The Effects of Integrated Treatment of UV Light and Low-dose Gamma Radiation on Escherichia coli O157:H7 and Salmonella enterica on Grape Tomatoes

Introduction: Foodborne disease is increasing worldwide, with millions of cases occurring each year.  In recent years considerable numbers of foodborne disease outbreaks associated with produce were reported and specifically tomatoes have been involved with a number of multistate outbreaks.

Purpose: The purpose of this study was to evaluate efficacy of integrated treatment of UVC and low dose Gamma radiation to inactivate inoculated mix strains of Escherichia coli O157:H7 and Salmonella enterica on whole grape tomatoes.

Methods: A mix bacterial cocktail composed of a three-strain mixture of E. coli O157:H7 (C9490, E02128 and F00475) and a three-serotype mixture of S. enterica (S. Montevideo G4639, S. Newport H1275, and S. Stanley H0558) were used for this study. Selection of these strains was based on their association with produce-related outbreaks. Spot inoculation (50-100 microliter) of tomatoes (surface and stem sites) was performed to achieve population of appropriately 10^7-8 CFU/tomato.  Inoculated tomatoes were subjected to UVC (253.7 nm) dose of 0.6 KJ/M² followed by four different low doses of gamma radiations ( 0.1 kGy, 0.25 kGy, 0.5 kGy, 0.75 kGy). The quality aspect (lycopene content, color, and texture) and the fate of background  microflora (mesophilic, psychrotrophic, mold and yeast counts) during storage at room temperature (~ 22 °C, 60% RH) over 21 days were also determined.

Results: Integrated treatment with UVC light plus low dose radiation significantly (P < 0.05) reduced the population of the target pathogens on whole grape tomato. Results indicate about 3.4 ± 0.27 and 3.0 ± 0.12 log CFU reduction of E. coli O157:H7 and S. enterica per tomato was achieved with UVC treatment (0.6 KJ/M²) combined with 0.25 kGy radiation. Even better, more than a 5 or 6 log CFU reduction per fruit was accomplished by combining UVC treatment with 0.5 kGy and 0.75 kGy radiation, respectively, for all tested pathogens. Furthermore, UVC light treatment with Gamma radiation significantly (P < 0.05) reduced the native microflora on tomatoes compared to control during storage.

Significance: Data obtained from this work suggests efficacious treatment strategy for produce indicating 5 or higher log reduction which is consistent with the recommendations of the National Advisory Committee on Microbiological Criteria for Foods.