Van der Waals Electrostatic and Hydrophobic

The droplet surfaces in many food emulsions acquire some hydrophobic character during their manufacture, storage, or consumption. A typical example is a whey-protein-stabilized emulsion which is subjected to heat (Monahan et al. 1996, Demetriades et al. 1997b). Heating

FIGURE 3.26 Influence of hydrophobic interactions on the interdroplet pair potential of electrostatically stabilized emulsions (r = 1 |lm, I = 0.1 M).

the emulsion above 65°C causes the protein molecules adsorbed to the oil-water interface to partially unfold and thus expose some of the nonpolar amino acids to the aqueous phase. The overall interdroplet pair potential for this type of system is given by:

w(h) = wvdw (h) + welectrostatic (h) + whydrophobic (h) (3.32)

The dependence of the overall interdroplet pair potential on droplet separation for the same system as shown in Figure 3.22 but for droplets which have different degrees of surface hydrophobicity is shown in Figure 3.26. As the hydrophobicity of the droplet surface increases, the hydrophobic attraction increases, which causes a decrease in the height of the energy barrier. When the surface hydrophobicity is sufficiently large, the energy barrier becomes so small that the droplets aggregate into the primary minimum. This accounts for the experimental observation that whey-protein-stabilized emulsions become more susceptible to aggregation when they are heated above a temperature where the protein molecules unfold (Demetriades et al. 1997b).

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