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8/10/2019 Beverage Emulsions Recent Developments in Formulation Production and Applications
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Accepted Manuscript
Beverage Emulsions: Recent Developments in Formulation, Production, and
applications
Daniel T. Piorkowski, David Julian McClements
PII: S0268-005X(13)00211-7
DOI: 10.1016/j.foodhyd.2013.07.009
Reference: FOOHYD 2311
To appear in: Food Hydrocolloids
Received Date: 13 May 2013
Revised Date: 8 July 2013
Accepted Date: 9 July 2013
Please cite this article as: Piorkowski, D.T., McClements, D.J., Beverage Emulsions: Recent
Developments in Formulation, Production, and applications,Food Hydrocolloids(2013), doi: 10.1016/
j.foodhyd.2013.07.009.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
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Graphical Abstract
BEVERAGE EMULSIONS: RECENT DEVELOPMENTS IN FORMULATION,
PRODUCTION, AND APPLICATIONS by D.T. Piorkowski and D.J. McClements
Food Hydrocolloids
The article provides an overview of recent research on the formation, stability, and
properties of beverage emulsions.
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BEVERAGE EMULSIONS:RECENT DEVELOPMENTS IN1
FORMULATION,PRODUCTION,AND APPLICATIONS2
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Daniel T. Piorkowski and David Julian McClements14
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Department of Food Science, University of Massachusetts, Amherst, MA 010036
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Journal:Food Hydrocolloids20
Submitted: March 201321
Revised: July 201322
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Abstract26
Soft drinks are one of the most widely consumed and profitable beverages in the world.27
This review article focuses on the utilization of emulsion science and technology for the28
fabrication of soft drinks by the beverage industry. A brief overview of the various high and low29
energy methods available for preparing this type of beverage emulsions is given, as well as a30discussion of the functional ingredients used to formulate these systems, including oil phases,31
emulsifiers, weighting agents, ripening inhibitors, and thickening agents. The influence of32
droplet characteristics on the physicochemical and sensory properties of beverage emulsions is33
reviewed, with special focus on their influence on product stability. Finally, we discuss recent34
developments in the soft drinks area, including fortification with vitamins, reduced calorie35
beverages, and all-natural products.36
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Keywords:beverages; soft drinks; nutraceuticals; flavors; emulsions; nanoemulsions38
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1. Introduction40
Globally, soft drinks are one of the most widely consumed and profitable beverages (Table41
1). Cola is the top soft drink flavor currently consumed in the United States, with lemon-lime42
and orange being the second and third. All three of these soft drink flavors contain hydrophobic43
citrus compounds extracted from fruit peels. Soft drinks may also contain a variety of other44
hydrophobic components, such as clouding agents, weighting agents, nutraceuticals, oil-soluble45
vitamins, and oil-soluble antimicrobials. The non-polar character of flavor oils and other46
hydrophobic ingredients means that these ingredients cannot simply be dispersed directly into an47
aqueous phase they would rapidly coalesce and separate through gravitational forces leading to48
a layer of oil on top of the product (Given, 2009). Instead they first have to be converted into a49colloidal dispersion consisting of flavor molecules encapsulated within small particles suspended50
within an aqueous medium, e.g., a microemulsion, nanoemulsion, or emulsion (McClements,51
2011; McClements & Li, 2010). These colloidal delivery systems must be carefully designed to52
provide desirable physicochemical, sensory, and biological attributes to the final product. A53
number of desirable attributes of colloidal delivery systems suitable for application in beverage54
products are highlighted below (McClements, Decker, & Weiss, 2007; McClements & Li, 2010):55
Composition:Ideally, the delivery systems should be fabricated entirely from label56
friendly food-grade ingredients that are economic and easy to handle.57
Fabrication:Ideally, the delivery systems should be fabricated using robust, reliable58
and inexpensive manufacturing methods that are easily implemented.59
Stability:The delivery systems should be designed to withstand all of the stresses60
that a product may experience during its production, storage, transport and61
utilization, such as temperature fluctuations, exposure to light and oxygen, exposure62
to mechanical forces (such as stirring flow through a pipe and vibrations)63
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Physicochemical and sensory properties: The delivery system should not adversely67
affect the optical properties, rheology, or flavor profile (aroma, taste, and mouthfeel)68of the beverage product into which it is incorporated.69
Biological activity:The delivery system should not adversely affect the biological70
activity of any encapsulated bioactive components, such as antimicrobials, vitamins,71
or nutraceuticals.72
This review article provides an overview of the current status of the design, formulation, and73production of emulsion-based delivery systems suitable for utilization within the beverage74
industry.75
2. Emulsion Science and Technology in the Beverage Industry76
Hydrophobic components (such as flavor oils, clouding agents, oil-soluble vitamins, and77nutraceuticals) can be incorporated into a variety of different colloidal delivery systems suitable78
for application within beverage products (McClements, 2012; McClements & Rao, 2011), with79
the most common being microemulsions, nanoemulsions, and emulsions (Figure 1). Each of80
these colloidal dispersions has particular benefits and limitations for the encapsulation of81
hydrophobic compounds. Microemulsions are thermodynamically stablesystems under a82
specific set of environmental conditions (e.g., composition and temperature), and are therefore83
easy to fabricate (often by simple mixing) and tend to have good long-term stability.84
Microemulsions typically contain very small particles (r< 25 nm) and therefore tend to be85
optically transparent, which is desirable for soft drinks that should be clear. On the other hand,86
the formation of microemulsions usually requires relatively high levels of synthetic surfactants87
and sometimes the use of cosurfactants/cosolvents, which can be undesirable for cost, taste, and88
labeling reasons. Microemulsions may also become thermodynamically unstable if89
environmental conditions are altered (such as temperature or composition).90
Conventional emulsions (r > 100 nm) and nanoemulsions (r < 100 nm) are both91
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and provide sufficient kinetic stabilitythroughout the lifetime of the product. Emulsions usually96
contain larger droplets than microemulsions and therefore they scatter light more strongly and97appear more turbid or cloudy. This is an advantage for soft drinks that are required to have a98
cloudy appearance, but a disadvantage for products where optical clarity is required.99
Nevertheless, recently it has been shown that emulsions with ultrafine droplets, often referred to100
as nanoemulsions, can be prepared that are optically transparent (McClements, 2012;101
McClements & Rao, 2011). A major advantage of emulsions and nanoemulsions is that the102
emulsifier-to-oil ratio required to formulate them is often much less than that required for103
microemulsions, and they can be formulated from all natural ingredients (such as proteins and104
polysaccharides) rather than synthetic surfactants (such as Tweens). In this article, we focus105
primarily on the utilization of emulsion systems (conventional emulsions and nanoemulsions) in106
the preparation of soft drinks but much of the material is also relevant to the formulation of107
microemulsions.108
It should be noted that the emulsions used in the beverage industry are typically divided into109
two groups:flavoremulsions and cloudemulsions. Flavor emulsions contain lipophilic110
compounds that are primarily present to provide taste and aroma to a beverage product (such as111
lemon, lime, or orange oils). On the other hand, cloud emulsions are used to provide specific112
optical properties to certain beverage products, i.e.,to increase their turbidity (cloudiness).113
Cloud emulsions are typically prepared using an oil phase that is highly water-insoluble and that114
is not prone to chemical degradation, such as flavorless vegetable oils. In addition, the size of the115
droplets within cloud emulsions is designed so that they have dimensions where strong light116
scattering occurs, but are not too large to undergo gravitational separation (e.g., r = 100-200117
nm). Cloud emulsions are often added to beverages that only contain a relatively low percentage118
of juice and provide a desirable cloudy appearance that hides sedimentation and ringing.119
In this article, we will use the term emulsion to refer to both nanoemulsions and120
conventional emulsions because they have similar structures and properties. Generally, an121
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whereas a system that contains water droplets dispersed in oil is called a water-in-oil (W/O)127
emulsion. It is possible to prepare more complex emulsion structures, e.g., oil-in-water-in-oil128(O/W/O), water-in-oil-in-water (W/O/W) or oil-in-water-in-water (O/W/W) emulsions129
(Benichou, Aserin, & Garti, 2004; Garti & Bisperink, 1998; van der Graaf, Schroen, & Boom,130
2005). Currently, almost all of the emulsions used in the beverage industry are of the O/W type,131
