Lateral phase separation in adsorbed binary protein films at the air-waterinterface

Lateral phase separation in two-dimensional mixed films of soy 11S/beta -ca sein, acidic subunits of soy 11 (AS11S)/beta -casein, and alpha -lactalbumi n/beta -casein adsorbed at the air-water interface has been studied using a n epifluorescence microscopy method. No distinct lateral phase separation w as observed in the mixed protein films when they were examined after 24 h o f adsorption from the bulk phase. However, when the soy 11S/beta -casein an d AS11S/beta -casein films were aged at the air-water interface for 96 h, p hase-separated regions of the constituent proteins were evident, indicating that the phase separation process was kinetically limited by a viscosity b arrier against lateral diffusion. In these films, beta -casein always forme d the continuous phase and the other globular protein the dispersed phase. The morphology of the dispersed patches was affected by the protein composi tion in the film. In contrast with soy 11S/beta -casein and AS11S/beta -cas ein films, no lateral phase separation was observed in the alpha -lactalbum in/beta -casein film at both low and high concentration ratios in the film. The results of these studies proved that proteins in adsorbed binary films exhibit limited miscibility, and the deviation of competitive adsorption b ehavior of proteins at the air-water interface from that predicted by the i deal Langmuir model (Razumovsky, L.; Damodaran, S. J. Agric. Food Chem. 200 1, 49, 3080-3086) is in fact due to thermodynamic incompatibility of mixing of the proteins in the binary film. It is hypothesized that phase separati on in adsorbed mixed protein films at the air-water and possibly oil-water interfaces of foams and emulsions might be a source of instability in these dispersed systems.