Particle size distribution data is often used to determine the concentration of droplets within discrete size classes, which is useful in determining the source of potential causes for instability in the emulsion (Piorkowski and McClements, 2014). Enhancing the packing fraction of oil droplets can also increase emulsion stability, which can be accomplished by increasing the oil-phase volume fraction. Appropriate emulsifiers are also used in order to prevent the flocs that occur due to attractive forces between the oil droplets (Meybodi et al., 2014).

13.5.2.4  Droplet Charge The charge on the droplets affects the stability of beverage emulsions. The droplet charge in beverage emulsions is due to the adsorption of ionic biopolymers and surfactants, such as proteins, ionic polysaccharides, ionic surfactants, phospholipids, fatty acids, and some small ions to oil surfaces (McClements, 2015). Zeta potential is often measured to characterize the droplet charge. The magnitude, range, and size of electrostatic interaction between droplets often determines the extent of droplet aggregation and the level of interaction with other charged species present in the emulsion. Ionic emulsifiers are effective as stabilizers as their utilization can prevent droplet aggregation by adsorbing to the droplet and creating electrostatic repulsion between droplets (Ozturk and McClements, 2016; Piorkowski and McClements, 2014). Oil droplet aggregation can be prevented if the droplet surface charge is lower than −30 mV or higher than 30 mV. The greater the absolute value of the zeta potential, the more adsorption of the emulsifier there is onto the oil droplet. This means that the emulsifier is strongly bound to the droplet and its stabilizing abilities will not degrade as easily under environmental stress (Meybodi et al., 2014).

13.6  Polysaccharide-Based Hydrocolloids 13.6.1 Adsorbing (Surface-Active) Hydrocolloids Adsorbing hydrocolloids are amphiphilic biopolymers that can s­ tabilize the oil droplet by directly adsorbing onto the oil-water interface (Fig.  13.3). Due to this property, these polymers are also called surface-active gums. Adsorbing hydrocolloids form a thick film around the oil droplet rather than projecting out into the aqueous phase of the emulsions, which allows them to gel and retain their hydrocolloid

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+

Oil droplet

Adsorbing polysaccharide-based hydrocolloied

Oil/adsorbing hydrocolloied complex

Fig. 13.3  Schematic illustration depicting the mechanism of surface-active hydrocolloid adsorption at the oil/water interface.

properties (Rezvani et  al., 2012). The thick film formed around the droplet creates a steric hindrance effect, making it impossible for other droplets to come into proximity and coalesce (Domian et al., 2015). The primary adsorbing hydrocolloids that will be focused on in this chapter are gum arabic and modified starch.

13.6.1.1  Gum Arabic Gum arabic (gum acacia) is derived from Acacia senegal and Acacia seyal trees. Gum arabic is highly branched arabinogalactan-proteins complex that is composed by d-galactose, l-arabinose, l-rhamnose, d-­glucuronic acid, and 4-O-methyl-d-glucuronic acid with a small fraction of proteins (Lopez-Torrez et al., 2015). The gum consists of a hydrophobic polypeptide chain that anchors the gum to the oil droplets in the emulsion and hydrophilic arabinogalactan blocks that provide steric and electrostatic stabilization by extending into the solution and prohibiting droplet aggregation. A high water solubility, low solution viscosity, good surface activity, and ability to form a protective film around emulsion droplets are all aspects of gum arabic that make it an attractive stabilizer. It is widely utilized for beverage emulsions, but the quantity needed and the price of the gum is a disadvantage compared to using other potential hydrocolloids (Phillips and Williams, 2009). Gum arabic is commonly used in beverage emulsions as an emulsifier and a stabilizer. It demonstrates the formerly mentioned characteristics of an ideal stabilizer, including its high water-solubility, low viscosity, and ability to not be greatly affected by changes in the environment (Raikos et al., 2017). Another unique and important quality of this hydrocolloid is its amphiphilicity. The hydrophobic polypeptide chains anchor to the oil droplets, while the hydrophilic regions extend throughout the aqueous phase to create a steric repulsion effect. The amphiphilic nature of gum arabic is an advantage because it can prevent coalescence, can lead to film formation, and display steric stabilization. This gum was discovered to have significant effects on

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v­ arious emulsion factors including opacity, specific gravity, zeta potential, and surface tension (Ozturk and McClements, 2016). Gum arabic has a significant impact on the specific gravity of the emulsion. Increasing the ratio of this hydrocolloid to water in turn increases the specific gravity. Opacity of the emulsion can also be affected by the ratio of gum to water. As this ratio is increased, opacity decreases. Increasing gum arabic concentrations leads to higher absolute value zeta potentials. The small surface charge on the surface of the gum produced by the polysaccharide tails creates an electrostatic mechanism that imparts greater stability to the emulsion. Gum arabic’s polysaccharide content assists in stabilizing emulsions by steric repulsion, and the polysaccharide tails face outwards toward the water phase while the polypeptide chains remain attached to the oil surface. Decreasing the surface tension of the emulsion can be accomplished by increasing the ratio of gum to water. Gum arabic could also have the ability to form a film around oil droplets, thereby inhibiting coalescence. Gum arabic acts as a surface protein and can lose its emulsifying abilities and surface activity if the protein is denatured in some way (Rezvani et al., 2012). Although surface-active, it demonstrates a low affinity for the oil-water interface relative to proteins, such as whey protein, which can be explained by its ability to form a film around the oil droplets as well. This means a high concentration of the hydrocolloid must be added to the emulsion for it to coat the oil droplet (Raikos et al., 2017; Tse and Reineccius, 1995). Increasing the oil phase in comparison to the water phase in emulsions containing gum arabic results in an increase in droplet size, which can be explained by an inadequate amount of gum to cover the oil droplets. In order to keep emulsions stable, it is imperative to maintain a small droplet size and a narrow size distribution. In regard to the flow behavior index, it has been thought that increasing the oil-phase and gum arabic concentration tends to decrease the index but the effect is not significant. Gum arabic does have a significant effect on the consistency coefficient; however, especially at higher concentrations of oil (Rezvani et al., 2012). The ability of gum arabic to stabilize emulsions mostly results from the formation of an interfacial film at the oil-water interface, creating a repulsive energy barrier that prevents contact of closely located droplets (Rezvani et al., 2012). Emulsions containing gum arabic are stable in a pH range of 2–8 with relatively constant droplet diameters (Qian et al., 2011). A thick coating of this hydrocolloid on the outside of the oil droplet provides steric repulsion as the polysaccharide molecules protrude into the aqueous phase. This also results in a reduction of the power of van der Waals forces between the droplets. The zeta potential of gum arabic stabilized emulsions remains stable at a pH range of 5–8 and negative throughout the entire pH range. A sharp drop in zeta