although there may be certain advantages to using other emulsion types for certain applications.132
For example, in principle it is possible to trap a hydrophilic bioactive component within the inner133
water phase of a W/O/W emulsion to protect it from chemical degradation or for taste masking.134
In practice, it is often difficult to formulate W/O/W emulsions that have sufficient stability for135
commercial applications, although this is still an active area of research.136
Emulsions are thermodynamically unfavorable systems that tend to break down over time137
though a variety of physicochemical mechanisms, including gravitational separation (creaming138
and sedimentation), droplet aggregation (flocculation and coalescence) and droplet growth139
(Ostwald ripening) (Dickinson, 1992a; Friberg, et al., 2004; McClements, 2005b). It is possible140
to form emulsions that are kinetically stable for a reasonable period of time by including141
substances known as stabilizers, e.g., emulsifiers, weighting agents, ripening inhibitors, or142
texture modifiers. It is important to clearly distinguish the different physicochemical143
mechanisms involved in promoting emulsion stability for these different categories of stabilizers.144
Emulsifiersare surface-active molecules that adsorb to the surface of freshly formed droplets145
during homogenization, forming a protective layer that prevents the droplets from aggregating.146
Weighting agentsare dense hydrophobic components added to low-density oils to prevent147
gravitational separation. Ripening inhibitorsare water-insoluble components added to polar oils148
to prevent Ostwald ripening. Texture modifiersare substances used to increase the viscosity or149
gel aqueous solutions, thereby retarding or preventing droplet movement. A more detailed150
description of different types of stabilizers that can be used in beverage emulsions is given in a151
later section. Selecting the most appropriate stabilizer(s) for a particular application is one of the152
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3. Controlling Droplet Characteristics155
The bulk physicochemical properties of beverage emulsions (such as optical properties,156
stability, rheology, molecular partitioning, and release characteristics) are largely determined by157
the properties of the droplets they contain (McClements, 2005b), such as composition,158
concentration, size, and charge (Figure 3). In this section, we discuss some of the most159
important droplet characteristics that can be controlled by beverage manufacturers in order to160
create products with specific desirable functional properties.161
3.1. Droplet composition162
The composition of the oil phase has a major influence on the formation and stability of163
beverage emulsions, which has often been overlooked in academic research. Beverage164
emulsions may contain a variety of different hydrophobic components, including flavor oils,165
essential oils, triacylglycerol oils, oil-soluble vitamins, nutraceuticals, weighting agents, and166
ripening inhibitors. These components vary in their molecular characteristics (such as molecular167
weight, molecular conformation, and functional groups), which leads to changes in their168
physicochemical properties (such as polarity, water-solubility, density, viscosity, refractive169
index, physical state, and melting point). Many of these molecular and physicochemical170
properties have a major influence on the formation, stability, and functionality of emulsions. For171
example, oil viscosity influences the efficiency of droplet disruption during high energy172
homogenization the closer the ratio of dispersed phase viscosity to continuous phase viscosity173
(D/C) is to unity, the more efficient is droplet disruption and the smaller is the particle size174
produced (Walstra, 1993, 2003). Oil density determines the rate of particle creaming or175
sedimentation within emulsions the greater the density contrast between the droplets and176surrounding fluid, the faster the rate of gravitational separation (McClements, 2005c). Oil177
refractive index determines the efficiency of light scattering by droplets in emulsions the178
greater the refractive index contrast between the droplets and surrounding fluid, the stronger the179
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high energy homogenization decreases as the interfacial tension decreases (Walstra, 1993).185
Second, the rate of droplet coalescence increases as the interfacial tension decreases (Kabalnov186& Wennerstrom, 1996). Third, the ability of emulsifiers to adhere to droplet surfaces decreases187
as the bare oil-water interfacial tension decreases (Chanamai, Horn, & McClements, 2002).188
Finally, the rate of droplet growth due to Ostwald ripening depends on the interfacial tension at189
the oil-water interface (Kabalnov, 2001).190
For flavor emulsions, it is important to control the type and concentration of the flavor191
molecules initially present in the oil phase. It is also important to be aware that the location of192
the flavor molecules within an emulsion is governed by their oil-water partition, which depends193
on carrier oil type (Choi, Decker, Henson, Popplewell, & McClements, 2009; Choi, Decker,194
Henson, Popplewell, & McClements, 2010b). The flavor profile of an emulsion may therefore195
change if the carrier oil type is altered, if the physical state of the carrier oil changes, or if an196
emulsion is diluted, since this will change the distribution of the flavor molecules in the oil,197
water and air (Choi, et al., 2009; Choi, et al., 2010b; Mei, et al., 2010).198
It is important for beverage manufacturers to understand the composition of the oil phases199
used to formulate commercial products, and to understand how specific lipophilic components200
influence the formation, stability, and properties of final products.201
3.2. Droplet concentration202
In general, the concentration of droplets in an emulsion influences its texture, stability,203
appearance, sensory attributes, and nutritional quality (McClements, 2005b; McClements & Rao,204
2011).Droplet concentration is usually characterized in terms of the dispersed phase volume205
fraction (), which is the volume of emulsion droplets (VD) divided by the total volume of206
emulsion (VE): = VD/VE. Practically, it is often more convenient to express the droplet207
concentration in terms of the dispersed phase mass fraction (m), which is the mass of emulsion208
droplets (mD) divided by the total mass of emulsion (mE): m= mD/mE. When the densities of the209
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facilitates handling and transport, but they are highly diluted when they are introduced into the215
final product (< 0.1% oil). The amount to which an emulsion concentrate is diluted influences216the appearance of a final product, since emulsion turbidity or cloudiness increases with oil217
droplet concentration. Dilution also influences the total amount of flavor molecules present in218
final products, as well as their partitioning between the oil and water phases (Choi, et al., 2009).219
In the concentrate, droplet concentration has a major impact on the rheological properties of the220
system. From a practical point of view, it may be important to have a high oil loading in the221
concentrate emulsion so as to reduce transport and storage costs, but not have the oil content so222
high that the product is unstable or cannot easily be dispersed into the final product.223
3.3. Droplet size distribution224
The size distribution of the droplets in a beverage emulsion has a strong impact on its225
physical stability (e.g., to gravitational separation, flocculation, coalescence and Ostwald226
ripening) and its optical properties (e.g., lightness and color) (McClements, 2005b).Beverage227
manufacturers must therefore specify the optimum droplet size distribution required for their228
particular product based on the properties required, e.g., optical clarity and shelf-life. They must229
then develop a formulation and manufacturing process that can reliably produce a beverage with230
this droplet size distribution. Immediately after the product has been manufactured it is usually231important to measure the droplet size distribution to ensure that it has met the specified quality232
criteria, e.g., using light scattering instruments. It may also be important to measure changes in233
the droplet size distribution of the product during storage or after an accelerated storage test to234
predict its long-term stability (McClements, 2007).235
The particle size distribution (PSD) of an emulsion specifies the concentration of droplets236
within different size classes, and can be conveniently measured using various commercially237
available instruments (McClements, 2005b). When presenting or interpreting PSD data on a238
beverage emulsion it is important to pay particular attention to the manner in which the particle239
concentration and particle size are presented The concentration of particles within a particular240
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widespread and informative way of presenting particle size data. Commercial beverage245
emulsions are always polydisperse systems that can be characterized as being "monomodal",246"bimodal" or "multimodal" depending on whether there are one, two, or more peaks in the247
particle size distribution. Typically, beverage manufacturers would like to produce a final248
product that has a narrow monomodal distribution, as this usually provides the best long-term249
stability.250
In many practical situations it is important to have knowledge of the full PSD of a beverage251
emulsion since this contains information about the size characteristics of all of the particles252
present, as well as providing insights into the possible origin and nature of any instability253
mechanisms. For example, it may be possible to detect a small population of large particles that254
