Most food emulsions contain droplets which are coated by a mixture of different types of surface-active components (e.g., proteins, polysaccharides, phospholipids, and surfactants), rather than a single chemically pure type (Dickinson 1992; Dalgleish 1996a,b; Walstra 1996b). Bulk physicochemical properties of emulsions, such as their ease of formation, stability, and texture, are governed by the nature of the interface, and therefore it is important for food scientists to understand the factors which determine the composition of the interfacial region (Dalgleish 1996a). Interfacial composition is determined by the type and concentration of surface-active components present, their relative affinity for the interface, the method used to prepare the emulsion, solution conditions (e.g., temperature, pH, and ionic strength), and the history of the emulsion. In this section, some of the most important factors which influence the interfacial composition of food emulsions are reviewed.
Interfacial composition depends on the relative adsorption rates of the various types of surface-active components which are present during the homogenization process, as well as any changes that may occur after homogenization (Dickinson 1992). Immediately after homogenization, the droplets tend to be coated by those surface-active molecules which absorb to the interface most rapidly under turbulent conditions (Section 5.7). Nevertheless, the interfacial composition may change with time after homogenization because some of the surface-active molecules that were initially present at the droplet surface are displaced by molecules in the bulk liquid that have a greater affinity for the surface. Alternatively, additional surface-active components may be added to the continuous phase of an emulsion after homogenization, and these may displace some of the emulsifier molecules at the droplet surface. The displacement of emulsifier molecules from an interface may be retarded if they are capable of undergoing some form of conformational change that enables them to bind strongly to their neighbors. For example, when an emulsion is heated under alkaline conditions, P-lactoglobulin is capable of unfolding and forming extensive covalent (disulfide) bonds with its neighbors, which makes it difficult to displace from an interface, even by more surface-active molecules (McClements et al. 1993d). The factors that determine the rate at
which emulsifier molecules are adsorbed to an interface are discussed in Section 5.7. In this section, the factors which determine the relative affinity of surface-active molecules for an interface are considered.
The affinity of an emulsifier molecule for an interface can be described by its adsorption efficiency and its surface activity (Dickinson 1992). The adsorption efficiency is a measure of the minimum amount of emulsifier required to saturate an interface, whereas the surface activity is a measure of the maximum decrease in interfacial tension achievable when an interface is completely saturated (i.e., nmax). Adsorption efficiencies and surface activities depend on the molecular structure of emulsifiers, as well as the prevailing environmental conditions. Amphiphilic biopolymers, such as proteins, tend to have higher adsorption efficiencies, but lower surface activities, than small-molecule surfactants (Figure 5.12).
Emulsifier molecules in solution partition themselves between the bulk solution and the interfacial region. The equilibrium constant between the adsorbed and unadsorbed states is proportional to eE/kT, where E is a binding energy (Hiemenz 1986, Dickinson 1992). A small molecule tends to have one fairly strong binding site (its hydrophobic tail), whereas biopolymer molecules tend to have a large number of relatively weak binding sites (nonpolar amino acid side groups). The overall binding energy of a biopolymer molecule tends to be greater than that of a small-molecule surfactant, and therefore it binds more efficiently (i.e., less emulsifier must be added to the bulk aqueous phase before the interface becomes completely saturated). For the same reason, small-molecule surfactants tend to rapidly exchange between the adsorbed and unadsorbed states, whereas biopolymer molecules tend to remain at an interface for extended periods after adsorption. The relatively slow desorption of biopolymer molecules from an interface compared to small-molecule surfactants is best illustrated by an analogy. Consider a flock of birds that are all connected to each other by a piece of string. At any particular time, an individual bird may decide to fly, but it cannot get very far from the ground because it is held back by the other birds. It is only when a large number of birds simultaneously decide to take off together that they are all able to leave the ground, which is statistically very unlikely. Thus, at low emulsifier concentrations, biopolymers have a greater affinity for an interface than small-molecule surfactants.
