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.

The purpose of this study was to explore the differences in nutritional components and whey protein components between sheep milk and milk from humans and other animal species used for dairy production. The whey protein components of milk from Hu sheep, East Friensian Milk sheep and their F1 hybrid at different stages of lactation were analyzed and compared with those of goat, cow and human milk. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Native-PAGE were used to identify landmark proteins that differed between milk samples. The contents of milk macronutrients like protein, fat and lactose significantly varied between sheep breeds, between lactation stages, and between sheep and other animal species (P < 0.05) and they showed a certain trend. SDS-PAGE showed that β-lactoglobulin and lactoferrin differed between sheep and human milk, α-lactalbumin differed between sheep and goat milk, and immunoglobulin G (IgG) heavy chain differed between sheep and cow milk. Native-PAGE indicated that there was one protein band that differed between sheep and goat whey as well as three ones that differed between sheep milk and human and cow whey.