FACTORS WHICH AFFECT OIL EXCHANGE BETWEEN OIL-IN-WATER EMULSION DROPLETS STABILIZED BY WHEY-PROTEIN ISOLATE - PROTEIN-CONCENTRATION, DROPLET SIZE AND ETHANOL

The ability of whey protein isolate (WPI) to enhance the exchange of oil molecules between droplets in an oil-in-water emulsion was investigated. Emulsions stabilized by WPI were prepared which initially contained a mixture of 10 wt.% n-hexadecane droplets and 10 wt.% octadecane droplets. The composition of the droplets changed with time because of interdroplet oil exchange. The extent of exchange was monitored by measuring the change in the crystallization behavior of the droplets, which is related to their composition, using differential scanning calorimetry, The rate of oil exchange increased when additional protein was added to the aqueous phase of the emulsions (0-4.4 wt.%) and was proportional to the specific surface area of the droplets (0.3-2.3 m2 g-1 of emulsion). Emulsions containing no additional WPI in the aqueous phase exhibited an appreciable rate of oil exchange. We propose that oil exchange occurs because of a dynamic equilibrium between oil in the emulsion droplets and that solubilized in the aqueous phase. The non-zero rate of oil exchange in the emulsions containing low WPI concentrations may occur because of the finite solubility of hydrocarbons in water, whereas the increase in interdroplet oil exchange with increasing protein concentration is probably because the proteins bind and transfer oil molecules. The addition of ethanol (0-30 wt.%) to the aqueous phase of the emulsions dramatically increased the rate of oil exchange, most likely due to the ability of ethanol to enhance the solubility of the hydrocarbons in water.