Survival of Listeria monocytogenes in Cottage Cheese.

Most studies on the behavior of Listeria monocytogenes in cheeses have focused on the soft, ripened types because of human listeriosis outbreaks linked to consumption of these foods, while observations in cottage cheese are limited and conflicting. Fresh market cottage cheese samples with and without sorbate, purchased in the United States and Canada, respectively, were contaminated with L. monocytogenes strain Scott A (103 CFU/g). Mean total plate counts/g, log10, in the fresh U.S. and Canadian cheeses were 3.54 and 5.22, respectively. After 24 days at 5°C, the respective numbers were 3.51 and 6.27. Mean pH values in the U.S. cheeses at freshness and after 24 days were 5.05 and 5.03, respectively, and values in the Canadian cheese were 4.89 and 4.77. Increases of >3 logs in cell numbers were seen at temperature abuse (10 and 20°C) in all cheese samples, although sorbate-containing cheese spoiled at a slower rate and samples were free of visible surface mold growth even after 21 days. Cottage cheese did not support growth of L. monocytogenes at any of the storage temperatures, and declines (0.35 and 0.81 log10, in sorbate-free and sorbate-containing cheese, respectively) were seen after 24 days at 5°C. Treatment with 3% sodium or calcium lactate did not affect listerial cell numbers. These findings demonstrate that sorbates in cottage cheese are effective as preservatives but not as antilisterials. Persistence of listeriae in cottage cheese emphasizes the need to prevent contamination during manufacturing and storage.