GROWTH AND SURVIVAL OF LISTERIA-MONOCYTOGENES IN MARKET CHEESES STORED AT 4-DEGREES-C TO 30-DEGREES-C

Forty-nine market cheeses representing 24 types and 28 brands were purchased from local supermarkets. Pieces of cheeses of approximately 1.5 x 0.5 cm were surface inoculated with log10 3.95 to 4.36 cells of a Listeria monocytogenes pool made up of five strains (Scott A, V7, RM-1, VPH1, VPH2) and placed in petri dishes. After wrapping with cellophane, the dishes were stored at 4, 8, and 30-degrees-C for up to 36 d. Of the cheeses, 36.7% supported growth equivalent to a mean inoculum increase of 1.4 log10 (range 0.21 to 3.58) in at least one storage temperature. They included soft Hispanic type (Queso Fresco, Panela Ranchero, pH 6.2-6.6), Ricotta (pH 5.9-6.1), Teleme (pH 5.9), Brie (pH 7.2-7.7), Camembert (pH 7.3), and cottage (pH 4.9-5.1) cheeses. Ricotta was the best and cottage the worst substrate for growth. Cheeses not supporting Listeria growth but causing gradual death at all temperatures include: Cotija (Hispanic hard cheese), cream, blue, Tillamook, Cracker Barrel, Monterey Jack, Swiss, Cheddar, Colby, string, Provolone, Muenster, Feta, and Kasseri with values of pH 4.3-5.6, process (American, Monterey Jack, Piedmont, pH 5.7-6.4), and Limburger (pH 7.2) cheeses. A highly significant (P < 0.005) correlation of Listeria growth with cheese pH values > 5.5 and absence of starter cultures during the cheese manufacturing was observed. Overall, the study demonstrated that cross-contamination of certain cheeses with L. monocytogenes originating from raw foods (meat, poultry, fish, vegetables), after opening of packages, may lead to significant growth of the pathogen during refrigerated storage.