Response of Listeria monocytogenes-inoculated leafy salad vegetables following irradiation and refrigerated storage under modified atmosphere. Section 2. Studies of post-irradiation survival of Listeria monocytogenes and L. Innocua on endive, regrowth under modified atmosphere packaging and product sensory qualities

This section summarizes research on the efficacy of ionizing radiation to eliminate the pathogen Listeria monocytogenes (ATCC 49594) or the non-pathogenic surrogate Listeria innocua (ATCC 51742) from endive (Cichorium endiva), the potential for combining irradiation with modified atmosphere packaging (MAP) to prevent re-growth of the pathogen during storage, and the effects of treatment on product sensory qualities. It was determined that the radiation sensitivity of the pathogen and the surrogate were similar, validating the use of the surrogate for studies of irradiation of salad vegetables. During refrigerated storage (in air) following irradiation, the population of L. monocytogenes on inoculated endive was briefly suppressed by 0.42 kGy, a dose calibrated to achieve a 99% (2 log) reduction. However, the pathogen regrew after 5 days until it exceeded the bacterial levels on the control after 19 days in storage. Treatment with 0.84 kGy, equivalent to 99.99% (4 log) reduction, suppressed L. monocytogenes throughout the course of refrigerated storage. Doses up to 1.0 kGy had no significant effect on color of endive leaf material, either taken from the leaf edge or the leaf midrib. The texture of leaf edge material was unaffected by doses up to 1.0 kGy, while the maximum dose tolerated by leaf midrib material was 0.8 kGy. In combination studies, cut pieces of endive were inoculated with L. monocytogenes, packaged in gas-impermeable bags in air, 5/5/90 or 10/10/80 percent CO2, O2 and N2 (AAir 0at, A5/5'' and A10/10'' respectively) and irradiated to 0.0 (control), 0.3 or 0.6 kGy. During refrigerated storage L. monocytogenes and background microflora regrew during storage on Air-0 samples, but not on 5/5 or 10/10 samples. In each of the three atmospheres, O2 declined and CO2 increased, irrespective of radiation dose. Irradiated leaf material in Air-0 tended to retain color attributes during storage better than non-irradiated; color retention was more variable under 5/5 and 10/10 packaging. After 8 days, maximum shear force relative to the initial level was significantly reduced in 5/5 at all radiation doses, was not significantly changed in Air-0, and was dose-dependent in 10/10. By 14 days, the texture of all samples had degraded significantly. These results indicate that irradiation and MAP can be combined to prevent the regrowth of L. monocytogenes during postirradiation refrigerated storage, thereby improving product safety. (author)