Fate of Listeria monocytogenes during the manufacture and ripening of Camembert cheese

The ability of Listeria monocytogenes to survive the Camembert cheese-making process and grow during ripening of the cheese was examined. Pasteurized whole milk was inoculated to contain about 500 L. monocytogenes [strain Scott A, V7, California, (CA) or Ohio (OH)] CFU/ml and made into Camembert cheese according to standard procedures. All wheels of cheese were ripened at 6°C following 10 d of storage at 15-16°C to allow proper growth of Penicillium camemberti . Duplicate wedge (pie-shaped), surface and interior cheese samples were analyzed for numbers of L. monocytogenes by surface-plating appropriate dilutions made in Tryptose Broth (TB) on McBride Listeria Agar (MLA). Initial TB dilutions were stored at 3°C and surface-plated on MLA after 2, 4, 6 or 8 weeks if the organism was not quantitated in the original sample. Selected Listeria colonies from duplicate samples were confirmed biochemically. Results showed that numbers of Listeria in cheese increased 5- to 10-fold 24 h after its manufacture. Listeria counts for strains Scott A, CA and OH decreased to