EFFECT OF PH AND TEMPERATURE ON PROTEIN UNFOLDING AND THIOL-DISULFIDEINTERCHANGE REACTIONS DURING HEAT-INDUCED GELATION OF WHEY PROTEINS

In heat-induced whey protein isolate (WPI) gels, polymerization of the constituent whey proteins, via intermolecular disulfide (S-S) bonding , was dependent on both the pH of the WPI solution and the temperature to which the solution was heated. At pH 9 and 11, polymerization as d etermined by SDS-PAGE occurred at room temperature (22 degrees C), whi le at pH 3, 5, and 7, polymerization was only evident after heating to 85, 75, and 70 degrees C, respectively. Measurement of total sulfhydr yl (SH) group content of gelling WPI solutions at each pH and temperat ure revealed that in the WPI solutions at pH 9 and 11 significant SH-S H oxidation to S-S occurred even at room temperature. In contrast, the total SH content of WPI solutions at pH 3 and 5 did not change with h eating, indicating that polymerization reactions involving SH/S-S inte rchange rather than SH/SH oxidation predominated. Estimation of the de gree of unfolding of the whey proteins by measuring the exposure of hy drophobic amino acid residues showed that at pH 9 and 11 extensive irr eversible unfolding of the protein molecules had occurred at room temp erature.