Nutrition and Health: Effects of Processing on Protein Quality of Milk and Milk Products

Milk proteins are very valuable as dietary sources of essential amino acids. In addition, many milk protein components exert distinct biological functions acting as enzymes, antimicrobial or antioxidant agents, and carriers of minerals or vitamins. These additional physiological functions are due to bioactive peptides encrypted within the intact proteins. These positive characteristics may be impaired by industrial processing, which almost always includes heat treatment, sometimes in combination with extreme pH conditions. Heat-induced deterioration reactions proceed during storage and can further affect nutritional quality of milk products with a long shelf life. Whey proteins are progressively denatured by heating. They lose solubility, and are partly bound to casein which, in turn, is more heat-stable. Upon heating, all milk proteins react with lactose, through the Maillard reaction (MR), with a progressive reduction of available lysine and, to a minor extent, of other amino acids. Lysine is rendered nutritionally unavailable also by β-elimination reactions, which lead to the formation of the unnatural amino acid lysinoalanine. Lysinoalanine, as well as some advanced products of MR, cross-links protein molecules, which therefore become less digestible. Furosine, the well-accepted indicator of the MR, and lysinoalanine represent reliable analytical markers allowing the assessment of type and extent of heat-induced covalent modifications of the protein components in drinking milk and in a variety of dairy products.