Chemical and Proteolysis-Derived Changes during Long-Term Storage of Lactose-Hydrolyzed Ultrahigh-Temperature (UHT) Milk

Proteolytic activity in milk may release bitter-tasting peptides and generate free amino terminals that react with carbohydrates, which initiate Maillard reaction. Ultrahigh temperature (UHT) heat treatment inactivates the majority of proteolytic enzymes in milk. In lactose-hydrolyzed milk a beta-galactosidase preparation is applied to the milk after heat treatment, which has proteolytic side activities that may induce quality deterioration of long-term-stored milk. In the present study proteolysis, glycation, and volatile compound formation were investigated in conventional (100% lactose), filtered (6096 lactose), and lactose-hydrolyzed (<1% lactose) UHT milk using reverse phase high-pressure liquid chromatography mass spectrometry, proton nuclear magnetic resonance, and gas chromatography mass spectrometry. Proteolysis was observed in all milk types. However, the degree of proteolysis was significantly higher in the lactose-hydrolyzed milk compared to the conventional and filtered milk. The proteins most prone to proteolysis were beta-CN and alpha(s1)-CN, which were clearly hydrolyzed after approximately 90 days of storage in the lactose-hydrolyzed milk.