Abstract: In order to study the effect of X-prolyl-dipeptide acyl-aminopeptidase (Pep X) activity of Lactobacillus on the antioxidant activity of Cheddar cheese, we obtained mutants with Pep X activity (named as UV-C3, UV-H3, UV-R3, and UV- P3, respectively) from Lactobacillus casei 1.0319, L. helveticus 1.0612, L. rhamnosus 1.0911, and L. plantarum 1.0202 by three rounds of UV mutagenesis and separately added them to Cheddar cheese. We analyzed the changes in Pep X activity and antioxidant activity during cheese ripening and established the correlation between them. Furthermore, we studied the effect of adding Pap X-producing Lactobacillus on cheese quality. The results showed that the activity of Pep X produced by Lactobacillus had a significant effect on the antioxidant activity and quality of Cheddar cheese. There was a significantly positive correlation between Pep X activity and the antioxidant activity of Cheddar cheese (P < 0.05). As maturation proceeded, the Pep X activity and antioxidant activity of cheese increased gradually up to the maximum on the 90th day. The activity of Pep X produced by UV-R3 was significantly higher than those produced by other strains (P < 0.05), and the maximum value was 10.16 U/g. The antioxidant capacity of the cheese inoculated with UV-R3 was significantly higher than those of the cheeses inoculated with other strains (P < 0.01), as evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging rate, hydroxyl radical scavenging rate, and reducing ability, and their maximum values were 68.43%, 67.56%, and 0.684 3 in the former sample, respectively. After the 120th day, as casein was hydrolyzed, bitter peptides were produced in a large amount, and the texture was remarkably lowered. The highest sensory flavor score of greater than 9.0 points was observed for all cheese samples, leading to the conclusion that they were edible at 120 days of ripening.

Key words: Lactobacillus; X-prolyl-dipeptide acyl-aminopeptidase; enzymatic activity; antioxidant activity; ripening period