ζ-Potentials of casein micelles from reconstituted skim milk heated at 120 °C

Abstract

Abstract Laser Doppler electrophoresis was used to determine the ζ-potentials of casein micelles in untreated, rennet treated or heated (pH 6.6 or 7.1/120 °C for 6, 15 or 25 min) reconstituted skim milk samples as a function of pH. Complex ζ-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 ζ-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 ζ-potentials than those from the samples heated at pH 7.1 over the entire pH range. Low levels of κ-casein were dissociated from the casein micelles on heat treatment at pH 6.6, whereas about 50% of the κ-casein was rendered soluble on heat treatment at pH 7.1. This dissociation of κ-casein, which was partially reversible on readjustment of the pH, may account for the observed differences in micellar ζ-potential between milk samples heated at pH 6.6 and 7.1.