Growth of Listeria monocytogenes on Vacuum-packaged Beef

Pieces of beef striploin (400 g) were inoculated with Listeria monocytogenes strain Murray B, vacuum packaged, and stored at either 0°C or 5.3°C. Growth of the organism on the beef depended on the temperature of storage, the pH of the lean, and the type of tissue. Growth was more rapid at 5.3°C than at 0°C, and faster on striploins of high pH (6.0-6.1) than on striploins of low pH (5.5-5.7). During storage, the population of L. monocytogenes was higher on fatty tissue than on lean principally because growth occurred earlier on the fat. When low pH striploins were held at 5.3°C, listeria grew from an initial count of 2-5×10³ CFU/cm² to 3×10⁷ CFU/cm² in 16 d on the fat, and in 20 d, to 10⁶ CFU/cm² on the lean and to 5×10⁷ CFU/ml in the purge fluid. After storage at 0°C for 76 d, the populations reached were 10⁶ CFU/cm² on the fat, 10⁴ CFU/cm² on the lean, and 3×10⁵ CFU/ml in the purge fluid. When high pH striploins were held at 0°C for 10 weeks, listeria grew from an initial population of 150-400 CFU/cm² to just over 10⁶ CFU/cm² on the fat, 2×10⁵ CFU/cm² on the lean, and 4×10⁶ CFU/ml in purge fluid.