Identification of critical concentrations determining foam ability and stability of β-lactoglobulin

To understand the properties of protein stabilized foam, quantitative parameters, such as the concentration dependence of the foam properties need to be determined. Recently, a concept was proposed that predicts the emulsifying ability (i.e. the droplet size in emulsions) based on different parameters, including the protein concentration. The aim of the present study is to investigate whether a similar concept can be applied to describe the foam ability and stability of protein stabilized foams. To achieve this, the foam, thin film and molecular properties of β-lactoglobulin (BLG) were determined at different concentrations and different pH values (pH 3-7). At each pH, a certain critical concentration for foam ability C_FA, could be identified above which the set foam volume was reached, while below that value the set volume was not reached. Furthermore, for all pH another critical concentration (C_crr32) at C > C_FA was identified as the point where the bubble radius (measured at the end of foam formation) reached a minimal value. The foam ability increased with increasing pH (pH 3-7). The difference in foam ability as a function of pH was reflected in the adsorption rate (slope Π/t^0.5 curve) of BLG. The foam stability increased with increasing concentration at each pH value but even in the protein rich regime where C > C_crr32 different foam stabilities were observed, which were highest at pH 7.