Submitted to: Journal of Radiation Physics and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2004
Publication Date: 1/5/2005
Citation: Niemira, B.A., Fan, X., Sokorai, K.J. 2005. Irradiation and modified atmosphere packaging of endive influences survival and regrowth of listeria monocytogenes and product sensory qualities. Journal of Radiation Physics and Chemistry. 72:41-48. Interpretive Summary: Listeria monocytogenes is a pathogenic bacterium that can be present on leafy salad vegetables such as endive. Irradiation is a process that uses radiant energy to eliminate bacteria from foods. Modified atmosphere packaging (MAP) is technique which replaces the normal mixture of oxygen, nitrogen and carbon dioxide in a package with a different mixture of gases. Used together, irradiation and MAP can effectively suppress the growth of microorganisms. Cut pieces of endive were inoculated with L. monocytogenes, and then packaged in bags that were flushed with varying mixtures of oxygen, nitrogen and carbon dioxide. Gamma radiation, 0.3 or 0.6 kGy, effectively reduced the levels of bacteria on all of the samples examined. The bacteria regrew in bags where oxygen was present, but not in bags where oxygen had been depleted. Irradiated leaf material tended to retain its quality better in packages with oxygen. These results indicate that irradiation and MAP can be combined so as to prevent the regrowth of L. monocytogenes during post-irradiation refrigerated storage, but at the cost of produce quality. These results will assist regulatory agencies and commercial food processors in the development of vegetable processing protocols that reduce the risk of food-borne illness so as to protect consumers, while preserving food quality.
Technical Abstract: Cut pieces of endive were inoculated with Listeria monocytogenes, packaged in gas-impermeable bags in air, 5/5/90 or 10/10/80 percent CO2, O2 and N2 ("Air-0", "5/5" and "10/10" respectively) and irradiated to 0.0 (control), 0.3 or 0.6 kGy. At various times during refrigerated storage, samples were taken and a determination made of a) total microflora, b) L. monocytogenes, c) headspace gas composition, d) color and e) texture. Irradiation reduced initial microbial counts. Bacteria 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 so as to prevent the regrowth of L. monocytogenes during post-irradiation refrigerated storage, but at the cost of produce quality.