Published Online: 15 JUL 2005
Copyright © 2005 John Wiley & Sons, Inc.
Bailey's Industrial Oil and Fat Products
How to Cite
McClements, D. J. and Weiss, J. 2005. Lipid Emulsions. Bailey's Industrial Oil and Fat Products. 3:14.
- Published Online: 15 JUL 2005
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
Many natural and processed foods contain small droplets of oil dispersed in an aqueous medium (e.g., milk, cream, fruit beverages, soups, cake batters, mayonnaise, cream liqueurs, sauces, deserts, salad cream, and ice cream) or small droplets of water dispersed in a lipid medium (e.g., butter and margarine). Despite the considerable diversity of physicochemical and sensory characteristics exhibited by these foods, they can all be considered to fall into a class of material called “emulsions” and their properties can be understood using the concepts and techniques of “emulsion science” (1-4). Emulsion science is a multidisciplinary subject that combines chemistry, physics, and engineering (5-12). The objective of emulsion scientists working within the food industry is to use the principles and techniques of emulsion science to improve the quality of the food supply and the efficiency of food production. The purpose of this chapter is to introduce the basic principles and techniques of emulsion science that are relevant for understanding, characterizing, and manipulating the properties of food products.
The wide diversity of physicochemical and organoleptic characteristics exhibited by food emulsions is the result of product formulation and processing conditions used to create them. The manufacture of an emulsion-based food product with specific desirable quality attributes depends on the selection of the suitable raw materials (e.g., water, oil, emulsifiers, thickening agents, minerals, acids, bases, vitamins, flavors, colorants, etc.) and optimization of processing conditions (e.g., mixing, homogenization, pasteurization, sterilization, etc.). The product must be transported and stored under appropriate conditions to maintain its desirable quality attributes prior to consumption (e.g., exposure to temperature variations, light, and mechanical agitation). Historically, the food industry relied largely on craft and tradition for the formulation of food products and the establishment of processing and storage conditions. Today, this approach is becoming increasingly unfeasible for the modern food industry, which must rapidly respond to changes in consumer preferences demanding a greater variety of cheaper, healthier, and more convenient foods (13-15). In addition, large-scale production operations are required for profitability of modern food companies. Large quantities of foods can consequently be produced at relatively low cost. The development of new foods, the improvement of existing foods, and the efficient operation of food processing operations require a more systematic and rigorous approach than was used previously (16). There have been considerable advances in our understanding of the physicochemical basis of the properties of food emulsions during the past decade. These advances are largely because of the availability of inexpensive but powerful personal computers that enable food scientists to carry out complex theoretical predictions and numerical simulations in a reasonably short time. The increasing availability of powerful new analytical instruments for probing the structure, interactions, and physicochemical properties of emulsions and their components further contribute to these advances.
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
An emulsion can be defined as a material that consists of small spherical droplets of one liquid dispersed in another liquid in which it is at least partly immiscible (Figure 1). Typically, the diameters of the droplets in food emulsions lie somewhere between 0.1 and 100 μm (1, 2, 17, 18). It is convenient to classify emulsions according to the relative organization of the oil and aqueous phases. A system that consists of oil droplets dispersed in an aqueous phase is called an oil-in-water or o/w emulsion, whereas a system that consists of water droplets dispersed in an oil phase is called a water-in-oil or w/o emulsion. The material within the emulsion droplets is usually referred to as the dispersed, internal, or discontinuous phase, whereas the material that makes up the surrounding liquid is usually referred to as the continuous or external phase. It is also possible to prepare multiple emulsions, e.g., oil-in-water-in-oil (o/w/o) or water-in-oil-in-water (W/O/W) type (19). For example, a W/O/W emulsion consists of water droplets dispersed within larger oil droplets, which are themselves dispersed in an aqueous continuous phase (20). These multiple emulsions may have advantages over traditional emulsions for certain applications, e.g., fat reduction, controlled ingredient release, or isolation of one ingredient from another (19).
The process of creating an emulsion from two separate immiscible liquids, or of reducing the size of the droplets in a preexisting emulsion, is called homogenization. In the food industry, this process is normally carried out using mechanical devices known as homogenizers, which subject the liquids to intense mechanical stresses that result in droplet disruption. An emulsion can be formed by homogenizing pure oil and pure water together, but the two phases rapidly separate into a system that consists of a layer of oil (lower density) on top of a layer of water (higher density). Phase separation occurs because droplets tend to merge with each other when they collide. The driving force for the phase-separation process is the fact that the contact between oil and water molecules is thermodynamically unfavorable, because of the hydrophobic effect (21). As a result, emulsions are considered to be thermodynamically unstable. However, it is possible to form emulsions that are kinetically stable (metastable) for a reasonable period of time (a few days, weeks, months, or years), by including two different classes of substances, emulsifiers and thickening agents, prior to homogenization. Emulsifiers are surface-active molecules that absorb at the surface of droplets that are generated during homogenization. There they form a protective membrane that prevents the hydrophobic water molecules from coming into direct contact with the hydrophilic lipid molecules, thus preventing aggregation. The most commonly used emulsifiers in the food industry are amphiphilic proteins, polysaccharides, small molecule surfactants, and phospholipids (22). The second class of compounds used to improve kinetic stability of emulsions is thickening agents. Thickening agents increase the viscosity of the continuous phase of emulsions, and they are used to modify emulsion texture and to enhance emulsion stability by retarding the movement of droplets. The most common thickening agents used in the food industry are polysaccharides, e.g., xanthan gum, alginate, carageenan, and guar gum (23). The term stabilizer is used to refer to any ingredient that can improve the stability of emulsions, and it may therefore be either an emulsifier or a thickening agent.
3 Droplet Characteristics
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
Many of the unique physicochemical and sensory properties of food emulsions are determined by the presence of the droplets they contain. The most important characteristics of emulsion droplets are therefore discussed below.
3.1 Droplet Concentration
The concentration of droplets in an emulsion is one of the key parameters influencing its appearance, texture, stability, and flavor. For example, opacity, viscosity, and creaming stability of emulsions usually increase as the droplet concentration increases. The droplet concentration is normally expressed in terms of the disperse-phase volume fraction (ϕ), which is equal to the volume of emulsion droplets (VD) divided by the total volume of the emulsion (VE): ϕ = VD/VE. Nevertheless, it can also be expressed in terms of the disperse-phase mass fraction (ϕm), which is equal to the mass of emulsion droplets (MD) divided by the total mass of the emulsion (ME): ϕm = MD/ME. The mass fraction can be related to the volume fraction using the following expression:
where ρ1 and ρ2 are the densities of the continuous and dispersed phases, respectively. If the densities of the two phases are equal, the mass fraction is equivalent to the volume fraction. The droplet concentration may also be presented as a mass (%ϕm = 100 ϕm) or as a volume (%ϕ = 100 ϕ) percentage. In many cases, the droplet concentration of an emulsion is known because the concentration of the ingredients used to prepare it is carefully controlled during emulsion production. Nevertheless, local variations in droplet concentration may occur within the emulsion, for example, when the droplets accumulate at the top or bottom of an emulsion because of gravitational separation. Such an emulsion will have properties that are significantly different from a homogeneous product. In addition, the droplet concentration of an emulsion may vary during the course of a processing operation, e.g., if a mixer or homogenizer is operating inconsistently. These operational inconsistencies may be caused by variations in homogenization pressure and temperature, volume-flow rate, or stirrer speed. It is therefore often important to be able to quantify the droplet concentration of an emulsion. The droplet concentration can be measured using traditional proximate analysis techniques (e.g., drying, solvent extraction, and density measurements) or by using more sophisticated modern analytical techniques (e.g., light scattering, electrical pulse counting, and ultrasonic spectroscopy) (4).
3.2 Droplet Size Distribution
The size of the droplets in an emulsion has a strong influence on many of its physicochemical and sensory properties, e.g., shelf life, appearance, texture, and flavor (1, 2, 4). For example, the stability of an emulsion to gravitational separation or droplet aggregation can be greatly improved by decreasing the droplet size. This is because the velocity of sedimentation is proportional to the square of the droplet size. The size of the droplets in an emulsion is largely determined by the emulsifier type and concentration, the physicochemical properties of the component phases, and the homogenization conditions (4). A food manufacturer normally specifies a preestablished desirable droplet size distribution for a particular product. If the product does not meet this specification, it typically must be reprocessed or even discarded.
An emulsion that contains droplets that all have the same size is referred to as being “monodisperse,” whereas an emulsion that contains droplets that have a range of different sizes is referred to as being “polydisperse.” The size of the droplets in a monodisperse emulsion can be completely characterized by a single number, such as the droplet diameter (d) or radius (r). Monodisperse emulsions, while difficult to produce, are sometimes prepared for use in fundamental studies because the interpretation of experimental measurements is usually much simpler than for polydisperse emulsions. In most industrial applications, food emulsions droplet sizes are always distributed, and so the specification of their droplet size is more complicated than for monodisperse systems. In some situations, it is important to have information about the full particle size distribution of an emulsion, i.e., the fraction of droplets in each specified size range. In most other situations, it is sufficient to simply know the mean size of emulsion droplets and the width of the distribution (9). Polydisperse emulsions can be characterized according to the general shape of the particle size distributions as being “monomodal,” “bimodal,” or “multimodal” depending on whether there are one, two, or more peaks in the distribution.
