Reduction of Normal Flora by Irradiation and Its Effect on the Ability of Listeria monocytogenes to Multiply on Ground Turkey Stored at 7°C When Packaged under a Modified Atmosphere†

Abstract

Listeria monocytogenes did not multiply faster during storage at 7°C on irradiated than on nonirradiated raw ground turkey, and there was a concentration-dependent inhibition of its multiplication by CO2. Ground turkey was gamma irradiated at 5°C to 0, 1.5, and 2.5 kGy and inoculated (∼100 CFU/g) after irradiation with a cocktail of L. monocytogenes ATCC 7644, 15313, 49594, and 43256. The meat was then packaged in air-permeable pouches or under atmospheres containing 30 or 53% CO2, 19% O2, and 51 or 24% N2 and stored at 7°C for up to 28 days. A dose of 2.5 kGy extended the time for the total plate count (TPC) to reach 107 CFU/g from 4 to 19 days compared to that for nonirradiated turkey in air-permeable pouches. Following a dose of 2.5 kGy at the end of the 28-day study, the TPCs were 106.42 and 104.98 under 25% and 50% CO2 atmospheres, respectively. Under air, 30% CO2, and 53% CO2 atmospheres, the populations of L. monocytogenes after 19 days incubation were 104.89, 103.60, and 102.67 CFU/g. The populations of lactic acid bacteria and anaerobic or facultative bacteria were also reduced by irradiation. Irradiating ground turkey did not decrease its safety when it was contaminated following processing with L. monocytogenes.