Purpose: This study investigated the genetic characteristics of E. coli isolates in Nappa cabbage and developed a dynamic model to predict the fates of E. coli in kimchi.
Methods: Kimchi ingredient samples [Napa cabbage (n=30), white radish (n=30), leek (n=20), ginger (n=20)] were analyzed to detect E. coli. Isolated colonies were identified by 16s rRNA analysis, and presences of stx1 and stx2 were investigated by PCR. O and H types were also determined. To develop a dynamic model, a 5-strain mixture of E. coli was inoculated in kimchi at 4 log CFU/g, followed by storage at 4 - 30°C up to 17 days. The Baranyi model was fitted to the microbiological data to calculate maximum specific growth rate (µmax; log CFU/g/h) and death rate (DR; log CFU/g/h). Polynomial equations were then fitted to the kinetic parameters, followed by developing a dynamic model. Root mean square error (RMSE) was calculated to evaluate the model performance.
Results: Of 30 Napa cabbage samples, one sample was contaminated with E. coli, and the E. coli was serotyped as O6:H34 with the presence of stx2 gene. At 4°C and 10°C, E. coli survived until 192 h and 132 h, respectively, and then it started to decrease. At 15 - 30°C, E. coli in cabbage kimchi grew initially and gradually started to decrease during fermentation. In growth phase, µmax increased as temperatures increased, and DR also increased at higher temperatures. The prediction of developed dynamic model was appropriate with 0.586 of RMSE.
Significance: The results indicate that E. coli from raw ingredients may contaminate kimchi, and the developed dynamic model should be useful in predicting the kinetic behavior of E. coli in cabbage kimchi.