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potential occurs as the pH changes from 2 to 4. Moving below the pKa value of the carboxyl groups causes the molecule to lose some of its charges, explaining why the zeta potential rises as the pH decreases. The consistently negative zeta potential can be attributed to negatively charged side groups on the backbone. This negative charge can cause unwanted side effects, such as lipid oxidation when transition metals are present (Qian et al., 2011). Addition of CaCl2 leads to a drastic decrease in the negative value of the zeta potential of the gum arabic coated droplets (Chanamai and McClements, 2002). Ionic strength is an important component in many food products, and it is essential to maintain stability in the emulsion despite the addition of salts. The particle size of gum arabic emulsions has been shown to stay relatively the same despite the addition of NaCl and CaCl2, which is explained by the fact that gum arabic primarily stabilizes emulsion systems by steric hindrance instead of electrostatic interactions (Qian et al., 2011).

13.6.1.2  Modified Starch Modified starches are the most commonly used substitute for gum arabic in beverage emulsions (Taherian et  al., 2007; Chanamai and McClements, 2001). Starches, sourced from potato, corn, rice, tapioca, and wheat, are modified for use in the food industry because natural starches consist of hydrophilic glucose backbones, which causes them to display poor surface activity. The process of modification attaches nonpolar side chains to increase their affinity to the oil-water interface (Piorkowski and McClements, 2014; McClements, 2015). The various classes of modified starch are detailed in Table 13.4. A common type of modified starch is octenyl-succinated anhydrate, or octenyl-succinated starch, which is produced by the esterification of starch with anhydrous octenyl-succinate acid (Taherian et  al., 2008). After modification, the resulting octenyl-succinated anhydrate is an example of an amphiphilic polysaccharide, or a hydrophobically modified starch (Chanamai and McClements, 2002; Taherian et al., 2006). Modified starch is an adsorbing polysaccharide, but displays lesser interfacial activity than proteins, so an excess of the polysaccharide must be added to ensure that all droplets are sufficiently coated. Unfortunately, this excess of polysaccharide can lead to depletion flocculation caused by osmotic pressure gradients (Chanamai and McClements, 2002; Piorkowski and McClements, 2014). Emulsions containing less than 1.7 wt% of modified starch are the most stable. This is because 1.7 wt% is the critical flocculation concentration for modified starch. The critical flocculation concentration is the concentration at which depletion flocculation begins to occur in hydrocolloid-stabilized emulsions and below which the emulsions are seen to be the most stable. As the modified starch concentration

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Table 13.4  Various Categories of Modified Starches and Their Uses. Starch Type

Category

Properties

Uses

Octenyl-succinated anhydrate (OSA) Hydroxypropyl starch Starch acetates

Stabilized starch

Amylopectin with nonpolar sidechains; mildly anionic Hydroxypropyl attached to glucosyl

Emulsifier

Monostarch phosphate

Stabilized starch

Distarch phosphate

Cross-linked starch

Distarch adipate

Cross-linked starch

Hydroxypropylated distarch phosphate

Cross-linked and stabilized starch

Phosphorylated distarch phosphate

Cross-linked and stabilized starch

Acetylated distarch phosphate Acetylated distarch adipate

Cross-linked and stabilized starch Cross-linked and stabilized starch

Stabilized starch Stabilized starch

Hydrophilic hydroxyl groups (on C2, C3, and C6) converted to hydrophobic acetyl groups Attachment of a phosphate via esterification to a single hydroxyl group on one anhydrous glucose unit Cross-linkage by same phosphate group between two hydroxyl groups on neighboring anhydrous glucose units Cross-linked with adipic acid

Glucose units cross-linked with phosphate, hydroxyl groups substituted with 2-hydroxypropyl Cross-linkage by same phosphate group between two hydroxyl groups on neighboring anhydrous glucose units; phosphate starch covalently linked Cross-linked with phosphate, esterified with vinyl acetate or acetic anhydride Cross-linked with adipic anhydride; esterified with acetic anhydride to covalently link starch groups

in emulsions increases, oil droplets flocculate and can potentially coalesce which will inherently increase the droplet size (McClements, 2015; Chanamai and McClements, 2001). Once the concentration of modified starch in the emulsion exceeds the critical flocculation concentration, the creaming rate increases rapidly; however, if the concentration is increased much higher, the viscosity of the continuous phase is then increased enough to slow the rise of the droplets and continue to stabilize the emulsion. Fortunately, beverage emulsions

Increase in viscosity; improved freeze-thaw stability Replacement for cellulose acetate Improvement of appearance and transparency; increase in viscosity Improvement of texture (depending on pH and cross-links) Increase in resistance to high temperatures, acidity, amyloglucosidase Increase in viscosity, stability during pH changes, and retrogradation resistance Increase in viscosity as pH value decreases

Increase in viscosity as pH value decreases Increase in viscosity as pH value increases

