Effect of thermophilic lactic acid bacteria on the fate of Enterobacter sakazakii during processing and storage of plain yogurt

Survival and growth of Enterobacter sakazakii during processing and storage of plain yogurt were investigated. Preheated rehydrated milk was inoculated with a cocktail culture of E. sakazakii (10³ cfu/mL of milk) and/or with thermophilic yogurt starter culture of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus salivarius ssp. thermophilus. The inoculated milk was incubated at 40C for 5 h, then the samples were cooled and subsequently stored at 4C for up to 7 days. The results showed that E. sakazakii grew at an early stage of fermentation but declined at the end of the process. There was no significant difference between the populations of E. sakazakii in the presence or absence of lactic acid bacteria during the first 4 h of the incubation period but there was significant difference during the last hour of the incubation period. The populations of E. sakazakii decreased significantly during cooling and storage of yogurt (pH 4.2-4.7) compared with nonfermented milk samples at 4C. The presence of E. sakazakii did not have a significant effect on the growth of LAB during fermentation and storage of yogurt. The results obtained from this study indicate that the pH of yogurt and storage temperature were critical to the survival and growth of E. sakazakii in the manufacture of plain yogurt.