ADSORPTION BEHAVIOR OF WHEY-PROTEIN ISOLATE AND CASEINATE IN SOYA OIL-IN-WATER EMULSIONS

A technique to determine directly the amount of adsorbed protein in protein-stabilized oil-in-water emulsions (20 wt%. soya oil, pH 7) has been developed employing sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and photometric densitometry. Using this technique the adsorption behaviour of whey protein isolate (WPI) and caseinate at the emulsion droplet surface has been studied. The minimum limiting surface concentrations required to stabilize emulsions containing caseinate or WPI were found to be 1 and 1.5 mg/m2 respectively. At protein concentrations >2.25 wt% in the aqueous phase, surface concentration of both proteins was at a high limit of approximately 3.2 mg/m2. WPI exhibited behaviour indicative of adsorption in distinct multilayers. Competitive adsorption of proteins in emulsions stabilized by mixtures of WPI and caseinate was studied using the same technique, and it was found that when protein concentration was the limiting factor, WPI and caseinate adsorbed to the same extent. However, with an excess of protein, caseinate adsorbed in preference to WPI.