Structure and rheological properties of oil-water and air-water interfaces stabilized with micellar casein isolate and whey protein isolate mixtures

The mechanical properties of oil-water (O-W) and air-water (A-W) interfaces stabilized with mixtures of micellar casein isolate and whey protein isolate at different ratios were investigated by large-amplitude oscillatory dilatational rheology (LAOD). The composition of the layer of proteins adsorbed at the O-W interfaces was investigated by multiphoton excitation microscopy. The structure of A-W interfaces was visualized by atomic force microscopy. The results obtained for O-W interfaces showed that casein preferentially adsorbs and dom-inates the interface even when present in mixtures in a very low proportion. The LAOD results show that the interfacial layer formed by casein is weaker and more brittle compared with the one formed by whey proteins. In case of A-W interfaces, casein is more surface active and displays faster diffusion, and leads to a decrease of surface tension to lower values compared with whey protein. Casein will co-adsorb with whey protein at the A-W interface, and the dilatational response of the viscoelastic solid-like network normally formed by pure whey proteins at the interface is significantly affected by casein, particularly at large deformation amplitudes. The interfacial layer appears to become more brittle with an increasing proportion of caseins in the mixture.