Abstract: Rubing cheese was manufactured by lactic acid bacterial fermentation or traditional direct acidification. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and liquid chromatography-mass spectrometry (LC-MS) were used to determine protein degradation and free amino acid contents in rubing cheese, respectively. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to identify peptides in rubing cheese. The biological activity of the hydrolysate of rubing cheese was characterized by in vitro antioxidant activity, α-glucosidase-inhibiting activity and angiotensin-converting enzyme (ACE)-inhibiting activity. According to the results of SDS-PAGE, both lactic acid bacterial fermentation and direct acidification could promote the degradation of milk proteins. Lactic acid bacterial fermentation caused a higher degree of degradation of whey protein, promoting the release of free amino acids. From the fermented rubing cheese and the directly acidified one, 23 and 13 potential ACE inhibitory peptides, six and four antioxidant peptides and five and one antiglycemic peptides were identified, respectively. Furthermore, lactic acid bacterial fermentation significantly improved the α-glucosidase-inhibiting activity and ACE-inhibiting activity of rubing cheese peptides (P < 0.05). The half-maximum inhibitory concentration (IC50) values of the fraction separated by ultrafiltration with a molecular mass of less than 3 kDa for α-glucosidase and ACE inhibitory activity were 1.250 and 0.416 mg/mL, respectively. In conclusion, lactic acid bacterial fermentation promotes protein degradation to release biologically active peptides and produce free amino acids, enhancing the biological value of rubing cheese.

Key words: rubing cheese; acidification; protein degradation; water-soluble extracts; bioactive peptides