Adsorption of soy globulin films at the air-water interface

The adsorption kinetics at the air-water phase interface of soy globulins (beta-conglycinin, glycinin, and reduced glycinin) has been studied. The adsorption kinetics was determined by surface tension measurements coupled with Brewster angle microscopy (BAM) as a function of time, protein concentration in the solution (within the range of 1-0.001%, wt/wt), and pH (2.0, 5.0, and 8.0). The ionic strength (0.05 M) and the temperature (20 degreesC) were maintained constant. The adsorption of soy globulins to the interface increases with the protein concentration in the solution, depending on the protein and, especially, on the pH. The adsorption decreases dramatically at pH 5.0, close to the isoelectric point of the protein. A lag period was observed at lower protein concentrations. The adsorption kinetics at the beginning of the adsorption is diffusion-controlled. However, the mechanism that controls the long-term adsorption is the penetration and unfolding of the protein. The molecular conformation of soy globulins, which depends on the pH, has an effect on the adsorption kinetics.