Surface shear viscometry as a probe of protein-protein interactions in mixed milk protein films adsorbed at the oil-water interface

Time-dependent surface viscosities are reported for films adsorbed from binary mixtures of the proteins α-lactalbumin, β-lactoglobulin and β-casein. The measurements were made at a planar interface between n-tetradecane and various protein solutions (10 −3 wt% of each protein, pH 7,25°C) using a Couette-type torsionwire surface viscometer operating at very low shear-rate. Differences in behaviour between simultaneous and sequential exposure of the pairs of proteins to the interface were investigated. Some experiments were performed with chemically modified β-lactoglobulin samples whose disulphide bonds had been cleaved and blocked. Displacement of one protein by another (e.g. α-lactalbumin by β-casein) is indicated by a sudden drop in surface viscosity immediately after addition of the second protein. In systems containing β-lactoglobulin, the long-time surface viscosity is very sensitive to the adsorption time of β-lactoglobulin prior to addition of the second protein. Blocking the disulphide bonds in β-lactoglobulin leads to a much faster approach to a steady-state surface viscosity. This is interpreted in terms of a much more rapid unfolding of the disordered molecules of modified β-lactoglobulin at the oil-water interface. We conclude that surface viscosity experiments give useful and sensitive information about competitive adsorption and cooperative interactions in mixed protein films.