Fate of Listeria monocytogenes during the souring of Ergo, a traditional Ethiopian fermented milk.

The growth potential of three strains of Listeria monocytogenes during the natural souring of raw milk into Ergo was studied to determine the effect of souring and container smoking on their inactivation. All three strains of L. monocytogenes grew well in sterile milk in unsmoked containers, reaching counts > 10(7) cfu/ml within 24 h at ambient temperature. Smoking of containers decreased the growth rate of the strains for 24 h but had varying effects on the strains. For all test strains, the count increased slightly (1.0 to 1.7 log units) until 12 h during the souring of Ergo in unsmoked containers. After 12 h of souring, the counts decreased steadily to undetectable levels at 48 or 60 h. Smoking of containers increased the rate of inactivation of the test organisms, and inactivation was complete at 36 h. Because Ergo is made by the natural souring of raw milk, consumers must be made aware of the potential dangers of acquiring listeriosis from fresh Ergo from raw milk.