Polymeric Steric Interactions 351 Polymeric Emulsifiers

In Section 3.3, we saw that van der Waals interactions always operate between emulsion droplets and that these interactions are strong enough to cause droplets to aggregate, unless there is a sufficiently strong repulsive interaction to prevent them from coming close together. When emulsion droplets are surrounded by a layer of electrically charged emulsifier molecules, they may be stabilized against aggregation by electrostatic repulsion (Section 3.4). Nevertheless, many food emulsions are stable to droplet aggregation, despite being surrounded by a layer of emulsifier molecules that has no electrical charge, which indicates that other types of repulsive interaction also play an important role in stabilizing these systems (Dickinson and Stainsby 1982, Dickinson 1992). The most important of these is polymeric steric stabilization, which is due to the presence of polymeric emulsifiers at the oil-water interface. To be effective at preventing droplet aggregation, steric interactions must be comparable, both in magnitude and range (but opposite in sign), to van der Waals and any other attractive interactions.

Most emulsifiers used in the food industry are either partly or entirely polymeric: proteins are polymers of amino acids, polysaccharides are polymers of sugars, and many nonionic surfactants have polar head groups which are polymers of oxyethylene (Chapter 4). The conformation of a polymeric emulsifier at an interface depends on the number, type, and sequence of monomers along its backbone (Dalgleish 1989, 1995, 1996a,b; Damodaran 1989, 1990, 1996; Dickinson and McClements 1995). An improved understanding of the relationship between the interfacial conformation of polymeric emulsifiers and their ability to stabi

FIGURE 3.14 Orientation of some polymeric emulsifiers at an oil-water interface: (a) small-molecule surfactants, (b) flexible biopolymers, and (c) globular biopolymers.

FIGURE 3.14 Orientation of some polymeric emulsifiers at an oil-water interface: (a) small-molecule surfactants, (b) flexible biopolymers, and (c) globular biopolymers.

lize emulsions is particularly important for food scientists because it enables them to create and select ingredients in a more systematic fashion.

An emulsifier tends to adopt an interfacial conformation which minimizes the free energy of the system (Damodaran 1989). The conformational free energy is determined by various enthalpic (intermolecular interaction energies) and entropic (configurational entropy and solvation) contributions. In practice, the major factor which determines the interfacial conformation of food emulsifiers is the hydrophobic effect (i.e., the tendency for a molecule to adopt an arrangement which minimizes the number of unfavorable contacts between polar and nonpolar groups) (Damodaran 1989, Dickinson and McClements 1995). Thus, small-molecule surfactants adopt a conformation in which the hydrocarbon tails protrude into the oil phase, while the hydrophilic head groups protrude into the aqueous phase (Figure 3.14a). Flexible biopolymers, such as casein or modified starches, exist as a series of loops, tails, and trains at an interface (Figure 3.14b). The predominantly hydrophilic or hydrophobic segments which protrude into the aqueous or oil phases (respectively) are referred to as loops or tails, whereas the predominantly neutral segments that lie flat against the interface are referred to as trains (Dickinson 1992). Immediately after adsorption, compact biopolymers (such as globular proteins) tend to adopt an orientation which maximizes the contact area between any hydrophobic patches on their surface and the oil phase (Damodaran 1989). After adsorption, the protein molecules may change their conformation in response to their new environment (Dickinson and McClements 1995; Corredig and Dalgleish 1995; Dalgleish 1996a,b). It must be stressed that many biopolymers take an appreciable time to undergo conformational changes, and therefore the interface may not be at thermodynamic equilibrium (which is assumed in many theories describing their behavior).

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