Survival of Listeria monocytogenes in Cold-Pack Cheese Food During Refrigerated Storage

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Abstract
Duplicate lots of cold-pack cheese food were manufactured, according to nine different formulations, inoculated to contain ca. 5 × 102 Listeria monocytogenes (strains Scott A, V7, California or Ohio) colony forming units (CFU)/g and stored at 4°C. L. monocytogenes in cheese food was enumerated by surface-plating appropriate dilutions made in tryptose broth containing 2% (w/v) sodium citrate (TBC) on McBride Listeria Agar (MLA). Initial TBC dilutions were stored at 3°C and surface-plated on MLA after 2, 4, 6 and 8 weeks if the organism was not quantitated by direct plating of the original samples. Selected Listeria colonies were confirmed biochemically. Populations of L. monocytogenes in cheese food manufactured without preservatives or acidifying agents generally decreased less than 10-fold after 182 d of storage. However, numbers of L. monocytogenes steadily decreased in cheese food containing 0.30% sorbic acid or 0.30% sodium propionate and which was acidified to pH 5.0 to 5.1 with lactic and/or acetic acid. In cheese food preserved with 0.30% sorbic acid, L. monocytogenes survived an average of 130 d in non-acidified cheese food and 112, 93 or 74 d in cheese food acidified with lactic acid, lactic plus acetic acid, or acetic acid, respectively. When 0.30% sodium propionate was substituted for sorbic acid, L. monocytogenes survived an average of 142 d in non-acidified cheese food and 118, 103 or 98 d in cheese food acidified with lactic, acetic, or lactic plus acetic acid, respectively.

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