Preparation, isolation, purification and identification of ACE inhibitory fractions from yak casein fermented by Bacillus cereus (XBMU-13).

Food-derived angiotensin-converting enzyme (ACE) inhibitory peptides have been shown to contribute to blood pressure reduction. In this study, a protease-producing bacterial strain was isolated and identified for fermentation of yak milk casein to produce ACE inhibitory peptides. Subsequently, fermentation conditions were optimized, and the resulting peptides were purified, and identified. The results showed that a Bacillus cereus strain, designated XBMU-13, was isolated from naturally fermented bean curd and exhibited a protease activity of 86.27 ± 3.82 U/mL. Using response surface methodology, the optimal fermentation parameters were obtained as follows: substrate concentration of 63.2 g/L, sucrose concentration of 6 g/L, inorganic salt concentration of 6 g/L, initial pH of 7.87, and fermentation temperature of 34 °C. Fraction G-10-1 exhibiting the highest ACE inhibitory ability was isolated through sequential ultrafiltration, Sephadex gel chromatography, and preparative liquid chromatography. Furthermore, nine peptide sequences in fraction G-10-1 were identified by liquid chromatography-mass analysis.