On the other hand, small-molecule surfactants tend to decrease the interfacial tension by a greater amount than biopolymer molecules at concentrations where the interface is completely saturated, because they pack more efficiently and therefore screen the unfavorable interactions between the oil and water molecules more effectively. Thus, at high emulsifier concentrations, small-molecule surfactants have a greater affinity for an interface than biopoly-
mers and will tend to displace them. This accounts for the ability of relatively high concentrations of surfactant molecules (e.g., 1% Tween 20) to displace proteins from the surface of oil droplets (Dickinson et al. 1993c; Courthaudon et al. 1991a,b,c,d; Dickinson and Tanai 1992; Dickinson 1992; Dickinson and Iveson 1993). Small-molecule surfactants are often added to ice cream premixes so as to displace the proteins from the surface of the milk fat globules prior to cooling and shearing (Goff et al. 1987). This causes the droplets to become more susceptible to partial coalescence, which leads to the formation of a network of aggregated droplets which stabilizes the air bubbles and gives the final product its characteristic shelf life (Berger 1976).
The ease with which proteins can be displaced from an oil-water interface often depends on the age of the interfacial membrane. Some globular proteins become surface denatured after adsorption to an interface because of the change in their environment (Dickinson and Matsumura 1991, McClements et al. 1993d). When the proteins unfold, they expose amino acids that are capable of forming disulfide bonds with their neighbors and thus form an interfacial membrane that is partly stabilized by covalent bonds. This accounts for the experimental observation that the ease with which P-lactoglobulin can be displaced from the surface of oil droplets decreases as the emulsion ages (Dalgleish 1996a).
The phase in which a surfactant is most soluble also determines its effectiveness at displacing proteins from an interface. For example, water-soluble surfactants have been shown to be more effective at displacing proteins from the surface of oil droplets than oil-soluble surfactants (Dickinson et al. 1993a,b).
Interfacial composition also depends on the relative concentration of the different types of emulsifiers present. A minor surface-active ingredient may make up a substantial fraction of the droplet membrane when the droplets are large (small total surface area), but have a negligible contribution when the droplets are small (large total surface area).
The thermal history of an emulsion often has an important influence on the composition of its interfacial layer. This is particularly true for globular proteins that are capable of undergoing conformational changes at elevated temperatures. The interfacial concentration of P-lactoglobulin in oil-in-water emulsions increases when they are heated to 70°C (Dickinson and Hong 1994). In addition, the protein becomes more difficult to displace from the interface using small-molecule surfactants. The most likely explanation for this behavior is the thermal denaturation of the P-lactoglobulin molecules at this temperature. Thermal denaturation causes the molecules to unfold and expose amino acids that were originally located in their interior, including amino acids with nonpolar and cysteine side groups. The increase in surface hydrophobicity of the protein enhances its affinity for the droplet surface, as well as for other protein molecules, which accounts for the increase in interfacial concentration. The ability of the protein to form disulfide bonds with its neighbors leads to a covalently bonded film, which accounts for the difficulty in displacing the protein with surfactants. Temperature has been found to influence the ability of small-molecule surfactants to displace P-casein from the surface of oil droplets (Dickinson and Tanai 1992). A minimum in the interfacial concentration of protein was observed at temperatures between 5 and 10°C.
The pH and ionic strength of the aqueous phase containing the emulsifier molecules also have an important influence on adsorption kinetics and interfacial composition (Hunt and Dalgleish 1994, 1996). Potassium chloride has been shown to affect the competition between proteins for an interface (Hunt and Dalgleish 1995). Increasing the concentration of KCl present in the aqueous phase prior to homogenization causes a decrease in P-lactoglobulin and an increase in a-lactalbumin at the interface of oil-in-water emulsions at pH 7, but has no influence on interfacial composition for emulsions at pH 3. Changes in the relative proportions of the different types of caseins were also observed for emulsions stabilized by caseinate when the KCl concentration was increased. Potassium chloride was also found to alter the competitive adsorption of proteins added after homogenization (Hunt and Dalgleish 1996).
The above discussion highlights the wide variety of factors which can influence interfacial composition, such as the emulsifier concentration, emulsifier type, solution conditions, temperature, and time. For this reason, a great deal of research is being carried out to establish the relative importance of each of these factors and to establish the relationship between interfacial composition and the bulk physicochemical properties of food emulsions.
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