Exchange reactions between whey proteins and caseins in heated soya oil-in-water emulsion systems — overall aspects of the reaction

The heat-induced exchange of caseins and serum proteins on the interfaces of oil-in-water emulsion droplets has been studied. At temperatures above 40°C, the proteins from whey protein isolate were capable of displacing adsorbed caseins. As the whey proteins β-lactoglobulin and α-lactalbumin adsorbed, αs1- and β-caseins were desorbed, principally the former. The αs2- and κ-caseins were not displaced. The rate of the displacement or exchange reaction was temperature-dependent, being almost undetectable at room temperature, but complete within 2 min at 80°C. The extent to which the caseins were displaced by whey proteins was also temperature dependent. The displacement reaction was neither affected by ionic strength, nor were any of the reactions apparently dependent on sulphydryl exchange reactions. On the other hand, the extents of displacement of the major caseins by the whey proteins were pH-dependent to a limited extent. It is evident that the behavior is not simply governed by equilibrium processes between adsorbed and non-adsorbed proteins, and this is discussed.