The number of droplets in most emulsions is extremely large, and so their size can vary continuously from some minimum value to some maximum value. When presenting particle size data, it is convenient to divide this size range into a number of discrete size classes and stipulate the number of droplets that fall into each class (9). The resulting data can then be represented in tabular form (Table 1) or plotted as a histogram that shows the number of droplets in each size class (Figure 2). Rather than presenting the number of droplets ni in each size class, it is often more informative to present the data as the number frequency, fi = ni/N, where N is the total number of droplets, or as the volume frequency, ϕi = vi/V, where vi is the volume of the droplets in the ith size class and V is the total volume of all the droplets in the emulsion. The shape of a particle size distribution changes appreciably depending on whether the fraction of droplets in each size category is presented as a number or a volume frequency (Figure 2). Hence, it is always important to clearly specify which parameter has been used when reporting particle size data. As the volume of a droplet is proportional to d3, the volume distribution is more representative of larger droplets present in the emulsion. The number distribution is more representative of droplets that may be small and therefore have low-volume fractions but may be present in large numbers.
|Size Class [μm]||di [μm]||Ni||fi [%]||ϕi [%]||C(di) [%]|
A particle size distribution can also be represented as a continuous curve, such as the distribution function F(di) or the cumulative function C(di). The (number) distribution function is constructed so that the area under the curve between two droplet sizes, di and di + δ di, is equal to the number of droplets ni in that size range, i.e., ni = F(di)δ di (9). This relationship can be used to convert a histogram to a distribution function, or vice versa. The cumulative function represents the percentage of droplets that are smaller than di and can be obtained through integration of the distribution function. The resulting curve has an S-shape that varies from 0% to 100% as the particle size increases. The particle size at which half the droplets are smaller and the other half are larger is known as the median droplet diameter, dm. The particle size distribution of an emulsion can also be modeled using mathematical theories (e.g., normal or log-normal distributions), which is convenient because the full data set can be described by a small number of parameters (9). Nevertheless, care should be taken to ensure that the mathematical model used realistically represents the actual data.
The size of droplets in a polydisperse emulsion may be expressed by one or two numbers, rather than stipulating the full particle size distribution (9). The most useful numbers are the mean diameter , which is a measure of the central tendency of the distribution, and the standard deviation, σ, which is a measure of the width of the distribution:
The above mean is also referred to as the mean length diameter, dL, because it represents the sum of the length of the droplets divided by the total number of droplets. It is also possible to express the mean droplet size in a number of other ways (Table 2). Each of these mean sizes has dimensions of length (meters), but stresses a different physical aspect of the distribution, e.g., the average length, surface area, or volume. For example, the volume-surface mean diameter is related to the surface area of droplets exposed to the continuous phase per unit volume of emulsion, AS:
|Name of Mean||Symbol||Definition|
This relationship is particularly useful as it allows one to calculate the total surface area of droplets in an emulsion, an important parameter that can be used to estimate the emulsifier concentration required to produce a kinetically stable emulsion. An appreciation of the various types of mean droplet diameter is also important because different experimental techniques used to measure droplet sizes are sensitive to different mean values (24). Consequently, it is always important to be clear about which mean diameter has been determined in an experiment when using or quoting droplet size data.
The importance of the particle size distribution in determining the physicochemical properties of food emulsions means that it is important to have analytical techniques to quantify this parameter. The size of the droplets in emulsions can be measured using a variety of different analytical methods, the most common being microscopy, light scattering, electrical pulse counting, sedimentation, and ultrasonic techniques (4). Each of these techniques has its own advantages and disadvantages. For example, some techniques are only suitable for analyzing very dilute emulsions (e.g., light scattering, electrical pulse counting), whereas others can be used to analyze concentrated emulsions in situ (e.g., ultrasonic spectrometry, NMR).
3.3 Droplet Charge
The electrical charge of emulsion droplets has an important impact on the stability of emulsion droplets and, in turn, influences physicochemical and organoleptic properties of food emulsions (1). The electrical charge on the droplets in food emulsions is usually the result of the adsorption of emulsifier molecules that contain ionized or ionizable groups, e.g., ionic surfactants, phospholipids, proteins, and polysaccharides (4). The magnitude and sign of the electrical charge at the droplet interface largely depends on type and concentration of surface-active molecules present at the interface, as well as pH and ionic composition of the aqueous phase. Droplet charge is a key parameter that determines not only how a droplet interacts with other charged species (e.g., emulsion droplets, polymers, mineral ions) but also how it behaves in the presence of an electrical field (which is the basis of experimental measurements of electrical charge). Charged species of opposite sign are attracted towards each other, whereas those of similar sign are repelled from each other. All of the droplets in an emulsion are usually coated with the same type of emulsifier and, as such, have the same electrical charge (if the emulsifier is ionized). When this charge is sufficiently large, the droplets are prevented from aggregating because of the electrostatic repulsion between them. The properties of emulsions stabilized by ionized emulsifiers are particularly sensitive to the pH and ionic strength of the aqueous phase. If the pH of the aqueous phase is adjusted so that the emulsifier loses its charge, or if salt is added to “screen” the electrostatic interactions between the droplets, the repulsive forces may no longer be strong enough to prevent the droplets from aggregating. Droplet aggregation is often undesirable in food emulsions because it can lead to an increase in emulsion viscosity and a decrease in creaming stability.
Electrostatic interactions also influence the interactions between emulsion droplets and other charged species, such as biopolymers, surfactants, vitamins, antioxidants, flavors, and minerals (2, 25-27). These interactions often have significant implications for the overall quality of an emulsion product. For example, volatility of a flavor may be reduced if flavor molecules are electrostatically attracted to the surface of emulsion droplet thereby altering the flavor profile of food emulsions (26). The susceptibility of oil droplets to lipid oxidation depends on whether the catalyst is electrostatically attracted to the droplet surface (27). In case of a repulsion of oxidation catalysts from the lipid–water interface, the extent of lipid oxidation in emulsions can be reduced. The accumulation of charged species at a droplet surface and the rate at which this accumulation takes place depends on the sign of their charge relative to that of the surface, the strength of the electrostatic interaction, their concentration, and the presence of any other charged species that might compete for the surface.
The electrical charge on an emulsion droplet can be manipulated by choosing emulsifiers with desirable charge characteristics (e.g., sign, magnitude, isoelectric point) and controlling the aqueous phase properties (e.g., pH and ionic strength). Consequently, it is possible to control the bulk physicochemical properties of emulsions by manipulating their electrical charge, e.g., aggregation stability, flavor distribution, and lipid oxidation. A variety of analytical techniques have been developed to measure the magnitude and sign of the charge on emulsion droplets, the most commonly used being particle electrophoresis and electroacoustics (4).
3.4 Droplet Crystallinity
Another parameter that influences the overall properties of the bulk emulsion is the physical state of the lipid droplets in an emulsion (17, 19, 28-31). Crystallization of lipid droplets in emulsions can be either beneficial or detrimental to product quality. Margarine and butter, the most common water-in-oil emulsions in the food industry, are prepared by a controlled destabilization of oil-in-water emulsions containing partly crystalline droplets. The stability of dairy cream to mechanical agitation and temperature cycling depends on the nature and extent of crystallization in milkfat globules. It should be noted that because the density of the phases can change as crystallization occurs, the rate at which milkfat droplets cream can be altered as droplets solidify. Emulsion manufacturers should therefore understand which factors influence the crystallization and melting of emulsified substances, and be aware of the effect that droplet phase transitions can have on the properties of emulsions.
The melting and crystallization behavior of emulsified substances can be quite different from that of the same substance in bulk (19). In particular, the degree of supercooling in emulsified materials is usually much greater than in bulk materials because the probability of finding a catalytic site that can promote nucleation is smaller in a particular droplet than in a bulk phase. A variety of experimental techniques are available for providing information about the crystallization and melting behavior of emulsion droplets, including microscopy, differential scanning calorimetry, NMR, and ultrasonics (4).
3.5 Droplet Interfacial Properties
The droplet interface is comprised of a narrow region (typically 2 nm to 20 nm thick) that surrounds each emulsion droplet and contains a mixture of oil, water, and emulsifier molecules (9, 10). The interfacial region typically does not contribute significantly to the total volume of an emulsion unless the droplet size is smaller than approximately 1 μm (Table 3). Contrary to this, the interfacial membrane does play a major role in determining bulk physicochemical and organoleptic properties of food emulsions. For this reason, food scientists are particularly interested in elucidating the factors that determine the composition, structure, thickness, and rheology of the interfacial region (2, 19, 32-35). The composition and structure of the interfacial region are determined by the type and concentration of surface-active species present, as well as by the events that occur both during and after emulsion formation, e.g., competitive adsorption. The thickness and rheology of the interfacial region influences the stability of emulsions to gravitational separation, coalescence, and flocculation, and determines the rate at which molecules leave or enter the droplets (4, 35). A variety of analytical techniques are available to provide information about the composition, thickness, and rheology of interfacial membranes. Some of these techniques can be directly applied to emulsions, whereas others can only be carried out at interfaces separating planar oil-water interfaces.
|Droplet Radius [μm]||No. of Droplets Per Gram Oil [g−1]||Droplet Surface Area Per Gram Oil [m2 g−1]||Percent Oil Molecules at Droplet Surface [%]|
|100||2.6 × 105||0.03||0.02|
|10||2.6 × 108||0.3||0.2|
|1||2.6 × 1011||3||1.8|
|0.1||2.6 × 1014||30||18|
3.6 Droplet-Droplet Interactions
Colloidal interactions govern whether emulsion droplets aggregate or remain as separate entities thereby impacting the characteristics of any aggregates formed, e.g., their size, shape, porosity, and deformability (2, 19, 36, 37). The rheological properties and creaming stability of many food emulsions depend on the extent of droplet aggregation and the characteristics of any aggregates formed (38, 39). The interactions between two emulsion droplets can be described in terms of an interdroplet pair potential (4). The interdroplet pair potential, w(h), is the energy required to bring two emulsion droplets from an infinite distance apart to a surface-to-surface separation of h (Figure 3). The overall interdroplet pair potential acting between two droplets is the sum of many different types of interactions, including van der Waals, steric, electrostatic, depletion, hydrophobic, and hydration interactions (4). These individual interactions can vary in their sign (attractive or repulsive), magnitude (weak to strong), and range (short to long) (Table 4). Each of the individual interactions usually has a simple monotonic dependence on surface-to-surface separation, but the sum of the interactions can exhibit a fairly complex behavior having both minima and maxima at certain separation distances (Figure 3). Generally, droplets tend to aggregate when attractive interactions dominate, but remain as individual entities when repulsive interactions dominate (4).
