zeta-Potentials of casein micelles from reconstituted skim milk heated at 120 degrees C

Laser Doppler electrophoresis was used to determine the I-potentials of casein micelles in untreated, rennet treated or heated (pH 6.6 or 7.1/120 degrees C for 6, 15 or 25 min) reconstituted skim milk samples as a function of pH. Complex zeta-potential-pH profiles were observed for untreated milk samples, with local maxima at pH 5.0 and 6.5 and corresponding minima at pH 5.5 and 7.5. Rennet treatment of milk reduced the micellar zeta-potential by approximately one-third. On heat treatment of the milk, the maxima and minima were either markedly reduced or eliminated. In addition, the casein micelles from the samples heated at pH 6.6 had higher zeta-potentials than those from the samples heated at pH 7.1 over the entire pH range. Low levels of kappa-casein were dissociated from the casein micelles on heat treatment at pH 6.6, whereas about 50% of the kappa-casein was rendered soluble on heat treatment at pH 7.1. This dissociation of kappa-casein, which was partially reversible on readjustment of the pH, may account for the observed differences in micellar zeta-potential between milk samples heated at pH 6.6 and 7.1. Copyright (C) 1996 Elsevier Science Limited