Effect of irradiation temperature on inactivation of Escherichia coli O157:H7 and Staphylococcus aureus.

The resistance of Escherichia coli O157:H7 and Staphylococcus aureus in ground beef to gamma radiation was significantly (P < 0.05) higher at subfreezing temperatures than above freezing. Ground beef was inoculated (ca. 2 x 10(8) CFU/g) with five isolates of either E. coli O157:H7 or S. aureus and subdivided into 25-g samples, vacuum packaged in barrier pouches, and tempered to 20, 12, 4, 0, -4, -12, -20, -30, -40, or -76 degrees C before gamma irradiation. The studies were repeated twice. The D10-values for both of these pathogens increased significantly at subfreezing temperatures, reaching maxima at approximately -20 degrees C. The D10-values for E. coli O157:H7 at 4 and -20 degrees C were 0.39 +/- 0.04 and 0.98 +/- 0.23 kGy, respectively. The D10-values for S. aureus at 0 and -20 degrees C were 0.51 degrees 0.02 and 0.88 +/- 0.05 kGy, respectively.