|Type||Sign||Magnitude||Range||Major Factors Influencing|
|Van der Waals||A||S||L||Refractive index, dielectric constant|
|Electrostatic||R or A||W–S||S–L||pH, ionic composition|
|Steric||R||S||S||Solvent quality, thickness|
|Depletion||A||W–M||M||Excluded species size and concentration|
|Hydrophobic||A||S||L||Surface hydrophobicity, temperature|
4 Emulsion Preparation
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
The process of converting two immiscible bulk-phase liquids into an emulsion, or of reducing the size of the droplets in a preexisting emulsion, is known as homogenization. The mechanical device designed to carry out this process is called a homogenizer (40). Homogenization can be separated into two categories depending on the nature of the starting material. The formation of an emulsion directly from two separate bulk liquids is referred to as primary homogenization, whereas the reduction in size of the droplets in an existing emulsion is referred to as secondary homogenization. The creation of a particular type of food emulsion may involve the use of either of these types of homogenization, or a combination of both. In large scale food processing operations, it is often more efficient to prepare an emulsion in two stages (1). First, the separate oil and water phases are converted to a coarse emulsion that contains fairly large droplets using one type of homogenizer (e.g., a high-speed blender). The droplets of the emulsion premix, having a low kinetic stability are further reduced in size using a different type of homogenizer (e.g., a high-pressure valve homogenizer). It should be noted that there is no clear distinction between most of the physical processes that occur during primary and secondary homogenization, e.g., mixing, droplet disruption, and droplet coalescence. Finally, some homogenizers are capable of producing emulsions with small droplet sizes directly from separate oil and water phases, e.g., high-intensity ultrasonicators, microfluidizers, or membrane homogenizers. As previously shown (see 3.2), many of the important characteristics and quality aspects of food emulsions depend on the size of the droplets they contain, including their stability, texture, appearance, and taste. Consequently, the major objective of homogenization is to create an emulsion in which the majority of droplets fall within an optimum size range that yields emulsions with properties specified by food manufacturers. We will therefore briefly discuss the major factors that determine the size of the droplets produced after the homogenization process.
4.2.1 Emulsifier Structure and Emulsifier Chemistry
One of the key factors to successfully produce a stable lipid emulsion is the addition of a suitable emulsifier. Although a detailed discussion of emulsifiers and emulsifier chemistry is beyond the scope of this chapter, a brief introduction to emulsifiers/surfactants seems appropriate. Surfactants are surface-active compounds that can adsorb to appropriate interfaces once dispersed in a solvent (41-48). Emulsifiers are those surfactants that are specifically used to stabilize emulsions. Surfactants are amphiphilic molecules (Figure 4). They have a hydrophilic and a hydrophobic area. The hydrophilic part is usually referred to as the head group of the surfactant, whereas the hydrophobic group is called the tail. The amphiphilic character of the molecule also causes the molecule to orient at interfaces, a reason for their surface activity. Surface or interfacial films (or layers) are therefore formed with the consequence that the surface tension is reduced. Hence, less energy is needed to disrupt the droplets further. Depending on the ratio of hydrophilic group to hydrophobic group, surfactants can have a higher solubility in either one of the two phases, oil or water. Surfactants are often much more complex than the simple example shown in Figure 4. Proteins, for example, are surface active and are classified as surfactants but they possess a highly complex three-dimensional structure that undergoes structural rearrangements upon adsorption at an interface. Other complex surfactants include modified starches or block copolymers. Specialty surfactants have been designed that contain not only one tail or head group but many to increase the steric stabilitization effect. There is no single classification scheme available to categorize all types of surfactants. The situation is also complicated by the fact that new surfactants are continuously developed. Nevertheless, there are a few characteristics listed below that can be used to classify the various surfactants:
Charge of the hydrophilic group (anionic, cationic, nonionic, or zwitterionic)
Nature of the lipophilic group (n-alkyl, iso-alkyl, (un)saturated alkyl)
Solubility in various solvents
Ratio of hydrophilic to hydrophobic group
One of the most important parameter is the charge of the head group. The charge of the surfactant can have a large impact on the chemical reactivity of the emulsion. For example, rates of lipid oxidation in emulsions that are susceptible to radical driven degradation processes differ dramatically depending on whether the emulsion was stabilized by a nonionic, cationic, or anionic emulsifier. This is because of repulsive or attractive interactions between the droplet interface and metal catalysts that may be present in the system. Another valuable classification tool involves the so-called HLB value. It is a measure of the ratio of the hydrophilic head group to the lipophilic tail. HLB values are very useful in order to select a surfactant for a particular application and are often listed by the surfactant manufacturer. Table 5 illustrates the ranges of HLB values that are most suited for a particular application.
4.2.2 Nonionic Emulsifiers
Nonionic surfactants are the principal surfactants encountered in food systems. They have several advantages over ionic surfactants. Nonionic surfactants can cover a wide range of HLB values. They are more environmentally friendly because they are easily biodegradable. The traditional source of the hydrophobic part of nonionic surfactants is fatty acid triglycerides, both from animal and plant sources. Primarily the higher members of the series such as palmitic, steric, oleic, and linoleic acid are used. Major utilization of fatty acids in surfactant chemistry involves the following reactions:
Esterification of fatty acids and polyhydric alcohols: The reaction of fatty acids with polyhydroxy compounds, such as ethylene glycol or glycerol, yields monoglycerides or polyglycerides. Prominent members of the higher polyol series, for example, include sorbitol and mannitol.
Alkanolamides of fatty acids: The condensation of fatty acids with monothanolamines or diethanolamines yields a group of products called alkanolamides. They usually contain a range of surface-active byproducts such as amino esters.
Oxyethylated surfactants: The multiple condensation of ethylene oxide with a hydrophobe that contains accessible hydrogen atoms yields a polyethyleneoxide with an attached hydrophobic tail group. Polyethyleneoxide surfactants constitute the major portion of nonionic surfactants. The ability to control the polymeric chain reaction has resulted in a group of surfactants that span a wide variety of HLB values. Often used in food applications are surfactants of the Tween® series that are obtained through a reaction of a sugar/fatty acid ester with ethylenoxide.
4.2.3 Anionic Emulsifiers
Anionic surfactants make about 75% of all the consumption of surface-active material. They are rarely encountered in the preparation of an actual food. The toxicity level of anionic surfactants is high and even small doses of anionic surfactants can cause allergic reactions and nausea. However, as they are very strong detergents, they can be used to solubilize components such as proteins. Due to their strong electrostatic repulsion, they are also very effective in stabilizing emulsions and are therefore often applied in technical emulsions such as emulsified lubricants.
The major subgroups of anionic surfactants include the alkali carboxylates (soaps), sulfates, sulfonates, and to a smaller degree, phosphates. The esterification of alcohol with sulfuric acid yields probably the best-studied surfactant, sodium dodecylsulfate or SDS. SDS, a sulfate ester, is an extremely effective emulsifier because of its high-electrostatic repulsion. Other sulfates are, for example, sulfated esters from fatty acids, sulfated ethers, and sulfated fats and oils. Sulfonates stem from the reaction of sulfonic acid with suitable substrates. Members of the class of sulfonates are, for example, sulfonic acid salts or aliphatic sulfonates. Other anionic surfactants include substances such as carboxylated soaps and esters of phosphoric acid.
4.2.4 Cationic Emulsifiers
Cationic surfactants are primarily recognized because of their strong bacteriostatic properties. While the total market share of cationic surfactants is less than 5%, they continue to play an important role as sanitizing and antiseptic agents, textile softeners, corrosion inhibitors, foam depressants, flotation chemicals, and as components in fungizides and germicides. Typically, cationic surfactants consist of a hydrophobic chain group derived from either fatty acid or petrochemical sources and a positively charged nitrogen atom. The hydrophobic group can be directly attached to the nitrogen or be indirectly linked via a bridging group such as a polyethyleneoxide. Alternatively, the nitrogen can also be part of a heterocyclic ring as is the case in alkylpyridinium salts.
4.3 Physical Basis of Homogenization
The physical processes that occur during homogenization can be highlighted by considering the formation of an emulsion from pure oil and pure water. When the two liquids are brought in contact, they tend to adopt the configuration that is thermodynamically most stable and has the lowest free energy, which consists of a layer of oil on top of a layer of water (Figure 5). This arrangement is adopted because it minimizes the unfavorable contact area between the two immiscible liquids, and because oil has a lower density than water. To create an emulsion, energy is needed to disrupt and intermingle oil and water phases. The energy is usually supplied in the form of some mechanical agitation (49). The oil droplets formed during the application of the mechanical agitation are constantly moving around and frequently collide and coalesce with neighboring droplets (49). If no more mechanical energy is supplied to the system, the droplets formed during the agitation process will move in the opposite direction of the gravitational field and eventually merge together to form a separate layer, i.e., they will phase separate (Figure 4). This process is enthalpically driven and favors the minimization of the contact area between the oil and water while the kinetics of the phase separation depends on the strength of the gravitational field and the nature of the two liquids.
To form an emulsion that is (kinetically) stable for a reasonable period of time, one must prevent the droplets from merging together after they have been formed (49, 50). This is achieved by having a sufficiently high concentration of emulsifier present during the homogenization process. Emulsifier molecules adsorb at the oil-water interface during homogenization to form a protective membrane that prevents droplets from coming into close contact required for coalescence. The size of droplets produced during homogenization, therefore, is a balance of two opposing physical processes: droplet disruption and droplet coalescence. The efficiency of the emulsification process increases if (1) the initial droplet size can be kept small and (2) the droplets are rapidly stabilized against coalescence once they are formed (Figure 5).
4.3.1 Droplet Disruption
The initial stages of primary homogenization involve the break-up and intermingling of the bulk oil and bulk aqueous phases so that fairly large droplets of one of the liquids become dispersed throughout the other liquid (49, 50). Later stages of primary homogenization, as well as the entire secondary homogenization, involves the disruption of larger droplets into smaller ones. Ultimately, the disruption of a droplet depends on a balance between interfacial forces that oppose enlargements of the interfacial area and disruptive forces generated within the homogenizer (49, 50). A thermodynamic consideration of a system with constant pressure, temperature, and composition but with varying interfacial areas yields the following expression for the free energy change (51):
Equation 5 illustrates that the overall free energy change of the system is a function of increases in the interfacial area and a system specific thermodynamic parameter, ∂G/∂A, also known as the interfacial tension γ. Equation (5) also indicates that the work required to deform and disrupt a droplet during homogenization must be significantly larger than γΔA (17, 50). This relationship also explains why emulsifiers, capable of readily adsorbing to the interfaces of emulsion droplets during homogenization and reducing their interfacial tension, will decrease the amount of work required for droplet disruption, thus improving homogenization efficiency.
