X-ray irradiation inactivation of Escherichia coli O157:H7, Salmonella enterica Serovar Typhimurium, and Listeria monocytogenes on sliced cheese and its bactericidal mechanisms

In the last two decades several foodborne disease outbreaks associated with cheese products were reported. The objective of this study was to investigate the efficacy of X-ray irradiation for the inactivation of foodborne pathogens on sliced cheese and to elucidate the underlying mechanisms of the lethal effect. In addition, the effect of the X-ray irradiation on product quality was determined. A mixed culture containing Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes was inoculated on the surfaces of cheese slices. The inoculated samples were re-packaged and treated with 0, 0.2, 0.4, 0.6, and 0.8 kGy of X-ray radiation. Approximately 5 log reductions in the viability of the three pathogens on samples were achieved at an irradiation dose of 0.6 kGy. Furthermore, the color values (L*, a*, and b*) and texture parameters of sliced cheeses were not altered significantly (all P > 0.05) after treatment at the maximum dose of 0.8 kGy. Various fluorescence staining methods were utilized to analyze the bactericidal mechanisms. The analyses confirmed that levels of depolarization of cell membranes, generation of reactive oxygen species, and intracellular enzyme inactivation were strongly related to the trends of microbial inactivation. The results of the present study suggest that X-ray irradiation may be an innovative antimicrobial intervention for various post-packaged dairy food products.