EFFECT OF PH ON THE STABILITY AND SURFACE-COMPOSITION OF EMULSIONS MADE WITH WHEY-PROTEIN ISOLATE

Emulsions (20 wt % soy oil) made with various concentrations (0.5 -2.5 wt %) of whey protein isolate (WPI) were most stable at pH 7 and least stable at pH 5.5. Emulsions made with imidazole buffer at pH 6 were stable, but those made with citrate buffer at the same pH were unstable. Emulsions prepared at pH 3, using citrate buffer, were stable. At pH 7 beta-lactoglobulin and alpha-lactalbumin adsorbed in proportion to their concentration, but at lower pH values alpha-lactalbumin was found to adsorb preferentially, depending on the protein concentration, pH, and buffer. When emulsions (2 wt % WPI) were acidified from pH 7 to 3 more alpha-lactalbumin became adsorbed and the emulsion was stable, but reducing the pH from 7 to 6 did not alter the interfacial composition of protein and the emulsion became unstable. The behavior of the whey proteins depends on variations of tertiary and quaternary structure with pH.