- Email: [email protected]
W protein-based emulsions
Accepted Manuscript Use of a mixer-type rheometer for predicting the stability of O/W protein-based emulsions A. Romero, M. Felix, V. Perez-Puyana, L. Choplin, A. Guerrero PII:
S0023-6438(17)30469-3
DOI:
10.1016/j.lwt.2017.07.008
Reference:
YFSTL 6366
To appear in:
LWT - Food Science and Technology
Received Date: 14 February 2017 Revised Date:
25 June 2017
Accepted Date: 1 July 2017
Please cite this article as: Romero, A., Felix, M., Perez-Puyana, V., Choplin, L., Guerrero, A., Use of a mixer-type rheometer for predicting the stability of O/W protein-based emulsions, LWT - Food Science and Technology (2017), doi: 10.1016/j.lwt.2017.07.008. 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.
ACCEPTED MANUSCRIPT 1
Use of a mixer-type rheometer for predicting the stability of O/W
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protein-based emulsions A. Romeroa,*, M. Felixa,V. Perez-Puyanaa, L. Choplinb, A. Guerreroa a
Departamento de Ingeniería Química, Universidad de Sevilla, Facultad de Química,
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41012 Sevilla, Spain b
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GEMICO, ENSIC, Université de Lorraine, Nancy, France
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Corresponding autor:
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*A. ROMERO
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Departamento de Ingeniería Química,
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Universidad de Sevilla, Facultad de Química,
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41012 Sevilla (Spain)
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E-mail: [email protected]
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Phone: +34 954557179; fax: +34 954556447.
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Abstract
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The present work illustrates the feasibility of performing Oil-in-Water (O/W) emulsions
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stabilized by different protein concentrates, as well as predicting the likelihood of
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emulsion destabilization over ageing time just after its preparation. To achieve this
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objective, four protein sources (rice, crayfish, potato and albumen) and four oil
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concentrations (450, 550, 650 and 750 g·kg-1) were used. The emulsification process
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was monitored by the use of a mixer-type rheometer. This rheometer was a valuable
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tool for understanding and controlling the emulsification process through the
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measurement of the viscosity of the different systems during the emulsification stage.
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ACCEPTED MANUSCRIPT Results reveal the importance of controlling the emulsification process to optimise the
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properties of the final emulsion, which is highly influenced by the oil concentration.
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Then, emulsions were characterized by means of flow properties and droplet size
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distribution (DSD). Eventually, a relationship was found that relates the rheological
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properties and the microstructure of the final emulsions during and after emulsification
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stage. These measurements have been demonstrated to be useful in order to predict
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the stability of protein-based emulsions.
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Keywords: Droplet size distribution; Emulsification; Oil concentration, Proteins;
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Viscosity.
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1. Introduction
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An emulsion is a mixture of two immiscible liquids in which one is dispersed in the other
34
in the form of droplets. Common food emulsions are formed by a mixture between oil
35
and water, and are called oil-in-water (O/W) emulsions or water-in-oil emulsions (W/O),
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depending on the dispersed or continuous phase.The final properties (i.e. stability,
37
texture or droplet size) of an O/W emulsion strongly depend on the specific
38
characteristics of each compound of an emulsion: the dispersed phase (oil), the
39
continuous phase (water) and the interface. Each one of them has its own complexity,
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which should be analysed to obtain useful information about further emulsion
41
properties. Among them, the interface became of particular interest in these types of
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kinetically-stabilized systems. The O/W interface of a food is usually stabilized by
43
proteins, low-molecular weight emulsifiers (mainly monoglycerides, phospholipid and
44
esters) or a combination of them (McClements, 2004).Characteristics of the interface
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depend on the type and concentration of the emulsifier, which tends to be adsorbed at
46
interfacedue to its amphiphilic character. Proteins are widely used as emulsifier in food
47
emulsions, being considered as a functional ingredient in the formation and
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stabilization of food emulsions and foams (McClements, 2004; Taherian, Fustier, &
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Ramaswamy, 2006). Proteins are quickly adsorbed at O/W interface of the droplet
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(Dickinson & McClements, 1995; Foegeding & Davis, 2011; Law & Kennedy, 1999).In
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recent years, egg yolk has been replaced by alternative protein systems such as
53
vegetable proteins, mainly soybean and wheat, or, even animal proteins systems.
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Among other alternatives, rice, potato and crayfish protein systems could be
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mentioned.
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Rice industry produces every year a large amount of by-products which involve an
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undesirable environmental effect. Thus, around 100 million tons of them are produced
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every year around the world (Li, Liu, Liao, & Yan, 2010). Unfortunately, these by-
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products are used in low-value-added applications, being mainly incinerated to obtain
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electricity or used for animal feeding (Njie & Reed, 1995).Another by-product from food
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industry comes from the potato protein corresponding to the potato skin. Potato protein
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concentrate from industrial wastes are generally subjected toextreme conditions to
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increase the protein solubility, as a consequence, proteins are extensively denaturated.
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However, some emulsifying of these potato concentrates have been found (Romero et
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al., 2011; van Koningsveld et al., 2006). On the other hand, protein surpluses not only
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may come from plants but also they may derive from an animal source. For example,
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proteins form the crayfish Procambarus clarkii. It was introduced in Andalusia (southern
68
region of Spain) some years ago and suffered a fast widespread growth, being
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considered as an invasive species (Kirjavainen & Westman, 1999). Nowadays, a
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strong local industry produces a big amount of surpluses and wastes. Emulsions have
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been made from this protein surplus (Felix, Romero, & Guerrero, 2017; Romero,
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Cordobes, & Guerrero, 2009).
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During the emulsification process, the strain and rupture of the droplets are controlled
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by a local capillary number (Ca), which establishes the relationship between the shear
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stress and the interfacial tension. Hence, there is a critical value for this relationship
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that depends on the viscosity ratio of disperse and continuous phases. Above the
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critical value, shear forces exceed surface tension and the breakup of droplets takes
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(Janssen & Meijer, 1995).
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On the other hand, the stability of an emulsion is the key factor considered for
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performing an emulsion since these are only kinetically stable. An emulsion is stable
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when the number, size distribution and spatial distribution of droplets do not change
83
over time. Thus, the destabilization of emulsions mainly depends on the initial size of
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droplets, the rheology of the continuous phase and the interactions among particles,
85
which are responsible for the flocculation and further coalescence of droplets (Sahin &
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Sumnu, 2006). Destabilization phenomena are related to the presence of attractive
87
interactions (van der Waals), electrostatic repulsions and steric interactions. Then, if
88
attractive interactions are weak, droplets tend to form a reversible structure which
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favours the stability. However, if attractive forces are strong the destabilization of
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emulsions may take place through the droplet flocculation and further coalescence
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(Tadros, 2013). At equilibrium, proteins adsorbed at O/W are able to form a film which
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has a double function: On one hand, it favours the electrostatic repulsions. In fact, high
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values of electrostatic potentials are desired for the stability of this kind of dispersed
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system (Franco, Guerrero, & Gallegos, 1995). On the other hand, the relatively high
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film thickness that may act as a mechanic barrier which avoids the droplet
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coalescence, due to its viscoelastic properties (Tadros, 2013; W. N. Zhang,
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Waghmare, Chen, Xu, & Mitra, 2015).
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Apart from the colloidal stability, the rheology of emulsions is influenced by several
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structural parameters, such as interparticle interactions, particle size, shape and
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polydispersity, continuous phase viscosity, etc. (Martínez, Partal, Muñoz, & Gallegos,
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2003). Among them, the nature of the interactions among particles has particular
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relevance. Moreover, other parameters such as the size and the shape of particles, the
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polydispersity and rheology of the continuous medium also determine the rheological
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response significantly (Ma & Barbosa-Cánovas, 1995). The stability of an emulsion can
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be determined through the use of rheological techniques, since the destabilization
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inversion or sedimentation provoke changes in the rheological properties of emulsions
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(Sanchez, Berjano, Guerrero, Brito, & Gallegos, 1998; Tadros, 2013). Rheological
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measurements are useful because it is possible to relate these properties withthe
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microstructure of the emulsions, allowing a prediction of emulsion stabilities. Especially
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interesting are flow measurements in steady and transitory states. Thus, since the first
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one contributes to describe the real response of emulsions during its transport, the
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second one reflects the way in which the structural construction or destruction takes
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place, reflecting also the shear effect (Gallegos, Franco, & Partal, 2004; Guerrero,
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Partal, Berjano, & Gallegos, 1996; Mezger, 2006). More specifically, the study of the
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rheological properties during the emulsification stage allows getting knowledge about
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the shear-induced droplet formation. This characterisation at an early stage is of crucial
118
importance to predict the properties and the long-term stability of the final emulsions.
