Surface dilatational behavior of β-casein at the solution/air interface at different pH values

The surface tension and dilatational visco-elasticity isotherms for β-casein determined at pH 7 and 9 are essentially the same, but differ remarkably from those measured at the isoelectric point (i.e.p.) of the protein at pH 5. A recently developed thermodynamic model is applied to the experimental data, which were not only obtained at equilibrium, but also under quasi-equilibrium conditions. It turned out that such a model can be adequately applied to data obtained not too far from the equilibrium state of a protein adsorption layer. The change in the model parameters allows to understand slow changes in the structure of the adsorption layer. Even at pH 5, where β-casein is most hydrophobic and in its most compact conformation, the data point to the fact that conformational changes may happen at the interface upon adsorption.