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are extensively diluted (300–2000 times), making the final modified starch concentration between 0.006 wt% and 0.04 wt%, a range too low to promote depletion flocculation in the final product (Domian et al., 2015; Chanamai and McClements, 2001). Due to the increased amount of flocculation at modified starch concentrations above the critical flocculation concentration, the viscosities of emulsions containing 1.7 wt% modified starch or greater were higher than those with modified starch concentrations less than 1.7 wt%. The viscosities increased appreciably above the critical flocculation concentration. It has been suggested through various studies that the addition of 0.3 wt% xanthan gum to modified starch-stabilized emulsions can reduce the viscosity of the emulsion to closely resemble fully stabilized emulsions that are not flocculated (Taherian et al., 2007, 2008; Chanamai and McClements, 2000, 2001). Due to the hydrophilic regions of modified starches, lipid droplets are relatively stable with respect to droplet size while in the pH range of 3–9 (Chanamai and McClements, 2002; Charoen et al., 2011; Qian et al., 2011). Despite the consistent stability, higher concentrations of modified starch increase droplet size because the nonpolar side chains can adsorb at the oil-water interface of multiple droplets. The modified starch coats the lipid droplet with a thick layer of polysaccharide that creates a steric, rather than electrostatic, repulsion against neighboring droplets, but the protruding side chains can still attach to other starch side chains. This brings together multiple oil droplets without allowing them to coalesce (Qian et al., 2011; Charoen et al., 2011; Chanamai and McClements, 2002; Taherian et al., 2007). The steric stabilization has been studied by Charoen et  al. (2011) and Chanamai and McClements (2002), who added calcium/sodium chloride to modified starch-containing emulsions. It was found that these charged species did not noticeably affect the stability and droplet size of the oil complex. Emulsions stabilized by electrostatic repulsion, in contrast to steric repulsion, would have their charges altered by the addition of the ions and therefore, the stability would be altered (Charoen et  al., 2011; Chanamai and McClements, 2002; Dickinson, 2003). Unlike droplet size, the zeta potential of the droplet is affected by the presence of salts. Modified starch has negatively charged carboxyl side groups on its backbone which contributes to the negative zeta potential seen at all pH values (Charoen et al., 2011; Padala et al., 2009). The zeta potential slightly reduces when the pH is below 5, a value below the pKa of the carboxyl groups and near the isoelectric point of whey protein, because the carboxyl groups lose some of their negative charges. Although modified starch-containing emulsions are known to be stabilized via steric repulsion, the presence of negatively charged side groups suggests that a small amount of the stabilization can be

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attributed to electrostatic repulsion (Charoen et al., 2011), a phenomenon that seemingly contradicts but really enhances prior studies. The droplets are stabilized enough by electrostatic repulsion due to the carboxyl groups on both the protein (whey) and the modified starch to be affected slightly by the counterions introduced by the sodium ions from sodium chloride (McClements, 2015).

13.6.2 Nonadsorbing Hydrocolloids There are many hydrocolloids that have little to no surface activity, but they have a potential to be used in combination with other natural emulsifiers (e.g., gum arabic or milk proteins) to stabilize the continuous phase of a beverage emulsion. These hydrocolloids are either polar or neutrally charged and therefore, cannot interact with the oil-water interface and emulsify; thus, they are referred to as nonadsorbing hydrocolloids. These hydrocolloids are mainly introduced to act as thickening and gelling agents to increase the viscosity of the aqueous phase, thereby stabilizing the oil droplets by inhibition of mobility (Khouryieh et al., 2015; Chen et al., 2010). Due to the lack of emulsifying capability, the hydrocolloids mentioned below are used less frequently than adsorbing hydrocolloids, such as gum arabic and modified starch. However, in the last decade, various types of biopolymers or a mixture of polysaccharide-protein complexes have been proposed as alternative sources for gum arabic or modified starches.

13.6.2.1  Xanthan Gum Xanthan gum is an anionic microbial polysaccharide produced by the bacterium, Xanthomonas campestris. It contains a glucose backbone bound by beta 1-4 linkages, and on every other glucose unit has a trisaccharide comprised of glucuronic acid between two mannoses. Pyruvate is also attached to around half of the outer mannose residues (Owens et al., 2018; Khouryieh, 2006). It is a popular polysaccharide used to stabilize oil-in-water emulsions; and, is especially favorable when combined with locust bean gum, because the two gums have been shown to display synergistic behavior when utilized together (Khouryieh et  al., 2015). Xanthan gum, even in low concentrations, stabilizes emulsions via electrostatic and steric repulsions as well as by increasing the viscosity (Liu et  al., 2016). The increased viscosity inhibits oxidation of oil droplets in the concentrated form of the beverage emulsions (Nasrabadi and Goli, 2016; Liu et al., 2015) by slowing the movements of pro-oxidating and metal chelating compounds. Although emulsions containing xanthan gum tend to display highly viscous solutions, these same emulsions also display pseudoplastic (shear-thinning) behavior. This is because the long xanthan gum chains will orient themselves with the flow as the force is decreased,

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which decreases the viscosity and resistance to flow (Mirhosseini and Tan, 2010; Taherian et al., 2007). Xanthan gum, as an anionic polysaccharide, encourages oil droplet repulsion due to its large (−15 mV) absolute value of zeta potential. The electrostatic repulsion of the droplets inhibits flocculation and coalescence, thereby increasing the stability of the emulsions. This increases the turbidity of the serum phase of the beverage because of the colloidal dispersion and refractive index effects. The electrostatic repulsion keeps droplet size small while increasing the opacity of the emulsions, a property desirable for beverage emulsions (Mirhosseini and Tan, 2010; Taherian et al., 2007). The zeta potential of xanthan gum-coated oil droplets signifies a large amount of adsorption onto the oil droplets, supporting the strong repulsion effects between oil droplets. If protein is present, the stabilizing effects of xanthan gum will become even stronger, especially if the protein is charged. This is because the level of adsorption onto the oil droplet when protein is present can be altered with changes in pH value. Emulsions with acidic pH levels (pH ≤ 4) tend to result in larger amounts of xanthan gum adsorption because the protein is now positively charged while the xanthan gum remains negatively charged (Liu et al., 2016). Food products are susceptible to wide ranges of environmental changes, including but not limited to heating and freezing. Therefore, it is important that the diluted form of a beverage is stabilized enough to withstand these fluctuations. Xanthan gum can form relatively thick layers around the oil droplets that prevent droplet coalescence, flocculation, and creaming. Emulsions with no added polysaccharides can form crystals when frozen, altering the overall quality once thawed again, yet xanthan gum can insulate the oil droplets and protect against environmental stresses such as these (Liu et al., 2016).

