Kefir grain tolerance to Escherichia coli contamination—industrial advantages

Abstract

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.

摘要

开菲尔粒在乳品工厂中常被用做发酵剂。本文评价了在18 °C条件下,开菲尔粒在污染了大肠杆菌(6.1 × 10² cfu.mL^-1)的巴氏杀菌奶中重复使用性。感染速率符合工厂的实际生产情况。采用菌落计数法测定了开菲尔粒、发酵剂和未接种开菲尔粒牛奶中的大肠杆菌。测定了污染的开菲尔粒的复制能力和发酵培养液酸化能力。评价污染奶对总乳酸菌(LAB)、发酵柠檬酸盐乳酸菌以及耐万古霉素乳酸菌的影响。从发酵乳中分离的开菲尔粒经水洗后没有检测到大肠杆菌的污染。在发酵剂中,大肠杆菌与菌种的生长速率相同,但是在未添加开菲尔粒的污染奶中,大肠杆菌以2倍对数值的速率生长。从污染牛乳和未污染的乳中分离出的开菲尔粒,通过连续5次接种到新鲜调制乳中,大肠杆菌表现出相似的生长动力学。最后一次接种中获得的发酵剂没有被大肠杆菌污染,而且表现出标准的酸乳特点。被大肠杆菌污染的牛乳并不能影响发酵柠檬酸盐乳酸菌、耐万古霉素乳酸菌、酵母进和总乳酸菌在开菲尔粒和发酵剂中总数。本研究结果发对开菲尔生产者来说有很重要的经济价值,但是应该考虑本研究只是针对一株菌,并且污染率较低的情况下得出的结论。