The nature of the disruptive forces that act on droplets during the homogenization process depend on the flow conditions they experience (i.e., laminar or turbulent), and therefore on the type of homogenizer used (49, 52). For a droplet to be broken up during homogenization, the magnitude of the disruptive forces must exceed that of the interfacial forces and their duration must exceed the time required to deform and disrupt the droplet (53, 54). The susceptibility of emulsion droplets to disruption can be characterized by the Weber Number (We), which is the ratio of the disruptive forces to the interfacial forces (50). Above a characteristic critical Weber number, droplets are disrupted, below this Weber number they remain intact. The value of the critical Weber number generally depends on the ratio of the viscosity of the dispersed phase to the continuous phase, ηD/ηC, and theoretical or semi-empirical expressions for the Weber number have been derived for a number of different flow conditions found in homogenizers, e.g., laminar flow and turbulent flow (54).
4.3.2 Droplet Coalescence
Droplet-droplet collisions occur frequently during homogenization because of the intense mechanical forces experienced by emulsions inside homogenizers. If emulsion droplets are not covered by a sufficiently strong interfacial membrane, they will tend to coalesce with one another during a collision (49). Immediately after the disruption of an emulsion droplet, the freshly formed interfaces are insufficiently covered by emulsifier molecules and therefore the new droplets are highly susceptible to coalescence when they collide with their neighbors. To prevent coalescence it is necessary to rapidly form a stable membrane of emulsifier molecules that is able to induce repulsive interactions between the droplets. The size of droplets produced in the homogenization process, therefore, depends not only on process parameters but also on the kinetics of the emulsifier adsorption at droplet interfaces (tadsorption) relative to the rate of droplet-droplet collisions (tcollision). Thus, the flow situation in the homogenizer, the bulk physicochemical properties of the oil and aqueous phases, and the nature of the emulsifier used all impact the resulting droplet size (51). Droplet coalescence during homogenization can therefore be reduced by using an emulsifier with rapid adsorption kinetics or by increasing the emulsifier concentration that ensures that τadsorption/τcollision ≪ 1. The importance of emulsifier adsorption kinetics on the size of the droplets produced during homogenization has been demonstrated experimentally (55). Under the same homogenization conditions, it has been shown that emulsifiers that adsorb rapidly produce smaller droplet sizes than those that adsorb slowly. Most food emulsifiers do not adsorb rapidly enough to completely prevent droplet coalescence, and so the droplet size achieved during homogenization is greater than that which is theoretically possible (56).
4.4 Homogenization Devices
A number of different types of homogenization devices are used to produce food emulsions (Table 6). Each of these devices has its own advantages and disadvantages and is often best suited for a particular type of product. In selecting a homogenizer, one needs to consider volume-flow rates, the nature of the starting materials, the desired droplet size distribution, the required physicochemical properties of the final product, availability of space, power, or pressure requirements, and the cost of purchasing and operating the equipment. The most commonly used homogenizers in the food industry at present are high-speed blenders, high-pressure valve homogenizers, and colloid mills (Figure 6). Ultrasonic homogenizers, while rarely used in industry, have proven to be very useful on a laboratory scale because of the small amounts of sample that can be processed and their low cost.
|Homogenizer||Throughput||Relative Energy Efficiency||Minimum Droplet Size||Sample Viscosity|
|High-speed blender||Batch||Low||2 μm||Low to medium|
|Colloid mill||Continuous||Intermediate||1 μm||Medium to high|
|High-pressure homogenizer||Continuous||High||0.1 μm||Low to medium|
|Ultrasonic probe||Batch||Low||0.1 μm||Low to medium|
|Ultrasonic jet homogenizer||Continuous||High||1 μm||Low to medium|
|Micro-fluidizer||Continuous||High||<0.1 μm||Low to medium|
|Membrane processing||Batch or Continuous||High||0.3 μm||Low to medium|
High-speed blender: The disruption of droplets in a blender occurs mainly due to the existence of a turbulent flow situation. The energy input per unit volume is unevenly distributed in the apparatus. This results in a broad droplet size distribution. The major droplet disruption occurs in the immediate vicinity of the rotating blades where shear forces are highest, e.g., due to the presence of Taylor vortexes. The effectiveness of droplet disruption depends on the geometry of the mixer and the rotational speed of the blades. Operational parameters include blade and vessel geometries and rotation speed of blades.
High-pressure valve homogenizer: Within a high-pressure valve homogenizer, extensive droplet disruption occurs. The homogenization valve can have various geometries with externally adjustable gap sizes. The premix is pumped into a valve with pressures between 10–100 MPa. Within the annulus of the valve, velocities exceed more than 200 m/s. The average residence time of the premix in the valve is less than a few milliseconds. Due to the fast acceleration of the liquid in the annulus, the hydrostatic pressure in the annulus can drop below the vapor pressure of the liquid. As a consequence, steam bubbles are formed. The formation of steam bubbles is transient and bubbles collapse in the rear part of the annulus where pressure and temperature increase again. The collapse of these cavitational bubbles is the primary source of the mechanical energy that causes oil droplet disruption. The effectiveness of the droplet disruption in a high-pressure valve homogenizer can be directly related to the applicable pressure difference.
Colloid Mill: Colloid mills are rotor-stator systems that can be used to reduce the particle size distribution of both liquid dispersions (emulsions) and solid dispersions (suspensions). The emulsion or suspension is pumped through a narrow gap that is formed by the rotating inner cone and the stationary outer cone. The width of the annulus can be adjusted by changing the relative position of the two cones. The principal size reduction in colloid mills is due to the high shear forces that are caused by the velocity difference between the rotor and the stator surfaces. To increase wall friction and reduce slip, surfaces are usually not smooth but are roughened or toothed, which, in turn, changes the flow conditions from laminar to turbulent, thereby increasing the shear forces in the annulus.
High-Intensity Ultrasonicator: Droplets are disrupted within a field of high-intensity ultrasonic waves. Droplet disruption occurs either due to cavitation or because the frequency of the ultrasonic wave equals the resonance frequencies of the droplets. This causes the droplets to oscillate vigorously. Eventually, the oscillation becomes supercritical and the droplets are disrupted. The effectiveness of sonication, therefore, depends on the nature of the continuous and dispersed phase. The type of oil, as well as the nature of the surfactant, is the limiting factor for the minimal droplet size that can be achieved.
4.5 Factors Influencing Droplet Size
From an operational point of view, food manufacturers need to be able to optimize both the composition of an emulsion and the required homogenization process to achieve a product that fulfills predetermined design criteria. As the majority of emulsion characteristics depend primarily on the droplet size distribution, the specific factors that impact droplet size need to be discussed.
4.5.1 Emulsifier Type and Concentration
For a fixed concentration of oil, water, and emulsifier, there is a maximum interfacial area that can be completely covered by an emulsifier. As homogenization proceeds, the size of the droplets decreases and the interfacial area increases. Once the emulsion interfacial area increases above a certain level, there may be insufficient emulsifier present to completely cover the surface of any newly formed droplets. This will not only increase the energy required for subsequent droplet disruption but also increase the probability for droplet coalescence. The minimum size of stable droplets that can be produced during homogenization is governed by the type and concentration of emulsifier present:
where Γsat is the excess surface concentration of the emulsifier at saturation (in kg m−2), ϕ is the disperse phase volume fraction, and cS is the concentration of emulsifier in the emulsion (in kg m−3). For a polydisperse emulsion, the radius rmin is the volume to surface mean radius. The minimum droplet size that can be produced during homogenization can be decreased by increasing the emulsifier concentration, decreasing the droplet concentration or using an emulsifier with a smaller Γsat. For a 10% oil-in-water emulsion containing 1% of emulsifier, the minimum droplet radius is about 60 nm (assuming Γsat = 2 × 10−6 kg m−2). While Equation (5) provides a first estimation to design a homogenization process, it should be noted that the actual mean diameter of emulsion droplets after homogenization is generally greater than the theoretical minimum.
In order to attain the theoretical minimum droplet size, a homogenizer must be capable of generating a pressure gradient that is large enough to disrupt any droplets that are greater than rmin. This pressure gradient is given by the LaPlace pressure where Δ p = 2 γ/r. Some types of homogenizer are not capable of generating such high-pressure gradients and are therefore not suitable for producing emulsions with small droplet sizes, even though there may be sufficient emulsifier present (50). The emulsion must also spend sufficient time within the homogenization zone for all of the droplets to be completely disrupted. In general, the residence time of droplets within the homogenization zone is distributed and this distribution is impacted by operational conditions (flow speed) and the geometry of the homogenization zone. Design of the homogenization zone is therefore of crucial importance to the efficiency of the homogenization process.
A large variety of emulsifiers are used in the food industry, and each of these exhibits different characteristics during homogenization, e.g., the speed at which they adsorb, the maximum reduction in interfacial tension, and the effectiveness of the interfacial membrane to prevent droplet coalescence. A food manufacturer must select the most appropriate emulsifier for each type of food product, taking into account their performance during homogenization, solution conditions, cost, availability, legal status, ability to provide long-term stability, and the desired physicochemical properties of the product. It is generally recommended that food manufacturers closely consult with emulsifier manufacturers to select an appropriate emulsifier that is optimized for their particular application. In addition, classification schemes for emulsifiers are available that aide in the selection of a suitable emulsifier.
