Growth Potential of Individual Strains of Listeria monocytogenes in Fresh Vacuum-Packaged Refrigerated Ground Top Rounds of Beef.

Separate inocula of four strains of Listeria monocytogenes were prepared at 4°C and inoculated (3.58 to 4.67 log colony forming units [CFU]/g) in top round ground beef (< 4.0% fat) patties (78.8 ± 6.7 g) of normal (5.47 ± 0.03) or high (6.14 ± 0.08) pH, which were stored (4°C) vacuum-packaged for 56 days and analyzed for L. monocytogenes , total aerobic plate counts (APC) and pH. In normal-pH ground beef, strain N-7143 (serotype 3a), multiplied from 4.25 ± 0.71 log CFU/g at day of inoculation to 6.53 ± 0.34 log CFU/g at 35 days of storage ( < 0.05); a 2.3 log CFU/g increase. Populations of strain Na-19 (serotype 3b) increased 1.8 log CFU/g in 35 days of storage, while numbers of strain Na-16 (serotype 1/2a) did not change ( > 0.05) during the 56 days of storage. Strain Scott A (serotype 4b) decreased in numbers from 4.00 ± 1.21 log CFU/g at day-0 to 2.72 ± 0.98 log CFU/g at 56 days. Populations of strain Scott A were lower ( < 0.05) than other strains after 21 days of storage. In high-pH ground beef, populations of strains Na-19, N-7143 and Na-16 increased ( < 0.05) by 2.87, 2.64 and 2.24 log CFU/g, respectively, in 28 days. Populations of strain Scott A did not change significantly ( > 0.05), and they were lower (< 0.05) than populations of other strains at 28 days. The results indicated that although growth of L. monocytogenes in vacuum-packaged, refrigerated ground beef was slow, it proceeded more rapidly in product of pH values above 6.0, and depended on strain of the pathogen tested.