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 degrees C), heating after ir radiation (0.8 kGy) and heating after irradiation and storage for 14 d ays at 2-3 degrees C on the destruction of Listeria monocytogenes and Salmonella typhimurium in artificially inoculated minced cook-chill ro ast beef and gravy was investigated. Inoculated minced roast beef samp les (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 the rmal D values for two strains of L. monocytogenes at 60, 65 and 70 deg rees 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-irr adiation were 44.0-46.4 s, 15.3-16.8 s and 5.5-7.8 s at 60, 65 and 70 degrees C, respectively. This reduction in D values provides evidence for radiation-induced heat-sensitisation in L. monocytogenes. There wa s some evidence of heat-sensitisation of S. typhimurium at 60 degrees C, but not at either 65 or 70 degrees C. The z values for L. monocytog enes and S. typhimurium in cook-chill roast beef and gravy were calcul ated to be 16.1-20.0 degrees C and 10.3 degrees C, respectively. In th e 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 degrees C). The ra diation-induced heat-sensitivity in L. monocytogenes was found to pers ist for up to 2 weeks storage at 2-3 degrees C prior to heating. As co ok-chill products are intended to be reheated prior to consumption the results of the present study suggest that any L. monocytogenes presen t 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 ma nufacture.