Heat stability of sheep's skim milk: Aggregation and interaction of proteins

Abstract

Sheep's milk proteins are susceptible to heat-induced coagulation, but the protein interactions under high heat treatment have not been determined. Heat stability and protein interactions of sheep's skim milk (SSM) at pH 6.2–7.2 were examined at 140 °C. SSM had the longest heat coagulation time at pH 6.9, but became very unstable at higher or lower pH. Protein aggregates formed consisted mainly of whey proteins and κ-casein (κ-CN)-depleted casein micelles. Modification of SSM pH alters ionic calcium concentration, dissociation of caseins and electrostatic interactions, resulting in different extents of protein interactions. The extent of dissociation of κ-CN from casein micelles increased with increasing pH (from ∼6.6 to 7.0) before and after heat treatment, contributing to κ-CN-depleted casein micelle aggregation. High ionic calcium concentrations, low levels of κ-CN on casein micelles and ready dissociation of κ-CN from casein micelles may be responsible for the low heat stability of sheep's milk.