Survival of Listeria monocytogenes in Refrigerated Cultured Milks and Yogurt

Skim milks containing Listeria monocytogenes were fermented at 21, 30, 37 or 42°C for 15 h with Streptococcus lactis, S. cremoris, S. thermophilus, Lactobacillus bulgaricus or L. bulgaricus plus S. thermophilus (LBST). Amounts of inocula were 5.0, 1.0, 0.5 or 0.1%. Yogurt mix was inoculated with L. monocytogenes and fermented at 45°C for 5 h. Cultured skim milks and yogurt were stored at 4°C and sampled weekly to monitor survival of L. monocytogenes and the pH. Yogurt was sampled ca. every 3 d for enumeration of L. monocytogenes and determination of pH. Results show that L. monocytogenes survived longest in skim milks fermented with S. thermophilus, ranging from 4 weeks in skim milk fermented at 42°C with a 5.0% inoculum (final pH 4.62) to 37 weeks in skim milk fermented at 37°C with a 1.0% inoculum (final pH 4.52). When skim milks were fermented with S. lactis, L. monocytogenes survived from 2 weeks (5.0% inoculum, 30°C incubation, final pH 4.20) to more than 13 weeks (0.1% inoculum, 21°C incubation, final pH 4.70). L. monocytogenes survived from 4 (final pH 4.36) to 13 (final pH 5.30) weeks in milk fermented with S. cremoris. L. bulgaricus was the most detrimental to L. monocytogenes; the pathogen survived only 3 d to 1 week in the skim milk fermented at 37°C with 0.1 % inoculum. Survival of L. monocytogenes in milk fermented with LBST culture ranged from 1 (final pH 3.93) to 12 (final pH 4.41) weeks in those skim milks in which the organism survived the fermentation process. L. monocytogenes survived yogurt manufacture and from 1 (final pH 4.13) to 12 (final pH 3.93) d during refrigerated storage of the product.