Combined effect of gamma radiation and heating on the destruction of Listeria monocytogenes and Salmonella typhimurium in cook-chill roast beef and gravy

The effect of heating alone (60, 65 or 70 °C), heating after irradiation (0.8 kGy) and heating after irradiation and storage for 14 days at 2–3 °C on the destruction of Listeria monocytogenes and Salmonella typhimurium in artificially inoculated minced cook-chill roast beef and gravy was investigated. Inoculated minced roast beef samples (5 g) were heated in Stomacher bags completely immersed in a water bath at each of the test temperatures. Survivors were enumerated and D and z values were determined for each of the pathogens. Observed thermal D values for two strains of L. monocytogenes at 60, 65 and 70 °C in the absence of pre-irradiation were 90.0–97.5 s, 34.0–53.0 s and 22.4–28.0 s, respectively, whereas thermal D values after pre-irradiation were 44.0–46.4 s, 15.3–16.8 s and 5.5–7.8 s at 60, 65 and 70 °C, respectively. This reduction in D values provides evidence for radiation-induced heat-sensitisation in L. monocytogenes. There was some evidence of heat-sensitisation of S. typhimurium at 60 °C, but not at either 65 or 70 °C. The z values for L. monocytogenes and S. typhimurium in cook-chill roast beef and gravy were calculated to be 16.1–20.0 °C and 10.3 °C, respectively. In the case of L. monocytogenes the z value also decreased as a consequence of pre-irradiation to a dose of 0.8 kGy (11.0–12.7 °C). The radiation-induced heat-sensitivity in L. monocytogenes was found to persist for up to 2 weeks storage at 2–3 °C prior to heating. As cook-chill products are intended to be reheated prior to consumption the results of the present study suggest that any L. monocytogenes present in a cook-chill product would be more easily killed during reheating if it were to be treated with a low dose of gamma radiation during manufacture.