Escherichia coli O157:H7 SURVIVAL IN TRADITIONAL AND LOW LACTOSE YOGURT DURING FERMENTATION AND COOLING PERIODS

Abstract The purpose of this study was to evaluate the behavior of E. coli O157:H7 during lactose hydrolysis and fermentation of traditional and low lactose yogurt. It also aimed to verify E. coli O157:H7 survival after 12 h of storage at 4 ºC ±1 ºC. Two different types of yogurts were prepared, two with whole milk and two with pre-hydrolyzed whole milk; in both groups one yogurt was inoculated with E. coli O157:H7 and the other one was not inoculated. The survival of E. coli and pH of yogurt were determined during fermentation and after 12-h refrigeration. The results showed that E. coli O157:H7 was able to grow during the fermentation period (from 4.34 log CFU.mL-1 to 6.13 log CFU.mL-1 in traditional yogurt and 4.34 log CFU.mL-1 to 6.16 log CFU.mL-1 in low lactose yogurt). The samples with E. coli O157:H7 showed gas formation and syneresis. Thus, E. coli O157:H7 was able to survive and grow during fermentation of traditional and low lactose yogurts affecting the manufacture technology. Moreover, milk contamination by E. coli before LAB addition reduces the growth of L. bulgaricus and S. thermophilus especially when associated with reduction of lactose content.

摘要 本研究旨在评估大肠杆菌O157:H7（E. coli O157:H7）在传统与低乳糖酸奶的乳糖水解及发酵过程中的行为，并验证其在4℃±1℃条件下冷藏12小时后的存活情况。
本研究制备了两类酸奶：2份以全脂牛乳为原料，2份以预水解全脂牛乳为原料；每组均设置接菌与未接菌两个样本。在发酵过程及12小时冷藏后，分别测定大肠杆菌的存活量与酸奶的pH值。
结果显示，大肠杆菌O157:H7可在发酵过程中增殖：传统酸奶中其菌落数从4.34 log 菌落形成单位（CFU）·mL⁻¹升至6.13 log CFU·mL⁻¹，低乳糖酸奶中则从4.34 log CFU·mL⁻¹升至6.16 log CFU·mL⁻¹。接种该菌的样本可观察到产气与乳清析出现象。
综上，大肠杆菌O157:H7可在传统与低乳糖酸奶的发酵过程中存活并增殖，对酸奶生产工艺产生不利影响。此外，在添加乳酸菌（LAB）前牛乳遭到该菌污染，会抑制保加利亚乳杆菌（L. bulgaricus）与嗜热链球菌（S. thermophilus）的生长，且该抑制效果在乳糖含量降低的情况下更为显著。