Survival of Listeria monocytogenes in Simulated Milk Cooling Systems

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Abstract
Survival of Listeria monocytogenes under conditions that might be found in milk cooling systems was studied. Sterile solutions of 0.1 and 0.01% peptone, 0.1 and 0.01% nonfat dry milk (NFDM), 30% propylene glycol, and 30% propylene glycol with 0.01 % NFDM were inoculated with 6000 L. monocytogenes Scott A/ml and were incubated at 4°C. The temperature was increased to 7°C when little growth was observed. At 7°C, populations approached 109 organisms/ml in NFDM and peptone. Growth was greater in the higher concentrations of each, and there was limited survival in the glycol media. Growth in minimal media, 0.01% peptone, 0.01% NFDM, 30% propylene glycol with 0.01% NFDM, and 1 % tryptic soy broth (TSB), was studied. These media were inoculated with 3500 L. monocytogenes Jalisco cheese/ml. At 4°C, more growth was observed in the NFDM than in the peptone, no survival was seen in the glycol media, and the most growth was observed in the TSB. Growth in sterile 10, 20, and 30% propylene glycol solutions (with 0.1 % NFDM) was studied by inoculation with 8800 L. monocytogenes Jalisco cheese/ml and incubation at 4°C. Growth in the 10% solution was observed. However, there was survival in the 20 and 30% solutions with no increase in numbers apparent over the time studied. Presence of L. monocytogenes in milk cooling systems may pose a hazard, especially in sweet water systems that might contain a small amount of milk.