4.5.2 Energy Input
The size of the droplets in an emulsion can be reduced by increasing the amount of energy supplied during homogenization (as long as there are a sufficient number of emulsifier molecules to cover the surfaces of the droplets formed). The energy input depends on the nature of the homogenizer. In a high-speed blender, the energy input can be enhanced by increasing the rotational speed or the length of time that the sample is blended. In a high-pressure homogenizer, it can be enhanced by increasing the homogenization pressure or recirculating the emulsion through the device, i.e., increasing the number of passes through the homogenizer. In a colloid mill, it can be enhanced by reducing the size of the gap between the stator and rotator system, increasing the rotational speed, using disks with roughened surfaces, or increasing again the number of passes through the homogenizer. In a high-intensity ultrasonicator, the energy input can be enhanced by increasing the intensity of the ultrasonic wave or by sonicating for a longer time. In a microfluidizer, the energy input can be increased by increasing the velocity at which the liquids are brought into contact with each other or by recirculating the emulsion. In a membrane homogenizer, the energy input can be enhanced by increasing the pressure at which the liquid is forced through the membrane. Under a given set of homogenization conditions (energy input, temperature, composition) there is a certain size below which the emulsion droplets cannot be reduced even with repeated homogenization, and therefore homogenizing the system any longer would be inefficient. It should be noted that increasing the energy input usually leads to increased manufacturing costs, which may offset the benefits gained through smaller droplet sizes. A food manufacturer should, therefore, always establish processing conditions that provide an optimum compromise between droplet size, processing time, and cost (50).
Under most circumstances, the droplet size will decrease as the energy input is increased. Nevertheless, there may be occasions when increasing the energy actually leads to an increase in droplet size because the effectiveness of the emulsifier is reduced by excessive heating or exposure to high pressures. This can be particularly important for protein-stabilized emulsions, because the molecular structure and functional properties of proteins are particularly sensitive to changes in their environmental conditions. For example, globular proteins, such as β-lactoglobulin, are known to unfold and aggregate when they are heated above a certain temperature, which reduces their ability to stabilize emulsions.
4.5.3 Properties of Component Phases
The composition and physicochemical properties of both the oil and aqueous phases influence the size of the droplets produced during homogenization (52). Variations in the type of oil or aqueous phase will alter the viscosity ratio, ηD/ηC, which determines the minimum size that can be produced under steady-state conditions. The interfacial tension of the oil-water interface depends on the chemical characteristics of the lipid phase, e.g., molecular structure or presence of surface-active impurities, such as free fatty acids, monoacylglycerols, or diacylglycerols. These surface-active lipid components tend to accumulate at the oil-water interface and lower the interfacial tension, thus lowering the amount of energy required to disrupt a droplet.
The aqueous phase of an emulsion may contain a wide variety of components, including minerals, acids, bases, biopolymers, sugars, alcohols, ice crystals, and gas bubbles. Many of these components will alter the size of the droplets produced during homogenization because of their influence on rheology, interfacial tension, coalescence stability, or adsorption kinetics. For example, the presence of low concentrations of short chain alcohols in the aqueous phase of an emulsion reduces the size of the droplets produced during homogenization because of the reduction in interfacial tension (57). The presence of biopolymers in an aqueous phase has been shown to increase the droplet size produced during homogenization due to their ability to suppress the formation of small eddies during turbulence (50). Proteinstabilized emulsions cannot be produced close to the isoelectric point of a protein or at high-electrolyte concentrations because the proteins are highly susceptible to aggregation.
Experiments have shown that the smallest droplet size that can be achieved using a high-pressure valve homogenizer increases as the disperse phase volume fraction increases (52). There are a number of possible reasons for this, (1) increasing the viscosity of an emulsion may suppress the formation of eddies responsible for breaking up droplets, (2) if the emulsifier concentration is kept constant, there may be insufficient emulsifier molecules present to completely cover the droplets, and (3) the rate of droplet coalescence is increased.
Temperature may influence the size of droplets produced during homogenization. The viscosity of both the oil and aqueous phases is temperature dependent, and therefore, the minimum droplet size that can be produced may be altered because of a variation in the viscosity ratio, ηD/ηC. Heating an emulsion usually causes a slight reduction in the interfacial tension between the oil and water phases, which would be expected to facilitate the production of smaller droplets. However, certain types of emulsifiers lose their ability to stabilize emulsion droplets against aggregation when they are heated above a certain temperature. For example, when small molecule surfactants are heated close to their phase inversion temperature, they are no longer effective at preventing droplet coalescence, or when globular proteins are heated above a critical temperature, they unfold and aggregate. Alterations in temperature also influence the competitive adsorption of surface-active components, thereby altering interfacial composition (58).
The temperature is also important because it determines the physical state of the lipid phase. It is practically impossible to homogenize a fat that is either completely or substantially solid because it will not flow through a homogenizer or because of the huge amount of energy required to break up the fat crystals into small particles. There are also problems associated with the homogenization of oils that contain even small amounts of fat crystals because of partial coalescence. The crystals from one droplet may penetrate the surface of another droplet leading to the formation of an aggregate. Extensive aggregation leads to the generation of large particles and to a dramatic increase in the viscosity that, in the most extreme case, can cause the homogenizer to become blocked. For this reason, it is usually necessary to warm a sample prior to homogenization to ensure that the lipid phase is completely liquid. For example, milkfat is usually heated to about 40°C to melt all the crystals prior to homogenization (52).
4.6 Other Processing Steps
Homogenization is only one of the processing operations involved in the production of a food emulsion. Postprocessing and preprocessing operations can have a direct impact on the properties of the final emulsions. One of the most common operations carried out prior to emulsion homogenization is to disperse the various ingredients into the phase in which they are most soluble. Oil-soluble ingredients, such as vitamins, colors, antioxidants, phospholipids, and lipophilic surfactants, are usually mixed with the oil, and water-soluble ingredients, such as proteins, polysaccharides, sugars, salts, vitamins, colors, antioxidants, and hydrophilic surfactants, are usually mixed with the aqueous phase. The intensity and duration of the mixing process depends on the time required to solvate and uniformly distribute the ingredients. Adequate solvation is important for the functionality of a number of food components, e.g., the emulsifying properties of proteins are often improved by allowing them to hydrate in water for a few hours prior to homogenization (59). If the lipid phase contains any crystalline material, it is necessary to warm it to a temperature where all the fat crystals melt prior to homogenization; otherwise it is extremely difficult to create a stable emulsion (52, 60). One of the most common operations after homogenization is thermal processing to improve the microbiological stability of the product, e.g., pasteurization or sterilization. Thermal processing of an emulsion may have a significant impact on its quality and long-term physical stability because many ingredients used in food emulsions are heat sensitive, e.g. proteins, surfactants, and polysaccharides. Food emulsions may also be subjected to a variety of other processing operations during their manufacture that impact their quality and shelf life, including mixing, pumping, freezing, and drying.
5 Physicochemical Properties of Food Emulsions
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
5.1 Emulsion Stability
The term “emulsion stability” is broadly used to describe the ability of an emulsion to resist changes in its properties with time. The properties of an emulsion may evolve over time due to a variety of physical, chemical, or biochemical processes. From a technological standpoint, it is important to identify the dominant processes occurring in the system of interest because effective strategies can then be rationally designed to overcome the problem. A number of the most important physical mechanisms responsible for the instability of emulsions are shown schematically in Figure 5.
5.1.1 Gravitational Separation
Gravitational separation is one of the most common forms of instability in food emulsions and may result in either creaming or sedimentation depending on the relative densities of the dispersed and continuous phases. Creaming is the upward movement of droplets due to the fact that their density is lower than that of the surrounding liquid, whereas sedimentation is the downwards movement of droplets due to the fact that they have a higher density than the surrounding liquid. Liquid edible oils normally have lower densities than water and, as a result, creaming is more prevalent in oil-in-water emulsions, whereas sedimentation is more prevalent in water-in-oil emulsions. If emulsions contain fully or partially crystalline fats, the density of the lipid phase may increase above the density of water causing sedimentation instead of creaming. The creaming velocity of an isolated rigid spherical particle suspended in a Newtonian liquid obeys Stokes’ law:
where, r is the radius of the particle, g is the acceleration due to gravity, ρ is the density, η is the shear viscosity, and the subscripts 1 and 2 refer to the continuous and dispersed phases, respectively. The sign of vStokes determines whether the droplet moves upwards (+) or downwards (−). To a first approximation, the stability of a relatively dilute food emulsion to creaming can be estimated using Stokes’ law. For example, an oil droplet (ρ2 = 910 kg m−3) with a radius of 1 μm suspended in water (η1 = 1 mPa s, ρ1 = 1000 kg m−3) should theoretically cream at a velocity of about 17 mm per day. An emulsion containing droplets of this size would not have a particularly long shelf life. Stokes’ law highlights a number of strategies that food manufacturers can use to retard gravitational separation in emulsions, i.e., decreasing the density contrast between the two phases, decreasing the droplet radius, or increasing the viscosity of the continuous phase. Each of these strategies is used in the food industry, with the most appropriate one or combination depending on the nature of the emulsion.
It should be stressed that Stokes’ law is inappropriate for accurately predicting the stability of many food emulsions due to gravitational separation because they do not exist as dilute suspensions of rigid spheres suspended in a Newtonian fluid. If the droplet concentration is high, single droplets will not move independent of each other and droplet movement may be retarded due to increased packing and hydrodynamic interactions. For this reason, the theory has been extended to take into account various other factors, such as droplet fluidity, droplet concentration, the interfacial membrane, and non-Newtonian continuous phases (4). A semiempirical equation that gives relatively good predictions of the creaming behavior of concentrated emulsions has been derived (Hunter, 1989):
Here, ϕc and k are parameters that depend on the nature of the spherical particles, i.e., their size, polydispersity, and colloidal interactions. Typically, the values of ϕc and k for nonflocculated monodisperse colloidal suspensions are around 0.5–0.6 and 5.4, respectively. Normally, ϕc is taken as the volume fraction at which the spherical particles become closely packed. This equation predicts that the creaming velocity decreases as the droplet concentration increases, until creaming is completely suppressed once a critical disperse phase volume fraction (ϕc) is exceeded (Figure 7). In general, the value of ϕc depends on the packing of the droplets within an emulsion, which is governed by their polydispersity and colloidal interactions. Polydisperse droplets are able to fill the available space more effectively than monodisperse droplets because the small droplets can fit into the gaps between the larger ones (61), and so ϕc is increased. When the droplets are strongly attracted to each other, they can form a particle gel at relatively low-droplet concentrations, which prevents any droplet movement (Figure 7). When the droplets are strongly repelled from each other, their effective size increases, which also causes complete restriction of their movement at lower values of ϕc (62).
