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D. W. THAYER, Food Safety Research Unit, USDA, ARS, ERRC, 600 East Mermaid Lane, Wyndmoor, PA 19038

Irradiation pasteurization of meat and poultry produces characteristic changes in the indigenous microflora of the product. These changes are so pronounced that microbiological methods have been proposed for the identification of irradiated foods. The most pronounced change is that the total plate count will be reduced in direct proportion to the radiation dose that's absorbed. In general, the population shifts from predominantly gram-negative to gram-positive bacteria; and the numbers of naturally occurring mesophiles, psychrotrophs, and anaerobes are greatly decreased. The populations of lactic acid bacteria are only minimally affected. The overall result of these changes is that the shelf life of properly refrigerated products will be extended; yet, if temperature abused, these irradiation-pasteurized meat or poultry products will spoil. Fortunately, most vegetative bacterial pathogens are relatively sensitive to ionizing radiation and will be inactivated at realistic levels of contamination. However, even those vegetative cells or spores (i.e., Bacillus, Clostridia, Salmonella, and Listeria) that may survive are radiation injured and are often more sensitive to food additives, good refrigeration, modified atmosphere packaging, and subsequent processing or cooking steps. The potential for the selection of viable and competitive mutant strains of bacteria or viruses with either increased radiation resistance or increased pathogenicity has been demonstrated to be minimal. If Clostridia, Salmonella, or Listeria should contaminate the meat or poultry either before or after irradiation processing, studies have demonstrated that the surviving indigenous microflora is adequate to prevent the unchecked multiplication of these pathogens.