Purpose: This study aimed to determine the potential for growth of L. monocytogenes and native populations of bacteria, yeast, and mold on cheese slices stored 42 days at 7°C.
Methods: Three LAND O’LAKES cheese products, two pasteurized process cheeses (New Yorker American and Reduced Sodium Deli American) and one natural cheese (Reduced Fat Swiss), were sliced using a Hobart model 2912 deli slicer, surface inoculated with a L. monocytogenes cocktail (ATCC 19115, 19118, 43257, 51779, and 51782) at 3.7 ± 0.1 log CFU/g, and air-dried 15 min. The slices were then folded and placed in a Whirl-pak bag. Periodically during 42 days storage at 7°C, three slices of each product were plated on Modified Oxford Agar to enumerate Listeria. Similarly, uninoculated cheese slices were plated on Plate Count Agar and Standard Methods Agar w/ Chloramphenicol to enumerate mesophilic aerobic bacteria, yeast and mold, respectively. Findings were analyzed using 2-sample t-test with equal variance (Minitab 17).
Results: After 42 days of storage at 7°C, populations of L. monocytogenes declined significantly (P < 0.05) to 3.1 ± 0.1, 3.2 ± 0.0 and 2.1 ± 0.3 log CFU/g in New Yorker American, Reduced Sodium Deli American, and Reduced Fat Swiss, respectively. Similarly, populations of native mesophilic bacteria significantly (P < 0.05) decreased in New Yorker American and Reduced Sodium Deli American. Bacterial counts were not performed on the Reduced Fat Swiss due to the presence of starter cultures.
Significance: This study supports the holding of opened packages of processed cheese in retail delis stored under proper refrigeration longer than the seven day limitation found in Food Code (2013).