Behavior of Listeria monocytogenes During the Manufacture and Ripening of Cheddar Cheese.

The ability of Listeria monocytogenes to survive the Cheddar cheesemaking process and persist during ripening of cheese was examined. Pasteurized whole milk inoculated to contain 5 ×10 cells of L. monocytogenes [strain Scott A, V7 or California (CA)]/ml was made into stirred-curd Cheddar cheese in a pilot-plant-sized vat. Cheese was ripened at 6 or 13°C. Listeria counts were obtained by surface-plating samples diluted in Tryptose Broth (TB) on McBride Listeria Agar (MLA). Initial TB dilutions were stored at 3°C and plated on MLA after 2, 4, 6 and 8 weeks if the organism was not detected with the original plating on MLA. Selected Listeria colonies from each sample were confirmed biochemically. During Cheddar cheese manufacture, Listeria counts remained relatively constant at ca. 5 x 102/ml of milk. After pressing the curd overnight, numbers of L. monocytogenes increased to about 1 × 10/g. Generally, greatest numbers of Listeria , about 5 × 10 cells/g, were detected in cheese after 14 d of ripening. Listeria counts for all 3 strains decreased during further ripening and except for strain V7, no appreciable difference in survival occurred in cheese aged at 6 or 13°C. Strains Scott A, CA and V7 survived for as long as 224, 154 and at least 434 d, respectively, in Cheddar cheese of normal composition. Strains V7 and CA were uniformly distributed throughout another set of cheese blocks and numbers of Listeria decreased uniformly throughout blocks of cheese during 98 d of storage.