may cause problems with creaming during long-term storage (i.e., ringing). In addition, by255
measuring changes in the PSD overtime it is sometimes possible to distinguish between different256
instability mechanisms (e.g.,coalescence versus Ostwald ripening). Nevertheless, in some257
situations it is more convenient to represent the full particle size distribution by a measure of its258
central tendency and spread. The mean, median, or modal particle sizes are often used as259
measures of the central tendency, whereas the relative standard deviation is often used as a260
measure of spread (Walstra, 2003). The mean particle size is the most widely used method of261
representing the central tendency of emulsion particle size distributions in the beverage industry.262
It is important to realize that a number of different mean particles sizes can be derived from263
a full PSD and each mean size can have a different magnitude and physical meaning264
(McClements, 2007). The three most commonly used mean particle sizes are the number-265
weighted mean diameter (dNor d10 = nidi/ ni), the surface-weighted mean diameter (dSor d32 =266
nidi3
/ nidi2
) and the volume-weighted mean diameter (dVor d43 = nidi4
/ nidi3
). Generally, the267
volume-weighted mean diameter is more sensitive to the presence of large particles than the268
number-weighted mean diameter, and so it often provides the most rigorous test of the physical269
stability of a beverage emulsion, i.e., if d43is small then the emulsion is more likely to remain270
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values should be treated with caution when used to represent highly polydisperse emulsions (e.g.,275
aggregated systems), and it is always useful to examine the full particle size distribution.276Commercial beverage manufacturers usually develop a set of standardized particle size277
criteria that they use to determine whether a particular batch of product has the desired278
physicochemical characteristics, e.g., long-term stability and optical properties. For example, a279
manufacturer might specify that that mean droplet diameter (d43) of a particular class of products280
should be < 500 nm, and that > 90% of the droplets should be smaller than 800 nm. The precise281
criteria used will depend on the product being manufactured (especially whether it should be282
clear or opaque).283
3.4. Droplet charge284
The droplets in most beverage emulsions have an electrical charge because of adsorption of285
ionic species to their surfaces, e.g., proteins, ionic polysaccharides, ionic surfactants,286
phospholipids, fatty acids, and some small ions (McClements, 2005b). The electrical287
characteristics of a droplet surface depend on the type, concentration and organization of the288
ionized species present, as well as the ionic composition and physical properties of the289
surrounding aqueous phase. The electrical charge on the oil droplets in a beverage may be290
important for a number of reasons: it determines the stability of the droplets to aggregation due291to its influence of the magnitude, range and sign of electrostatic interactions; it determines the292
interactions of droplets with other charged species in an emulsion e.g., ions (such as calcium or293
iron), or polyelectrolytes (such as proteins or polysaccharides); it influences how the droplets294
interact with electrically charged surfaces, such as storage vessels, bottles, cups, and the mouth;295
it influences the behavior of the droplets in an electrical field, which is important for measuring296
their charge using electrophoresis.297
The electrical characteristics of a droplet in an emulsion are usually characterized in terms298
of its surface charge density (), electrical potential (0), and/or potential () (Hunter, 1986).299
Th f h d i i th t f l t i l h it f hi h300
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the electrical potential decreases with increasing ionic strength due to these effects. Thezeta-305
potential () is the electrical potential at the "shear plane", which is defined as the distance away306from the droplet surface below which the counter-ions remain strongly attached to the droplet307
when it moves in an electrical field. Practically, the -potential is a better representation of the308
electrical characteristics of an oil droplet because it inherently accounts for the adsorption of any309
counter ions or ionic species to the droplet surface. In addition, the -potential is more310
convenient to measure than the surface charge density or electrical potential(Hunter, 1986).311Typically, the electrical characteristics of the droplets in an emulsion are determined by312
measuring the -potential versuspH under appropriate measurement conditions (such as ionic313
composition).314
Droplet aggregation is inhibited in many beverage emulsions by using ionic emulsifiers that315
adsorb to the droplet surfaces and prevent them from coming close together because of316
electrostatic repulsion (Dickinson, 1992b; Friberg, et al., 2004; McClements, 2005b).317
Electrostatic repulsion plays a major role in determining the aggregation stability of fat droplets318
coated by charged emulsifiers that only form thin layers that generate short range steric319
repulsion, such as globular proteins and ionic surfactants. On the other hand, electrostatic320
repulsion is less important in systems where the fat droplets are coated by emulsifiers that form321
thick interfacial layers that generate long range steric repulsion, such as polysaccharides (gum322
arabic and modified starch). For electrostatically-stabilized emulsions, the magnitude of the -323
potential should be greater than about 20 mV to produce systems that are stable during long-term324
storage. For sterically-stabilized emulsions, the droplet charge may not be important in terms of325
their physical stability, but it may still be important in systems where chemical reactions occur326
within the oil droplets that are induced by water-soluble ionic species, such as oxidation of -3327
fatty acids by transition metals.328
3.5. Interfacial properties329
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2011), and is therefore particularly important in beverage emulsions since they usually contain335
droplets considerably smaller than this size . The interfacial region can influence many336important physicochemical and sensory properties of beverages emulsions, including their337
stability, rheology, mouthfeel, and flavor. For this reason, it is often important to have338
knowledge about the interfacial properties of the droplets in a beverage emulsion, and to339
establish the major factors that influence them. Some of the most important properties of the340
interfacial region are: composition; structural organization; thickness; rheology; interfacial341
tension; and charge. These properties are determined by the type, concentration and interactions342
of any surface-active species present, as well as by the events that occur before, during, and after343
emulsion formation, e.g., complexation, competitive adsorption, layer-by-layer formation344
(Dickinson, 2003). As mentioned earlier, the electrical charge on the droplet interface influences345
its interaction with other charged molecules, as well as its stability to aggregation. The thickness346
and rheology of the interfacial region influences the stability of emulsions to gravitational347
separation, coalescence and flocculation, and determines the rate at which molecules leave or348
enter the droplets (Dickinson, 2003; McClements, 2005b). For example, the ability of interfacial349
coatings to prevent droplet flocculation is strongly influenced by their thickness.350
Beverage manufacturers should therefore be aware of the nature of the interfacial region351
surrounding the oil droplets in their products, and the fact that they may be able to manipulate its352
properties to improve product performance.353
3.6. Colloidal interactions354
The attractive and repulsive colloidal interactions that operate between the oil droplets in355
beverage emulsions determine their stability to flocculation and coalescence, which in turn356
influences their creaming stability and rheology (Friberg, et al., 2004; McClements, 2005b). The357
colloidal interactions between two oil droplets can be described in terms of an interaction358
potential (w(h)), which is the energy required to bring two droplets from an infinite distance359
apart to a surface to surface separation of h (Fig re 4) The overall interaction potential is made360
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has a simple dependence on surface-to-surface separation, but the sum of the interactions can365
exhibit a more complex dependence. For example, the interaction potential between two oil366droplets coated by a layer of charged polymer molecules would have a number of maximum and367
minimum values at certain separations, such as short- and long-range energy barriers, and368
primary and secondary minima (Figure 4). Generally, droplets tend to aggregate when attractive369
interactions dominate, but remain as individual entities when repulsive interactions dominate370
(McClements, 2005b).371
It is particularly important for scientists working in the beverage industry to identify and372
understand the major colloidal interactions operating between the droplets in their particular373
product. This knowledge can then be used to establish the optimum approach for maintaining374
product stability during production, transport and storage. For example, if a beverage emulsion375
is stabilized by a protein-based emulsifier, then electrostatic repulsive interactions will play an376
important role in preventing droplet aggregation. In this situation, the system will be sensitive to377
environmental changes that reduce the magnitude and range of the electrostatic repulsion acting378
