Survival of Escherichia coli O157:H7, Listeria monocytogenes 4b and Yersinia enterocolitica O3 in different yogurt and kefir combinations as prefermentation contaminant.

Abstract

To compare microbiological safety of yogurt, kefir and different combinations of yogurt and kefir samples by using three foodborne pathogenic strains (Escherichia coli O157:H7, Listeria monocytogenes 4b and Yersinia enterocolitica O3) as indicators. Fresh yogurt and kefir drinks were added to pasteurized milk at a 5% rate either separately or together, and then incubated at different temperatures (43 degrees C for yogurt and 30 degrees C for kefir), depending on appropriate growth temperature of their starter microflora. While traditional yogurt was found to be the least suppressive on the three pathogenic micro-organisms, samples obtained from two subsequent fermentation process (samples fermented at 43 degrees C for 3 h and at 30 degrees C for 21 h) were more suppressive than that of traditional kefir. There was no significant survival difference between E. coli O157:H7 and L. monocytogenes 4b in samples tested (P > 0.05), but Y. enterocolitica O3 was more susceptible than other two test strains (P < 0.05). The microbiological safety of the dairy product fermented at two consecutive periods was superior than that of traditional yogurt or kefir alone. These experiments may mimic what happens when yogurt and kefir starter micro-organisms are combined in a milk fermentation process with different time and temperature periods.