Kefir grain tolerance to Escherichia coli contamination—industrial advantages

Kefir grains are used in dairy plants for mother or bulk starter culture production. The aim of the study was to evaluate the possibility of the re-use of kefir grains grown at 18 °C for 24 h in pasteurized Escherichia coli contaminated milk up to 6.1 × 10² cfu.mL⁻¹. The contamination rate was adapted to a realistic situation which may take place in plant. E. coli was enumerated in kefir grains, postharvest broth, and in milk incubated without kefir grains. Determination of the replication ability of the affected grains and acidification activity of postharvest broth was performed. The impact of milk contamination on total lactic acid bacteria (LAB), citrate-fermenting and vancomycin-tolerant LAB, and yeasts of the kefir grains and postharvest broth was evaluated. The grains separated from fermented milk and washed with water were not E. coli-contaminated. In the postharvest broth (starter culture), E. coli was found at inoculation rate, whereas in contaminated milks incubated without grains, E. coli population increased at least by 2 log. Separated from contaminated and uncontaminated milk, grains displayed similar growth dynamics in five subsequent transfers into freshly prepared milk. The postharvest broth obtained from last transfer was not contaminated and showed standard acidification milk profile. E. coli contamination of milk did not influence adversely the ratio of citrate-fermenting, vancomycin-tolerant LAB, yeasts, and total LAB counts in kefir grains and postharvest broth. These findings would have economical value for kefir producers, but it should be considered that only one strain with low contamination rate was tested.