In vitro Antioxidant and α-Glucosidase Inhibitory Activities of Lactobacillus spp. Isolated from Indonesian Kefir Grains

In this study, nine Lactobacillus kefiri and one Lactobacillus rhamnosus isolates with kefir grain origin have been demonstrated to include potentials as probiotics. The aim of this study was to investigate ability of the isolates to produce compounds with α-glucosidase inhibitory and antioxidant activities and identify peptides with MW of ≤ 3 kDa in cell-free supernatants. Material and Methods: All isolates were cultured in de Man, Rogosa and Sharpe broth media at 37 oC for 24 h. Assessment of α-glucosidase inhibitory and antioxidant activities was carried out on cell-free supernatants. Assessment of optimum incubation time was carried out on two isolates with the highest α-glucosidase inhibitory and antioxidant activities. The two isolates were used to ferment reconstituted skim milk. Cell-free supernatant of the fermented skim milk was fractionated using filters of 10 and 3 kDa. Then, peptides in fractions of ≤ 3 kDa were identified. Results and Conclusion: The highest α-glucosidase inhibitory activity was seen in Lactobacillus rhamnosus BD2 and Lactobacillus kefiri YK4 as 73.58 and 64.31%, respectively. The highest antioxidant activity was observed in Lactobacillus kefiri JK5 and Lactobacillus kefiri JK17 as 44.31 and 41.57%, respectively. When Lactobacillus rhamnosus BD2 and Lactobacillus kefiri YK4 were cultured in reconstituted skim milk, their α-glucosidase inhibitory activities respectively decreased to 25.72 and 36.16% while the antioxidant activities respectively increased to 74.53 and 80.92%. Fractionation of the cell-free supernatants from fermented reconstituted skim milk of Lactobacillus kefiri YK4 showed that the highest antioxidant activity was included in fractions greater than 10 kDa. Although fractions of 3 kDa or less exhibited quite high antioxidant activities. Identification of peptides in fractions of 3 kDa or less showed that the peptides were mostly derived form β-casein. Of these peptides, two peptides with sequences of FPPQSV and YQEPVLGPVRGPFPIIV have been reported to include antioxidant activities