Purpose: This study was conducted to determine the potential for Lm growth in a wide variety of dairy products during refrigerated storage.
Methods: Thirty-seven dairy products, including varieties of pasteurized process cheeses, cream cheeses, cultured aged cheeses, cultured mozzarella cheeses, feta cheese, cottage cheese and sour cream were inoculated with 100-500 CFU/g of a 6-strain cocktail of Lm and incubated at 45 °F for various time periods, depending on the shelf life of each product. Products were tested for Lm counts on modified Oxford Agar at varying intervals throughout the shelf life to determine if the organism was able to survive and grow in the products. Growth was considered an increase in counts of greater than or equal to 1 log level. The pH, water activity, fat, moisture, and salt of the uninoculated products were determined at the initiation of the study. Aerobic plate count testing was also performed using All Purpose Agar with Tween at the same test intervals throughout the study.
Results: The majority of products did not support the growth of Lm, and counts of the organisms decreased over the product shelf life in most cases. Only certain products were able to support the growth of Lm at refrigeration temperatures: certain pasteurized process cheeses and higher-pH aged cheese such as Gouda. Product formulation and presence of active cultures were important factors in determining the likelihood of Lm growth.
Significance: This information will be useful in determining the refrigerated dairy products having the potential to be contaminated post-process that are at risk for Lm growth during storage and distribution. The data can be used to determine which monitoring programs may be necessary in production facilities.