Electron Beam Irradiation for Improving the Safety of Fruits and Vegetables

NON-TECHNICAL SUMMARY: While the application of Good Agricultural Practices (GAP) seems to be effective at preventing the introduction of bacterial pathogens in produce in most cases, other factors not yet understood may favor contamination in some instances. Therefore, contaminated fruits and vegetables may reach consumers before the time it takes to understand the factors that promote the presence, spreading or magnification of pathogens in crops along the processing chain. This makes it necessary for consumers to rely on a treatment that ensures the safety of fruits as well as meeting the government's nutritional goals which include an increase in the consumption of fresh fruits and vegetables. Even though electron beam (e-beam) irradiation can reduce or eliminate the pathogens, the quality of the product may be affected. To develop meaningful e-beam treatments it is imperative to determine the D-value of Salmonella, Escherichia coli O157:H7 and hepatitis A virus (HAV), which are the 3 foodborne pathogens that most frequently are found to cause outbreaks of foodborne diseases linked to produce. It is also paramount to determine the maximum dose that each commodity can withstand without physical damage. The effect of postharvest irradiation of produce on the alteration, if any, of produce-borne agrochemicals is also important in evaluating the consequences of e-beam irradiation of produce. Therefore, the objectives of this project are: to determine D-values for Salmonella and E. coli O157:H7 on lettuce, spinach, fresh-cut cantaloupes, fresh-cut tomatoes, and of HAV on green onions and fresh strawberries; to determine the maximum dose of e-beam energy that can be irradiated onto each of these commodities without changes in firmness, color, respiration rate, and overall microbiological counts; and to determine the effect of post-harvest e-beam irradiation on the molecular stability of produce-borne agrochemicals. For determining D-values, produce will be spiked with the selected pathogens and then subjected to e-beam irradiation at increasing doses. Plate counts will be conducted on samples of irradiated produce. HAV enumeration will be conducted using the Virus Plaque Assay. Non-inoculated batches of the same commodities will be irradiated at increasing doses up to 5 kGy and subjected to instrumental analysis including firmness, color and respiration rate tests, and microbiological analysis including aerobic plate counts, lactic acid bacteria and yeasts and molds counts. The final objective will be achieved by testing produce from crops that have field-incurred agrochemicals to determine the pre- and postirradiation chemical signatures of these compounds using mass spectrometry interfaced to both gas and liquid chromatography. This study should permit collecting useful data for further development of strategies for reducing pathogens in fresh and fresh-cut produce after GAP and Good Manufacturing Practices are applied.

<P>
APPROACH: Five strains each of both pathogens will be separately inoculated onto fresh romaine lettuce, baby spinach, cantaloupes and roma tomatoes. Lettuce and spinach will be separated in leaves whereas the cantaloupes will be peeled using sterile knives. Tomatoes and peeled cantaloupes will be cut in pieces and then sliced to fit the attenuators used for irradiation. All commodities will then be inoculated with a suspension of each pathogen and placed in sealed containers with a thickness fitting the height of the attenuators to minimize maximum/minimum dose absorption ratio. These will be subjected to e-beam irradiation at increasing doses between 0 (control) and 1 kGy, with increments of 0.1 kGy. After treatment, plate counts of each pathogen will be conducted to determine the dose effect. D-value for hepatitis A virus (HAV) on green onions and fresh strawberries, will be determined by inoculating these commodities with a suspension of HAV. Individual strawberries or green onions will be inoculated with a viral suspension and allowed to dry and then placed in sterile weighing boats and bagged so as to prevent leaks. The bags will then be subjected to e-beam irradiation at increasing doses between 0 (control) and 10 kGy, with increments of 1 kGy. After treatment, each piece of strawberry or green onion will be washed and the rinse fluid will be collected in its entirety and diluted for HAV enumeration by the Virus Plaque Assay. To determine the maximum dose of e-beam energy that can be irradiated onto each of these commodities without changes in firmness, color, respiration rate, and microbial counts, non-inoculated fresh romaine lettuce leaves, baby spinach leaves, cut cantaloupes and tomatoes, whole green onions and whole strawberries, will be irradiated at 0, 1.5 and then up to 5 kGy with 0.5 kGy increases. Irradiated produce will then be subjected to instrumental analysis consisting of color by Hunter Lab, firmness using a TA.XT2i texture analyzer, and respiration rate by CO2 analysis using a PIR-2000 infrared CO2 gas analyzer (IRGA), as well as microbiological analysis, testing for aerobic plate counts (APC), lactic acid bacteria (LAB), yeast and molds by plating appropriate dilutions in PetrifilmTM APC, LAB or Yeast/Mold products. These analyses will be conducted immediately after treatment and at 3-day intervals during 12 days of storage at 4 to 5C. To determine the effect of post-harvest e-beam irradiation on the molecular stability of produce-borne agrochemicals, samples of crops that have field-incurred pesticides and other agrochemicals such as fertilizers will be collected and separated in two groups. One group will be subjected to irradiation and the other will be used as control. Then, the pre- and post-irradiation chemical signatures of produce will be followed through gas chromatography/mass spectrometry analysis (GC/MS) with sample extracts prepared by Quechers. Those chemicals not amenable to GC/MS analysis will be tested by liquid chromatography/mass spectrometry.