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In a previous manuscript, the mixer-type rheometer was used as a tool to evaluate the
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influence of agitation speed, oil and protein concentrations as well as pH on the
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properties of egg albumen-based emulsions over emulsification (Romero, Perez-
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Puyana, Marchal, Choplin, & Guerrero, 2017). The aim of this study was to compare
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the use of four different proteins (rice, crayfish, potato and albumen) in mayonnaise
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type O/W emulsion stabilization at four different oil concentrations (450, 550, 650 and
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750 g oil per kg emulsion) over and after processing by a mixer-type rheometer, as well
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as predicting the likelihood of emulsion destabilization over ageing time just after its
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preparation. Therefore, characterization of the emulsions, particularly its viscosity and
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particle size distribution, was carried out to accomplish this objective.
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2. Materials and methods
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2.1. Materials
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Different protein systems were used. The rice protein concentrate, from rice husks, was
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provided by Remy Industries (Leuven-Wijgmaal, Belgium), the crayfish flour was
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supplied by Protastar (Reus, Barcelona, Spain) and blbumen protein isolate was
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delivered by Proanda (Barcelona, Spain). Table 1 shows the composition for each
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protein concentrate system supplied. Sunflower oil was purchased in a local market
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and used without further purification. In addition, more information about molecular
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weight distribution, isolelectric point and protein solubility as a function of pH values as
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well as interfacial properties of these protein systems can be found in the literature
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(Felix, Martin-Alfonso, Romero, & Guerrero, 2014; Romero et al., 2012; Romero,
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Beaumal, et al., 2011; Romero, Cordobes, Guerrero, & Cecilia Puppo, 2011).
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2.2. Emulsification Process
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Emulsions were prepared in a mixer-type rheometer. This device consists of a
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cylindrical vessel equipped with a double helical ribbon impeller installed in a RS150
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controlled stress rheometer, as described by (Nzihou, Bournonville, Marchal, &
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Choplin, 2016). Different O/W protein-based emulsions were prepared by means an
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Ultra Turrax T-25 homogenizer from IKA at 15,000 rpm to apply mechanical energy for
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the emulsification process. In fact, this homogenizer was introduced into the mixer-type
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rheometer. At the same time, the mixer-type rheometer, rotating at 5 rad/s, was used in
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order to evaluate the evolution of viscosity. These values (agitation and rotation speed
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correspond to homogenizer and mixer-type rheometer, respectively) were selected
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according to previous studies (Romero et al., 2017). On the other hand, the final pH of
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all emulsions was 3.0, and the protein concentration used was 30 g·kg-1. These values
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(pH and protein concentrations) correspond to typical values found in food emulsions
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and were also selected according to previous studies (Romero et al., 2017). Emulsions
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were characterized in terms of the influence of the type of protein concentrate used as
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well as the oil concentration (450, 550, 650 and 750 g·kg-1). The emulsification
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sequence consists of 30 s with only Ultra Turrax (UT) agitation, 420 s with UT
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homogenization in order to stabilize the viscosity value.
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2.3. Characterization of emulsions
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2.3.1. Rheological measurements
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During emulsification
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A mixer-type rheometer was used to determining rheological changes over the
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emulsification process, more specifically, viscosity (η) along the emulsification time.
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This mixer-type rheometer has been already validated for the in-situ measurement of
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the viscosity during the emulsification process (Romero et al., 2017). Note that
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parameters are included in an analytical method, based on the Couette analogy, which
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allows as to extract absolute viscosity-shear rate data in non-conventional geometries
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(Aït-Kadi, Marchal, Choplin, Chrissemant, & Bousmina, 2002; Nzihou et al., 2016).
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After emulsification
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Steady state shear flow tests were carried out in a RF II rheometer from
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Rheometric Scientific (USA) the day after emulsion preparation and 30 days later. The
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measurements were performed at 25 ºC from 0.1 to 10 s-1and the geometry used was
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parallel plates (dia: 25 mm) with a rough surface, avoiding sample slide, and a gap
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between plates of 1 mm.The data obtained were fitted to a Power-law model (Ostwald
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de Waele), obtaining the viscosity ( ) as a function of the shear rate ( ). The following
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expression was used for this model:
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Where
and
=
1
are the consistency and flow index, respectively. The value of viscosity,
at 1 s-1 was selected in order to compare emulsions.
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2.3.2. Droplet Size Distribution (DSD) measurements
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Measurements of DSD were performed in a laser light scattering apparatus (Malvern
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Mastersizer 2000, UK). For this purpose, 0.5 mL of emulsion was taken and diluted in
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11.5 of 0.05 mol/L, pH Tris-HCl buffer with 1 g SDS/ 100 g of buffer. SDS was used as
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a disaggregating material in order to facilitate disruption of floccules (drops
186
agglomeration), often achieved by repulsive droplet/droplet interactions (Hasenhuettl &
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Hartel, 2008; Nzihou et al., 2016). Values of the Sauter mean diameter (d3,2) which is
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inversely proportional to the specific surface area of droplets, were obtained as follows: =
∑ ∑
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Where
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Uniformity parameter ( ) also was calculated from DSD measurements. This
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parameter is an index of polydispersity of the DSD and it is defined as follows:
∑ |
, 0,5 − , 0,5 ∑
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is the number of droplets with a diameter
, 0,5 is the median for the distribution, and
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Where
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a diameter
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Both studies (rheological and DSD measurements)were carried out along time (1 and
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30 days after emulsification) in order to study the effect ofprotein concentrates and oil
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concentration on the emulsion stability. Emulsions were stored at 5 ºC, being subjected
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to the same thermorheological history before performing any tests.
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2.4. Statistical analysis
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Three replicates of each rheological and DSD measurement as well as emulsification
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process were carried out. All the data were reported as means ±95% confidence limits.
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Statistical analyses were performed using t-test and one-way analysis of variance
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(ANOVA, p<0.05).
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3. Results
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3.1. Emulsification process
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Figure 1 shows the results of viscosity obtained from the mixer-type rheometerover
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emulsification process for all the protein systems studied (rice, crayfish, potato and
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albumen) at 650 g·kg-1 oil concentration. As observed, the viscosity of emulsions has a
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ACCEPTED MANUSCRIPT quite marked dependence on the emulsification stage. Thus, over the first 30 s, where
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the only component is the protein concentrate dispersed into water, the viscosity does
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not change over mixing time. In fact, no significant differences between proteins
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solutions were found. However, after starting the addition of oil (from 30s), the viscosity
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of the systems increased in each case until reaching a maximum value. In this stage,
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two different behaviours can be found. On one hand, rice and albumen-based
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emulsions reached a plateau value before finishing the oil addition. On the other hand,
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potato and crayfish-based emulsions increase their viscosity as the amount of oil
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increases. The following 60 s with UT agitation did not yield any significant change in
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emulsion’s viscosity with the exception of the crayfish-based system that suggested
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that longer emulsification time would be required. Finally, the latest 290 s without
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homogenization induce a decrease in viscosity only for the potato-based emulsion,
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probably as a consequence of a damping process after the significant rise during the oil
221
addition. The profiles of viscosity reveal significant differences not only on the
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emulsification stage, but also on the protein used. Thus, the viscosity of the potato-
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based emulsion is the highest, whereas the one found for rice-based emulsions is the
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lowest. Crayfish and albumen-based emulsions showed an intermediate behaviour.