5.1.2 Droplet Aggregation
The droplets in emulsions are in continual motion because of the effects of thermal energy, gravity, or applied mechanical forces, and as they move about, they frequently collide with their neighbors (63, 64). After a collision, emulsion droplets may either move apart or remain aggregated, depending on the relative magnitude of the attractive and repulsive interactions between them. Droplets aggregate when there is a minimum in the interdroplet pair potential that is sufficiently deep and accessible to the droplets (Figure 3). The three major types of aggregation in food emulsions are flocculation, coalescence, and partial coalescence (1, 2, 17, 18).
Droplet flocculation is the process whereby two or more droplets come together to form an aggregate in which the droplets retain their individual integrity. It may be either advantageous or detrimental to emulsion quality depending on the nature of the food product. Flocculation accelerates the rate of gravitational separation in dilute emulsions, which is undesirable because it reduces their shelf life (65). It also causes a pronounced increase in emulsion viscosity and may even lead to the formation of a gel (66, 67) (Figure 8). Some food products are expected to have a low viscosity and therefore flocculation is detrimental. In other products, a controlled amount of flocculation may be advantageous because it leads to the creation of a desirable texture. Improvements in the quality of emulsion-based food products, therefore, depends on a better understanding of the factors that determine the degree of floc formation, the structure of the flocs formed, the rate at which flocculation proceeds, and the effect that flocculation has on the bulk physicochemical properties of emulsions (4).
Flocculation may occur in emulsions through a variety of different processes, described below, that either increase the attractive forces or decrease the repulsive forces between the droplets.
Reduced electrostatic repulsion. Electrostatically stabilized emulsions may flocculate when the electrostatic repulsive interactions between the droplets are reduced. This can be achieved by altering the pH so that the electrical charge on the droplets is reduced, adding multivalent counter ions that bind to the surface of the droplets and reduce the droplet charge, or increasing the ionic strength of the surrounding medium to screen the electrostatic interactions (21).
Increased depletion attraction. The presence of nonadsorbing colloidal particles, such as biopolymers or surfactant micelles, in the continuous phase of an emulsion causes an increase in the attractive force between the droplets due to an osmotic effect associated with the exclusion of colloidal particles from a narrow region surrounding each droplet. This attractive force increases as the concentration of colloidal particles increases, until eventually, it may become large enough to overcome the repulsive interactions between the droplets and cause them to flocculate (68-72). This type of droplet aggregation is usually referred to as depletion flocculation (17, 18).
Increased hydrophobic attraction. This type of interaction is important in emulsions that contain droplets that have some nonpolar regions exposed to the aqueous phase. A good example of this type of interaction is the effect of thermal processing on the flocculation stability of oil-in-water emulsions stabilized by globular proteins (66, 67, 73). At room temperature, whey-protein-stabilized emulsions (pH 7, 10 mM NaCl) are stable to flocculation because of the large electrostatic repulsion between the droplets, but when they are heated above 70°C they become unstable. The globular proteins adsorbed to the surface of the droplets unfold above this temperature and expose nonpolar amino acids that were originally located in the interior of the proteins (74, 75). Consequently, the droplet surface becomes more hydrophobic, which increases the hydrophobic attraction between droplets favoring flocculation (32, 67, 74).
Formation of biopolymer bridges. The addition of biopolymers to the aqueous phase of emulsions may promote flocculation by forming bridges between two or more droplets (64). Biopolymers may adsorb either directly to the bare surfaces of droplets or to the adsorbed emulsifier molecules that form the interfacial membrane (17). However, biopolymers will only bind to droplet surfaces if there is a sufficiently strong attractive interaction between segments of the biopolymer and the droplet surface. The principle molecular interactions that may be responsible for binding are hydrophobic and electrostatic (2, 76). For example, a positively charged biopolymer might adsorb to the surface of two negatively charged emulsion droplets, causing them to flocculate (77).
The development of a suitable strategy to prevent droplet flocculation in an emulsion, therefore, depends on identification of the physicochemical origin of flocculation in this particular system. In the most general terms, flocculation can be prevented by ensuring that the repulsive forces dominate the attractive forces.
Coalescence is the process whereby two or more liquid droplets merge together to form a single larger droplet (Figure 5). Coalescence is the principal mechanism by which an emulsion eventually attains its thermodynamically most stable state because the contact area between the oil and water phases decreases over the course of the process. Coalescence also causes emulsion droplets to cream or sediment more rapidly because of the droplet size increase. In oil-in-water emulsions, coalescence eventually leads to the formation of a separate oil layer, a process that is referred to as oiling off. In water-in-oil emulsions, it leads to the merging, sedimentation, and finally phase separation of water droplets.
Coalescence requires that the molecules of liquid within two or more emulsion droplets come into direct contact (12, 17, 18, 78). Droplets, therefore, need to be in close proximity, which is the case in highly concentrated emulsions or in flocculated emulsions or creamed layers, for example. In a subsequent step, a disruption of the interfacial membrane must occur to allow the liquid molecules to come into direct contact. The rate at which coalescence proceeds and the physical mechanism by which it occurs is highly dependent on the nature of the emulsifier used to stabilize the system. Coalescence is an extremely complex process because it depends not only on the extent of gravitational, colloidal, hydrodynamic, and mechanical forces that act on the droplets but the intrinsic properties of the droplet membrane as well. Improving the stability of an emulsion to coalescence may be achieved by preventing droplet flocculation, preventing formation of a creamed layer, reducing the droplet concentration, and altering the rheological properties of the interfacial membrane to improve rupture resistance.
220.127.116.11 Partial Coalescence
Partial coalescence occurs when two or more partially crystalline oil droplets come into contact and form an irregularly shaped aggregate. It is initiated when a solid fat crystal from one droplet penetrates the interface to the liquid phase of a second oil droplet (17, 28-31). Consequently, the lipid crystal is surrounded by lipid molecules instead of water molecules, which is thermodynamically favored, i.e., the fat crystal is better wetted by liquid oil rather than water. Over time the droplets may continue to merge to further reduce the surface area of lipid that is exposed to water. Nevertheless, the aggregates partly retain the shape of droplets from which they were formed due to the low mobility of molecules in fat crystal networks (17, 19, 28, 60).
Partial coalescence only occurs in emulsions that contain partially crystalline regions. This is because one of the key requirements for partial coalescence is penetration into the liquid phase. (30). If all droplets were completely liquid, they would undergo normal coalescence. If all droplets were completely solid, they would undergo flocculation rather than partial coalescence because of the lack of liquid lipid regions that had sufficient molecular mobility required for merging. Thus, one can expect an “optimum” solid fat content at which partial coalescence would be highest. Indeed it has been found that increasing the solid fat content of the droplets causes an initial increase in the partial coalescence rate until a maximum value is reached, after which the partial coalescence rate decreases (30). The solid fat content at which this maximum rate occurs depends on the morphology and location of the crystals within the droplets, as well as the magnitude of the applied shear stresses (28, 31).
Partial coalescence is particularly important in dairy products, because milkfat globules are partly crystalline over a fairly wide temperature range (60, 79). The application of shear forces in combination with temperature cycling to cream-containing partly crystalline milkfat globules can cause partial coalescence, which leads to a marked increase in solution viscosity (28, 80, 81). Partial coalescence is an essential process in the production of ice cream, whipped toppings, butter, and margarine (1, 82-85). Oil-in-water emulsions are cooled to a temperature where the droplets are partly crystalline and a shear force is applied, which leads to droplet aggregation via partial coalescence (60). In butter and margarine, aggregation results in phase inversion (85), whereas in ice cream and whipped cream, the aggregated fat droplets form a network that surrounds incorporated air pockets thereby improving ice cream stability and texture (86, 87).
5.1.3 Ostwald Ripening
Ostwald ripening is the process whereby large droplets grow at the expense of smaller ones because of diffusion driven mass transport of dispersed phase molecules from one droplet to another through the intervening continuous phase (88, 89). Ostwald ripening has traditionally not been recognized as a significant cause of emulsion instability. For example, it has been argued that the low solubility of triacylglycerols in water result in low mass transport rates (1). More recently however, it has been demonstrated that Ostwald ripening may in fact be the primary source of emulsion instability in oil-in-water emulsions that contain lipids with higher solubility (90) or in emulsions that contain alcohol, e.g., cream liquors (91). Due to the nature of the process, it should be noted that traditional methods to improve emulsion stability, such as increasing the emulsifier concentration or choosing an alternative emulsifier, may not be effective. Ostwald ripening occurs because the solubility of the molecules in a spherical droplet in the surrounding aqueous phase in the vicinity of the droplet interface increases as the curvature of the interface decreases i.e., the size of the droplet decreases (88):
Here, Vm is the molar volume of the solute, γ is the interfacial tension, S(∞) is the solubility of the solute in the continuous phase for a droplet with infinite curvature (a planar interface), and S(r) is the solubility of the solute above a curved interface of radius r. Hence, there is a higher concentration of dissolved lipid molecules around a small droplet than around a larger one. Solubilized lipid molecules will move from the smaller droplets to the larger droplets because of this concentration gradient. Once steady state has been achieved, the rate of Ostwald ripening ω is given by (88).
where D is the diffusion coefficient of the solute and is the mean size of the emulsion. This equation indicates that the change in droplet size with time becomes more rapid as the solubility of the molecules in the continuous phase increases (Figure 9). Therefore, Ostwald ripening may be reduced by reducing the solubility of the lipid in the aqueous phase or by reducing the surface tension (90).
5.1.4 Phase Inversion
Phase inversion is the process whereby a system changes from an oil-in-water emulsion to a water-in-oil emulsion, or vice versa (Figure 5). Phase inversion is an essential step in the manufacture of a number of important food products, including butter and margarine (1, 60, 85). In most other foods, phase inversion is undesirable because it has an adverse effect on the products appearance, texture, stability, and taste and should therefore be avoided.
