Competitive effects in the adsorbed layer of oil-in-water emulsions stabilised by β-lactoglobulin–Tween 20 mixtures

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Abstract
Competitive adsorption has been investigated in oil-in-water emulsions stabilised using mixtures of the protein β-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 β-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 : β-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 β-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 β-lactoglobulin that occurs during homogenisation.