Destruction of Escherichia coli and Salmonella typhimurium in Microwave-cooked Soups

Abstract

Single serving (i.e. 200 ml) portions of tomato soup, vegetable soup, and broth inoculated with Escherichia coli or Salmonella typhimurium at about 107 organisms/ml were exposed to 915 MHz microwaves. After various timed exposures the temperature of the top, middle, and bottom regions as indicated by changes in previously positioned assemblies of temperature sensitive paper strips were noted and aliquots were removed from the same regions for standard plate count determination of survivors. For any given exposure time, the temperature of the middle region was warmest; that of the bottom, intermediate; and the temperature of the top, coolest. Despite the relative temperatures of the regions, however, the consistent finding was that, for any exposure time, the closer the sampled organisms were to the top, the lower their level of survival. In terms of temperature, it was noted that organisms in the top had declined to a given level of survival at a temperature lower than the temperature corresponding to the same survival level in the middle or bottom soup regions. These data suggest that the heat generated during the microwave exposure alone is inadequate to fully account for the nature of the lethal effects of microwaves for microorganisms.