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These differences are probably due to not only the different protein amount and type of
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protein but also the presence of other components in the composition (i.e. lipids or
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starches) as well as the different behaviour at the pH value used (3.0). Anyway,
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viscosity of emulsions is a key factor for the stability of these systems, since it
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decrease the destabilization kinetic (Tadros, 2013). Thus, systems with higher viscosity
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would lead to emulsions with higher long-term stability. In this sense, depending on the
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protein system, it is possible to optimize the emulsification time as the maximum
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viscosity is reached.
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3.2. Emulsion characterisation
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potato and albumen) at four different oil concentration: 450 g·kg-1 (A), 500 g·kg-1(B),
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650 g·kg-1 (C) and 750 g·kg-1 (D) the day after emulsion preparation. All steady state
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flow curves exhibit a very shear-thinning behaviour, which is characteristic for O/W
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emulsions (Tadros, 2004). As for the influence of the dispersed phase, this figure
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reveals that the concentration of the oil has a marked effect on the viscosity of the
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system. Thus, an increase in oil concentration induces an increase in the viscosity of
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the protein-based emulsions in any case. This oil-dependence of the viscosity has
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been previously found by other authors (Pearce & Kinsella, 1978; Taherian et al.,
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2006), and it is related to the increase of droplet flocculation. The increase of oil
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droplets favours the formation of droplets floccules which do not allow the free
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movement of droplets, increasing the emulsion viscosity (Pal, 1992). In addition, the
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increase of viscosity due to the increase in oil concentration induces an increase in the
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degree of shear-thinning behaviour (lower flow indices)(Pal, 2000), which can be
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attributed to the structuration of the protein-based emulsions. As regards the influence
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of protein nature, Figure 2 also evidences the high influence of the protein used on the
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final viscosity of the emulsion. Thus, rice-based emulsions are the less viscous in any
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case, although the carbohydrate concentration, mainly starches, could be expected to
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increase the emulsion viscosity. However, although the viscosity of emulsions depends
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on the nature of the protein used, the viscosity mainly depends on the oil concentration.
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In this sense, it may point account that the crayfish and albumen-based emulsions
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contain a more percentage of oil due to the presence of lipids in the composition, but
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this increase is always lower than 1% of the total lipid content. On the other hand, the
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presence of lipids could lead to a relevant surface activity but lipids in the samples
258
have been isolated and checked that these do not show relevant surface activity.
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Figure 3 shows DSD for all protein-based emulsions analysed (rice, crayfish, potato
260
and albumen) at four different oil concentrations: 450 g·kg-1 (A), 550 g·kg-1 (B), 650
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g·kg-1 (C) and 750 g·kg-1 (D). Generally, DSD profiles exhibit unimodal profile, whose
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used. The increase of oil tends to forward the peak, which in turn means the formation
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of smaller droplets. The DSD profile is highly influenced by the protein used, since as a
265
general rule, albumen-based emulsions have the highest sizes and potato based
266
emulsions have the lowest ones. The results obtained of viscosity can be related to the
267
DSD, since generally, higher viscosity values induce lower droplet sizes. These results
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are related with the Capillary number described in the introduction section, where it
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was described that the viscosity of the continuous phase influence the shear stress
270
applied, limit the recoalescence process and, consequently, decrease the droplet sizes.
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In fact, the viscosity of shear-thinning emulsions is strongly influenced by the droplet
272
size; a significant increase in the viscosity occurs when the droplet size is reduced.
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However, these emulsions showed higher viscosity in comparison with Pal (2000)
274
using surfactant as emulsifier. These deviations can be attributed to the higher
275
viscosity of the continuous phase of protein solutions (proteins and higher emulsifier
276
concentration was used). On the other hand, the use of different amounts of oil seems
277
to indicate that there is a more uniformity profile at intermediate oil concentration, which
278
depends on the protein system. Thus, potato and rice-based emulsions exhibit wider
279
peaks when the lowest or the highest oil concentration (450 or 750 g·kg-1) are used.
280
3.3. Emulsion stability
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Table 2 shows parameters from flow curves (flow index,
282
corresponding to the viscosity at 1 s-1 shear rate), as well as parameters from DSD
283
measurements (Sauter diameter, d3,2, and uniformity parameter,
284
parameters were evaluated for all protein-based emulsions studied (rice, crayfish,
285
potato and albumen) at four different oil concentration (450, 550, 650 and 750 g·kg-1)
286
the day after the emulsion preparation and after 30 days.
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The viscosity (
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concentration. The increase in oil provokes a marked increase in emulsion viscosity.
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, and viscosity,
,
). All these
) confirms the high dependence of the emulsions on the oil
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290
values of viscosity tends to have shorter changes of relative viscosity values over
291
ageing time. However, viscosity is a very sensitive parameter in order to estimate
292
emulsion stability. As may be observed, as a general rule, the flow index ( ) tends to
293
increase over the ageing time, which indicates that the shear-thinning behaviour
294
decreases. Therefore, the destabilization processes (mainly coalescence), which
295
emulsions suffers over storage time, induce the loss of structure, decreasing not only
296
the viscosity but also the shear-thinning behaviour (higher n values).
297
As for d3,2 diameter from DSD profiles, Table 2 confirms that the Sauter diameter
298
depends on the protein used. However, in any case, as it could be deduced in Figure 3,
299
the increase in oil concentration induces the decrease of droplet sizes and,
300
consequently, leads to emulsions are more stable over the ageing time studied. In fact,
301
when the 750 g·kg-1 oil concentration is used, no significant changes in droplet sizes
302
are observed for all the protein systems evaluated. This long term stability is related to
303
initial high viscosity value, as well as lower droplet sizes. Moreover, the two protein-
304
based emulsions whose viscosity increased until finishing the oil addition (crayfish and
305
potato) always lead lower d3,2 diameter, confirming the relation between the rheological
306
properties of the emulsification stage and the microstructure of the final emulsions. In
307
addition, in general, when the viscosity of emulsion measured in mixer-type rheometer
308
is higher than approximately 50 Pa·s, emulsions were stable after 30 days, according
309
to Sauter diameter results. On the other hand, interestingly, emulsion stability is
310
confirmed by the uniformity parameter ( ). The lowest values were obtained for
311
albumen-based emulsions, apart from them, the increase in oil concentration generally
312
leads to lower
313
general rule, the more unimodal emulsions (low
314
are, since polydispersity systems are considered unstable since larger droplets grow
315
up at the expense of smaller droplets (Amid & Mirhosseini, 2012; Y. Zhang et al.,
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values, which are related to the thinner peaks above mentioned. As a values), the more stable emulsions
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2008). In fact,
317
emulsions, at the most oil concentrated emulsions studied (750 g·kg-1),
318
does not generally exhibit the lower values but evidencing the lower evolution, as a
319
consequence of the increase of emulsions stability found. However, once again, in
320
these systems, it is demonstrated that stability of emulsions seems to be governed by
321
the viscosity, as a consequence of the increase in oil and this parameter (
322
be the more appropriate in order to predict emulsion destabilization since their change
323
are more evident and significant between different emulsions evaluated.
324
4. Conclusions
325
Results confirm the high importance of the characterisation of the emulsification stage
326
in order to follow the emulsion viscosity. Thus, it is possible to optimize the
327
emulsification time with the corresponding power saving, selecting the proper time and
328
oil content in order to reach a proper viscosity. In fact, the increase of oil addition leads
329
to a non-lineal increase in viscosity, because a lot of new droplets are not formed.
330
Therefore, the use of a mixer-type rheometer allows us to reach final emulsions with
331
proper properties, rheological properties and microstructure, which are related to the
332
stability of the emulsions.