13.6.2.2 Galactomannans Mannans are heterogeneous plant polysaccharides that serve as energy reserves for leguminous seeds (locust bean gum, from Ceratonia siliqua; guar gum, from Cyamopsis tetragonolobus) and the tubers of Amorphophallus konjac (konjac glucomannan). The plant polysaccharides may also serve as structural components of cell walls in softwoods as seen with the water-soluble galactoglucomannan (Picea abies). Galactomannans (guar gum, locust bean gum) are comprised of a (1→4)-β-d-mannopyranosyl backbone with attached single-unit (1→6)-α-d-galactopyranosyl residues to mannose C-6 sites. Guar gum and locust bean gum have mannose/galactose ratios of 1.5 and 3.5 respectively. Konjac glucomannan has a chain consisting of (1→4)-β-d-glucopyranosyl and (1→4)-β-d-mannopyranosyl units which carry acetyl substitutions. It has a mannose/glucose ratio of

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16 and a 5% degree of acetylation. Galactoglucomannan has all three structural elements and thus has a mannose/glucose/galactose ratio of around 4:1:0.5 with a 30% degree of acetylation (Mikkonen et  al., 2009). Other commercial gums in this class of hydrocolloids include tara gum, cassia gum, etc.; however, although used in emulsions as a whole, they are not commonly used in beverage emulsions. The distribution of the galactose units throughout the main chain, as well as the galactose content strongly influences the physicochemical properties of galactomannans. Stronger synergistic interactions and greater functionality are seen in galactomannans that contain longer galactose side chains. Specifically, locust bean gum and xanthan gum have been known to form a strong synergistic relationship that results in highly viscous gels at a low total polysaccharide concentration (Khouryieh et al., 2015). Studies have also been conducted analyzing the effect of environmental conditions on the physical and chemical stability of emulsions containing whey protein-xanthan-locust bean gum complexes. Results concluded that emulsion stability is highly dependent on pH as well as the type of biopolymer (Owens et al., 2018). Below the isoelectric point of whey protein, which occurs around pH 5.0, the negatively charged xanthan-locust bean gum mixture and positively charged whey protein isolate will form a protein-polysaccharide complex increasing electrostatic interaction between the biopolymers. At a neutral pH, both whey protein isolate and xanthan-locust bean gum are negatively charged, leading to electrostatic repulsion. Xanthan-locust bean gum at pH 7 increases the overall stability by forming a secondary layer around droplets coated with whey protein isolate, beyond the improved stability obtained from the synergistic effect of xanthan-locust bean gum (Owens et al., 2018). The synergistic relationship between xanthan gum and guar gum, in contrast, is much weaker; however, enzyme-modified guar gum was found to have similar co-gelation as locust bean gum when it contained similar mannose/galactose ratios. To make the structure of guar gum more similar to locust bean gum’s skeleton, an enzyme-mediated ­reaction is used to remove some of the side-chain (1→6)-α-linked d-galactosyl residues without major cleavage of the backbone. A suitable enzyme for this task is α-galactosidase (Chityala et al., 2016). Galactomannans are used as stabilizers as they are able to modify the rheological properties of the aqueous phase between dispersed particles (Dickinson, 2003), yet they can also be considered emulsifiers as they have strong interfacial activity (Mikkonen et al., 2009; Garti and Reichman, 1994). Their interfacial tension lowering characteristics have been attributed to the presence of a small fraction of protein that is closely related to the polysaccharide structure, similar to gum arabic (Brummer et al., 2003). It has also been suggested that the protein fraction does not play a notable part in the gum activity. Instead,

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it is argued that a “salting-out” effect, which helps oil droplets gain precipitation of a gum film, is the reason that galactomannans have strong interfacial activity (Garti and Reichman, 1994). The presence of these two contradicting suggestions is due to the method of separation of the protein fraction from the galactomannan molecule. One method uses enzyme hydrolysis, which results in purified gum that is less surface active than unpurified gum, whereas another method uses physical separation, also referred to as ultracentrifugation (Brummer et al., 2003; Garti and Reichman, 1994). Physical methods cannot easily separate the protein from the molecule and are susceptible to enzyme hydrolysis (Bouyer et al., 2012).

13.6.2.3 Carrageenan “Carrageenan” is a broad term commonly used to classify a family of polysaccharides extracted from various families of red seaweed. As a whole, the family is utilized for its gel forming, emulsifying, and thickening properties. Carrageenan is most commonly used in dairy dessert products because of its interaction with milk proteins (Saha and Bhattacharya, 2010). The carrageenan backbone comprises of alternating α-(1→3) and β-(1→4) glycosidic linkages interspersed with galactose and 3,6-anhydrogalactose. The ester content can range from 18% to 40%. Carrageenan comes in three main forms—kappa, iota, and lambda—that differ in the number and position of ester sulfate groups (Stone and Nickerson, 2012; Wüstenberg, 2014; Necas and Bartosikova, 2013; Imeson, 2010). Carrageenan classes with a higher amount of esterification are water-soluble at lower temperatures but form weaker gels. Lambda contains the highest degree of esterification and exhibits little to no gelling, unlike kappa and iota. The lack of gelling along with its expense are two reasons lambda carrageenan is inefficient for utilization in food products (Wüstenberg, 2014; Necas and Bartosikova, 2013; Stone and Nickerson, 2012; Gu et  al., 2005). The physical characteristics and gelling properties of the three main classes of carrageenan are given in Table 13.5. Kappa Carrageenan Kappa carrageenan, as an ionic carrageenan polysaccharide, is highly influenced by the presence of other charged species in solution with it that can reorient the conformation from ordered to disordered (Necas and Bartosikova, 2013). It displays great sensitivity to potassium by way of forming bridges with the cation at its d-galactose sulfate group and interacting electrostatically with an anhydro-O-3,6 ring of another d-galactose ring (Stone and Nickerson, 2012). This is further demonstrated by the gels kappa carrageenan forms with κ-casein via the interaction between the positively charged regions and negatively

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Table 13.5  Properties and Functions of the Three Carrageenan Classes. Sulfate group Esterification 3,6-Anhydrogalactose content Water solubility (80°C) Water solubility (20°C) Gel consistency Gelation in presence of cations Synergism Interaction with proteins

Kappa (κ-)

Iota (ι-)

Lambda (λ-)

1 22% 33% Soluble Only in presence of Na+ Brittle Strong gels with K+ Galactomannans κ-Casein

