This study explored changes in the acidity of sheep milk across different lactation periods and the key factors affecting the acidity of sheep milk. The pH, titratable acidity, fat content, protein content and composition, total sugar content, mineral content, density, lactose content, fatty acid composition and content were analyzed, and the correlation between physicochemical indexes and acidity was investigated. The results indicated that the acidity of raw sheep milk was closely related to lactation stages. The highest acidity was observed in the early lactation period, which then gradually decreased as lactation progressed. Acidity changes were primarily associated with the protein content, fat content, fatty acid composition, total sugar content, lactose content, mineral content, and density of the milk. Among these, the protein content showed the strongest correlation with acidity changes, whereas fatty acid content had a relatively low correlation with acidity variations.

Bioactive peptides were prepared from sheep casein by neutral protease hydrolysis. The effect of hydrolysis time on the degree of hydrolysis and angiotensin converting enzyme (ACE) inhibitory activity of hydrolsates was investigated. The peptides were analyzed and identified by liquid chromatography-mass spectrometry (LC-MS) to select the ones with potential ACE inhibitory activity. The results showed that the highest degree of hydrolysis of 9.65% as well as a percentage of ACE inhibition of 84.55% was attained after 300 min, and the concentration of the peptide YYQQRP required for 50% ACE inhibition was 5–10 μmol/L. The sequence analysis of the peptides was performed by ultra-high pressure liquid chromatography-time of flight mass spectrometry (UPLC-TOF-MS). YYQQRP was identified as a new peptide with potential ACE inhibitory activity by comparison with the protein database. It was hypothesized that the structure of YYQQRP might be similar to that of the substrate and could reduce the catalytic activity of ACE by binding to the active site of ACE.