Phase inversion can be triggered by some alteration in the composition or environmental conditions of an emulsion, e.g., disperse phase volume fraction, emulsifier type, emulsifier concentration, solvent conditions, temperature, or mechanical agitation (2, 92). Only certain types of emulsion are capable of undergoing phase inversion, rather than being completely broken down into their component phases. These emulsions are capable of existing in a kinetically stable state after the phase inversion has taken place. It is usually necessary to agitate an emulsion during the phase inversion process, otherwise it will separate into its component phases. The physicochemical basis of phase inversion is believed to be extremely complex, involving aspects of flocculation, coalescence, partial coalescence, and emulsion formation.
5.1.5 Chemical Instability
Molecular species present in an emulsion can be subject to chemical or biochemical reactions that alter their perceived quality. One of the most important types of chemical changes in food emulsions is the result of the oxidation of unsaturated lipids (93, 94). In some foods, a limited amount of lipid oxidation is desirable because it leads to the development of a characteristic taste or smell, e.g., cheeses (94). On the other hand, lipid oxidation is undesirable in many foods because it leads to the development of undesirable off-flavors (“rancidity”) and potentially toxic reaction products (95). The high susceptibility of polyunsaturated lipids to lipid oxidation has restricted their incorporation into many food products, which is unfortunate because greater consumption of polyunsaturated lipids is beneficial to health (96). Consequently, there is considerable interest in development of effective strategies for retarding the oxidation of unsaturated lipids in food emulsions.
A great deal of research has been carried out to elucidate lipid oxidation mechanisms in bulk fats and oils (97, 98). This research has provided important insights into the factors that influence lipid oxidation and strategies to control it. Nevertheless, the application of this knowledge to food emulsions is often limited because the lipids are dispersed as discrete phases dispersed in structurally and compositionally heterogeneous matrices (99). In these foods, the organization of the lipid molecules within the system, as well as their interactions with other types of molecules in their immediate vicinity, has a pronounced influence on their susceptibility to lipid oxidation (25). Recent experimental work has shown that the susceptibility of emulsified lipids to oxidation depends on a variety of factors. These include the chemical structure of the unsaturated lipids; the type, concentration, and location of antioxidants and proxidants; the oxygen concentration; the temperature; the interfacial characteristics of the droplets, and the purity of the ingredients (27). Studies of the influence of structure and composition of emulsions on the rate of chemical reactions are likely to be an important area of research in the future.
5.2 Emulsion Rheology
The application of a stress to a material causes it to deform or to flow (100, 101). The extent of the deformation and flow depends on the physicochemical properties of the material. Rheology is the science that is concerned with the relationship between applied stresses and the deformation and flow of matter (101). Most rheological tests involve the application of a stress to a material and a measurement of the resulting flow or deformation (100). Sophisticated and sensitive analytical techniques are available for characterizing the rheological behavior of complex food emulsions, which are widely used in industrial, government, and university research laboratories. The knowledge gained from application of these techniques is important to food scientists for a number of reasons (1, 102-105). Many of the sensory attributes of food emulsions are directly related to their rheological properties, e.g., creaminess, thickness, smoothness, spreadability, pourabilty, flowability, brittleness, and hardness. A food manufacturer, therefore, must be able to design and produce a product that has the rheological properties expected by the consumer. The shelf life of many food emulsions depends on the rheological characteristics of the component phases, e.g., the creaming of oil droplets depends on the viscosity of the aqueous phase. Information about the rheology of food products is used by food engineers to design processing operations that depend on the way that a food behaves when it flows through a pipe, is stirred, or is packed into containers. Rheological measurements are also used by food scientists as an analytical tool to provide fundamental insights about the structural organization and interactions of the components within emulsions (11, 106).
5.2.1 Mathematical Modeling of Emulsion Rheology
Food emulsions are compositionally and structurally complex materials that can exhibit a wide range of different rheological behavior, ranging from low-viscosity fluids (such as milk and fruit juice beverages) to solids with elastic moduli (such as refrigerated margarine or butter). Our ability to control the rheological properties of food emulsions depends on a quantitative understanding of the relationship between rheology, composition, and microstructure. A variety of theories have been used to relate the rheological properties of emulsions to their composition and microstructure. In general, the apparent viscosity of an emulsion can be described by the following equation:
where η1 is the viscosity of the continuous phase, η2 is the viscosity of the dispersed phase, ϕ is the dispersed phase volume fraction, r is the droplet radius, w(h) is the interaction potential between the droplets, and τ is the applied shear stress. The precise nature of the equation used to describe the rheological properties of an emulsion depends on the characteristics of the system, e.g. droplet concentration, droplet interactions, and continuous phase rheology. Exact expressions of the relationship between the rheology of colloidal suspensions and their composition/structure are only available in certain limiting cases, such as Einsteins’ equation for a dilute suspension of rigid spherical particles given below.
This equation illustrates that the rheology of a dilute emulsion is proportional to the rheology of the continuous phase and increases with increasing droplet concentration. In concentrated emulsions, the rheology is influenced by hydrodynamic interactions associated with the relative motion of neighboring particles. At low-particle concentrations, hydrodynamic interactions mainly occur between pairs of particles, but as the particle concentration increases, three or more particles may be involved (107). As the particle concentration increases, the measured viscosity becomes larger than that predicted by the Einstein equation because these additional hydrodynamic interactions lead to a greater degree of energy dissipation. The Einstein equation can be extended to account for the effects of these interactions by including additional volume fraction terms (108):
The value of the constants, a, b, c, etc., can either be determined experimentally or theoretically (107). For a colloidal dispersion of rigid spherical particles the value of a is 2.5. Therefore, Equation 13 equals the Einstein equation (Eq. 12) at low-volume fractions. A rigorous theoretical treatment of the interactions between pairs of droplets has established that b = 6.2 for rigid spherical particles. Experiments have shown that Equation 13 can be used up to particle concentrations of about 10% with a = 2.5 and b = 6.2 for colloidal dispersions in the absence of long-range colloidal interactions (107). It is difficult to theoretically determine the value of higher order terms in Equation 13 because of the mathematical complexities involved in describing interactions between three or more particles. Instead, a semiempirical approach is used to develop equations that describe the viscosity of concentrated colloidal dispersions. One of the most widely used equations was derived by Dougherty and Krieger and is applicable across the whole volume fraction range (Figure 9) (9, 109).
where, [η] is the intrinsic viscosity and ϕc is the maximum packing volume fraction, which is usually taken to be an adjustable parameter that is determined experimentally. Physically, ϕc is related to the particle volume fraction at which the spheres become close packed. The intrinsic viscosity is 2.5 for spherical particles, but may be appreciably larger for nonspherical or aggregated particles (8). Typically, the value of ϕc is between about 0.6 and 0.7 for spheres that do not interact via long-range colloidal interactions (9), but it may be considerably lower for suspensions in which there are strong long-range attractive or repulsive interactions between the droplets. This is because the effective volume fraction of the particles in the colloidal dispersion is greater than the actual volume fraction of the particles, so that the maximum packing volume fraction is reached at lower particle concentrations (62).
5.2.2 Factors Influencing Emulsion Rheology
A variety of factors determine the rheological properties of food emulsions. Some of the most important of these factors are highlighted below.
Continuous phase rheology. The viscosity of most food emulsions is dominated by the rheology of the continuous phase (Equation 13). One of the most effective means of modifying the rheology of an emulsion is, therefore, to add a thickening or gelling agent to the continuous phase. The main exception to this rule is in systems that contain a network of aggregated particles. In these systems, the rheological properties are largely determined by the number and strength of the attractive forces between the aggregated particles.
Disperse phase volume fraction. The viscosity of food emulsions tends to increase with increased disperse phase volume fraction (Figure 10). The viscosity increases relatively slowly, with ϕ at low droplet concentrations, but increases steeply when the droplets become closely packed together. At higher droplet concentrations, the particle network formed has predominantly elastic characteristics.
Droplet-droplet interactions. The nature of the colloidal interactions between the droplets in an emulsion is one of the most important factors determining its rheological behavior. When the interactions are long range and repulsive, the effective volume fraction of the dispersed phase may be significantly greater than its actual volume fraction (ϕeff = ϕ (1 + δ/r)3), and so the emulsion viscosity increases. When interactions between the droplets are sufficiently attractive, the effective volume fraction of the dispersed phase is increased because of droplet flocculation, which results in an increase in emulsion viscosity. The rheological properties of an emulsion therefore depend on the relative magnitude of the attractive (mainly van der Waals, hydrophobic, and depletion interactions) and repulsive (mainly electrostatic, steric, and thermal fluctuation interactions) colloidal interactions.
Droplet size. Droplet size may influence the rheology of emulsions in a variety of ways. First, the viscosity of relatively concentrated suspensions (>30%) tends to decrease with increasing droplet size due to Brownian motion effects (108-110). This effect also causes emulsions to exhibit shear thinning behavior. At low-shear stresses, the particles have a three-dimensional isotropic and random distribution because of their Brownian motion (11). As the shear stress increases the particles become more ordered along the stream lines to form “strings” or “layers” of particles that offer less resistance to the fluid flow and therefore a decrease in viscosity (Figure 11). The viscosity decreases from a high constant value at low shear-stresses (η0) to a low constant value at high-shear stresses (η∞). The shear thinning behavior of an emulsion is characterized by a critical shear stress, which corresponds to the stress where the viscosity has decreased by 50% between the low and high shear stress values. This critical shear stress increases with decreasing particle size, i.e., Brownian motion effects are more important for smaller droplets. Second, the droplet size influences the relative importance of the attractive and repulsive interactions between droplets, which may influence the rheology because it changes the effective volume fraction of the droplets (see above).
5.3 Emulsion Appearance
The appearance of an emulsion is one of the most important factors influencing its perceived quality, as it is usually the first sensory impression that a consumer makes of a product (4, 111). A better understanding of the factors that determine emulsion appearance would therefore aid in the design of emulsion-based food products with improved quality. When a light beam is incident upon the surface of an emulsion, a portion of the incident light beam is transmitted through the emulsion while another portion is reflected. The relative proportions of light transmitted and reflected at different wavelengths depend on the scattering and adsorption of the light wave by the emulsion. Light scattering and absorption depend on size, concentration, refractive index, and spatial distribution of droplets, as well as the presence of any chromophoric materials (e.g., dyes). Hence, the overall appearance of an emulsion is influenced by its structure and composition. Scattering is largely responsible for the “turbidity,” “opacity,” or “lightness” of an emulsion, whereas absorption is largely responsible for “chromaticity” (blueness, greenness, redness, etc). It should be stressed that the overall appearance of an emulsion also depends on the nature of the light source and detector used (112-114). Finally, color of emulsion is also impacted by the nature of the lipids themselves that is the number and position of double bonds.
