Inactivation of foodborne pathogens on fresh produce by combined treatment with UV-C radiation and chlorine dioxide gas, and mechanisms of synergistic inactivation

This study was conducted to evaluate the antimicrobial effect of the combined treatment of UV-C radiation (UVC) and chlorine dioxide (ClO2) gas against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on spinach leaves and tomato surfaces and to clarify the mechanisms of the synergistic effect of this combined treatment. In the case of spinach leaves, as treatment time increased the combined treatments of UVC and ClO2 gas showed additive effects: the total microbial inactivation of the combined treatment was not significantly (p > 0.05) different from the sum of individual treatments. On tomatoes, synergistic effects in inactivating E. coli O157:H7 and S. Typhimurium were observed after combination treatment of UVC and ClO2 gas (10 ppmv) for 15 min or more. For both pathogens, inactivation achieved with the combination treatment was significantly (p < 0.05) higher than the sum of UVC and ClO2 gas (10 ppmv) inactivation. In the case of L. monocytogenes, the synergistic effect was observed after the combination treatment of UVC and ClO2 gas (10 ppmv) for 20 min. Measuring leakage of UV-absorbing substances and analyzing transmission electron microscopy images provide evidence that damage to the cell membrane and changes to membrane permeability are involved in the synergistic lethal effect of the combination treatment of UVC and ClO2 gas. Combined treatment of UVC and ClO2 gas (10 ppmv) did not significantly (p > 0.05) affect the color and texture of samples during 7 days of storage.