Comparative study of the antimicrobial activity of kefir grown under different substrate

Abstract

Kefir is an ancient beverage, slightly acidic and alcoholic fermented that originated in the Caucasian region of Asia. Kefir is a natural fermented product comprised of a probiotic bacteria and yeast complex that coexist in symbiotic association. Kefir consumption has been associated with many advantageous properties to general health, including as an antioxidative, anti-obesity, anti-inflammatory, anti-microbial, and anti-tumor moiety. Generally, kefir may be identified depending on the type of substrate used for fermentation, which are dairy and non-dairy kefir. The different manufacturing conditions of kefir (agitation; the inoculum concentration; as well as the fermentation time and temperature) may alter the original characteristics of the microbial composition, hence affecting their health-giving properties. Therefore, this study aims to comparative the antimicrobial activity of kefir grown under different substrate.Kefir drinks were prepared from three different substrates: 0% milk fat (4.9 g% carbohydrates), water (4.8g% Muscovado sugar) and LB lactose (4.5g% lactose). A total of 3 g of kefir grains were inoculated in 30 mL of each substrate (10% w/v). Erlenmeyer flasks were incubated at 28°C and 100 rpm. Samples were taken at 24 (T1), 48 (T2), 72 (T3), 120 (T4) and 168 h (T5). The supernatants were obtained by centrifugation at 10,000 xg for 10 minutes. Antimicrobial activity was determined by diffusion in agar on Petri dishes containing the LB for bacteria and potato-glucose agar for fungi. The target strains used were: E. coli, Staphylococcus aureus, Fusarium sp., Aspergillus sp. and a fungus isolated from bread without identified yet.The antimicrobial activity varied according to the type of kefir and the fermentation time, and was found after T3. The supernatants of water kefir presented the best results of microbial activity, followed by the milk kefir, while the kefir that grew in LB-lactose did not show activity. The water kefir supernatant inhibited four of the five target strains. The most sensitive microorganisms to it were E. coli, followed by Aspergillus sp., the mold isolated from bread and Fusarium sp. Staphylococcus was not inhibited by any of the supernatants. The pH of the three types of kefirs decreased as the incubation time increased. The antimicrobial activity of milk and water artificially acidified with acetic acid and lactic acid was evaluated as control but no inhibitory activity was obtained for any of the target strains. Therefore, supernatants from kefir could be attributable to antimicrobial metabolites in supernatants rather than the low pH. Further research is necessary to study the compounds responsible for these functional properties and their stability for its use as food additive.