Survival kinetics of heat-stressed Escherichia coli O157:H7 and Listeria monocytogenes cells as post-fermentation contaminants in kefir during refrigerated storage

The survival kinetics of heat-adapted (43 °C 18 h, 45 °C 18 h) and heat-shocked (45 °C 2 h) Escherichia coli O157:H7 and Listeria monocytogenes cells inoculated into kefir was investigated during refrigerated storage. Inactivation data was analyzed using log-linear, log linear-shoulder, and Weibull models. The non-linear models fit better to obtained survival data. Survival curves of two pathogens that heat-adapted at 43 °C displayed downward concavity with a lag phase. Heat adaptation of L. monocytogenes at 43 °C resulted in 12 h shorter lag time but 76 h longer 4D reduction time, while those of E. coli O157:H7 displayed 59 h shorter lag time than controls. After heat shocking at 45 °C (2 h), 4D reduction time of E. coli O157:H7 was 17 h longer than control, whereas the survival curve of L. monocytogenes was concave upward with a 4D reduction time 93 h shorter than control. Heat adaptation at 45 °C for 18 h resulted in concave upward survival curves. The 4D reduction times of L. monocytogenes and E. coli O157:H7 were 73 and 85 h shorter than controls, respectively. In conclusion, the response of the cells to exposure elevated temperatures prior to inoculation into kefir affected the kinetics of the survival curves.