Ensuring safety of complex foods.

Foodborne illness outbreaks and product recalls are occasionally associated with ready-to-eat (RTE) sandwiches and other complex RTE food products. Ionizing radiation (IR) can inactivate pathogens found in many foods. However, less data are available on the ability of low-dose IR, doses under 5 kGy that are typically used for pasteurization purposes, to inactivate pathogenic bacteria on complex multicomponent RTE products.

USDA-ARS scientists have investigated the ability of IR to inactivate pathogenic bacteria, including Salmonella spp., L. monocytogenes, S. aureus, E. coli O157:H7 and Y. enterocolitica on RTE foods, such as a frankfurter on a roll, a beef cheeseburger and a vegetarian cheeseburger. They determined that IR may be an effective way to inactivate common foodborne pathogens found in complex RTE products.

The studies have included the use of IR to inactivate pathogens in an aerobic environment, or in the presence of modified atmospheres. The individual bacterial species--three isolate mixtures--were surface-inoculated (8 log10 CFU per g) onto irradiated products using a self-contained irradiator at 0.095 kGy per min at 4 C.

The surviving bacteria were enumerated using standard microbiological techniques for determining the D-10 value. As you may know, the D-10 value for a microorganism is defined as the UV dose necessary to effect a 90% reduction in colony forming units (CFUs). The average D-10 value, the radiation dose needed to inactivate 1 log10 of pathogen on the three products, was 0.61, 0.54, 0.47, 0.36 and 0.15 kGy for Salmonella spp., S. aureus, L. monocytogenes, E. coli O157:H7 and Y. enterocolitica, respectively.

A low IR dose of 1.25 kGy was able to inactivate more than 2 log10 of Salmonella spp., the most radiation-resistant of the pathogens tested, on each of the three products tested. An IR dose of 2.5 kGy was able to inactivate more than 4 log10 of Salmonella spp. The presence of modified atmospheres did not affect the radiation resistance of the pathogens.

Further information. Christopher Sommers, USDA-ARS Eastern Regional Research Center, Room 0022, 600 E. Mermaid Lane, Wyndmoor, PA 19038; phone: [phone]; fax: [phone]; email: [e-mail].