Bacterial survival and thermal responses of beef loaf after microwave processing

The effects of wrapping product in polyvinylidene chloride (PVDC) film, load size (150, 600 and 1200 g loaves), microwave output power (713 +/- 5 W and 356 +/- 3 W at 2450 MHz) and processing time on bacterial survival were evaluated in beef loaf. Specifically the survival of aerobic flora, and inoculated Staphylococcus aureus (ATCC 6538) and Escherichia coli (O157:H7) were assessed. Results showed that beef loaf wrapped in PVDC film had lower levels of survival for aerobic bacteria, S. aureus and E. coli than unwrapped beef loaf. When beef loaf was exposed to the same microwave dose (watts x minutes processed/grams of food), the percent of survival remained the same, regardless of load size or microwave output power. Exposed microwave dose, which correlated highly to percent of bacterial survival (R2 = 0.8-0.83) and end point temperature (R2 = 0.84), is suggested in place of output power, time or temperature as a predictor of microbial quality in foods that have been microwave processed.