Fate of Escherichia coli O157:H7 during the processing and storage of Ergo and Ayib, traditional Ethiopian dairy products

The long-term survival of E. coli O157:H7 in acid foods is well documented. This prompted us to evaluate the organismʹs survival during the making of Ergo, a traditional Ethiopian sour milk and Ayib, a traditional Ethiopian cottage cheese and during storage of these products at both ambient and refrigeration temperatures. E. coli O157:H7 test strains were separately inoculated into milk at initial levels of log10 3 cfu/ml. Levels of E. coli O157:H7 in the absence of lactic acid bacteria (LAB) reached log10 8.4 cfu/ml at 24 h as the pH dropped to 5.6±1.0. In milk inoculated with LAB (log10 6.4 cfu/ml) and E. coli O157:H7 (log10 3 cfu/ml), levels of LAB were log10 9.6 cfu/ml and 9.4 cfu/ml at 24 h and 72 h, respectively, and the pH values were 3.5 and 3.9, respectively. In the presence of LAB, E. coli O157:H7 grew to log10 6.5 cfu/ml at 24 h, with the levels decreasing to log10 3.2 cfu/ml at 72 h. Post-fermentation inoculation of E. coli O157:H7 in Ergo at an initial level of log10 3 cfu/ml, resulted in complete elimination of test organisms at 6 h at ambient temperature storage, but they were recovered until 72 h at refrigerated storage. At a higher initial inoculum level (log10 6 cfu/ml) of the E. coli O157:H7, the counts decreased by 4 logs within 12 h (pH 4.2) at ambient temperature storage, and complete elimination was observed at 36 h (pH 4.0). At refrigeration temperature, counts at 72 h were between 2.2 and 3.5 log cfu/ml for the different strains. During Ayib processing, E. coli O157:H7 in souring milk increased by 3 logs in 24 h, with a slight reduction being observed at 36 h. The pH dropped to 4.3 during this time. The numbers of E. coli O157:H7, immediately after curd cooking, were below detectable levels, but could be recovered by enrichment. Complete inactivation was observed on 24 h after curd cooking. When E. coli O157:H7 was inoculated into steam-treated Ayib (pH 4.2–4.3) at levels of log10 6.0–6.7 cfu/g and maintained at ambient temperatures, there was a gradual decrease in numbers to 3.7–4.3 logs by day 3. After day 7, pH values were 3.8–3.9 and E. coli O157:H7 was only detectable after enrichment. E. coli O157:H7 strain MF-1847 was completely inactivated by day 9, but the other two strains could still be recovered by enrichment at this time. At refrigeration storage, decrease in count of the test strains was gradual and counts at day 8 and 9 were >4 log cfu/g. As Ergo is preferably consumed soon after (24 h) fermentation, traditional fermentation of Ergo would not guarantee that E. coli O157:H7 can be controlled and, therefore, Ergo can be a potential health hazard if prepared from milk contaminated with E. coli O157:H7.