333
The long term stability is related to the increase of emulsion viscosity found during
334
emulsification, as well as the decrease of droplet sizes. Thus, the initial high viscosity
335
(after emulsification) and small droplet sizes are related to more stable emulsions. In
336
this sense, on one hand, the nature of the protein used highly affects to the viscoelastic
337
and microstructure of the final emulsions since the protein system can affect the
338
viscosity of emulsion. On the other hand, the analysis of the oil amount revels that the
339
increase in oil tends to increase the viscosity of emulsions, and decrease the Sauter
340
diameter increasing the stability of final emulsions, being the viscosity the most
341
sensitive parameter in order to predict the emulsion stability.
parameter usually increases over aging time. However, for these
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parameter
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) seems to
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ACCEPTED MANUSCRIPT Although mixer-type rheometer has been demonstrated as a useful tool in order to
343
optimize emulsions stabilized by different protein systems and oil concentration,
344
although further research will be necessary taking into account other factors such as
345
the protein concentration, the influence of other minor components or the pH value.
346
5. Acknowledgements
347
This work is part of a research project sponsored by “Ministerio de Economía y
348
Competitividad”,
349
MINECO/FEDER, EU) and University of Seville for the grant of the VPPI-US. The
350
authors gratefully acknowledge their financial support.
351
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FIGURE CAPTIONS
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Figure 1: Rheological characterization of o/w emulsions over emulsification stage at
474
650 g·Kg-1 oil concentration and for all protein systems studied:
476 477
potato and
rice,
crayfish,
albumen.
Figure 2: Flow curves for all protein-based emulsions studied ( potato and
crayfish,
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rice,
albumen) at four different oil concentrations: (A) 450 g·kg-1, (B) 550
g·kg-1, (C) 650 g·kg-1 and (D) 750 g·kg-1 the day after preparation.
479
Figure 3: Droplet Size Distributions (DSD) for all protein-based emulsions analysed (
481
rice,
crayfish,
potato and
albumen) at four different oil
concentrations: (A) 450 g·kg-1, (B) 550 g·kg-1, (C) 650 g·kg-1 and (D) 750 g·kg-1.
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TABLE CAPTIONS
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Table 1: Composition of different protein type systems: Rice, Crayfish, Potato and
485
Albumen.
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Table 2: Parameters from flow curves (
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and ) as well as from DSD profiles (d3,2 and
) for all protein-based emulsions studied (rice, crayfish, potato and albumen) at four
488
different oil concentrations (450, 550, 650 and 750 g·kg-1) the day after the emulsion
489
preparation and after a storage of 30 days.
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ACCEPTED MANUSCRIPT Table 1
Protein (g·kg-1)
Carbohydrates (g·kg-1)
Ash (g·kg-1)
Lipids (g·kg-1)
Moisture (g·kg-1)
Rice
782 ± 12
64 ± 3
52 ± 1
24 ± 3
78 ± 4
Crayfish
647 ± 12
2±1
134 ± 2
183 ± 8
34 ± 3
Potato
801 ± 23
59 ± 6
8±1
Albumen
730 ± 11
4±1
60 ± 3
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Protein System
101 ± 2
130 ± 5
76 ± 2
AC C
EP
TE D
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31 ± 4
ACCEPTED MANUSCRIPT
750
d3,2 (µm)
30 days
1 day
30 days
0.3 ± 0.1a 6.9 ± 0.1A 6.1 ± 0.2α 23.1 ± 0.2П 3.9 ± 0.1c 50.4 ± 0.2C 43.2 ± 0.4γ 46.9 ± 0.3Ѓ
0.1 ± 0.1b 2.9 ± 0.2B 2.4 ± 0.1β 14.5 ± 0.2Ф 2.1 ± 0.1d 19.5 ± 0.3D 32.3 ± 0.5δ 38.8 ± 0.2Σ
21.7 ± 0.2e 91.3 ± 0.5E 202.2 ± 2.6ε 90.1 ± 0.3Ω 138.9 ± 5.8g 460.9 ± 2.7G 282.3 ± 3.6η 179.7 ± 4.5Γ
13.8 ± 0.7f 43.0 ± 0.5F 174.4 ± 4.3ζ 71.6 ± 0.7Θ 119.5 ± 3.7h 330.5 ± 2.4H 267.9 ± 2.9 η 132.8 ± 2.2Ξ
0.47 ± 0.02a 0.40 ± 0.04A 0.41 ± 0.01α 0.34 ± 0.03П 0.37 ± 0.02b 0.31 ± 0.02B 0.21 ± 0.02γ 0.19 ± 0.02Ѓ 0.27 ± 0.02c 0.35 ± 0.01C 0.31 ± 0.02δ 0.09 ± 0.01Ω 0.31 ± 0.03c 0.16 ± 0.04D 0.38 ± 0.02ε 0.05 ± 0.01Γ
0.49 ± 0.03a 0.37 ± 0.01A 0.49 ± 0.02β 0.44 ± 0.02Ф 0.37 ± 0.03b 0.36 ± 0.02C 0.24 ± 0.02 γ 0.24± 0.01Ѓ 0.32 ± 0.02c 0.28 ± 0.02C 0.34 ± 0.03δ 0.13 ± 0.01Θ 0.32 ± 0.02c 0.25 ± 0.03D 0.37 ± 0.02ε 0.09 ± 0.02Ξ
U
1 day
30 days
1 day
30 days
13.42 ± 0.04a 7.37 ± 0.19A 6.89 ± 0.05α 16.79 ± 0.19П 7.43 ± 0.02c 5.29 ± 0.08C 5.73 ± 0.04γ 13.70 ± 0.63Ѓ
18.72 ± 0.52b 15.01 ± 0.50B 20.12 ± 0.04β 22.99 ± 0.45Ф 8.10 ± 0.06d 6.34 ± 0.01D 6.46 ± 0.01δ 15.27 ± 0.34Ѓ
1.83 ± 0.09a 1.28 ± 0.60A 1.16 ± 0.05α 0.37 ± 0.01П 0.43 ± 0.01c 0.76 ± 0.03C 0.81 ± 0.01γ 0.24 ± 0.02Ф
2.59 ± 0.06b 1.56 ± 0.01B 2.45 ± 0.01β 0.35± 0.01П 0.55 ± 0.01d 1.75 ± 0.01D 0.86 ± 0.01δ 0.41± 0.02Ѓ
5.27 ± 0.03e 4.46 ± 0.06E 4.29 ± 0.03ε 9.69 ± 0.14Ω 4.84 ± 0.05f 2.44 ± 0.02F 2.37 ± 0.03η 8.43 ± 0.03Γ
5.31 ± 0.02e 4.61 ± 0.01E 4.36 ± 0.01ζ 9.70± 0.04 Ω 4.93 ± 0.04f 2.57 ± 0.03F 2.43 ± 0.01η 8.73± 0.06 Γ
0.67 ± 0.01e 0.87 ± 0.18C 0.85 ± 0.14δ 0.37 ± 0.02П 1.11 ± 0.01f 0.62 ± 0.01F 1.02 ± 0.06α 0.26 ± 0.01Ф
0.68 ± 0.01e 0.96 ± 0.01E 1.00 ± 0.01α 0.44± 0.01Ѓ 1.15 ± 0.01f 0.78 ± 0.01C 1.07 ± 0.03α 0.45± 0.01Ѓ
SC
1 day
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650
n
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550
Rice Crayfish Potato Albumen Rice Crayfish Potato Albumen Rice Crayfish Potato Albumen Rice Crayfish Potato Albumen
η1 (Pa·s)
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450
Protein system
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Oil Concentration (g·kg-1)
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Table 2
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ACCEPTED MANUSCRIPT Highlights Oil-in-Water emulsions were stabilized by four different protein concentrates
•
Oil content influenced emulsion viscosity depending on the nature of the protein used
•
Rice-based emulsions showed the lowest viscosity regardless of oil concentration
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Higher oil concentration produced an increase in the viscosity of emulsions
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Albumen-based and potato-based emulsions showed the highest and lowest Droplet
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S0023-6438(17)30469-3
DOI:
10.1016/j.lwt.2017.07.008
Reference:
YFSTL 6366
To appear in:
LWT - Food Science and Technology
Received Date: 14 February 2017 Revised Date:
25 June 2017
Accepted Date: 1 July 2017
Please cite this article as: Romero, A., Felix, M., Perez-Puyana, V., Choplin, L., Guerrero, A., Use of a mixer-type rheometer for predicting the stability of O/W protein-based emulsions, LWT - Food Science and Technology (2017), doi: 10.1016/j.lwt.2017.07.008. 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.
