Interfacial behavior and rheology of oxidized proteins and lipids

Food proteins such as dairy proteins are widely used in food applications, for example, in emulsion or foam-based products. Often production processes for such products involve thermal or mechanical stresses, which can damage the native structure of proteins, and affect their functionality. In addition, in the presence of other reactive molecules (e.g., oxidizing unsaturated lipids), proteins
can be subjected to chemical modifications (such as oxidation). However, little is known on how such modifications of proteins affect their interfacial behavior and their emulsifying properties. We investigated the effects of oxidation of dairy proteins, and of lipid oxidation products, on protein adsorption kinetics and surface rheological properties of oil-water interfaces stabilized by whey
proteins, using a drop tensiometer. The effects on dilatational properties were determined in large amplitude oscillatory deformations, and analyzed using Lissajous curves. Interfacial films formed from oxidized whey proteins were less elastic at small deformation values, compared to native whey proteins, and displayed strain hardening in compression and strain thinning in extension, upon increasing deformation. The presence of oxidized lipids also greatly decreased the elasticity of native protein-based interfaces. Protein and lipid oxidation can thus alter the formation and properties of oil-water interfaces, which may in turn affect the quality of food emulsions.