Growth of Listeria monocytogenes on Vacuum-packaged Beef

Abstract

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.