Abstract: In order to explore the change of lipids in raw milk during cold storage at 4 ℃, acid value (AV) and peroxide value (PV) were measured to determine the degree of fat oxidation in raw milk. Ultra-high performance liquid chromatography quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) was used to analyze the differences and changes of lipid metabolites during cold storage. The results showed that both AV and PV continued to increase during 6 days of cold storage, and the rate of increase was the fastest from day 0 to day 1. The analysis of differential metabolites showed that the changes of the metabolic system in raw milk were very complicated, and the types and changes of lipid metabolites were the most prominent among the six significantly differential metabolites identified. The number of lipid metabolites increased from 9 to 24. Further analysis found that the expression levels of free fatty acids and phospholipids fluctuated greatly on day 3 to 4 and day 6, respectively. Based on these results combined with KEGG pathway analysis, it was found that fatty acid biosynthesis in which free fatty acids are involved changed significantly during cold storage (P < 0.05). On day 3, the most lipid metabolism pathways (9) were identified. Therefore, the third day of cold storage?could be seen as the key point for quality control of raw milk.

Key words: raw milk; refrigeration; lipid metabolites; metabolomics