INTERFACIAL PROPERTIES OF MILK PROTEIN-STABILIZED EMULSIONS AS INFLUENCED BY PROTEIN-CONCENTRATION

Interfacial area, protein load, and interfacial energetics were determined on emulsions produced from sodium caseinate (CAS) and whey protein isolate (WPI). Protein concentration in the aqueous phase varied from 0.125 to 1%. At low concentrations, whey protein emulsion showed larger interfacial area (smaller oil droplets) than casein emulsions. However, for concentrations higher than 0.5% the reverse trend was observed. Protein load increased with protein concentration, but leveled off at a concentration of 0.5% for whey protein emulsions. From contact angle measurements on hydrated layers, the interfacial interaction energy between oil droplets was calculated. As protein concentration increased, the short-range polar (AB) contribution to interaction energy became repulsive while the long-range nonpolar (LW) contribution remained attractive. The degree of orientation of the hydration layer was calculated from the AB contribution to oil droplet surface energy. Increasing protein concentration reduced water molecule orientation at the surface of oil droplets. The hydration layer of casein-stabilized oil droplets showed a lower degree of orientation than the hydration layer of whey protein-stabilized oil droplets. Results are discussed in relation to emulsion stability.