ACCEPTED MANUSCRIPT 1
Use of a mixer-type rheometer for predicting the stability of O/W
2
protein-based emulsions A. Romeroa,*, M. Felixa,V. Perez-Puyanaa, L. Choplinb, A. Guerreroa a
Departamento de Ingeniería Química, Universidad de Sevilla, Facultad de Química,
4
41012 Sevilla, Spain b
5
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GEMICO, ENSIC, Université de Lorraine, Nancy, France
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Corresponding autor:
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*A. ROMERO
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Departamento de Ingeniería Química,
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Universidad de Sevilla, Facultad de Química,
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41012 Sevilla (Spain)
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E-mail: [email protected]
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Phone: +34 954557179; fax: +34 954556447.
14
Abstract
15
The present work illustrates the feasibility of performing Oil-in-Water (O/W) emulsions
16
stabilized by different protein concentrates, as well as predicting the likelihood of
17
emulsion destabilization over ageing time just after its preparation. To achieve this
18
objective, four protein sources (rice, crayfish, potato and albumen) and four oil
19
concentrations (450, 550, 650 and 750 g·kg-1) were used. The emulsification process
20
was monitored by the use of a mixer-type rheometer. This rheometer was a valuable
21
tool for understanding and controlling the emulsification process through the
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measurement of the viscosity of the different systems during the emulsification stage.
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ACCEPTED MANUSCRIPT Results reveal the importance of controlling the emulsification process to optimise the
24
properties of the final emulsion, which is highly influenced by the oil concentration.
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Then, emulsions were characterized by means of flow properties and droplet size
26
distribution (DSD). Eventually, a relationship was found that relates the rheological
27
properties and the microstructure of the final emulsions during and after emulsification
28
stage. These measurements have been demonstrated to be useful in order to predict
29
the stability of protein-based emulsions.
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Keywords: Droplet size distribution; Emulsification; Oil concentration, Proteins;
31
Viscosity.
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1. Introduction
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An emulsion is a mixture of two immiscible liquids in which one is dispersed in the other
34
in the form of droplets. Common food emulsions are formed by a mixture between oil
35
and water, and are called oil-in-water (O/W) emulsions or water-in-oil emulsions (W/O),
36
depending on the dispersed or continuous phase.The final properties (i.e. stability,
37
texture or droplet size) of an O/W emulsion strongly depend on the specific
38
characteristics of each compound of an emulsion: the dispersed phase (oil), the
39
continuous phase (water) and the interface. Each one of them has its own complexity,
40
which should be analysed to obtain useful information about further emulsion
41
properties. Among them, the interface became of particular interest in these types of
42
kinetically-stabilized systems. The O/W interface of a food is usually stabilized by
43
proteins, low-molecular weight emulsifiers (mainly monoglycerides, phospholipid and
44
esters) or a combination of them (McClements, 2004).Characteristics of the interface
45
depend on the type and concentration of the emulsifier, which tends to be adsorbed at
46
interfacedue to its amphiphilic character. Proteins are widely used as emulsifier in food
47
emulsions, being considered as a functional ingredient in the formation and
48
stabilization of food emulsions and foams (McClements, 2004; Taherian, Fustier, &
49
Ramaswamy, 2006). Proteins are quickly adsorbed at O/W interface of the droplet
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ACCEPTED MANUSCRIPT formed and form a film which protects droplets against destabilization phenomena
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(Dickinson & McClements, 1995; Foegeding & Davis, 2011; Law & Kennedy, 1999).In
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recent years, egg yolk has been replaced by alternative protein systems such as
53
vegetable proteins, mainly soybean and wheat, or, even animal proteins systems.
54
Among other alternatives, rice, potato and crayfish protein systems could be
55
mentioned.
56
Rice industry produces every year a large amount of by-products which involve an
57
undesirable environmental effect. Thus, around 100 million tons of them are produced
58
every year around the world (Li, Liu, Liao, & Yan, 2010). Unfortunately, these by-
59
products are used in low-value-added applications, being mainly incinerated to obtain
60
electricity or used for animal feeding (Njie & Reed, 1995).Another by-product from food
61
industry comes from the potato protein corresponding to the potato skin. Potato protein
62
concentrate from industrial wastes are generally subjected toextreme conditions to
63
increase the protein solubility, as a consequence, proteins are extensively denaturated.
64
However, some emulsifying of these potato concentrates have been found (Romero et
65
al., 2011; van Koningsveld et al., 2006). On the other hand, protein surpluses not only
66
may come from plants but also they may derive from an animal source. For example,
67
proteins form the crayfish Procambarus clarkii. It was introduced in Andalusia (southern
68
region of Spain) some years ago and suffered a fast widespread growth, being
69
considered as an invasive species (Kirjavainen & Westman, 1999). Nowadays, a
70
strong local industry produces a big amount of surpluses and wastes. Emulsions have
71
been made from this protein surplus (Felix, Romero, & Guerrero, 2017; Romero,
72
Cordobes, & Guerrero, 2009).
73
During the emulsification process, the strain and rupture of the droplets are controlled
74
by a local capillary number (Ca), which establishes the relationship between the shear
75
stress and the interfacial tension. Hence, there is a critical value for this relationship
76
that depends on the viscosity ratio of disperse and continuous phases. Above the
77
critical value, shear forces exceed surface tension and the breakup of droplets takes
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ACCEPTED MANUSCRIPT place. However, below this critical value the recoalescence process is more relevant
79
(Janssen & Meijer, 1995).
80
On the other hand, the stability of an emulsion is the key factor considered for
81
performing an emulsion since these are only kinetically stable. An emulsion is stable
82
when the number, size distribution and spatial distribution of droplets do not change
83
over time. Thus, the destabilization of emulsions mainly depends on the initial size of
84
droplets, the rheology of the continuous phase and the interactions among particles,
85
which are responsible for the flocculation and further coalescence of droplets (Sahin &
86
Sumnu, 2006). Destabilization phenomena are related to the presence of attractive
87
interactions (van der Waals), electrostatic repulsions and steric interactions. Then, if
88
attractive interactions are weak, droplets tend to form a reversible structure which
89
favours the stability. However, if attractive forces are strong the destabilization of
90
emulsions may take place through the droplet flocculation and further coalescence
91
(Tadros, 2013). At equilibrium, proteins adsorbed at O/W are able to form a film which
92
has a double function: On one hand, it favours the electrostatic repulsions. In fact, high
93
values of electrostatic potentials are desired for the stability of this kind of dispersed
94
system (Franco, Guerrero, & Gallegos, 1995). On the other hand, the relatively high
95
film thickness that may act as a mechanic barrier which avoids the droplet
96
coalescence, due to its viscoelastic properties (Tadros, 2013; W. N. Zhang,
97
Waghmare, Chen, Xu, & Mitra, 2015).
98
Apart from the colloidal stability, the rheology of emulsions is influenced by several
99
structural parameters, such as interparticle interactions, particle size, shape and
100
polydispersity, continuous phase viscosity, etc. (Martínez, Partal, Muñoz, & Gallegos,
101
2003). Among them, the nature of the interactions among particles has particular
102
relevance. Moreover, other parameters such as the size and the shape of particles, the
103
polydispersity and rheology of the continuous medium also determine the rheological
104
response significantly (Ma & Barbosa-Cánovas, 1995). The stability of an emulsion can
105
be determined through the use of rheological techniques, since the destabilization
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107
inversion or sedimentation provoke changes in the rheological properties of emulsions
108
(Sanchez, Berjano, Guerrero, Brito, & Gallegos, 1998; Tadros, 2013). Rheological
109
measurements are useful because it is possible to relate these properties withthe
110
microstructure of the emulsions, allowing a prediction of emulsion stabilities. Especially
111
interesting are flow measurements in steady and transitory states. Thus, since the first
112
one contributes to describe the real response of emulsions during its transport, the
113
second one reflects the way in which the structural construction or destruction takes
114
place, reflecting also the shear effect (Gallegos, Franco, & Partal, 2004; Guerrero,
115
Partal, Berjano, & Gallegos, 1996; Mezger, 2006). More specifically, the study of the
116
rheological properties during the emulsification stage allows getting knowledge about
117
the shear-induced droplet formation. This characterisation at an early stage is of crucial
118
importance to predict the properties and the long-term stability of the final emulsions.