between droplets, such as altering the pH or adding salts (particularly multivalent counter-ions).379
On the other hand, if the beverage emulsion is stabilized by a polysaccharide-based emulsifier,380
then steric repulsive interactions will be most important for preventing droplet aggregation. In381
this case, the product will be much less sensitive to droplet aggregation when the pH or ionic382
strength is changed. In this latter case, emulsion stability depends on the thickness and383
hydrophilicity of the interfacial layer, which will depend on the molecular characteristics of the384
polysaccharide molecules. A summary of the major colloidal interactions in beverage emulsions385
is given in Table 2.386
4. Physicochemical Properties387
The physicochemical properties of beverage emulsions play an important role during the388
manufacturing process, as well as in determining the perceived quality attributes of the final389
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4.1. Optical properties393
The first cue that a consumer uses to judge the quality or desirability of a finished beverage394product is its visual appearance (provided it is packaged or poured into a transparent container,395
such as a bottle or cup). Each type of beverage product is expected to have a particular396
appearance depending on its nature, e.g., a dark brown cola, a cloudy orange juice, or a clear397
green lime juice. From a scientific viewpoint, emulsion appearance is categorized in terms of398
their opacity and color, which can be quantitatively described using tristimulus color coordinates,399
such as theL*a*b* system (McClements, 2005b). In this color system,L*represents the400
lightness, and a*and b*are color coordinates: where +a*is the red direction, -a*is the green401
direction; +b*is the yellow direction, -b*is the blue direction; lowL*is dark and highL*is402
light. The opacity of an emulsion can therefore by characterized by the lightness (L*), while the403
color intensity can be characterized by the chroma: C = (a*2+ b*2)1/2. The color intensity is404
usually inversely related to the lightness, so that the chroma decreases (fades) when the lightness405
increases. The optical properties of emulsions are mainly determined by the relative refractive406
index, concentration, and size distribution of the droplets they contain (Chanamai &407
McClements, 2002b; Danviriyakul, McClements, Decker, Nawar, & Chinachoti, 2002;408
McClements, 2005b). The lightness of an emulsion tends to increase with increasing refractive409
index contrast and increasing droplet concentration, and has a maximum value at a particular410
droplet size. This has important implications for the development of beverage products that411
should be either clear or opaque. In general, the lightness of emulsions increases steeply as the412
oil droplet concentration increases from about 0 to 5 wt%, but then increases more gradually at413
higher droplet concentrations (Figure 5).414
As mentioned earlier, some beverages are expected to have optical clarity, whereas others415
are expected to be cloudy. Optimizing the initial particle size distribution of a beverage416
emulsion, as well as inhibiting any changes in the particle size during storage, is therefore a417
particularly important part of designing a commercial product with the desired optical properties.418
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transparent and cloudy products. For cloudy products, the majority of droplets should be424
between about 200 and 400 nm in diameter so that the light scattering is very strong425
(McClements, 2002). In this case, the scattering efficiency of the individual oil droplets will426
determine the minimum amount of a clouding emulsion required to reach a particular turbidity in427
the final product.428
4.2. Rheology429
The rheological properties of beverage emulsions are also an important factor determining430
their manufacture and utilization. Most beverage emulsions are initially manufactured in a431
concentrated form, which is diluted appreciably during the production of the final beverage432
product. The droplet concentration in the beverage concentrate typically ranges from 3 to 30%,433
while that in the final product is typically < 0.1%. Industrially, the rheology of the beverage434
concentrate is important since it influences the ease of mixing, flow through a pipe, and435packaging. A manufacturer typically wants to have as high an oil loading as possible, without436
the product becoming too viscous or gel-like to handle easily. This requires careful control of437
the total droplet concentration in the system. The droplet concentration in the final beverage438
concentration is usually so low that the rheology is dominated by the properties of the aqueous439
continuous phase (see discussion below).440
The rheology of dilute colloidal dispersions is normally characterized by the shear viscosity441
(Genovese, Lozano, & Rao, 2007; McClements, 2005b). When the droplet concentration is less442
than about 5% (< 0.05), the shear viscosity can be described by Einsteins equation:443
444
( ) 5.210 += (1)445
446
Here, is the viscosity of the overall system, 0 is the viscosity of the continuous phase, and447
is the disperse phase volume fraction. This equation under-predicts the viscosity of colloidal448
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dilute emulsion increases linearly with droplet concentration, but that the most important factor454
affecting the overall rheology is the viscosity of the continuous phase. Thus the most effective455
means of controlling the viscosity of a dilute beverage emulsion is to change the viscosity of the456
continuous phase, e.g., by adding sugars or polymer thickening agents.457
The viscosity of concentrated emulsions can be described by a semi-empirical equation that458
takes into account droplet-droplet interactions (Berli, Deiber, & Quemada, 2005; McClements,459
2005b; Quemada & Berli, 2002):460
2
c
0 1
=
(2)461
Here, is the disperse phase volume fraction, and c(0.63) is a critical disperse phase462
volume fraction above which the droplets are so closely packed together that they cannot easily463
flow past each other. This equation shows that the viscosity of an emulsion increases with464
increasing droplet concentration, gradually initially and then steeply as the droplets become more465
closely packed (Figure 5). Around and above the droplet concentration where close packing466
occurs, the emulsion becomes highly viscosity and may exhibit solid-like characteristics, such as467
visco-elasticity and plasticity (Berli, et al., 2005; McClements, 2005b; Quemada & Berli, 2002).468
In flocculated systems the critical concentration where the system becomes highly viscous or469
solid-like may be much lower than in a non-flocculated system. It is therefore important for470
beverage manufactures to consider the influence of droplet concentration and interactions on the471
rheological properties of emulsion concentrates (Genovese, et al., 2007; McClements, 2005b;472
Walstra, 2003).473
4.3. Molecular distribution and release characteristics474A beverage emulsion may contain a number of constituents that partition into different475
phases within the product, e.g.,oil, aqueous, interfacial, or gas phases (McClements, 2005b).476
The physical location of some of these constituents may have a major impact on the quality477
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(Choi, et al., 2009; Choi, et al., 2010b). This would suggest that it is better to keep citral under483
neutral conditions or within the emulsion concentrate as long as possible before the final dilution484
into the acid phase is carried out. The perceived flavor profile of beverage emulsions depends on485
the distribution of volatile molecules between the liquid and gas phases. Increasing the oil486
content of an emulsion decreases the concentration of hydrophobic (KOW> 1) volatiles in the487
headspace and therefore reduces the perceived flavor profile (Figure 6). This phenomenon is488
important to take into account when reformulating a beverage product so that it contains a489
different fat concentration, e.g., fortification with a bioactive lipid such as -3 oils.490
The location of a constituent within a beverage emulsion is governed by its equilibrium491
partition coefficients (e.g., oil-water, oil-air, oil-interface) and its mass transport kinetics through492
the system (McClements, 2005b). When a beverage emulsion is placed in the mouth there is a493
redistribution of flavor molecules, with some of the aroma compounds leaving the product and494
entering the nasal cavity. The rate at which flavor molecules leave the droplets in beverage495
emulsions is usually extremely quick (< 0.1 s for KOW< 1000), and therefore droplet dimensions496
tend to have little impact on the flavor release profile (McClements, 2005b). Nevertheless, it497
may be possible to encapsulate oil droplets within hydrogel matrices to slow down the release of498
flavor molecules within the mouth.499
5. Beverage Emulsion Shelf-Life500
One of the most important factors determining the commercial viability of beverage501
emulsions is their ability to resist changes in their physical and chemical properties after their502
production. Beverage emulsions experience a range of environmental stresses during their503
manufacture, transport, storage, and utilization that may reduce their shelf lives: mechanical504
forces (e.g., stirring, flow through a pipe, centrifugation, vibrations, and pouring); temperature505
variations (e.g.,freezing, chilling, warming, pasteurization, and sterilization); exposure to light506
(e g natural or artificial visible or ultraviolet waves); exposure to oxygen; variations in solution507
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hydrolysis); acceleration of physical instability mechanisms, (e.g.,flocculation, coalescence or512
Ostwald ripening). In this section, a brief overview of some of the major instability mechanisms513