2 32% 26% Soluble Only in presence of Na+ Elastic Strong gels with Ca2+ Gum arabic Only in acidic environments

3 37% Little to none Soluble Soluble No gelling No gelling None Only in acidic environments

charged regions on the protein and carrageenan, respectively. Due to this strong interaction, small concentrations of kappa carrageenan are needed to stabilize the emulsions, making it a convenient hydrocolloid for beverages (Saha and Bhattacharya, 2010). At low concentrations, kappa carrageenan interacts with casein micelles to form aggregated particles that can later be spray dried and used for various food products. The concentration is below the concentration required for gelling, so the resulting powdered milk product can be used to form drinks that do not display too high of viscosities (Martin et al., 2006; Ji et al., 2008a; Flett et al., 2010). The large particle size of these kappa-carrageenan/ casein aggregates are thought to aid in the perceived creaminess of dairy products (Flett et al., 2010; Ji et al., 2008b). Kappa carrageenan also forms a strong interaction with locust bean gum and konjac glucomannan, but this interaction is not desirable for beverage due to the resulting high viscosities (Stone and Nickerson, 2012; Imeson, 2010). Iota Carrageenan Iota carrageenan has been shown to display an interaction with gum arabic. The interaction between these two hydrocolloids is termed an associative interaction. An associative interaction results in increasing emulsion stability with increasing polymer concentration. High concentrations of the iota carrageenan/gum arabic mixture in whey protein emulsions leads to high adsorption onto oil droplets, a second layer formed around the oil, and a bridging effect between several droplets (Samhouri et al., 2009; Dickinson, 2009).

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Various studies have been completed regarding iota carrageenan and its interaction with protein coated oil droplets (Harnsilawat et al., 2006; Gu et al., 2004; Mun et al., 2010). As the iota-carrageenan concentration increases, the zeta potential of the droplets switch from positive to negative, reaching a plateau at 0.004% with a potential around −40 mV indicating a strong iota-protein interaction. More specifically, iota-carrageenan concentrations between 0.006% and 0.020% fully saturate the oil droplets and prevent droplet flocculation. This occurrence is strongest at pH 3 due to the anionic backbone of iota carrageenan creating a strong charge density that can interact strongly with a positively charged protein such as β-lactoglobulin.

13.6.2.4 Pectin Pectin is a polysaccharide found in plants, such as sugar beets and in the peels of citrus fruits. It consists of α-(1→4) linked d-galacturonic acid units regularly interspersed with rhamnogalacturonan regions (Chen et al., 2016). Pectin is frequently found in beverage emulsions, most commonly as a thickener, emulsifier, gelling agent, texturizer, and clouding agent (Mirhosseini and Tan, 2010; Mesbahi et al., 2005). Acetyl groups and protein fragments found in pectin chains allow the polymer to stabilize oil droplets by lowering the oil-water surface tension. In addition to the acetyl groups, carboxyl groups found on the pectin backbone influence the stabilizing characteristics of the polymer, depending on the extent to which the carboxyl groups are esterified, in that they ionize in mildly acidic environments and protonate in alkaline environments (Mirhosseini and Tan, 2010; Bai et al., 2017). The anionic nature of pectin allows for stabilization even after pH changes and additions of various salts (Piorkowski and McClements, 2014; Harnsilawat et al., 2006). Sugar beet pectin is amphiphilic, which contributes to the pectin’s ability to strongly adsorb onto the oil droplet in the emulsion and prevent aggregation (Bai et  al., 2017; Chen et  al., 2016). The amphiphilicity of pectin is due to hydrophilic polysaccharide regions that interact with the aqueous phase and amphiphilic protein regions that adsorb onto the oil droplet (Piorkowski and McClements, 2014; Bai et  al., 2017). It has also been suggested that the proteins associated with pectin contain methyl, acetyl, and ferulic acid esters that contribute to the hydrophobicity of the amphiphilic polysaccharide. The protein regions on the pectin backbone permit pectin to be categorized as both an emulsifier and a stabilizer, but it does not display as much surface activity as modified starch, guar gums, and various proteins. The acid group of the ferulic acid found on the backbone of sugar beet pectin is esterified, leaving the hydroxyl group of the ferulic acid free to act as an antioxidant by interacting with free radicals in the aqueous phase (Xu et al., 2012; Ngouémazong et al., 2015). Citrus pectin, also

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an emulsifying category of pectin, displays varying degrees of methoxylation, which affect the efficacy at stabilizing and emulsifying beverage emulsions (Ozturk and McClements, 2016; Schmidt et al., 2015). The nonpolar, hydrophobic protein regions of pectin adsorb directly onto the oil droplet while the polar, hydrophilic regions protrude into the aqueous environment. This lowers the surface tension at the oil-water interface, as well as creates both steric and electrostatic repulsions (McClements and Gumus, 2016; Bai et  al., 2017; Chen et  al., 2016). The electrostatic repulsion is enhanced by the highly anionic carboxyl groups, which have a pKa of 3.5 (Piorkowski and McClements, 2014). Sugar beet pectin exhibits high viscosities in emulsions, indicating that a small amount (~1.5%) of the polysaccharide is required to stabilize the emulsion. This is helpful in beverage emulsions because the emulsion will remain stabilized even after dilution when the pectin concentration is low (Chen et al., 2016). As the concentration of pectin in an emulsion is increased, the viscosity values display a sharp increase. This could be caused by strong attractive polysaccharide-­ polysaccharide interactions between multiple pectin-coated oil droplets in the emulsions. However, emulsions with high pectin concentrations are more susceptible to depletion flocculation, which can eventually lead to creaming and ringing, all of which are undesirable traits for beverage emulsions (Bai et  al., 2017). This reiterates that ­pectin-containing beverage emulsions retain higher quality with a lower pectin concentration. When mixed with xanthan gum, pectin displays highly repulsive droplet stabilization when present in concentrations under 4.5 wt%. The zeta potential of these emulsions have an absolute value of 25 mV or greater, signifying the absence of droplet flocculation (Mirhosseini and Tan, 2010), and therefore, an increase in stabilization. The turbidity of emulsions with high concentrations of pectin is found to be high, indicating that there are free pectin molecules within the aqueous phase that were not able to adsorb onto an oil droplet due to saturation of the oil-water interface.

