A comprehensive study of the instability of direct-steam-infusion UHT milk: Insights from metabolomics and proteomics

Direct-steam-infusion UHT (dUHT) milk retains more nutrients compared to traditional UHT milk but challenged by age gelation, impacting shelf life. To understand this instability, raw milk, dUHT milk, and age gelation milk were comprehensively studied using metabolomics and proteomics. 2496 metabolites were detected, including amino acids and derivatives, nucleotides and nucleotide derivatives, lipids and fatty acids, and organic acids and derivatives. The most significant change after age gelation was in amino acids, particularly L-lysine, which emerged as the most important indicator related to protein hydrolysis. Proteomics revealed increased calcium-binding proteins, protein fragments, and various proteases in age gelation milk. Ribonuclease A family, cathepsin D, carboxypeptidases, serine/threonine-protein kinase, PLK serine protease 23 contributed to metabolite changes and protein hydrolysis during age gelation. These findings suggested that the hydrolysis of various enzymes altered the overall composition of milk, leading to age gelation, which provided new insights into the instability of dUHT milk.