COMPETITIVE EFFECTS IN THE ADSORBED LAYER OF OIL-IN-WATER EMULSIONS STABILIZED BY BETA-LACTOGLOBULIN-TWEEN 20 MIXTURES

Competitive adsorption has been investigated in oil-in-water emulsions stabilised using mixtures of the protein beta-lactoglobulin and the surfactant Tween 20 and correlated with changes in the composition and physical properties of the adsorbed layer in oil-water-oil thin films. The emulsifying activity of a fixed concentration (3 mg ml-1) of beta-lactoglobulin was studied as a function of increasing Tween 20 concentration by measurement of the specific surface area (SSA) of the emulsion. The SSA remained relatively constant up to a Tween 20 : beta-lactoglobulin molar ratio (R) of 4. The SSA decreased sharply at higher R values. An increase in the level of incorporation of Tween 20 into the adsorbed layer of emulsions was also observed at R = 4. Thin films formed between oil droplets in the continuous phase of emulsions containing fluorescent-labelled beta-lactoglobulin revealed that surface diffusion was observed in samples of composition R = 4 or greater. The difference between observations made here at the oil/water interface and previous studies at the air/water interfaces where transitions in composition and diffusion were observed at R = 1 appears to be explained by a shear-induced conformational change in the beta-lactoglobulin that occurs during homogenisation.