Reduction of Listeria monocytogenes in Precooked Vacuum-Packaged Beef Using Postpackaging Pasteurization

Precooked beef loin chunks were inoculated separately with three strains of Listeria monocytogenes (Scott A, 101M, and 103M). Uninoculated chunks served as controls. All chunks were vacuum packaged after inoculation. Half were not pasteurized and half were pasteurized in 85°C water for 16 min. All samples were stored at 4°C for up to 85 d and examined periodically. Pasteurization reduced all microflora and significantly reduced populations of three strains of L. monocytogenes on the surface and in the broth of the precooked beef chunks for 85 d of refrigerated storage as determined by direct plating procedures. All three strains of L. monocytogenes were recovered from the inoculated pasteurized beef using enrichment. Uninoculated chunks were positive for Listeria spp. which were primarily Listeria welshimeri . Without pasteurization, microflora increased 5- to 6-fold within 14 d of storage. Populations of Scott A and 101M strains of L. monocytogenes increased significantly within 7 d. After 14 d, populations of all three strains did not differ from initial levels and remained unchanged for the remainder of the storage period.