in beverage emulsions is given, and some suggestions for preventing them from occurring are514
provided.515
5.1. Physical Stability516
Emulsions are thermodynamically unfavorable systems that tend to break down over time517
due to a variety of physicochemical mechanisms (Figure 2), including gravitational separation,518
flocculation, coalescence and Ostwald ripening (Dickinson, 1992a; Friberg, et al., 2004;519
McClements, 2005b). All of these instability mechanisms lead to a change in the structural520
organization of the various components within the system, rather than in the type of molecules521
present. Nevertheless, changes in the chemical structure of active components can lead to522
changes in physical stability, and vice versa.523
5.1.1. Gravitational Separation524
Gravitational separation is one of the most common forms of physical instability in525
commercial beverage emulsions, and it may take the form of either creaming or sedimentation526
depending on the relative densities of the oil droplets and the surrounding aqueous phase.527
Creaming is the upward movement of droplets when they have a lower density than the aqueous528
phase, whereas sedimentation is the downwards movement of droplets when they have a higher529
density than the aqueous phase. The oil phases used in beverage emulsions consist primarily of530
triacylglycerol and/or flavor oils, which have lower densities than water and so creaming is more531
prevalent (Table 3). However, if a beverage emulsion contained an excess of weighting agent532
within the oil phase then it may be prone to sedimentation. A beverage emulsion is also prone to533
sedimentation if it contains very small oil droplets covered by relatively thick and dense534
interfacial layers (see below) (McClements, 2011).535
One of the most common problems reported in beverage emulsions is ringing, which is the536
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0
0
2
9
)(2
=
particleparticlegrv (3)542
where, v is the creaming velocity, rparticleis the particle radius, particleis the particle543
density,0is the aqueous phase density, 0is the aqueous phase viscosity, and gis the544
acceleration due to gravity. This equation shows that the rate of droplet creaming should545
decrease as the droplet size decreases, the density contrast decreases, or the aqueous phase546
viscosity increases. Gravitational forces cause droplets to move either upwards or downwards547
depending on their density relative to the surrounding aqueous phase. Hence, if only548
gravitational forces operated, then the droplets would accumulate at either the top or the bottom549
of an emulsion. In practice, droplets may also move because of Brownian motion associated550
with the thermal energy of the system. Brownian motion favors the random distribution of the551
droplets throughout the entire volume of the emulsion, rather than their accumulation at either552
the top or bottom. Gravitational forces tend to dominate droplet movement in emulsions553
containing relatively large droplets (r> 100 nm), whereas Brownian motion forces tend to554
dominate droplet movement in emulsions containing smaller droplets (McClements, 2011).555
Consequently, emulsions become more stable to creaming or sedimentation as the particle size556
decreases because the creaming velocity decreases (vr2) and because Brownian motion effects557
increase.558
The above calculations assume that the particles in beverage emulsions are homogeneous559
spheres consisting entirely of oil phase. In practice, the particles in beverage emulsions actually560
have a core-shell structure, consisting of an oil core and an interfacial shell. In this case, the561
overall particle radius is given by rparticle= rcore+ , and the overall particle density (particle)562
depends on the densities of the core (C) and shell (S) materials and the volume fraction of the563
shell (S):564
CSSSparticle )1( += (4)565
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sufficiently thick and dense emulsifier layers. Thus, it should be possible to produce density571
matched particles in beverage emulsions by controlling the oil core size and the thickness of the572
adsorbed emulsifier layer.573
The above discussion has highlighted a number of approaches that can be used to inhibit or574
prevent gravitational separation in beverage emulsions. First, gravitational separation can be575
prevented by matching the density of the dispersed (oil) and continuous (aqueous) phases. The576
density of the aqueous phase typically varies from about 1000 to 1050 kg m-3
, depending on the577
amount of sugars and other solutes present (Table 3). The density of most oil phases is less than578
this value, and therefore oil droplets will tend to move upwards. As already mentioned, the579
density of the core-shell particles within a beverage emulsion can be matched to the surrounding580
aqueous phase by adding a weighting agent to the oil phase, or by controlling the thickness and581
density of the emulsifier layer. Second, gravitational separation can be inhibited by reducing the582
size of the droplets in the emulsion, since the creaming velocity is proportional to the droplet size583
squared (Stokes Law). If the droplets are sufficiently small, then Brownian motion effects will584
dominate and the system will remain stable to creaming or sedimentation. Third, gravitational585
separation can be inhibited by increasing the viscosity of the aqueous phase, e.g., by adding586
thickening or gelling agents. This approach may not always be viable since it will also influence587
the texture and mouthfeel of the final product.588
Another approach some beverage manufacturers have used to mask the undesirable effects589
of creaming (ringing) on the appearance of a product is to design the packaging so as to590
obscure the effect, e.g., with appropriate placement of the labels or cap.591
5.1.2. Droplet Aggregation592
The aggregation state of the droplets in a beverage emulsion is important because it593
influences the stability of the product to gravitational separation. Changes in particle size during594
storage may also influence other important quality attributes of beverage products, such as their595
appearance (cloudiness or homogeneity) The tendency for droplet aggregation to occur in a596
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interfacial shell characteristics (such as thickness, charge, packing, rheology and601
hydrophobicity), and the properties of the intervening fluid (such as pH, ionic strength, osmotic602
pressure, and temperature). To a first approximation the overall colloidal interactions between a603
pair of droplets in a beverage emulsion can be described by the sum of the van der Waals (wVDV),604
electrostatic (wE), and steric (wS) interactions (McClements, 2005b):605
606
w(h) = wVDV(h) + wE(h)+ wS(h) (5)607
608
The van der Waals interactions are attractive, whereas the steric and electrostatic609
interactions are usually repulsive (Table 2). The van der Waals attraction operates between all610
kinds of droplets and would always cause aggregation if there were no opposing repulsive forces.611
The magnitude and range of the steric repulsion depend on the thickness and chemistry of the612
interfacial layer, whereas the magnitude and range of the electrostatic repulsion depend on the613
droplet charge (-potential) and the ionic composition of the aqueous phase. To design a product614
that is stable to droplet aggregation one must assure that the repulsive interactions dominate the615
attractive interactions. This is usually achieved by using an emulsifier that generates repulsive616
interactions between the droplets. The emulsifiers used in the beverage industry typically617
stabilize the droplets against aggregation by generating steric and/or electrostatic repulsive618
interactions. Emulsifiers that form relatively thick open interfaces (such as polysaccharides and619
non-ionic surfactants with large hydrophilic head-groups) can generate a steric repulsion that is620
sufficient strong and long range to overcome the attractive van der Waals interactions, and621
thereby stabilize the system against aggregation. Emulsifiers that form highly charged interfaces622
(such as proteins and ionic surfactants) can generate a strong electrostatic repulsion between623
droplets that prevent aggregation. However, emulsifiers that can only stabilize emulsions due to624
electrostatic interactions may be prone to instability when the pH or ionic strength is changed.625
Some emulsifiers use a combination of electrostatic and steric repulsion to stabilize the system,626
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Droplets aggregate when there is a primary or secondary minimum in the interaction potential632
that is sufficiently deep and accessible to the droplets (Figure 4). The two major types of633
aggregation in beverage emulsions are flocculation and coalescence.634
5.1.2.1. Flocculation635
Droplet flocculation is the process whereby two or more droplets come together to form an636
aggregate in which the droplets retain their individual integrity (Figure 2). Droplet flocculation637
is usually detrimental to beverage emulsion quality because it accelerates the rate of gravitational638separation thereby reducing their shelf-life. Flocculation can also cause an appreciable increase639
in the viscosity of beverage emulsion concentrates, and may even lead to the formation of a gel.640
This may be undesirable since it would influence the transport, handling and dispersibility of the641
product. Flocculation may occur in beverage emulsions through a variety of different processes642
that either increase the attractive forces or decrease the repulsive forces operating between the643
droplets. The mechanism that is important in a particular emulsion depends largely on the nature644
of the emulsifier used and the solution conditions (e.g.,pH, ion type and concentration, and645
