Effect of Evaporative Cooling, Fat Content and Food Type on Pathogen Survival during Microwave Cooking


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Virginia Tech


Due to the rapid nature of microwave heating, the microbiological safety of foods prepared in the microwave has been in question for several years. Because foods are heated from the inside out and are strictly governed by their own internal properties such as ionic content, moisture level and specific heat, work must be done to further master control of such properties so microwave cooking can be more predictable, controlled and ensure control pathogens.

This study concentrated on the effect of fat content, evaporative cooling and food type on the rate of food borne pathogen survival rates in microwave heated foods. Foods investigated in this study included fresh, raw broccoli spears; a regular, whole muscle breaded chicken patty and a fat free, breaded, formed chicken patty; and raw ground beef patties at three differing fat percentages. All foods were tested in triplicate. A Sharp® 1000W Light-Duty Commercial Microwave Oven was used to treat inoculated samples according to their recommended cooking times. Two sets of samples were treated, one wrapped with Saran™ Wrap and the other without wrap.

F-values were determined for each product. Raw ground beef patties at fat contents of 30%, 15% and 7%, heated for the same time had F-values ranging from 0.03 to 126.20. The lower the fat content, the lower the lethality. Regular and fat free chicken tenders had similar patterns. F-values for fresh broccoli indicted that vegetative pathogens survived the recommended microwave process. Covering in Saran™ Wrap had some preventive effect on evaporative cooling depending on the food tested and significantly (p < 0.05) increased most F-values.

Inoculated pack studies were performed in triplicate on each food with Listeria monocytogenes, Salmonella and Escherichia coli O157:H7. Survival was determined by presence or absence of growth of each pathogen after enrichment. Listeria monocytogenes survived in all samples except for the 30% fat ground beef patties. The Salmonella species had a lower survival rate; however, it was still present in uncovered 15% fat ground beef, covered 7% fat ground beef, uncovered chicken patties (both types) and in all broccoli samples tested. E. coli O157:H7 survived in all samples except the 30% fat ground beef samples.

Results indicate that higher fat contents seem to ensure lower rates of pathogen survival. This was especially true for the raw ground beef, which had received no prior processing other than the grinding of the whole muscle. There were fewer survival differences in the preprocessed, frozen chicken patties. Both were shown to support no pathogen survival in covered samples, except the fat free chicken patties. Listeria monocytogenes was shown to consistently survive the suggested cooking time in these samples. This is consistent with expectations that fat free food samples would display more survival than regular fat samples.

Overall, covering samples with Saran™ had little effect on pathogen survival rates. There were survival differences in some covered and uncovered samples consistent with expectations that covered samples would show less survival than uncovered, but further work including more samples would be necessary to ensure that the covered or uncovered variable made the true difference in pathogen survival. Finally, broccoli demonstrated consistent pathogen survival in all categories of testing. This indicates microwave oven prepared vegetables could be a prime source of pathogen transmission to consumers. Further work needs to concentrate on determining the correct processing times and parameters that need to be met to ensure safe food.



microwave, food borne pathogens