13.6.2.5 Celluloses Cellulose, a semicrystalline and water-insoluble polymer found in plants, is the most abundant polysaccharide found naturally. Many derivatives of cellulose are found in food and beverage emulsions, the most common of which are hydroxypropyl methylcellulose and carboxymethylcellulose. Both of these can be produced via the addition of hydroxy-alkyl and alkyl groups, respectively, to unmodified cellulose (Haleem et  al., 2014; Mirhosseini et  al., 2008a). When ­unmodified cellulose is added to stabilize an emulsion, it is referred to as “cellulose nanocrystals” because of its crystalline structure.

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Another class of c­ ellulose is made from the bacteria Komagataeibacter sucrofermentans, and is dubbed “bacteria cellulose.” Bacterial cellulose has differing advantages from both carboxymethylcellulose and hydroxypropyl methylcellulose, regardless of the fact that it has the same chemical structure. Bacterial cellulose has a lower density and higher water-holding capacity, mechanical strength, and crystallinity (Paximada et al., 2016a,b). The backbone of carboxymethylcellulose is anionic and amphiphilic, two characteristics that explicitly aid in stabilization. The anionic property of this derivative aids in the stabilization of oil droplets by electrostatic repulsion, while the amphipathic property allows it to act as both an emulsifier and a stabilizer (Mirhosseini et al., 2008a). Carboxymethylcellulose may also increase the viscosity of emulsions as well as improve the perceived texture of the final product, which makes it a viable substitution for modified starch in beverage emulsions. At pH 7, carboxymethylcellulose creates a viscous network with whey protein-stabilized oil droplets, but not because of the polysaccharide/protein interaction. Since whey is anionic at pH 7, it cannot interact strongly with the anionic carboxymethylcellulose; rather, the carboxymethylcellulose gels the aqueous phase that surrounds the oil droplets, stabilizing them by preventing movement via an increase in viscosity (Malinauskytė et al., 2014). However, manufacturers must be careful with the amount of carboxymethylcellulose added to the product because of the highly viscous solutions the cellulose derivative can produce. When carboxymethylcellulose is present in low concentrations, the chains are able to extend to the maximum length, creating a viscous solution that can still exhibit shear thinning. The spacious arrangement enables the extended chains to align themselves with the direction of the shearing force. When the derivative is present in high concentrations, the chains overlap and result in an entangled network of carboxymethylcellulose that can resist increasing shear stress (Arancibia et al., 2013). This mechanism has similar stabilizing effects to a Pickering emulsion, which is stabilized by solid particles that adsorb directly to the oil-water interface. Since carboxymethylcellulose is not a solid particle, an emulsion containing it cannot be dubbed a Pickering emulsions; however, when cellulose nanocrystals are present in the emulsion rather than carboxymethylcellulose, the oil droplet is then stabilized by a solid particle and then henceforth be named a Pickering emulsion (Shen et al., 2016; Kalashnikova et al., 2012).

13.6.2.6 Alginate β-d-Mannuronate and α-l-guluronate are the copolymers which comprise the linear backbone of the alginate family; the amount of either of these polymers along the backbone is highly variable. These

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residues can even combine to form a dimeric alginate. This linear backbone is highly anionic and contains no side chains allowing the polysaccharide to lay flat against the oil droplet. The alginate can then be esterified to produce products, such as propylene glycol alginate (Phillips and Williams, 2009). Alginates are found in various brown seaweeds. The ratio of copolymers can differ in these seaweeds due to the growing season or environmental conditions, so it may be desirable to utilize a specific seaweed if a high concentration of one residue is preferred. Alginates can also be isolated from bacteria if a pure mannuronate alginate is needed. Total ionic strength, pH, and the content of gelling ions can all have important effects on the solubility of alginates. Alginates tend to precipitate out as the pH is lowered, although there are some exceptions, such as the alginates obtained from Ascophyllum nodosum that are soluble even when lowered to a pH of 1.4. Addition of salt has a profound effect on solubility, with less than 0.1 M NaCl required in order to reduce the solubility of the alginate. The content of gelling ions, such as Ca2+ can also reduce solubility. The content of Ca2+ above 3 mM dramatically reduces the amount of alginate in solution, only leaving 1%–3% still dissolved. However, this can be combated with some complexing agents (Phillips and Williams, 2009). There are two types of alginates, propylene glycol alginate and ­sodium alginate, but only propylene glycol alginate is commonly utilized in the food industry. The only alginate that is in high demand in the beverage industry is propylene glycol due to sodium alginate’s inability to function at pH levels below 4. However, sodium alginate may be useful in conjugation with other emulsion stabilizers, such as β-lactoglobulin and modified starch (Cheong et al., 2014). Propylene glycol alginate is an alginate derivative that is formed by esterifying the alginate and propylene oxide. Producing propylene glycol alginate in this manner confers several benefits including the enhancement of the polysaccharide’s emulsifying ability at lower pH values while maintaining a high solubility. Propylene glycol alginate can also produce emulsions with enhanced viscosity, stabilization, and film formation. Research on alginates as beverage emulsion components is lacking although some studies have been performed. A study by Cheong et al. (2014) described the impact of adding propylene glycol alginate to soursop beverage emulsions. In terms of creaming stability, emulsions containing propylene glycol alginate display decreased stability as the temperature is raised. This decreased stability is due to the inability of this alginate to form a film of high surface shear viscosity by adsorbing at the oil droplet surface which can be noted by observation of lower interfacial activity. The oil droplets then coalesce due to this incomplete absorption. This lower surface activity also leads to larger droplet sizes especially after storage (Paraskevopoulou et al., 2007). When

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used in conjugation with β-lactoglobulin, sodium alginate proves to be stable to creaming at both 0 and 50 mM NaCl. These emulsions became unstable at 100 mM, which suggests the electrostatic screening effect of the salt overcame the repulsion between the negatively charged droplets, leading to droplet aggregation. This reduced stability could also be due to desorption of the polysaccharide from the oil droplet, however, the zeta potential was not drastically reduced for these emulsions which implies that the polysaccharide has not fully desorbed (Harnsilawat et al., 2006).