functional ingredients).646
Reduced electrostatic repulsion: Electrostatically stabilized emulsions may flocculate when647
the electrostatic repulsion between the droplets is reduced. A number of physicochemical648
changes may cause this reduction in electrostatic repulsion (Israelachvili, 2011): (i) the pH is649
altered so that the net charge on the droplets is reduced; (ii) counter-ions bind to the surface of650
the droplets and reduce their charge (charge neutralization); (iii) the ionic strength of the651
aqueous phase is increased to screen the electrostatic interactions (electrostatic screening).652
Protein-coated oil droplets are particularly sensitive to flocculation due to reduction in the653
electrostatic repulsion between them when the pH or ionic composition is altered (Demetriades,654
Coupland, & McClements, 1997a; McClements, 2004).655
Increased depletion attraction: The presence of non-adsorbing colloidal entities in the656
continuous phase of an emulsion such as biopolymers or surfactant micelles generates an657
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causes them to flocculate. This type of droplet aggregation is usually referred to as depletion662
flocculation. The presence of relatively high concentrations of non-adsorbed biopolymer663
emulsifiers (gum arabic and modified starch) have been shown to induce depletion flocculation in664
model beverage emulsions (Chanamai & McClements, 2001). Depletion flocculation may also be665
promoted by other kinds of biopolymers that might be used in beverages, such as maltodextrin,666
pectin, xanthan gum, and carrageenan (Cao, Dickinson, & Wedlock, 1990; Cho & McClements,667
2009; Gu, Decker, & McClements, 2004; Gunning, Hibberd, Howe, & Robins, 1988).668
Increased hydrophobic interactions: This type of interaction is important in emulsions that669
contain droplets that have some non-polar regions exposed to the aqueous phase. A good670
example of this type of interaction is the effect of thermal processing on the flocculation stability671
of oil-in-water emulsions stabilized by globular proteins (Demetriades, Coupland, &672
McClements, 1997b). At room temperature, whey protein stabilized emulsions (pH 7) are stable673
to flocculation because of the large electrostatic repulsion between the droplets, but when they674
are heated above 70oC they become unstable. The globular proteins adsorbed to the surface of675
the droplets unfold above this temperature and expose non-polar amino acids that were originally676
located in their interior. Exposure of these non-polar amino acids increases the hydrophobic677
character of the droplet surface and therefore leads to flocculation because of the increased678
hydrophobic attraction between the droplets.679
Formation of biopolymer bridges: Many types of biopolymer promote flocculation by680
forming bridges between two or more droplets. Biopolymers may adsorb either directly to the681
bare oil surfaces of the droplets or to the adsorbed emulsifier molecules that form the interfacial682
layer. To be able to bind to the droplets there must be a sufficiently strong attractive interaction683
between segments of the biopolymer and the droplet surface. The most common types of684interaction that operate in food emulsions are hydrophobic and electrostatic (Dickinson, 2003).685
For example, a positively charged biopolymer (such as chitosan) might adsorb to the surface of686
two negatively charged emulsion droplets causing them to flocculate (Ogawa, Decker, &687
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flocculation in that system. In general, flocculation can be prevented by ensuring that the693
repulsive forces dominate the attractive forces, and that there are no additives that can promote694
bridging.695
5.1.2.2. Coalescence696
Coalescence is the process whereby two or more liquid droplets merge together to form a697
single larger droplet (Figure 2). Coalescence causes emulsion droplets to cream or sediment698
more rapidly because of the increase in their particle size. In beverage emulsions, coalescence699eventually leads to the formation of a layer of oil on top of the material, which is referred to as700
oiling off. This process is one of the main reasons for the shiny oily layers often seen on top of701
unstable beverage emulsions.702
The susceptibility of a beverage emulsion to droplet coalescence is highly dependent on the703
nature of the emulsifier used to stabilize the system, since this instability mechanism involves704
two or more droplets fusing together. In general, the susceptibility of oil droplets to coalescence705
is determined by the nature of the forces that act between the droplets (i.e. gravitational,706
colloidal, hydrodynamic and mechanical forces) and the resistance of the interfacial layer to707
rupture. The stability of emulsions to coalescence can be improved by preventing the droplets708
from coming into close proximity for extended periods, e.g.,by preventing droplet flocculation,709
preventing the formation of a creamed layer, or having too high droplet concentrations710
(McClements, 2005b). Alternatively, one can control the properties of the interfacial layer711
surrounding the oil droplets to make it more resistant to rupture, e.g., by selecting an appropriate712
emulsifier or other additives that alter surface properties.713
5.1.3. Ostwald Ripening714
This susceptibility of a beverage emulsion to Ostwald ripening (OR) is mainly determined715
by the solubility of the oil phase in the aqueous phase: the higher the solubility, the more716
unstable the emulsion. Oil phases with very low water-solubilities (such as the vegetable oils717
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the water-solubility of an oil contained within a spherical droplet increases as the radius of the723
droplet decreases, which means that there is a higher concentration of solubilized oil molecules724
in the aqueous phase surrounding a small droplet than surrounding a larger one (Kabalnov &725
Shchukin, 1992; McClements, 2005b). The presence of this concentration gradient means that726
solubilized oil molecules tend to move from the immediate vicinity of smaller droplets to that of727
larger droplets. This leads to an increase in mean droplet size over time, which can be described728
by the following equation once steady state conditions have been achieved (Kabalnov &729
Shchukin, 1992):730
731
DtStdtd == 93233 )0()( (6)732
733
Here, d(t)is the number-weighted mean droplet diameter at time t, d0is the initial number-734
weighted mean droplet diameter, is the Ostwald ripening rate, =2Vm/RT, Sis the water-735
solubility of the oil phase in the aqueous phase, Dis the translational diffusion coefficient of the736
oil molecules through the aqueous phase, Vmis the molar volume of the oil, is the oil-water737
interfacial tension,Ris the gas constant, and Tis the absolute temperature.738
The most important factor determining the stability of a beverage emulsion to OR is the739
water-solubility of the oil phase (S) (Weiss, Herrmann, & McClements, 1999). For this reason740
OR is not usually a problem for emulsions prepared using oils with a very low water-solubility,741
such as long chain triglycerides (e.g., corn, soy, sunflower, or fish oils). On the other hand, OR742
may occur rapidly for emulsions prepared using oils with an appreciable water-solubility, such as743
flavor oils and essential oils (Li, Le Maux, Xiao, & McClements, 2009; McClements, et al.,744
2012; Wooster, Golding, & Sanguansri, 2008b). OR can be retarded in these systems by adding745
a substance known as a ripening inhibitor. A ripening inhibitor is a non-polar molecule that is746
soluble in the oil phase but insoluble in the water phase, e.g., a long chain triacylglycerol (such747
as corn oil) This type of molecule can inhibit OR by generating an entropy of mixing effect that748
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droplets after OR occurs. Differences in the composition of emulsion droplets are754
thermodynamically unfavorable because of the entropy associated with mixing: it is more755
favorable to have the two lipids distributed evenly throughout all of the droplets, rather than to756
be located in particular droplets. Consequently, there is a thermodynamic driving force that757
operates in the opposite direction to the OR effect. The change in droplet size distribution with758
time then depends on the concentration and solubility of the two components within the oil759
droplets. This approach has previously been used to improve the stability of food-grade760
nanoemulsions, such as those containing short chain triglycerides, essential oils, and flavor oils761
(Li, et al., 2009; McClements, et al., 2012; Wooster, et al., 2008b). An example of this effect is762
shown in Figure 7 which shows that droplet growth in orange oil-in-water emulsions during763
storage can be inhibited by adding a sufficiently high concentration of corn oil (the ripening764
inhibitor) (McClements, et al., 2012),. Orange oil (4-fold) has a relatively high solubility in765
water, and therefore is highly prone to OR, which leads to an appreciable increase in mean766
droplet size during storage. On the other hand, corn oil has a very low solubility in water, and767
therefore it can retard OR if it is incorporated into the oil phase prior to homogenization. These768
results show that incorporating 10% corn oil into the oil phase was sufficient to inhibit OR in769
these systems (Figure 7). OR may also be retarded by adding certain kinds of weighting agents770
(such as ester gums) since these substances also have a very low water solubility and therefore771
act as ripening inhibitors (Lim, et al., 2011).772
5.2. Chemical Stability773
A number of lipophilic compounds that may be present in beverage emulsions can undergo774
chemical degradation during storage, which leads to a loss of color, flavor and/or nutrients. A775
