LAUSR

Abstract Dry heating of cow's milk protein in the presence of the milk sugar lactose leads to a loss of solubility and digestibility. Most studies that investigated the loss of solubility in milk protein powders suggested that, besides structural changes, hydrophobic interaction, hydrogen bonds, disulphide bonds, and Maillard reaction-induced crosslinking are responsible for this. However, little is known about the direct contribution of these inter- and intramolecular interactions on loss of solubility and protein digestibility. Low temperature (60 °C) and high temperature (130 °C) dry heating of cow's milk protein in the presence of lactose was applied after which both the soluble and insoluble fractions were analysed with SDS-PAGE and LC-MS/MS. The Maillard reaction was monitored by quantification of Ne-carboxymethyllysine, Ne-carboxyethyllysine, and pentosidine with LC-MS/MS. Loss of solubility was analysed with solvent solubility tests. Protein hydrolysis after simulated infant in vitro digestion, and after hydrolysis with single enzymes, was monitored using SDS-PAGE and the o-phthaldialdehyde assay. The results indicated that caseins are the main proteins that become insoluble upon dry heating, independent of the heating temperature. The decreased solubility of low temperature dry heated cow's milk protein is induced by hydrogen bonds and hydrophobic interactions and did not impair protein hydrolysis. At the same time, covalent protein crosslinking is an important determinant in protein solubility and digestibility of high temperature dry heated cow's milk protein.