Surface Properties of Protein Alcoholic Solutions: II. Surface Dilational Rheology

Dynamic surface dilational properties of alcoholic solutions of proteins (Bovine Serum Albumine (BSA) and β-casein) were studied by the damped longitudinal waves (0.1-1 Hz) method as a function of the ethanol content. At low ethanol contents (0-5%, v/v) the BSA solutions and the β-casein at 12%, v/v, exhibited viscoelastic behavior. At high ethanol content, the BSA solution surface displayed an elastic behavior. Absolute surface rheological coefficients (high-frequency elasticity ETo, low-frequency elasticity E(0), the constant rate of relaxation kr , and the intrinsic surface viscosity ηs) of the surface at its quasiequilibrium were obtained from the frequency spectra of the waves' characteristics. We have introduced two models: one based on the adsorption-desorption of protein aggregates and another based on a conformational change surface reaction, which could fit our results. The decrease of ETo, kr, and ηs when ethanol content increases has been related to the decrease of surface coverage by protein.