Effects of ultra-high-temperature processes on metabolite changes in milk.

Processing can affect milk properties and alter the composition of milk metabolites, which has corresponding effects on milk flavor and quality. It is quite important to study the safe quality control of milk processing. Therefore, the purpose of this study was to identify metabolites at different steps of ultra-high-temperature-sterilized (UHT) milk processing using gas chromatography-mass spectrometry (GC-MS). These steps included raw milk, pasteurized milk (80°C for 15 s), semi-finished milk (after pasteurizing, it was homogenized at 75°C with pressure of 250 bar), UHT milk (at 140°C for 10 s), and finished milk (homogenized UHT milk). A total of 66 metabolites were identified across all samples, including 30 metabolites in the chloroform layers of the milk samples and 41 metabolites in the water layers; 5 metabolites were found in both layers. The metabolites were primarily fatty acids, amino acids, sugars, and organic acids. For example, pasteurized and ultra-high-temperature-sterilized kinds of milk had lactose contents similar to those of raw milk, with increases in saturated fatty acids such as hexadecanoic acid and octadecanoic acid. Additionally, these findings indicated that these methods of processing can affect the contents of some components of milk. Therefore, from the perspective of milk's nutritional value and consumer health, the excessive heating of dairy products should be avoided and the milk heat treatment process should be standardized from the source.