Protein Stability in Sterilised Milk and Milk Products

Abstract

Sterilised milk and milk products are produced by intense heat treatments and are stored at ambient temperature for several months. These conditions present a considerable challenge to the stability of their protein components. Several changes occur during the heat treatment, usually more during in-container sterilisation than UHT treatment, and some changes continue during storage. Most are time- and temperature-dependent and some are pH-dependent. The major chemical changes are denaturation of whey proteins, covalent cross-linking of proteins, redistribution of proteins between the colloidal and serum phases, lactosylation and subsequent Maillard reactions, degradation of individual amino acids via deamidation and dephosphorylation as well as generation of volatile sulfur compounds, and proteolysis by indigenous and bacterial proteinases. Physical changes also occur, including deposit formation during heating, and further sediment formation and gelation during storage. The mineral fraction plays an important role in some of these reactions. The changes to proteins have implications for the nutritional status of the product. There is a reduction in bioavailability of essential amino acids, particularly lysine, and formation of potentially toxic compounds resulting from Maillard reactions and protein cross-linking. However, the proteins in some sterilised products may be more digestible due to physical changes causing increased accessibility to digestive proteinases.