Interfacial adsorption and rheological behavior of β-casein at the water/hexane interface at different pH

The equilibrium and dynamic interfacial properties of β-casein (βCS) adsorbed at the water/hexane interface have been studied systematically over a wide range of pH. The results are analyzed in the context of a recently developed thermodynamic model, which is not only applied to characterize the adsorption behavior of βCS layers, but at the same time the visco-elastic properties of the surface layer. The equilibrium model can be adequately used for the adsorption layer when they are in a quasi-equilibrium state, however, it turned out that reliable information is provided only in the vicinity of equilibrium. Thus, the interfacial structure attained by the protein at the water/hexane interface resulting from the cohesive interaction between oil phase and the hydrophobic parts of the protein molecules is satisfactorily accounted in view of the results presented. The dilational viscosity of the adsorbed βCS layers is found to be very small and therefore, the interfacial layers behave essentially elastic. The analysis of the interfacial elasticity provides information on the conformational transitions within the interfacial layer. The selection of the model parameters is based on the slow structural change of βCS adsorption layers with increasing pH of the solution. The maxima encountered in the dynamic elasticity values even at pH 5, where βCS is in its most compact conformation, clearly suggest that the interfacial layer undergoes structural changes depending on the age of the adsorption layer.

Surface pressure isotherms at different pH used for a possible further improvement of the thermodynamic adsorption model of β-casein at the water/hexane interface.Figure optionsDownload as PowerPoint slide