few representative examples of chemical degradation of lipophilic components in oil-in-water776
emulsions are given below.777
Citrus Degradation. Several mechanisms lead to the chemical decomposition of citrus flavor778
t ( h it l d li d it ll l) i l di id ti h d l ti779
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components and increasing the concentration of undesirable flavor components (Tan, 2004;784
Ueno, Masuda, & Ho, 2004). The beverage industry would therefore like to identify effective785
strategies for preventing these undesirable chemical degradation reactions.786
There has been a great deal of research on establishing the major factors that influence the787
chemical degradation of citral because this is one of the most important flavor compounds found788
in commercial beverages. The degradation rate of citral in aqueous solutions has been shown to789
increase with decreasing pH (Choi, et al., 2009) (Figure 8). Most commercial beverages have790
acidic aqueous phases and are therefore highly susceptible to flavor loss during storage due to791
this acid-catalyzed mechanism. The chemical stability of citral has been shown to be much792
higher when it is located within an oil phase than in an aqueous phase (Choi, et al., 2009).793
Consequently, the chemical degradation of citral in beverage emulsions can be improved by794
ensuring that the citral molecules are located primarily in an oil phase rather than in the aqueous795
phase. Indeed, studies have shown that citral stability can be improved by increasing the oil796
droplet concentration (Choi, et al., 2009) or by adding surfactant micelles to the aqueous phase797
(Choi, et al., 2010b), although these strategies may not be practical for most commercial798
products. It was proposed that citral stability may be improved by encapsulating it within solid799
lipid particles rather than within liquid oil droplets, however the opposite was found to be true800
experimentally, which was attributed to the expulsion of the citral molecules into the aqueous801
phase after droplet crystallization (Mei, et al., 2010). Addition of various kinds of natural802
antioxidants to flavor oil emulsions has also been shown to improve the stability of citral to803
chemical degradation (Yang, Tian, Ho, & Huang, 2011). The oil droplets in beverage emulsions804
are surrounded by a coating of emulsifier molecules, and so it may be possible to improve the805
stability of the citral molecules within them by engineering the properties of the interfacial layer806(Decker & McClements, 2001; Given Jr., 2009). Indeed, studies have shown that citral807
degradation was faster in flavor oil droplets coated by an anionic surfactant than those coated by808
a non-ionic or cationic surfactant, which was attributed to differences in the accumulation of809
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Polyunsaturated Lipid Degradation. There has been great interest in the beverage industry815
in fortifying products with -3 lipids (such as flax, fish and algal oils) since these lipids have816
been claimed to have health benefits and are currently under-consumed by the general817
population. Nevertheless, there are many technical difficulties associated will incorporating818
these lipids into beverage products due to their high susceptibility to oxidation. Lipid oxidation819
affects the quality of emulsion-based products, influencing their flavor, odor, and nutritive value820
(Frankel, Satu-Gracia, Meyer, & German, 2002). The oxidation of polyunsaturated lipids is a821
highly complex series of chemical reactions that is initiated when a lipid interacts with an822
oxygen reactive species, and proceeds through molecular cleavage and oxygen addition reactions823
to the formation of a wide variety of volatile compounds (McClements & Decker, 2000; Waraho,824
McClements, & Decker, 2011). The rate at which oxidation takes place is dependent on several825
factors: the molecular structure of the lipids; storage conditions; the presence of pro-oxidants and826
antioxidants; and the structural organization of the system. Based on this knowledge a variety of827
strategies have been developed to inhibit or prevent lipid oxidation in emulsified products:828
addition of oil-soluble and water-soluble antioxidants; chelation of pro-oxidant transition metals;829
engineering the interface to prevent pro-oxidants from coming into close proximity to lipid830
substrates; controlling environmental conditions, such as exposure to heat, oxygen, or light.831
Carotenoid degradation. Carotenoids are natural compounds found in many fruits and832
vegetables that are may be used in foods an colorants or nutraceuticals because of their potential833
health benefits (Mayne, 1996; Ryan, O'Connell, O'Sullivan, Aherne, & O'Brien, 2008). One of834
the major factors currently limiting the incorporation of carotenoids into many food and835
beverage products is their high susceptibility to chemical degradation. In particular, carotenoids836
have a conjugated polyunsaturated hydrocarbon chain that makes them highly prone to837autoxidation (Boon, McClements, Weiss, & Decker, 2009). A number of factors have previously838
been shown to promote the oxidation of carotenoids, including highly acidic environments839
(Konovalov & Kispert, 1999), light (Mortensen & Skibsted, 1996), heat (Mader, 1964), singlet840
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Woodall, et al., 1997; Yamauchi, Miyake, Inoue, & Kato, 1993). The chemical degradation of845
carotenoids leads to color fading, and may reduce their beneficial health properties.846
Recent studies have examined the influence of interfacial properties (i.e.,emulsifier type),847
storage conditions (i.e., pH, ionic strength, and temperature) and antioxidant addition (i.e.,848
vitamin E, Coenzyme Q10, EDTA and ascorbic acid) on the chemical degradation of -carotene849
encapsulated within oil-in-water nanoemulsions (Qian, Decker, Xiao, & McClements, 2012).850
The rate of -carotene degradation was found to increase with decreasing pH and increasing851
temperature, was faster for a non-ionic surfactant (Tween 20) than for a protein (-852
lactoglobulin), and decreased with increasing antioxidant addition to either the oil or aqueous853
phase.854
5.3. Defining the End of Shelf Life855
The end of the shelf life of a product can be defined as the time when it becomes856unacceptable to consumers, which depends on the rate of the various physical and chemical857
instability mechanisms occurring. A product may become unacceptable when a ring of oil858
droplets is visible at the top of the bottle, when the flavor components decompose/oxidize and859
create an unacceptable flavor profile, when the color changes beyond an acceptable level, or860
when the product is microbiologically unsafe to consume. A beverage manufacture should861
establish quantitative criteria that can be used to establish the end of the shelf life of their862
particular product. They should then develop a systematic testing scheme that can be used to863
predict the shelf life of products.864
6. Beverage Emulsion Manufacture865
Beverage emulsions are usually prepared using a two-step process: a beverage emulsion866
concentrate (3 30 wt% oil) is prepared, which is then diluted extensively to create the finished867
product (< 0.1 wt% oil) (Tan, 2004). In this section, we briefly describe the major characteristics868
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aqueous phase often has to be heated and mechanically agitated to facilitate dissolution and874
dispersion of water-soluble components (such as emulsifiers, thickening agents, buffers, minerals875
and other functional ingredients). Similarly, the oil phase may also have to be heated and876
mechanically agitated to facilitate the melting and dispersion of any antioxidants, weighting877
agents, ripening inhibitors, or colors. Once the oil and aqueous phases have been prepared they878
are blended together using a high-shear mixer to form a coarse emulsion (d1 to 10 m), which879
is then homogenized using a mechanical device to form a fine emulsion (d0.1 to 1 m). When880
beverages are prepared using low energy homogenization methods a different approach may be881
taken (see below). In this case, water-soluble surfactants and some other water-soluble882
components may initially be incorporated into the oil phase, which is then mixed with the883
aqueous phase. This process can lead to the spontaneous formation of a microemulsion,884
nanoemulsion, or emulsion depending on system composition and preparation procedure. After885
preparation the beverage emulsion concentrate is often pasteurized to reduce the microbial load,886
and then stored or transported to the place where it will be used.887
Finished Product: The finished product is created by diluting the beverage emulsion888
concentrate with another aqueous phase, which may contain various other ingredients, such as889
colors, flavors, preservatives, pH regulators etc. Typically, the concentrate is diluted 500-1000890
times to produce a final product that often has an oil concentration < 20 mg per liter for a ready-891
to-drink product (Given, 2009). The final product may be homogenized again to ensure that any892
non-polar colors, flavors, and preservatives are incorporated into the oil droplets. Appropriate893
selection of ingredients and processing conditions may lead to beverage products with shelf lives894
longer than 12 months. However, the perceived quality of a product may deteriorate after895
extended storage due to detrimental changes in its physical or chemical properties. Beverage896emulsions are susceptible to various physical instability mechanisms that can lead to undesirable897
changes in appearance, such as ringing and oiling off (see earlier section). Beverage emulsions898
are also liable to undesirable quality changes due to chemical degradation, e.g.,changes in flavor899
