Growth and Survival of Listeria monocytogenes in Market Cheeses Stored at 4 to 30°C

Abstract

Forty-nine market cheeses representing 24 types and 28 brands were purchased from local supermarkets. Pieces of cheeses of approximately 1.5 × 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°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.