TIME-DEPENDENT COMPETITIVE ADSORPTION OF MILK-PROTEINS AND SURFACTANTS IN OIL-IN-WATER EMULSIONS

Competitive adsorption of pure milk proteins and non-ionic surfactants has been studied in model oil-in-water emulsions (4 g kg-1 beta-lactoglobulin or beta-casein, 200 g kg-1 n-hexadecane) as a function of the age of the adsorbed protein layer at the oil-water interface. With beta-lactoglobulin-stabilised emulsions containing oil-soluble surfactant C12E2 (diethylene glycol n-dodecyl ether), there is found to be a steadily increasing amount of protein associated with the emulsion droplets over a few hours following emulsification. Addition of water-soluble surfactant Tween 20 (polyoxyethylene (20) sorbitan monolaurate) to a beta-lactoglobulin-stabilised emulsion (with or without C12E2) leads to less protein displacement if the emulsion is aged prior to addition of Tween 20. Moderate additions of C12E2 or Tween 20 produce no time dependence in the competitive adsorption in beta-casein-stabilised emulsions, although some time dependence is observed when C12E2 and a high concentration of Tween 20 are present together. Crystallisation of the oil phase in beta-casein-stabilised emulsions at pH 7 leads to a lowering of the measured protein surface concentration, especially in the presence of C12E2, and a reduction in the surfactant to protein molar ratio required for complete protein displacement by water-soluble surfactant (Tween 20 or octaethylene glycol n-dodecyl ether). Under more acidic conditions of pH 5 or pH 3, the surface coverage and ease of displacement of beta-lactoglobulin at the surface of liquid emulsion droplets is substantially different from that under neutral pH conditions.