X-ray irradiation inactivation of Escherichia coli O157:H7, Salmonella enterica Serovar Typhimurium, and Listeria monocytogenes on sliced cheese and its bactericidal mechanisms
Introduction
In the dairy industry, cheeses have become a major consumer product. More than one-third of the milk produced in the United States is used to make cheese (Velasco et al., 2016). Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes are important pathogens of concern to the dairy industry. In 2017, eight people were infected with L. monocytogenes in four US states after consuming cheese. All eight people were hospitalized and, two died (CDC, 2017). The commercial cheeses made with pasteurized milk between from October 2006 through February 2007 also caused listeriosis outbreak in Germany (Koch et al., 2010). In 2010, cheeses contaminated with E. coli O157:H7 caused an outbreak of infections in five US states. Fifteen persons became ill and required hospitalization and one developed hemolytic uremic syndrome (HUS) (CDC, 2010). In the US, there was a recent E. coli O157:H7 outbreak which delivered by cheddar cheeses and caused 7 illnesses and 1 hospitalization (CDC, 2016). Outbreaks of, salmonellosis associated with cheese consumption occurred in Canada and the US (Ha et al., 2017; Kim et al., 2016). In 2008, the outbreaks of Salmonella enterica traced to cheddar cheeses, resulting in 70 illnesses in the US (CDC, 2008).
In the process of production, cheeses can be contaminated by pathogens after pasteurization, cutting, packaging, and during transport. Silva et al. (2003) isolated L. monocytogenes in pasteurized milk samples, drainage, wooden shelves, the floor of the cheese refrigeration room, and finished cheese samples in a cheese processing facility. Therefore, additional control methods are needed to inactivate pathogens on cheese surfaces after the packaging step. A non-thermal technique can protect foods from contamination with pathogens while maintaining the nutritional and sensory characteristics of the food products, and thus has been increasingly applied to food processing (Ha et al., 2016). To inactivate pathogens on cheeses, several new non-thermal processing have been evaluated, including dielectric barrier discharge plasma (Lee et al., 2012; Yong et al., 2015) and high pressure (Evert-Arriagada et al., 2012). However, dielectric barrier discharge plasma treatment can change the color, flavor, odor, and acceptability of sliced cheese (Lee et al., 2012; Yong et al., 2015). The high pressure treatment can change sensory characteristics of cheese, such as color and firmness (Evert-Arriagada et al., 2012).
Ionizing radiation is an efficient non-thermal pathogen control treatment. According to a 1980 joint study report issued by the Food and Agriculture Organization, International Atomic Energy Agency, and the World Health Organization (FAO/IAEA/WHO), ionizing radiation of any food up to a dose level of 10 kGy presents no toxicological hazard and does not cause nutritional or microbiological problems. In 1992, a joint study group report by the WHO and the International Organisation of Consumers Unions (IOCU; now Consumers International) reaffirmed the stability of irradiated foods. A 1999 WHO report described that irradiated foods are safe and have no toxicological risks. Among the forms of ionizing radiation, X-rays obtained by bombarding a metal target, such as tantalum, with high velocity electrons has continued to receive attention as an attractive alternative to gamma-ray or E-beam irradiation (Jung et al., 2015; Mahmoud, 2009). X-rays have the advantages of higher penetration power than E-beams and absence of harmful radioactive sources, such as Cobalt-60 or Cesium-137 associated with gamma-rays (Janatpour et al., 2005; Jeong et al., 2010; Song et al., 2016). Moreover, the rising costs of radioactive sources (Jung et al., 2015) and negative consumer perception of gamma-ray irradiated foods (Deliza et al., 2010) are driving the demand for new ionizing radiation sources.
Recently, several studies reported that treatment with X-rays is effective in inactivating pathogens without affecting the quality of the treated foods. Mahmoud (2009) reported that X-ray treatment significantly (P < 0.05) reduced the content of viable Cronobacter to less than detectable limits (ca. 108–109 log reduction) in skim milk, low-fat milk, and whole-fat milk. X-ray inactivation of E. coli O157:H7, L. monocytogenes, Salmonella enterica, and Shigella flexneri on spinach leaves was investigated by Mahmoud et al. (2010), who concluded that this irradiation did not significantly affect the color of spinach leaves, while reducing the viable populations of the pathogens to less than their detectable limits. In addition, the effectiveness of pathogen inactivation and changes in quality factor in foods irradiated with X-rays were not significantly different when compared to those of other ionizing radiation such as gamma-rays and E-beams (Jung et al., 2015; Song et al., 2016). However, to the best of our knowledge, there are no published data on the inactivation of pathogens by X-ray treatment on packaged cheese products. Furthermore, no previous study has identified the mechanisms of bacterial inactivation by X-ray radiation.
The study investigated the inactivation of E. coli O157:H7, S. Typhimurium, and L. monocytogenes on packaged sliced cheese using X-ray irradiation, evaluated the change of quality factors following X-ray treatment, and sought to elucidate the mechanism of the bacterial inactivation.
Section snippets
Bacterial strains
Three strains each of E. coli O157:H7 (ATCC 35150, ATCC 43889, and ATCC 43890), S. Typhimurium (ATCC 19585, ATCC 43971, and DT 104), and L. monocytogenes (ATCC 15313, ATCC 19111, and ATCC 19115), were obtained from the bacterial culture collection of Hankyong National University (Anseong, South Korea) and were used in the experiments. All strains were stored frozen at −80 °C in tryptic soy broth (TSB; MB Cell, Los Angeles, CA, USA) containing 20% glycerol. Working cultures were streaked onto
Inactivation of pathogenic bacteria by X-ray treatment
Table 1 shows the actual measured doses obtained using alanine dosimeters. The actual irradiated doses displayed the same trends as target doses (Table 1). Reductions in the viable counts of E. coli O157:H7, S. Typhimurium, and L. monocytogenes on sliced cheese surfaces during X-ray treatment are summarized in Table 2. The bactericidal effect of X-rays increased with increasing the X-ray dose. Treatment at a dose of 0.2 kGy resulted in 2.47, 1.93, and 2.28 log reductions of E. coli O157:H7, S.
Discussion
Common packaged ready-to-eat (RTE) foods, such as sliced cheeses have been implicated in outbreaks of foodborne pathogens (Coillie et al., 2004). Cross-contamination occurs frequently during processing as slicing, cut, and packing (Lianou and Sofos, 2007; Rød et al., 2012). RTE foods are consumed directly, without a final sterilization step (Guenther et al., 2009; Ha et al., 2017). An additional superficial antimicrobial intervention step may become essential in order to control pathogens on
Acknowledgments
This work was supported by a research grant from Hankyong National University in 2017. We would like to thank Dr. Beom-Seok Song and Dr. Jong-Heum Park in the Korea Atomic Energy Research Institute for their assistance with the experiments.
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