13.7  Protein-Based Hydrocolloids Proteins are frequently used in all emulsion types as the primary emulsifier for the oil droplet. The polypeptide chains of proteins contain both hydrophilic (e.g., glutamine and asparagine) and hydrophobic (e.g., phenylalanine, leucine, and isoleucine) amino acids that create an amphiphilic protein. With this characteristic, proteins adsorb onto the oil with the hydrophobic regions and interact with the aqueous phase with the hydrophilic regions. Due to the amphiphilic nature, proteins can both emulsify and stabilize the oil droplets (Ozturk and McClements, 2016; Lam and Nickerson, 2013; Hasenhuettl and Hartel, 2008). Milk proteins, casein, and whey, are the most commonly utilized protein-based emulsifiers for emulsions.

13.7.1 Milk Proteins Dairy or milk proteins are chemically and structurally versatile biopolymers (Tavares et al., 2014). According to Kimpel and Schmitt (2015), they are able to serve as a natural transport for hydrophobic bioactives (e.g., oil droplets) due to the many hydrophobic regions in their structures. Milk proteins are also inexpensive, have a wide range of availability, and have GRAS status (generally recognized as safe). There are two main classes of milk proteins, caseins and whey proteins, that can be divided into several segments. Whey protein is globular and can be broken down into α-­ lactalbumin, β-lactoglobulin, serum albumin (BSA), and immunoglobulins. They are able to form high levels of secondary, tertiary, and in many cases, quaternary structures (Singh, 2011). Whey protein has a characteristic isoelectric point around pH 5, indicating that above pH 5 whey protein is negatively charged (deprotonated) and below pH 5 the protein is positively charged (protonated). The isoelectric point of whey protein is an important quality since it dictates the stability of the emulsion based on where the pH level is in relationship to the isoelectric point (McClements, 2015; Lam and Nickerson, 2013). Whey protein, because of its globular folding, is especially good at emulsifying

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and encapsulating hydrophobic ingredients, such as hydrophobic vitamins and oils. This is due to the high concentration of nonpolar regions on the amphiphilic protein chain (Tavares et al., 2014; Hong et al., 2012; Tippetts et al., 2012). Another aspect of whey protein’s emulsifying capabilities includes the ability to inhibit lipid oxidation in oil-in-water emulsions through a variety of mechanisms. These mechanisms, as defined by Hu et al. (2003), include whey proteins ability to: repel transition metals by forming cationic charges on emulsion droplet surfaces, reduce lipid-hydroperoxide-transition metal interactions by forming thick viscoelastic films at emulsion droplet interfaces, chelate prooxidative metals, and deactivate free radicals by their sulfhydryl groups and other amino acids. Caseins can be fractioned into four different phosphoproteins: αs1-, αs2-, β-, and κ-casein (Singh and Ye, 2009); sodium and calcium caseinates are also related to the casein family as commercial milk-protein products. Caseins precipitate around pH 4.6, are considered fundamentally unstructured properties, and tend to form casein micelles which are highly hydrated colloidal particles (Tavares et  al., 2014). They are able to exhibit efficient emulsifying properties due to their characteristic nonpolar regions scattered throughout their polypeptide backbones which allow them to adsorb onto the oil droplet. The backbone also consists of residues of anionic phosphoseryl, which promotes electrostatic bridging with cationic species (McClements, 2015). Due to its capability to bind with cationic molecules, casein is a natural delivery molecule for calcium and calcium phosphates, which adds to the nutritional element of beverage emulsions. Another advantage to this ability to bind with cationic molecules is the ability to stabilize iron, which, if left free, can cause oxidative instability in the emulsion. The unstructured and porous folding of caseins increases the protein’s ability to encapsulate minerals, flavor compounds, and other proteins. The encapsulation mechanism offered by casein protects enriched foods (Tavares et al., 2014; Anema and de Kruif, 2012; Sangeetha and Philip, 2012; Sahlan and Pramadewi, 2012; Haham et al., 2012). The loosely structured chain of casein promotes the protein to form a thick interfacial layer around the oil droplet being stabilized. Loops of the hydrophilic regions of the protein project out into the aqueous phase, which creates both a steric and electrostatic stabilization effect (Raikos et al., 2017).

13.7.2 Gelatin Gelatin is not utilized as often in beverage emulsions as the other hydrocolloids mentioned previously. Gelatin is derived from the collagen of various animals, such as cows, pigs, and fish. At relatively high temperatures this protein undergoes a conformational helix-to-coil

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transition; however, due to its thermos-reversible properties, it transitions back to the helical conformation when cooled. This allows gelatin to be utilized as an efficient gelling agent. Gelatin can be modified in two ways: type “A,” which has an isoelectric point between pH 7 and 9, is produced by sweltering animal tissues in acid, and type “B,” which has an isoelectric point around pH 5, is produced by the same process as type “A” but in the presence of alkaline species (McClements, 2015). A study performed by Taherian et  al. (2011) examined the properties of beverage emulsions containing gelatin types A and B with and without conjugation with whey protein isolate. The addition of fish gelatin to the oil-in-water emulsions created positively charged droplets. Type “A” may be more suitable for food emulsions due to its ability to create positively charged droplets and therefore, potentially repel iron ions that could cause oxidation. Thus, type “A” emulsions may have higher oxidative stability than type “B” emulsions, especially over the pH range typically found in foods (Taherian et al., 2011; McClements, 2015). An increase in pH leads to an increase in the net anionic charge found on the droplets, especially in emulsions where fish gelatin is in conjugation with whey protein isolate. This is attributed to the high isoelectric of the gelatin relative to the isoelectric point of the whey protein around pH 5. At the oil/water interface, gelatin adsorbs less readily in comparison to the protein concentration at the interface of emulsions containing only whey protein. This is due in part to the high molecular weight of the gelatin compared to the whey protein (Liu et al., 2014; McClements, 2015). Even when gelatin was added to emulsions in conjunction with whey protein, the whey protein was preferentially adsorbed. Emulsions containing droplets coated in the mixture of whey protein and gelatin showed a reduced rate of coalescence and particle growth when compared to emulsions containing only whey protein-coated droplets at the same pH. This may be due to charge repulsion, but the fish gelatin was also shown to have a much higher viscosity than the whey protein, which leads to the possibility of steric hindrance playing a role in preventing coalescence (Taherian et al., 2011). Emulsions containing whey protein and fish gelatin in conjugation have been shown to have higher viscosities, higher gel strengths, and the appearance of high molecular weight bands during SDSPAGE testing when compared to emulsions containing only gelatin or whey. This is most likely due to the cross-linking that can occur when the two proteins are utilized together. Utilizing fish gelatin alone is superior to using only whey protein in terms of rates of coalescence at lower pH values (Taherian et al., 2011). At higher pH values closer to the isoelectric point of the gelatin, whey protein emulsions are

