The influence of two levels of fat (11.l to 13.9% [low-fat] and 27.1 to 27.9% [high-fat]) and temperature (frozen [-17 to -15°C] and refrigerated [3 to 5°C]) on gamma irradiation (60CO) inactivation of Listeria monocytogenes and Staphylococcus aureus in raw ground beef patties was investigated. Ground beef patties inoculated with stationary growth phase cells of five-strain mixtures of L. monocytogenes or S. aureus were treated with seven mean gamma irradiation doses up to 2.062 or 2.147 kGy, respectively. D10 values ranged from 0.507 to 0.610 kGy and 0.435 to 0.453 kGy for L. monocytogenes and S. aureus , respectively. Neither the fat content of beef nor the temperature during irradiation treatment influenced inactivation rates of the two pathogens. Regression coefficients were high for all treatment conditions, the lowest being 0.984 for L. monocytogenes and 0.990 for S. aureus in high-fat frozen beef. Based on the highest D10 value obtained, a dose of 2.50 kGy would theoretically kill 4.10 log10 L. monocytogenes and 5.12 log10 S. aureus per gram of ground beef. The fact that this investigation was done under commercial processing and irradiation treatment conditions, to the extent possible, makes the significance of the results more meaningful to the beef industry.
Keywords: Gamma irradiation; bacterial pathogens; ground beef.