Influence of modified atmosphere and varying time in storage on the irradiation sensitivity of Salmonella on sliced roma tomatoes

Salmonella contamination of tomatoes is a recurrent food safety concern. Irradiation inactivates pathogens on fresh and fresh cut produce. However, the interaction of time in refrigerated storage and modified atmosphere packaging (MAP) may influence the response of pathogens to irradiation. Roma tomatoes were sliced and inoculated with a cocktail of outbreak strains of Salmonella. The inoculated tomatoes were packaged under one of four atmospheres: air, 10/90 O2/N2, 5/95 O2/N2 or 100% N2. The packages were kept in refrigerated storage (10 degree C) for various times after inoculation, to simulate the potential time delay between packaging and irradiation treatment. Tomatoes were irradiated immediately (0 h), or after 24 or 48 h in storage. The surviving populations were recovered and enumerated. Irradiation effectively reduced Salmonella at all times. Estimated D10 value (the dose necessary for 1 log reduction) varied significantly among the combinations of time and MAP, ranging from 0.165a0.335 kGy. Tomatoes packaged in air, irradiated at 0 h, had a D10 of 0.165 kGy; all other combinations showed significantly higher D10. Reduced oxygen generally resulted in higher D10 values, with the highest D10 of 0.335 kGy obtained for 100% N2, 0 h. Time in storage pre-irradiation tended to increase D10 for air and 5/95 O2/N2, but not for 10/90 O2/N2 or 100% N2. These results suggest that time required for refrigerated holding of processed Roma tomatoes or shipment to an off-site irradiation service provider may alter the efficacy of irradiation if reduced oxygen MAP is used.