119
In a previous manuscript, the mixer-type rheometer was used as a tool to evaluate the
120
influence of agitation speed, oil and protein concentrations as well as pH on the
121
properties of egg albumen-based emulsions over emulsification (Romero, Perez-
122
Puyana, Marchal, Choplin, & Guerrero, 2017). The aim of this study was to compare
123
the use of four different proteins (rice, crayfish, potato and albumen) in mayonnaise
124
type O/W emulsion stabilization at four different oil concentrations (450, 550, 650 and
125
750 g oil per kg emulsion) over and after processing by a mixer-type rheometer, as well
126
as predicting the likelihood of emulsion destabilization over ageing time just after its
127
preparation. Therefore, characterization of the emulsions, particularly its viscosity and
128
particle size distribution, was carried out to accomplish this objective.
129
2. Materials and methods
130
2.1. Materials
131
Different protein systems were used. The rice protein concentrate, from rice husks, was
132
provided by Remy Industries (Leuven-Wijgmaal, Belgium), the crayfish flour was
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134
supplied by Protastar (Reus, Barcelona, Spain) and blbumen protein isolate was
135
delivered by Proanda (Barcelona, Spain). Table 1 shows the composition for each
136
protein concentrate system supplied. Sunflower oil was purchased in a local market
137
and used without further purification. In addition, more information about molecular
138
weight distribution, isolelectric point and protein solubility as a function of pH values as
139
well as interfacial properties of these protein systems can be found in the literature
140
(Felix, Martin-Alfonso, Romero, & Guerrero, 2014; Romero et al., 2012; Romero,
141
Beaumal, et al., 2011; Romero, Cordobes, Guerrero, & Cecilia Puppo, 2011).
142
2.2. Emulsification Process
143
Emulsions were prepared in a mixer-type rheometer. This device consists of a
144
cylindrical vessel equipped with a double helical ribbon impeller installed in a RS150
145
controlled stress rheometer, as described by (Nzihou, Bournonville, Marchal, &
146
Choplin, 2016). Different O/W protein-based emulsions were prepared by means an
147
Ultra Turrax T-25 homogenizer from IKA at 15,000 rpm to apply mechanical energy for
148
the emulsification process. In fact, this homogenizer was introduced into the mixer-type
149
rheometer. At the same time, the mixer-type rheometer, rotating at 5 rad/s, was used in
150
order to evaluate the evolution of viscosity. These values (agitation and rotation speed
151
correspond to homogenizer and mixer-type rheometer, respectively) were selected
152
according to previous studies (Romero et al., 2017). On the other hand, the final pH of
153
all emulsions was 3.0, and the protein concentration used was 30 g·kg-1. These values
154
(pH and protein concentrations) correspond to typical values found in food emulsions
155
and were also selected according to previous studies (Romero et al., 2017). Emulsions
156
were characterized in terms of the influence of the type of protein concentrate used as
157
well as the oil concentration (450, 550, 650 and 750 g·kg-1). The emulsification
158
sequence consists of 30 s with only Ultra Turrax (UT) agitation, 420 s with UT
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ACCEPTED MANUSCRIPT homogenization and oil addition, followed by 60 s with UT agitation and 290 s without
160
homogenization in order to stabilize the viscosity value.
161
2.3. Characterization of emulsions
162
2.3.1. Rheological measurements
163
During emulsification
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A mixer-type rheometer was used to determining rheological changes over the
165
emulsification process, more specifically, viscosity (η) along the emulsification time.
166
This mixer-type rheometer has been already validated for the in-situ measurement of
167
the viscosity during the emulsification process (Romero et al., 2017). Note that
168
parameters are included in an analytical method, based on the Couette analogy, which
169
allows as to extract absolute viscosity-shear rate data in non-conventional geometries
170
(Aït-Kadi, Marchal, Choplin, Chrissemant, & Bousmina, 2002; Nzihou et al., 2016).
171
After emulsification
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Steady state shear flow tests were carried out in a RF II rheometer from
173
Rheometric Scientific (USA) the day after emulsion preparation and 30 days later. The
174
measurements were performed at 25 ºC from 0.1 to 10 s-1and the geometry used was
175
parallel plates (dia: 25 mm) with a rough surface, avoiding sample slide, and a gap
176
between plates of 1 mm.The data obtained were fitted to a Power-law model (Ostwald
177
de Waele), obtaining the viscosity ( ) as a function of the shear rate ( ). The following
178
expression was used for this model:
180
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Where
and
=
1
are the consistency and flow index, respectively. The value of viscosity,
at 1 s-1 was selected in order to compare emulsions.
181
2.3.2. Droplet Size Distribution (DSD) measurements
182
Measurements of DSD were performed in a laser light scattering apparatus (Malvern
183
Mastersizer 2000, UK). For this purpose, 0.5 mL of emulsion was taken and diluted in
184
11.5 of 0.05 mol/L, pH Tris-HCl buffer with 1 g SDS/ 100 g of buffer. SDS was used as
7
ACCEPTED MANUSCRIPT 185
a disaggregating material in order to facilitate disruption of floccules (drops
186
agglomeration), often achieved by repulsive droplet/droplet interactions (Hasenhuettl &
187
Hartel, 2008; Nzihou et al., 2016). Values of the Sauter mean diameter (d3,2) which is
188
inversely proportional to the specific surface area of droplets, were obtained as follows: =
∑ ∑
2
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,
189
Where
190
Uniformity parameter ( ) also was calculated from DSD measurements. This
191
parameter is an index of polydispersity of the DSD and it is defined as follows:
∑ |
, 0,5 − , 0,5 ∑
|
(3)
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=
.
SC
is the number of droplets with a diameter
, 0,5 is the median for the distribution, and
192
Where
193
a diameter
194
Both studies (rheological and DSD measurements)were carried out along time (1 and
195
30 days after emulsification) in order to study the effect ofprotein concentrates and oil
196
concentration on the emulsion stability. Emulsions were stored at 5 ºC, being subjected
197
to the same thermorheological history before performing any tests.
198
2.4. Statistical analysis
199
Three replicates of each rheological and DSD measurement as well as emulsification
200
process were carried out. All the data were reported as means ±95% confidence limits.
201
Statistical analyses were performed using t-test and one-way analysis of variance
202
(ANOVA, p<0.05).
203
3. Results
204
3.1. Emulsification process
205
Figure 1 shows the results of viscosity obtained from the mixer-type rheometerover
206
emulsification process for all the protein systems studied (rice, crayfish, potato and
207
albumen) at 650 g·kg-1 oil concentration. As observed, the viscosity of emulsions has a
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is the volume of droplets with
8
ACCEPTED MANUSCRIPT quite marked dependence on the emulsification stage. Thus, over the first 30 s, where
209
the only component is the protein concentrate dispersed into water, the viscosity does
210
not change over mixing time. In fact, no significant differences between proteins
211
solutions were found. However, after starting the addition of oil (from 30s), the viscosity
212
of the systems increased in each case until reaching a maximum value. In this stage,
213
two different behaviours can be found. On one hand, rice and albumen-based
214
emulsions reached a plateau value before finishing the oil addition. On the other hand,
215
potato and crayfish-based emulsions increase their viscosity as the amount of oil
216
increases. The following 60 s with UT agitation did not yield any significant change in
217
emulsion’s viscosity with the exception of the crayfish-based system that suggested
218
that longer emulsification time would be required. Finally, the latest 290 s without
219
homogenization induce a decrease in viscosity only for the potato-based emulsion,
220
probably as a consequence of a damping process after the significant rise during the oil
221
addition. The profiles of viscosity reveal significant differences not only on the
222
emulsification stage, but also on the protein used. Thus, the viscosity of the potato-
223
based emulsion is the highest, whereas the one found for rice-based emulsions is the
224
lowest. Crayfish and albumen-based emulsions showed an intermediate behaviour.
