Antioxidant Activity of Skimmed Cow and Soy Milks Fermented by Lactic Isolates of Kefir Granules

Background and Objective: The proteolytic system of lactic acid bacteria hydrolyzes milk protein into several peptides, including those with antioxidative activities. The aim of this study was to assess antioxidant activities in cow and soy milks fermented by Lacticaseibacillus rhamnosus BD2, Lentilactobacillus kefiri YK4 and Lentilactobacillus kefiri JK17 previously isolated from kefir granules and their correlations with the peptide contents.Material and Methods: Reconstitutes of skimmed cow and soy milks were fermented by the highlighted isolates at 37 celcius for 0, 24, 48 and 72 h. Fermented products were analyzed for the isolates, pH, total titratable acidity, antioxidant activity (% radical 2,2-diphenyl-1picrylhydrazil inhibition and antioxidant capacity expressed in mu g AAE.ml-1 whey) and total peptides. Fermented skimmed cow and soy milks with the highest antioxidant activity were then partially fractionated using molecular filters of 10 and 3 kDa. Fractions with the highest activity were analyzed further for peptide identification. Statistical analysis was carried out using one-way analysis of variance and Duncan multiple range tests (p<0.05) using SPSS software v.16.0.Results and Conclusion: All isolates were able to grow in reconstituted skimmed cow and soy milks, while total count of reached to 9 log CFU.ml-1 with significant (p<0.05) increases in titratable acidity and total peptides and decreased pH. Growth of the three isolates was mildly slower in soy milk than in skimmed cow milk. The maximum antioxidant activities were seen after 72-h fermentation of cow and soy milks. No differences were observed in antioxidant activity of cow milk fermented by the three isolates; however, Lacticaseibacillus rhamnosus BD2 produced the highest antioxidant activity in soy milk. In general, increases in antioxidant activities correlated with increases in the peptide contents. Fraction of less than 3 kDa of the two milks fermented by Lacticaseibacillus rhamnosus BD2 showed the highest antioxidant activity. Analyses of peptides present in these fractions using high resolution LC-MS/MS and in silico identification of peptides with antioxidant activity have been reported in this study. The present study suggests that the three isolates can be used as starter cultures in fermenting cow and soy milks to increase their antioxidant activities. Peptides with molecular weight of less than 3 kDa play key roles in antioxidant activity.