Evaluation of Proteolysis in milk fermented by proteolytic lactic acid bacteria and antibacterial activities of their peptide extract

In this study, acidification and proteolysis in reconstituted skim milk fermented using proteolytic lactic acid bacteria were investigated during fermentation and after 7 days of cold storage. Also, antibacterial activities of crude peptide extracts obtained from the fermented samples were evaluated. Lactobacillus delberueckii subsp. bulgaricusL. reuteri and Lactococcus lactis subsp. lactis were used as starter in single or co-culture. Lactococcus lactis subsp. lactis and lactobacillus reuteri showed the fastest and slowest acidification, respectively, and the co-cultures were intermediate. During fermentation, the highest proteolytic activity was found in single culture of Lactococcus lactis subsp. lactis and triple co-culture(peptide content of 1.30 and 1.35 mg/ml) which was followed by a reduction after 7d storage. Peptide content from Lactobacillus reuteri fermentation was not significantly different from that of acidified control milk (ACM). ACM showed no antibacterial activity at the tested concentrations (5-40 mg/ml), while all the fermented milk samples showed antibacterial activities against all five tested pathogens at 40 mg/ml. After fermentation (day 0), the minimum inhibitory concentration of the most of fermented samples was calculated as 20 mg/ml, except for that of triple co-culture-fermented milk which was 40 mg/ml. The antibacterial activity of peptide extracts after 7d storage were varying and starter culture dependent. In overall, in spite that proteolysis resulted in antibacterial activity in the product, no positive correlation was found between proteolytic activity and antibacterial activity.