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favorable due to the lack of charge present on the gelatin-coated droplets which can encourage coalescence. In emulsions made with both whey protein and fish gelatin, the difference between apparent viscosities between day 1 and day 21 measurements was decreased despite being measured at the isoelectric points of both proteins (Taherian et al., 2011).

13.8  Protein-Polysaccharide Interactions Although efficient emulsifiers, proteins do not produce emulsions that are stable under broad environmental conditions (e.g., pH, ionic strength, etc.) as polysaccharides, and therefore, typically are combined with a polysaccharide (Owens et al., 2018; McClements, 2015; Lam and Nickerson, 2013; Tippetts and Martini, 2012; Dalgleish, 2004). The emulsion stability can be improved by adding polysaccharide due to forming an interfacial complex with the adsorbed protein layer, that is, forming “bilayer” coated droplets (Fig. 13.4). The mechanism by which the added polysaccharide interacts with and stabilizes the protein-oil complex is critical in determining how effective the polysaccharide is for the final beverage. The stable formation mechanism for these beverage emulsions is the multilayer approach or layer-by-layer deposition. Multilayer emulsions provide an extra layer of protection to oxidation and degradation, which demonstrates high resistance to environmental stresses and unwanted conditions (Raikos et  al., 2017; Zhang et  al., 2015). Many studies have proposed using polysaccharide-protein complexes as alternative sources for gum arabic or modified starches in beverage emulsions (Zhang et al., 2015; Harnsilawat et al., 2006).

Add polysaccharide

Add emulsifier

+

+ Oil

Protein

Polysaccharide

Single layer

Double layer

Fig. 13.4  Schematic illustration of oil droplet stabilization by two layers. Protein is added first to the emulsion to act as a primary emulsifier, and then an oppositely charged polysaccharide is added to form a second layer.

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Further Reading Diop, C.I.K., Li, H.L., Xie, B.J., Shi, J., 2011. Effects of acetic acid/acetic anhydride ratios on the properties of corn starch acetates. Food Chem. 126 (4), 1662–1669. EFSA ANS Panel (EFSA Panel on Food Additives and Nutrient Sources added to Food), 2017. Scientific opinion on the re-evaluation of oxidised starch (E 1404), ­monostarch phosphate (E 1410), distarch phosphate (E 1412), phosphated distarch phosphate (E 1413), acetylated distarch phosphate (E 1414), acetylated starch (E 1420), acetylated distarch adipate (E 1422), hydroxypropyl starch (E 1440), hydroxypropyl distarch phosphate (E 1442), starch sodium octenyl succinate (E 1450), acetylated oxidised starch (E 1451) and starch aluminium octenyl succinate (E 1452) as food additives. EFSA J. 15 (10), 4911–5007.

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Kapelko, M., Zieba, T., Michalski, A., Gryszkin, A., 2015. Effect of cross-linking degree on selected properties of retrograded starch adipate. Food Chem. 167 (15), 124–130. Lewandowicz, G., Walkowski, A., Blaszczak, W., 2004. Degree of substitution of crosslinked starches vs. functionality in food products. In: Yuryev, V.P., Tomasik, P., Ruck, H. (Eds.), Starch: From Starch Containing Sources to Isolation of Starches and Their Applications. Nova Science Publishers, New York. McClements, D.J., Decker, E.A., Park, Y., Weiss, J., 2009. Structural design principles for delivery of bioactive components in nutraceuticals and functional foods. Crit. Rev. Food Sci. Nutr. 49 (6), 577–606. Mirhosseini, H., Tan, C.P., Aghlara, A., Hamid, N.S.A., Yusof, S., Chern, B.H., 2008b. Influence of pectin and CMC on physical stability, turbidity loss rate, cloudiness and flavor release of orange beverage emulsion during storage. Carbohydr. Polym. 73, 83–91. Polnaya, F.J., Haryadi, D.W., Marseno, Cahyanto, M.N., 2013. Effects of phosphorylation and cross-linking on the pasting properties and molecular structure of sago starch. Int. Food Res. J. 20 (4), 1609–1615. Schramm, L.L. (Ed.), 1992. Emulsions: Fundamentals and Applications in the Petroleum Industry. American Chemical Society, Washington, DC. Shukri, R., Shi, Y., 2017. Structure and pasting properties of alkaline-treated phosphorylated cross-linked waxy maize starches. Food Chem. 214 (1), 90–95. Sołowiej, B., Dylewska, A., Kowalczyk, D., Sujka, M., Tomczyńska-Mleko, M., Mleko, S., 2016. The effect of pH and modified maize starches on texture, rheological properties and meltability of acid casein processed cheese analogues. Eur. Food Res. Technol. 242 (9), 1577–1585. Tecante, A., Santiago, M.D.C.N., 2012. Solution properties of κ-carrageenan and its interaction with other polysaccharides in aqueous media. In: De Vincente, J. (Ed.), Rheology. In Tech, pp. 241–264. Tuschhoff, J.V., 1986. Hydroxypropylated starches. In: Wurzburg, O.B. (Ed.), Modified Starches: Properties and Uses. CRC Press, Boca Raton, FL. Yeh, A., Yeh, S., 1993. Some characteristics of hydroxypropylated and cross-linked rice starch. Cereal Chem. 70 (5), 596–601. Zhao, J., Schols, H.A., Chen, Z., Jin, Z., Buwalda, P., Gruppen, H., 2012. Substituent distribution within cross-linked and hydroxypropylated sweet potato starch and potato starch. Food Chem. 133, 1333–1340.