Reconstituted skim milk samples at pH between 6.5 and 7.1 (heating pH) were heated at 80degreesC, 90degreesC or 100degreesC for 30 min (heating temperature). The particle size of the casein micelles was measured at pH 4.75-7.1 (measurement pH) and at temperatures of 10 C, 20degreesC and 30degreesC (measurement temperature) using photon correlation spectroscopy. The particle size of the casein micelles, at a measurement pH of 6.7 and a measurement temperature of 20degreesC, was dependent on the heating pH and heating temperature to which the milk was subjected. The casein micelle size in unheated milk was about 215 nm. At a heating pH of 6.5, the casein micelle size increased by about 15, 30 and 40 nm when the milk was heated at 80degreesC, 90degreesC or 100degreesC, respectively. As the heating pH of the milk was increased, the size of the casein micelles decreased so that, at pH 7.1, the casein micelles were similar to20 nm smaller than those from unheated milk. Larger effects were observed as the heating temperature was increased from 80degreesC to 100degreesC. The size differences as a consequence of the heating pH were maintained at all measurement temperatures and at all measurement pH down to the pH at which aggregation of the micelles was observed. For all samples, size measurements at 10degreesC showed no aggregation at all measurement pH. Aggregation occurred at progressively higher pH as the measurement temperature was increased. Aggregation also occurred at a progressively higher measurement pH as the heating pH was increased. The particle size changes on heating and the aggregation on subsequent acidification may be related to the pH dependence of the association of whey proteins with, and the dissociation of K-casein from the casein micelles as milk is heated. (C) 2004 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.