225
These differences are probably due to not only the different protein amount and type of
226
protein but also the presence of other components in the composition (i.e. lipids or
227
starches) as well as the different behaviour at the pH value used (3.0). Anyway,
228
viscosity of emulsions is a key factor for the stability of these systems, since it
229
decrease the destabilization kinetic (Tadros, 2013). Thus, systems with higher viscosity
230
would lead to emulsions with higher long-term stability. In this sense, depending on the
231
protein system, it is possible to optimize the emulsification time as the maximum
232
viscosity is reached.
233
3.2. Emulsion characterisation
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ACCEPTED MANUSCRIPT Figure 2 shows flow curves for all protein-based emulsions studied (rice, crayfish,
235
potato and albumen) at four different oil concentration: 450 g·kg-1 (A), 500 g·kg-1(B),
236
650 g·kg-1 (C) and 750 g·kg-1 (D) the day after emulsion preparation. All steady state
237
flow curves exhibit a very shear-thinning behaviour, which is characteristic for O/W
238
emulsions (Tadros, 2004). As for the influence of the dispersed phase, this figure
239
reveals that the concentration of the oil has a marked effect on the viscosity of the
240
system. Thus, an increase in oil concentration induces an increase in the viscosity of
241
the protein-based emulsions in any case. This oil-dependence of the viscosity has
242
been previously found by other authors (Pearce & Kinsella, 1978; Taherian et al.,
243
2006), and it is related to the increase of droplet flocculation. The increase of oil
244
droplets favours the formation of droplets floccules which do not allow the free
245
movement of droplets, increasing the emulsion viscosity (Pal, 1992). In addition, the
246
increase of viscosity due to the increase in oil concentration induces an increase in the
247
degree of shear-thinning behaviour (lower flow indices)(Pal, 2000), which can be
248
attributed to the structuration of the protein-based emulsions. As regards the influence
249
of protein nature, Figure 2 also evidences the high influence of the protein used on the
250
final viscosity of the emulsion. Thus, rice-based emulsions are the less viscous in any
251
case, although the carbohydrate concentration, mainly starches, could be expected to
252
increase the emulsion viscosity. However, although the viscosity of emulsions depends
253
on the nature of the protein used, the viscosity mainly depends on the oil concentration.
254
In this sense, it may point account that the crayfish and albumen-based emulsions
255
contain a more percentage of oil due to the presence of lipids in the composition, but
256
this increase is always lower than 1% of the total lipid content. On the other hand, the
257
presence of lipids could lead to a relevant surface activity but lipids in the samples
258
have been isolated and checked that these do not show relevant surface activity.
259
Figure 3 shows DSD for all protein-based emulsions analysed (rice, crayfish, potato
260
and albumen) at four different oil concentrations: 450 g·kg-1 (A), 550 g·kg-1 (B), 650
261
g·kg-1 (C) and 750 g·kg-1 (D). Generally, DSD profiles exhibit unimodal profile, whose
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ACCEPTED MANUSCRIPT maximum value and peak width depends on the oil concentration and protein system
263
used. The increase of oil tends to forward the peak, which in turn means the formation
264
of smaller droplets. The DSD profile is highly influenced by the protein used, since as a
265
general rule, albumen-based emulsions have the highest sizes and potato based
266
emulsions have the lowest ones. The results obtained of viscosity can be related to the
267
DSD, since generally, higher viscosity values induce lower droplet sizes. These results
268
are related with the Capillary number described in the introduction section, where it
269
was described that the viscosity of the continuous phase influence the shear stress
270
applied, limit the recoalescence process and, consequently, decrease the droplet sizes.
271
In fact, the viscosity of shear-thinning emulsions is strongly influenced by the droplet
272
size; a significant increase in the viscosity occurs when the droplet size is reduced.
273
However, these emulsions showed higher viscosity in comparison with Pal (2000)
274
using surfactant as emulsifier. These deviations can be attributed to the higher
275
viscosity of the continuous phase of protein solutions (proteins and higher emulsifier
276
concentration was used). On the other hand, the use of different amounts of oil seems
277
to indicate that there is a more uniformity profile at intermediate oil concentration, which
278
depends on the protein system. Thus, potato and rice-based emulsions exhibit wider
279
peaks when the lowest or the highest oil concentration (450 or 750 g·kg-1) are used.
280
3.3. Emulsion stability
281
Table 2 shows parameters from flow curves (flow index,
282
corresponding to the viscosity at 1 s-1 shear rate), as well as parameters from DSD
283
measurements (Sauter diameter, d3,2, and uniformity parameter,
284
parameters were evaluated for all protein-based emulsions studied (rice, crayfish,
285
potato and albumen) at four different oil concentration (450, 550, 650 and 750 g·kg-1)
286
the day after the emulsion preparation and after 30 days.
287
The viscosity (
288
concentration. The increase in oil provokes a marked increase in emulsion viscosity.
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, and viscosity,
,
). All these
) confirms the high dependence of the emulsions on the oil
11
ACCEPTED MANUSCRIPT This increase in emulsion stability involves lower changes over time, since higher initial
290
values of viscosity tends to have shorter changes of relative viscosity values over
291
ageing time. However, viscosity is a very sensitive parameter in order to estimate
292
emulsion stability. As may be observed, as a general rule, the flow index ( ) tends to
293
increase over the ageing time, which indicates that the shear-thinning behaviour
294
decreases. Therefore, the destabilization processes (mainly coalescence), which
295
emulsions suffers over storage time, induce the loss of structure, decreasing not only
296
the viscosity but also the shear-thinning behaviour (higher n values).
297
As for d3,2 diameter from DSD profiles, Table 2 confirms that the Sauter diameter
298
depends on the protein used. However, in any case, as it could be deduced in Figure 3,
299
the increase in oil concentration induces the decrease of droplet sizes and,
300
consequently, leads to emulsions are more stable over the ageing time studied. In fact,
301
when the 750 g·kg-1 oil concentration is used, no significant changes in droplet sizes
302
are observed for all the protein systems evaluated. This long term stability is related to
303
initial high viscosity value, as well as lower droplet sizes. Moreover, the two protein-
304
based emulsions whose viscosity increased until finishing the oil addition (crayfish and
305
potato) always lead lower d3,2 diameter, confirming the relation between the rheological
306
properties of the emulsification stage and the microstructure of the final emulsions. In
307
addition, in general, when the viscosity of emulsion measured in mixer-type rheometer
308
is higher than approximately 50 Pa·s, emulsions were stable after 30 days, according
309
to Sauter diameter results. On the other hand, interestingly, emulsion stability is
310
confirmed by the uniformity parameter ( ). The lowest values were obtained for
311
albumen-based emulsions, apart from them, the increase in oil concentration generally
312
leads to lower
313
general rule, the more unimodal emulsions (low
314
are, since polydispersity systems are considered unstable since larger droplets grow
315
up at the expense of smaller droplets (Amid & Mirhosseini, 2012; Y. Zhang et al.,
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values, which are related to the thinner peaks above mentioned. As a values), the more stable emulsions
12
ACCEPTED MANUSCRIPT 316
2008). In fact,
317
emulsions, at the most oil concentrated emulsions studied (750 g·kg-1),
318
does not generally exhibit the lower values but evidencing the lower evolution, as a
319
consequence of the increase of emulsions stability found. However, once again, in
320
these systems, it is demonstrated that stability of emulsions seems to be governed by
321
the viscosity, as a consequence of the increase in oil and this parameter (
322
be the more appropriate in order to predict emulsion destabilization since their change
323
are more evident and significant between different emulsions evaluated.
324
4. Conclusions
325
Results confirm the high importance of the characterisation of the emulsification stage
326
in order to follow the emulsion viscosity. Thus, it is possible to optimize the
327
emulsification time with the corresponding power saving, selecting the proper time and
328
oil content in order to reach a proper viscosity. In fact, the increase of oil addition leads
329
to a non-lineal increase in viscosity, because a lot of new droplets are not formed.
