Impact of interfacial composition on co-oxidation of lipids and proteins in oil-in-water emulsions: Competitive displacement of casein by surfactants

In this study, the interfacial composition of protein-stabilized emulsions was manipulated by adding a small molecule surfactant to induce displacement of the adsorbed protein. The rate and extent of lipid and protein oxidation was then measured for emulsions with different interfacial compositions. Lipid and protein co-oxidation were studied in walnut oil-in-water (O/W) emulsions (5% v/v oil, 1.0% w/v protein, 0–1.0% w/v Tween 20, pH 7). The interfacial tension, protein surface load, surface potential, and mean particle size of the emulsions decreased as the surfactant concentration increased, suggesting that caseinate was displaced by Tween 20. Emulsions stabilized solely by caseinate exhibited relatively slow lipid oxidation when incubated in the dark at 45 °C for up to 8 days, as determined by lipid hydroperoxides and 2-thiobarbituric acid-reactive substances (TBARS). In contrast, the caseinate itself was rapidly oxidized, as shown by carbonyl formation, intrinsic fluorescence loss, and electrophoresis measurements. Competitive displacement of adsorbed caseinate by Tween 20 reduced protein oxidation but promoted lipid oxidation, indicating that adsorbed proteins were more sensitive to oxidation themselves but also more efficient at protecting lipids from oxidation than non-adsorbed ones. These results demonstrate the important role that interfacial composition plays on the oxidative stability of food emulsions containing mixtures of emulsifiers.