Survival of Escherichia coli O157:H7 and Staphylococcus aureus during the fermentation and storage of kefir

Abstract The survival of Escherichia coli O157: H7 and Staphylococcus aureus, which are foodborne pathogens was investigated during fermentation and storage of kefir. The kefir grains were inoculated with 5% of the whole fat UHT cow milk and the E. coli O157: H7 and S. aureus cultures were added separately to give 105 cfu mL-1 and left to incubate at 22 °C for 24 hours at first stage. During the incubation, the bacteria and yeast counts, pH and titratable acidity amounts were determined. In the second stage, E. coli O157: H7 and S. aureus cultures were separately added to produce 105 cfu mL -1 of kefir and was stocked at 5 °C for 3 days. Every day samples were taken and the same tests were done. During fermentation, in all samples the initial while TA was 0.14%, the fermentation was 1.0% and 1.01% at the end. The kefir samples were pH 4.5 at the end of the 3 day period of storage. S.aureus was significantly inhibited while E.coli was not inhibited during the fermentation process. During storage, E. coli O157: H7 and S. aureus were inhibited within 2 days. The kefir has the quality of being a healthy product during fermentation and storage.

摘要 本研究探究了食源性致病菌大肠埃希氏菌O157:H7（Escherichia coli O157:H7）与金黄色葡萄球菌（Staphylococcus aureus）在开菲尔（kefir）发酵及贮藏过程中的存活情况。实验以5%比例的全脂超高温灭菌（UHT）牛乳接种开菲尔粒，随后分别接入大肠埃希氏菌O157:H7与金黄色葡萄球菌菌液，使其初始浓度达到10⁵ cfu·mL⁻¹，先于22℃下进行第一阶段培养24小时。培养期间，测定样品的细菌与酵母菌计数、酸碱度（pH）及滴定酸度。第二阶段，分别向开菲尔体系中接入上述两种菌液至浓度为10⁵ cfu·mL⁻¹，随后于5℃下贮藏3天。每日取样并开展相同指标检测。发酵阶段，所有样品初始滴定酸度为0.14%，发酵结束时滴定酸度分别为1.0%与1.01%；3天贮藏期结束后，开菲尔样品的pH值为4.5。发酵过程中，金黄色葡萄球菌受到显著抑制，而大肠埃希氏菌未受抑制。贮藏阶段，大肠埃希氏菌O157:H7与金黄色葡萄球菌均在2天内受到抑制。开菲尔在发酵与贮藏过程中均具备健康食品的品质。