330
Therefore, the use of a mixer-type rheometer allows us to reach final emulsions with
331
proper properties, rheological properties and microstructure, which are related to the
332
stability of the emulsions.
333
The long term stability is related to the increase of emulsion viscosity found during
334
emulsification, as well as the decrease of droplet sizes. Thus, the initial high viscosity
335
(after emulsification) and small droplet sizes are related to more stable emulsions. In
336
this sense, on one hand, the nature of the protein used highly affects to the viscoelastic
337
and microstructure of the final emulsions since the protein system can affect the
338
viscosity of emulsion. On the other hand, the analysis of the oil amount revels that the
339
increase in oil tends to increase the viscosity of emulsions, and decrease the Sauter
340
diameter increasing the stability of final emulsions, being the viscosity the most
341
sensitive parameter in order to predict the emulsion stability.
parameter usually increases over aging time. However, for these
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parameter
AC C
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) seems to
13
ACCEPTED MANUSCRIPT Although mixer-type rheometer has been demonstrated as a useful tool in order to
343
optimize emulsions stabilized by different protein systems and oil concentration,
344
although further research will be necessary taking into account other factors such as
345
the protein concentration, the influence of other minor components or the pH value.
346
5. Acknowledgements
347
This work is part of a research project sponsored by “Ministerio de Economía y
348
Competitividad”,
349
MINECO/FEDER, EU) and University of Seville for the grant of the VPPI-US. The
350
authors gratefully acknowledge their financial support.
351
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FIGURE CAPTIONS
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Figure 1: Rheological characterization of o/w emulsions over emulsification stage at
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650 g·Kg-1 oil concentration and for all protein systems studied:
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potato and
rice,
crayfish,
albumen.
Figure 2: Flow curves for all protein-based emulsions studied ( potato and
crayfish,
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rice,
albumen) at four different oil concentrations: (A) 450 g·kg-1, (B) 550
g·kg-1, (C) 650 g·kg-1 and (D) 750 g·kg-1 the day after preparation.
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Figure 3: Droplet Size Distributions (DSD) for all protein-based emulsions analysed (
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rice,
crayfish,
potato and
albumen) at four different oil
concentrations: (A) 450 g·kg-1, (B) 550 g·kg-1, (C) 650 g·kg-1 and (D) 750 g·kg-1.
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TABLE CAPTIONS
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Table 1: Composition of different protein type systems: Rice, Crayfish, Potato and
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Albumen.
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Table 2: Parameters from flow curves (
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and ) as well as from DSD profiles (d3,2 and
) for all protein-based emulsions studied (rice, crayfish, potato and albumen) at four
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different oil concentrations (450, 550, 650 and 750 g·kg-1) the day after the emulsion
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preparation and after a storage of 30 days.
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ACCEPTED MANUSCRIPT Table 1
Protein (g·kg-1)
Carbohydrates (g·kg-1)
Ash (g·kg-1)
Lipids (g·kg-1)
Moisture (g·kg-1)
Rice
782 ± 12
64 ± 3
52 ± 1
24 ± 3
78 ± 4
Crayfish
647 ± 12
2±1
134 ± 2
183 ± 8
34 ± 3
Potato
801 ± 23
59 ± 6
8±1
Albumen
730 ± 11
4±1
60 ± 3
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Protein System
101 ± 2
130 ± 5
76 ± 2
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750
d3,2 (µm)
30 days
1 day
30 days
0.3 ± 0.1a 6.9 ± 0.1A 6.1 ± 0.2α 23.1 ± 0.2П 3.9 ± 0.1c 50.4 ± 0.2C 43.2 ± 0.4γ 46.9 ± 0.3Ѓ
0.1 ± 0.1b 2.9 ± 0.2B 2.4 ± 0.1β 14.5 ± 0.2Ф 2.1 ± 0.1d 19.5 ± 0.3D 32.3 ± 0.5δ 38.8 ± 0.2Σ
21.7 ± 0.2e 91.3 ± 0.5E 202.2 ± 2.6ε 90.1 ± 0.3Ω 138.9 ± 5.8g 460.9 ± 2.7G 282.3 ± 3.6η 179.7 ± 4.5Γ
13.8 ± 0.7f 43.0 ± 0.5F 174.4 ± 4.3ζ 71.6 ± 0.7Θ 119.5 ± 3.7h 330.5 ± 2.4H 267.9 ± 2.9 η 132.8 ± 2.2Ξ
0.47 ± 0.02a 0.40 ± 0.04A 0.41 ± 0.01α 0.34 ± 0.03П 0.37 ± 0.02b 0.31 ± 0.02B 0.21 ± 0.02γ 0.19 ± 0.02Ѓ 0.27 ± 0.02c 0.35 ± 0.01C 0.31 ± 0.02δ 0.09 ± 0.01Ω 0.31 ± 0.03c 0.16 ± 0.04D 0.38 ± 0.02ε 0.05 ± 0.01Γ
0.49 ± 0.03a 0.37 ± 0.01A 0.49 ± 0.02β 0.44 ± 0.02Ф 0.37 ± 0.03b 0.36 ± 0.02C 0.24 ± 0.02 γ 0.24± 0.01Ѓ 0.32 ± 0.02c 0.28 ± 0.02C 0.34 ± 0.03δ 0.13 ± 0.01Θ 0.32 ± 0.02c 0.25 ± 0.03D 0.37 ± 0.02ε 0.09 ± 0.02Ξ
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13.42 ± 0.04a 7.37 ± 0.19A 6.89 ± 0.05α 16.79 ± 0.19П 7.43 ± 0.02c 5.29 ± 0.08C 5.73 ± 0.04γ 13.70 ± 0.63Ѓ
18.72 ± 0.52b 15.01 ± 0.50B 20.12 ± 0.04β 22.99 ± 0.45Ф 8.10 ± 0.06d 6.34 ± 0.01D 6.46 ± 0.01δ 15.27 ± 0.34Ѓ
1.83 ± 0.09a 1.28 ± 0.60A 1.16 ± 0.05α 0.37 ± 0.01П 0.43 ± 0.01c 0.76 ± 0.03C 0.81 ± 0.01γ 0.24 ± 0.02Ф
2.59 ± 0.06b 1.56 ± 0.01B 2.45 ± 0.01β 0.35± 0.01П 0.55 ± 0.01d 1.75 ± 0.01D 0.86 ± 0.01δ 0.41± 0.02Ѓ
5.27 ± 0.03e 4.46 ± 0.06E 4.29 ± 0.03ε 9.69 ± 0.14Ω 4.84 ± 0.05f 2.44 ± 0.02F 2.37 ± 0.03η 8.43 ± 0.03Γ
5.31 ± 0.02e 4.61 ± 0.01E 4.36 ± 0.01ζ 9.70± 0.04 Ω 4.93 ± 0.04f 2.57 ± 0.03F 2.43 ± 0.01η 8.73± 0.06 Γ
0.67 ± 0.01e 0.87 ± 0.18C 0.85 ± 0.14δ 0.37 ± 0.02П 1.11 ± 0.01f 0.62 ± 0.01F 1.02 ± 0.06α 0.26 ± 0.01Ф
0.68 ± 0.01e 0.96 ± 0.01E 1.00 ± 0.01α 0.44± 0.01Ѓ 1.15 ± 0.01f 0.78 ± 0.01C 1.07 ± 0.03α 0.45± 0.01Ѓ
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Rice Crayfish Potato Albumen Rice Crayfish Potato Albumen Rice Crayfish Potato Albumen Rice Crayfish Potato Albumen
η1 (Pa·s)
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ACCEPTED MANUSCRIPT Highlights Oil-in-Water emulsions were stabilized by four different protein concentrates
•
Oil content influenced emulsion viscosity depending on the nature of the protein used
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Rice-based emulsions showed the lowest viscosity regardless of oil concentration
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Higher oil concentration produced an increase in the viscosity of emulsions
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Albumen-based and potato-based emulsions showed the highest and lowest Droplet
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