Direct evidence for the role of Maillard reaction products in protein cross-linking in milk powder during storage

Protein cross-linking, a possible cause of loss of solubility of milk protein concentrate (MPC) powder, may occur via advanced Maillard reaction products (e.g., methylglyoxal) or dehydroalanine. This study was designed to determine the relevance of these cross-linking pathways with the aid of proteomic techniques. Cross-linked proteins in the insoluble fraction of reconstituted MPC80 contained all major caseins, with αS1-casein predominating, and a small amount of β-lactoglobulin. To investigate the mechanism of protein cross-linking, model heating experiments were carried out on αS1-casein. Dephosphorylation of αS1-casein did not prevent protein cross-linking, suggesting that dehydroalanine was not involved in the cross linking. However, densitometric analysis showed that the amount of cross-linked αS1-casein was considerably enhanced by adding lactose or methylglyoxal. Protein cross-linking in MPC induced by methylglyoxal was also shown on 2-dimensional electrophoresis gels, confirming a possible pathway of protein cross-linking in milk powders involving advanced Maillard reaction products (e.g., methylglyoxal).