Effect of some fermentation conditions on antibacterial activity of fermented milk by kefir grains

Abstract

The present study aimed to investigate the effect of type of milk, fermentation time, temperature, and stirring conditions on the antibacterial activity of fermented milk by kefir grains. All fermentation conditions had a significant effect on the antibacterial activity of kefir samples against four bacteria, except for the effect of temperature on the diameter of the inhibition zone of Shigella dysenteriae. Considering all factors and their interactions, fermented samples in full-fat milk compared to nonfat milk, at 37°C compared to 25°C and under stirred conditions compared to non-stirred samples showed higher antibacterial activity. Also, fermented samples for 48, 72, and 120 hr showed higher antibacterial activity against Escherichia coli, Staphylococcus aureus, and Bacillus cereus, respectively, and fermented samples for 48 and 120 hr showed higher antibacterial activity against S. dysenteriae. Practical applications Several studies have investigated the antibacterial activity of kefir. But the novelty of the present study is that we have also investigated the effect of some fermentation conditions on the antibacterial activity of kefir samples. Furthermore, in this study, the interactions of these variables in the antibacterial activity of kefir samples using Design-Expert software have been analyzed. The fermentation conditions had a significant effect on the antibacterial activity of fermented milk samples. In general, to achieve the most of antibacterial activity, is recommended fermentation in full-fat milk, at 37°C, and under stirred conditions. In addition to these conditions, to achieve the most antibacterial activity against E. coli, S. aureus, B. cereus, and S. dysenteriae, is recommended fermentation times of 48, 72, 120, and 120 hr, respectively.