5.3.1 Mathematical Modeling of Emulsion Color
Human beings have great difficulty objectively quantifying the color of objects, so color is normally quantified instrumentally in terms of “tristimulus coordinates,” such as the L * a* b* system specified by the Commission International de l’Eclairage (CIE) (114). The advantage of using the tristimulus coordinate system is that the color of an object can be described in terms of just three mathematical variables. It is then possible to determine whether an object meets some predefined quality criteria in a quantitative manner. For example, in the L* a* b* color space, L* is lightness, and a* and b* are color coordinates: where L* = 0 is black, L* = 100 is white, + a* is the red direction, −a* is the green direction, +b* is the yellow direction, and −b* is the blue direction, (114). The overall color intensity of a product can be characterized in terms of its chroma, C = (a2 + b2)1/2. One of the major advances in recent years has been the development of a theoretical approach to relate the tristimulus color coordinates of emulsions to their composition (dye and droplet concentration) and microstructure (particle size distribution) (115). This approach has led to the development of relationships given below.
where α (λ) is the absorption spectra of the dye solution, c is the concentration of dye present, ϕ is the disperse phase volume fraction, r is the droplet radius, and n is the ratio of the refractive indices of the dispersed to the continuous phases. These equations can be used to predict the influence of emulsion composition and microstructure on product appearance, which reduces the number of time-consuming and laborious experiments required in a laboratory. A number of systematic experimental studies have recently been carried out to determine the influence of composition and microstructure on the color of oil-in-water emulsions (116-121). These measurements are in excellent qualitative agreement with predictions made using the light scattering theory mentioned above. However, the quantitative agreement is still fairly poor, mainly because of problems associated with accounting for the optical measurement system, although a number of methods of overcoming this problem have been proposed (115).
5.3.2 Factors Influencing Emulsion Color
Disperse phase volume fraction. As the droplet concentration is increased, more and more light is scattered. Emulsion lightness (L) increases and emulsion chromaticity (C) decreases with increasing disperse phase volume fraction (Figure 12). L and C change steeply when the droplet concentration is increased from 0 wt% to 5 wt%, but then remain relatively constant at higher droplet concentrations (117-119).
Droplet size. The magnitude and direction of light scattered by a particle depend on the ratio of its radius to the wavelength of light. Emulsion lightness increases with droplet radius from 0 nm to 100 nm, has a maximum value around 100 nm, and then decreases as the droplet radius is increased further (119).
Relative refractive index. The scattering efficiency of a particle increases as the contrast in refractive index between the particle and the surrounding liquid increases. Emulsion lightness, therefore, is high when the refractive index of the droplets is either much smaller or much greater than the refractive index of the continuous phase, but decreases when the refractive index ratio tends towards unity. Indeed, it is possible to prepare optically transparent emulsions with high droplet concentrations by matching the refractive index of the continuous phase to that of the disperse phase by adding water-soluble solutes, such as glycerol or sucrose (62, 116). Refractive index matching is particularly useful in some emulsion studies because it means that the emulsions can be analyzed using spectroscopic techniques that require optically transparent solutions, such as UV-visible spectrophotometry, fluorescence, and CD.
5.4 Emulsion Flavor
Flavor plays a critical role in determining the quality of a food emulsion during consumption. The term “flavor” refers to those volatile components in foods that are sensed by receptors in the nose (aroma) and those nonvolatile components that are sensed by receptors in the tongue and the inside of the mouth (taste) (122, 123). In addition, certain components in foods may also contribute to flavor because of their influence on the perceived texture (mouthfeel) (124). The flavor of a food is therefore a combination of aroma, taste, and mouthfeel, with aroma usually being the most important (122). Flavor perception is an extremely complicated process that depends on a combination of physicochemical, biological, and psychological phenomena (125). Before a food is placed in the mouth, its flavor is perceived principally through those volatile components that are inhaled directly into the nasal cavity. After the food is placed in the mouth, the flavor is determined by nonvolatile molecules, which leave the food and are sensed by receptors on the tongue and the inside of the mouth, as well as by those volatile molecules that are drawn into the nasal cavity through the pharynx at the back of the mouth (122). The interactions between flavor molecules and human receptors that lead to the perceived flavor of a food are extremely complicated and are still poorly understood (123). In addition, expectations and eating habits vary from individual to individual, so that the same food may be perceived as tasting different by two separate individuals or by a single individual at different times. This section focuses on the physicochemical aspects of flavor partitioning and release in foods, because these are the most relevant topics to emulsion science.
5.4.1 Flavor Partitioning
The perception of a flavor depends on the precise location of the flavor molecules within an emulsion. The aroma is determined by the presence of volatile molecules in the vapor phase above an emulsion (122, 126). Most flavors are perceived more intensely when they are present in the aqueous phase, rather than in the oil phase (127, 128). Certain flavor molecules may associate with the interfacial region, which alters their concentration in the vapor and aqueous phases (129). It is therefore important to establish the factors that determine the partitioning of flavor molecules within an emulsion. An emulsion system can be conveniently divided into four phases between which the flavor molecules distribute themselves: the interior of the droplets, the continuous phase, the oil-water interfacial region, and the vapor phase above the emulsion. The relative concentration of the flavor molecules in each of these regions depends on their molecular structure and the properties of each of the phases (130, 131).
A number of the most important factors that influence the equilibrium distribution of flavors in food emulsions are listed below.
Flavor partition coefficients. The equilibrium distribution of a particular flavor molecule between two phases (e.g., oil-water, air-water, or air-oil) is characterized by an equilibrium partition function. These partition coefficients determine the distribution of the flavor molecules between the oil, water, and head space phases of an emulsion.
Surface activity. Many flavor molecules are amphiphilic in character, having both nonpolar and polar regions. These molecules will tend to accumulate at an oil-water interface.
Droplet concentration. The concentration of flavor molecules in the headspace of an emulsion depends on the disperse phase volume fraction, i.e., the ratio of oil to water. Previous studies have shown that there is a decrease in the fraction of a nonpolar flavor in the vapor phase as the oil content increases, whereas the amount of a polar flavor is relatively unaffected. Thus, nonpolar flavors in an emulsion become more odorous as the fat content is decreased, whereas the polar flavors remain relatively unchanged. This has important consequences when deciding the type and concentration of flavors to use in low-fat analogs of existing emulsion-based food products.
Flavor Binding. Many proteins and carbohydrates are capable of binding flavor molecules, and therefore altering their distribution within an emulsion (131-135). Flavor binding can cause a significant alteration in the perceived flavor of a food. This alteration is often detrimental to food quality because it changes the characteristic flavor profile, but it can also be beneficial when the bound molecules are off-flavors. A flavor chemist must therefore take binding effects into account when formulating the flavor of a particular product.
Solubilization. Surfactants are normally used to physically stabilize emulsion droplets against aggregation by providing a protective membrane around the droplet. Nevertheless, there is often enough free surfactant present in an aqueous phase to form surfactant micelles. These surfactant micelles are capable of solubilizing the nonpolar molecules in their hydrophobic interior, which increases the affinity of nonpolar flavors for the aqueous phase. By a similar argument, reverse micelles in an oil phase are capable of solubilizing polar flavor molecules.
5.4.2 Flavor Release
Flavor release is the process whereby flavor molecules move out of a food and into the surrounding saliva or vapor phase during mastication (126, 127). The release of flavors from a food material occurs under extremely complex and dynamic conditions (136). A food usually spends a relatively short period (typically 1 to 30 seconds) in the mouth before being swallowed. During this period, it is diluted with saliva, experiences temperature changes, and is subjected to a variety of mechanical forces. Mastication may therefore cause dramatic changes in the structural characteristics of a food emulsion.
During mastication, nonvolatile flavor molecules must move from within the food, through the saliva to the taste receptors on the tongue, and the inside of the mouth, whereas volatile flavor molecules must move from the food, through the saliva and into the gas phase, where they are carried to the aroma receptors in the nasal cavity. The two major factors that determine the rate at which these processes occur are the equilibrium partition coefficient (because this determines the initial flavor concentration gradients at the various boundaries) and the mass transfer coefficient (because this determines the speed at which the molecules move from one location to another). A variety of mathematical models have been developed to describe the release of flavor molecules from oil-in-water emulsions.
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
Over the past few years, there has been a growing emphasis on the understanding of the colloidal basis of properties of food emulsions, rather than just treating them as a “black box” whose properties could be characterized in terms of certain empirical parameters. Researchers are attempting to quantitatively relate the properties of food emulsions to the characteristics, interactions, and spatial distribution of the droplets they contain. A wide variety of analytical, mathematical, and computational techniques are being developed and utilized to achieve this objective. Powerful commercial instruments are widely available to quantify the colloidal characteristics of both dilute and concentrated emulsions, e.g., droplet size, concentration, and electrical charge (137-139). Theoretical, computational, and experimental work is providing a much better understanding of the various types of colloidal interactions that operate between emulsion droplets (140, 141). The cost, sensitivity, and range of commercial rheometers are continually improving (109). New analytical instruments are being developed that will enable researchers to measure changes in structure and bulk physicochemical properties. Traditional microscopic techniques are being refined so that they can be used to characterize the microstructure of delicate materials, such as emulsions (142). In addition, new microscopic technologies are being developed to characterize the organization of molecules at an interface (143-145). Advances in our understanding of the relationship between emulsion properties and colloidal characteristics are also being made through development of more comprehensive physical theories (107, 146, 147) and the utilization of powerful computational techniques (36, 37, 148). The application of these new concepts and tools will eventually lead to a much better understanding of the colloidal basis of emulsion properties. This knowledge will enable food manufacturers to design foods in a more rational fashion, which should eventually lead to improvements in product quality and reductions in manufacturing costs.
- Top of page
- Droplet Characteristics
- Emulsion Preparation
- Physicochemical Properties of Food Emulsions
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