Behavior of Listeria monocytogenes in Skim Milk and in Yogurt Mix during Fermentation by Thermophilic Lactic Acid Bacteria

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Abstract
Behavior of Listeria monocytogenes in skim milk and in yogurt mix during fermentation with thermophilic lactic starters was determined. Sterile skim milk was inoculated with ca. 103 L. monocytogenes cells/ml and with 5.0, 1.0 or 0.1% of a milk culture of Streptococcus thermophilus, Lactobacillus bulgaricus or a mixture of the two species. The milk was incubated at 37 or 42°C for 15 h, followed by refrigeration at 4°C. Yogurt mix was inoculated with ca. 5 × 103 L. monocytogenes cells/ml of mix and then was incubated at 45°C for 5 h, followed by refrigeration at 4°C. L. monocytogenes survived the 15-h fermentation with S. thermophilus in all combinations of level of inoculum and temperature of incubation, but inhibition of growth ranged from 96 to 100%. When incubated with L. bulgaricus, L. monocytogenes survived only between 9 and 15 h of incubation; a decrease in pH to below 4.0 was accompanied by rapid death of the pathogen. The combination of the two species was more inhibitory to L. monocytogenes than was S. thermophilus alone but less inhibitory than was L. bulgaricus alone. In yogurt mix, L. monocytogenes grew during the fermentation and increased in number by about one order of magnitude.