Inhibitory power of kefir: The role of organic acids

Milk and MRS broth fermented with kefir grains from different households we re examined for inhibitory activity toward gram-negative and gram-positive strains. Fermented milk obtained with 10 g per 100 ml of inoculum (final pn 3.32 to 4.25) and MRS broth fermented with 1 and 10 g per 100 ml of inocul a (final pH 4.18 to 5.25) had inhibitory power demonstrated by spot test an d agar well diffusion assay. This inhibitory effect could be assigned to th e undissociated form of lactic and acetic acid produced during the fermenta tion process. Kefir supernatants inhibited the growth of Escherichia coli 3 in nutrient broth at 37 degrees C for 24 h. However, supernatants of yogur t or milk artificially acidified with lactic and acetic acids allowed the g rowth of E. coli 3 in the same conditions. A bacteriostatic effect of milk fermented with kefir grains over E. coli 3 was also demonstrated.