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homogenization method and operating conditions), and storage conditions (e.g., exposure to905
elevated temperatures, light, and oxygen).906
A number of different methods can be used to form beverage emulsion concentrates. In907
general, these approaches can be categorized as either high-energy or low-energy approaches908
depending on the underlying physical principle of droplet formation (Acosta, 2009; Anton &909
Vandamme, 2009; Leong, Wooster, Kentish, & Ashokkumar, 2009; Pouton & Porter, 2006;910
Tadros, Izquierdo, Esquena, & Solans, 2004). High-energy approaches utilize mechanical911
devices (homogenizers) that generate intense forces capable of disrupting and intermingling912
the oil and aqueous phases leading to the formation of very fine oil droplets (Figure 9). The913
most commonly used homogenizers utilized in the beverage industry for forming emulsions are914
high pressure valve homogenizers, but microfluidizers and ultrasonic methods may also be used915
(Gutierrez, et al., 2008; Leong, et al., 2009; Velikov & Pelan, 2008; Wooster, et al., 2008b).916
High-energy approaches are probably the most common method used for preparing beverage917
emulsions at present because they are capable of large-scale production, and they can be used to918
prepare emulsions from a variety of different starting materials. Low energy approaches rely on919
the spontaneous formation of fine oil droplets within mixed surfactant-oil-water systems when920
the solution or environmental conditions are altered (Anton, Benoit, & Saulnier, 2008;921
Bouchemal, Briancon, Perrier, & Fessi, 2004; Chu, Ichikawa, Kanafusa, & Nakajima, 2007;922
Freitas, Merkle, & Gander, 2005; Tadros, et al., 2004; Yin, Chu, Kobayashi, & Nakajima, 2009).923
A number of different low energy approaches have been developed, and some of these are924
suitable for utilization within the beverage industry, e.g., phase inversion and spontaneous925
emulsification methods (Figure 10). The minimum particle size that can be produced using926
either approach depends on many different factors, which are highlighted in the sections below.927
6.1. High-Energy Approaches928
The size of the droplets generated by high energy approaches is determined by a balance929
b t t i i ithi th h i d l t di ti d930
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2004). The smallest droplet size that can be produced by a particular high-energy device935
depends on homogenizer design (e.g., flow and force profiles), homogenizer operating936
conditions (e.g., energy intensity, duration), environmental conditions (e.g. temperature), sample937
composition (e.g., oil type, emulsifier type, concentrations), and the physicochemical properties938
of the component phases (e.g., interfacial tension, viscosity) (Kentish, et al., 2006; Wooster, et939
al., 2008b).940
High energy homogenizers are widely used to produce beverage emulsions because they can941
be utilized with a wide variety of different types of oils and emulsifiers. Once the942
homogenization conditions have been optimized, beverage emulsions can be produced using943
triacylglycerol oils or flavor oils as the oil phase, and proteins, polysaccharides, phospholipids,944
or surfactants as emulsifiers. Thus, high-energy methods are suitable for producing both cloud945
emulsions and flavor emulsions. Even so, the size of the droplets produced depends strongly on946
the characteristics of the oil and emulsifier used (see below). For example, it is usually easier to947
produce very small droplets when the oil phase has a low viscosity and/or interfacial tension948
(e.g., flavor oils) than when it has a high viscosity and/or interfacial tension (e.g., triacylglycerol949
oils).950
6.1.1. High Pressure Valve Homogenizers951
High pressure valve homogenizers are currently the most common high-energy method of952
producing beverage emulsions. Initially, a coarse emulsion is produced using a high shear mixer953
and then this is fed directly into the inlet of the high pressure valve homogenizer. The954
homogenizer has a pump that pulls the coarse emulsion into a chamber on its backstroke and955
then forces it through a narrow valve at the end of the chamber on its forward stroke (Figure 9).956
As the coarse emulsion passes through the valve it experiences a combination of intense957
disruptive forces that cause the larger droplets to be broken down into smaller ones. Different958
nozzle designs are available to increase the efficiency of droplet disruption. The droplet size959
produced using a high pressure valve homogenizer usually decreases as the number of passes960
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emulsifier present to cover all the new droplet surfaces formed during homogenization, and to965
use an emulsifier that can rapidly adsorb to the droplet surfaces to prevent re-coalescence (Jafari,966
et al., 2008). Usually, there is a linear relationship between the logarithm of the homogenization967
pressure (P) and the logarithm of the droplet diameter (d): log d log P, with the constant of968
proportionality depending on homogenizer type (McClements, 2005b). To reduce the droplet969
size to the level required in beverage emulsions it is sometimes necessary to operate at extremely970
high pressures and to use multiple passes through the homogenizer. Even then, it is only971
possible under certain circumstances to obtain droplets less than 100 nm in radius ( e.g.,high972
emulsifier levels, low interfacial tensions, and appropriate viscosity ratios).973
6.1.2. Microfluidizers974
The formation of beverage emulsions using a microfluidizer also involves forcing a coarse975
emulsion through a narrow orifice under high pressure to facilitate droplet disruption. However,976
the design of the channels through which the emulsion is made to flow within a microfluidizer is977
different from that of a high pressure valve homogenizer (Figure 9). The microfluidizer divides978
an emulsion into two streams that are then made to impinge on each other in an interaction979
chamber. Intense disruptive forces are generated within the interaction chamber when the two980
fast moving streams of emulsion collide, leading to highly efficient droplet disruption.981
A number of studies have examined the potential application of microfluidizers for the982
production of model beverage emulsions (Dalgleish, West, & Hallett, 1997; Henry, Fryer, Frith,983
& Norton, 2010; Klein, Aserin, Svitov, & Garti, 2010). These studies have shown that small984
droplets can be produced provided that conditions are optimized to facilitate droplet disruption985
and inhibit droplet coalescence. The droplet size tends to decrease with increasing986
homogenization pressure, number of passes, emulsifier concentration, and decreasing disperse-987
to-continuous phase viscosity ratio (Wooster, et al., 2008b). Again, there is usually a linear988
relationship between the logarithm of homogenization pressure and the logarithm of the droplet989
di t l d l P R t t di h id tifi d f th j f t i fl i th990
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appreciably steeper for the surfactant (-0.57) than for the protein (-0.29), which was attributed to995
the fact that the protein may adsorb more slowly to the droplet surfaces, and that it may form a996
viscoelastic coating that inhibits further droplet breakup. The dependence of the mean droplet997
diameter on viscosity ratio (D/C) was also examined by preparing emulsions using different oil998
phase and aqueous phase compositions. For the ionic surfactant there was a distinct decrease in999
mean droplet diameter with decreasing viscosity ratio, which suggested that droplet disruption1000
within the homogenizer became easier as the viscosity of the two phases became more similar.1001
On the other hand, little change was found in mean droplet size with viscosity ratio when a1002
globular protein was used as an emulsifier, which again may be due to the relatively slow1003
adsorption of the protein and its ability to form a coating that inhibits further droplet disruption.1004
6.1.3. Ultrasonic Homogenizers1005
Beverage emulsions can also be formed continuously using ultrasonic homogenizers. This1006
type of homogenizer utilizes high intensity ultrasonic waves to generate intense disruptive forces1007
(mainly generated by cavitation) that break the oil and water phases into very small droplets1008
(Kentish, et al., 2006; Leong, et al., 2009; Lin & Chen, 2008). Batch and continuous ultrasonic1009
homogenizers are available for producing emulsions (Leong, et al., 2009). However, continuous1010
ultrasonic homogenizers are probably the most commonly used methods for the large scale1011
production of fine emulsions (Figure 9). The size of the droplets produced using these devices1012
tends to decrease as the intensity of the ultrasonic waves is increased or the residence time in the1013
disruption zone is increased (Abismail, Canselier, Wilhelm, Delmas, & Gourdon, 1999; Maa &1014
Hsu, 1999). The homogenization efficiency also depends on the type and amount of emulsifier1015
present, and the viscosity of the oil and aqueous phases (Jafari, He, & Bhandari, 2006; Kentish,1016
et al., 2006; Leong, et al., 2009; Maa & Hsu, 1999). Ultrasonic homogenizers are particularly1017
suitable for low-viscosity fluids, but are less suitable for more viscous systems.1018
6.2. Low-Energy Approaches1019
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methods (Figure 10) (Anton, et al., 2008; Anton & Vandamme, 2009; Fernandez, Andre, Rieger,1025
& Kuhnle, 2004; Maestro, Sole, Gonzalez, Solans, & Gutierrez, 2008). Some of these low1026
energy methods are already used in the beverage industry for forming oil-in-water emulsions,1027
whereas others may also be for certain applications.1028
Low energy approaches are often more effective at producing small droplet sizes than high1029
energy approaches, but they are often more limited in the types of oil