INFLUENCE OF EMULSIFIER ON COMPETITIVE ADSORPTION OF ALPHA-S-CASEIN PLUS BETA-LACTOGLOBULIN IN OIL-IN-WATER EMULSIONS

The influence of Tween 20 [polyoxyethylene(20) sorbitan monolaurate] on the competitive adsorption of two milk proteins, alpha(s)-casein and beta-lactoglobulin, has been investigated in hydrocarbon oil-in-water emulsions (20 wt.% n-tetradecane, pH 7, 0.5 wt.% protein in aqueous phase, 0.4 wt.% protein in total emulsion). For emulsions made with alpha(s)-casein + beta-lactoglobulin (1:1 weight ratio), the presence of emulsifier during homogenization leads to more displacement of beta-lactoglobulin than alpha(s)-casein, but preferential displacement of the globular protein by the disordered protein does not occur when emulsifier is added after homogenization. Addition of alpha(s)-casein to emulsions made with beta-lactoglobulin does not lead to any displacement of the globular protein from the oil-water interface, irrespective of whether Tween 20 was present or not during emulsion formation. The results indicate that the competitive adsorption behaviour of beta-lactoglobulin with another protein is strongly dependent on whether both proteins are exposed simultaneously to the interface during emulsification - as opposed to beta-lactoglobulin being adsorbed first during emulsification and the second protein being added some time later. It is clear that the kinetics of competitive protein exchange in emulsions between the interface and the bulk aqueous phase is influenced significantly by the presence of emulsifier.