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.

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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

beverage emulsions recent developments in formulation production and applications

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