126.96.36.199 Spores and Vegetative Bacteria
Al-Bachir  investigated the effect of irradiation (0 to 2.5 kGy) on the quality of two cultivars of Syrian grapes (Vitis vinifera) stored at 1 to 2°C for two weeks. The irradiation treatment decreased spoilage caused by Botrytis cinerea and improved the quality of both varieties. The optimum doses were 0.5 to 1.0 kGy for Helwani grapes and 1.5 to 2.0 kGy for Baladi grapes. The storage periods were extended by 50% after irradiation at optimal doses for both varieties.
Aziz and Moussa  studied the effect of irradiation on the viable population of fungi and production of mycotoxins in randomly collected fruits that included strawberries, apricots, plums, peaches, grapes, dates, figs, apples, pears, and mulberries. Analysis of these fruits detected the mycotoxins penicillic acid, patulin, cyclopiazonic acid, citrinin, ochratoxin A, and aflatoxin B1. Irradiation of fruits at doses of 1.5 and 3.5 kGy significantly decreased the total fungal counts compared with unirradiated controls. The corresponding occurrence of mycotoxins in fruits decreased with increasing irradiation dose and was not detected after treatments at 5.0 kGy.
Niemira et al.  irradiated frozen broccoli, corn, lima beans, and peas at subfreezing temperatures ranging from —20 to —5°C and determined the influence of irradiation temperature on quality factors of frozen vegetables as well as irradiation sensitivity of inoculated L. monocytogenes. The irradiation resistance of L. monocytogenes changed significantly with the type of vegetable and the treatment temperature. The levels of irradiation necessary to reduce the bacterial population by 90% (D values) for L. monocytogenes increased with decreasing temperature for all the vegetables that were evaluated. D values ranged from 0.505 kGy for broccoli to 0.613 kGy for corn at —5°C and from 0.767 kGy for lima beans to 0.916 kGy for peas at —20°C.
Lettuce inoculated with 1 107 CFU/g of acid-adapted E. coli 0157:H7 was chlorinated at 200 mg/ml and irradiated at 0.15, 0.38, or 0.55 kGy by Foley et al. . The viability of E. coli 0157:H7, aerobic mesophiles, yeast, and molds was measured over 10 days. Chlorination alone reduced counts of E. coli 0157:H7 by 1 to 2log10 CFU/g. Chlorination combined with irradiation at 0.55kGy produced 5.4log10 reductions in E. coli 0157:H7 levels. When stored at 1 and 4°C after irradiation at 0.55 kGy, standard plate counts and yeast and mold counts were reduced by 2.5log10 CFU/g for samples storage on day 17 without obvious softening of the lettuce or any other adverse effect on sensory quality.
Niemira et al.  irradiated leaf pieces and leaf homogenate of endive (Cichorium endiva) inoculated with L. monocytogenes (pathogen) or Listeria innocua (nonpathogenic surrogate). Similar radiation sensitivity was obtained for the two strains, but L. innocua was more sensitive to irradiation in leaf homogenate than on the leaf surface. A dose of 0.42 kGy reduced L. monocytogenes on inoculated endive by 99%; however, the pathogen grew after 5 days of refrigerated storage until it exceeded the bacterial levels of the control after 19 days of storage, but a dose of 0.84 kGy, equivalent to a 99.99% reduction, suppressed L. monocytogenes throughout refrigerated storage. When increasing the doses up to 1.0 kGy, no significant change of color was observed for endive leaves taken either from the leaf edge or the leaf midrib. Dose tolerances for acceptable texture of leaf edge and midrib material were a maximum of 1.0 and 0.8 kGy, respectively.
Niemira et al.  also irradiated orange juices with varying levels of turbidity and inoculations with Salmonella Anatum, Salmonella Infantis, Salmonella Newport, or Salmonella Stanley at 2°C. Neither the resistance of each isolate (D value) nor the pattern of relative resistance among isolates was altered in orange juice. S. Anatum (D = 0.71 kGy) was significantly more resistant than the other species in orange juice, followed by S. Newport (D = 0.48 kGy), S. Stanley (D = 0.38 kGy), and S. Infantis (D = 0.35 kGy).
Van Gerwen  analyzed the irradiation resistance of both spores and vegetative bacteria based on the data available from the literature. As expected, spores were found to have significantly higher D values with an average of 2.48 kGy compared to the average D value for most vegetative bacteria of 0.76 kGy. The notoriously radiation-resistant nonpathogenic vegetative bacterium Deinococcus radiodurans, had the highest D value of 10.4 kGy. The average irradiation resistances for spores and vegetative bacteria were further estimated to be 2.11 and 0.42 kGy after excluding specific conditions showing extreme D values.
According to Dubey et al. , outbreaks of cyclospora-associated gastroenteritis in humans have been epidemiologically linked to the ingestion of fecally contaminated fruits (raspberries), vegetables (lettuce), or herbs (basil). Also, cryptosporidium oocysts have been demonstrated on vegetables. Unsporulated and sporulated T. gondii oocysts were used as a model system to determine the effect of irradiation on fruits contaminated with other coccidia such as cyclospora or cryptosporidium. Unsporulated oocysts of T. gondii irradiated at 0.4 to 0.8 kGy sporulated, but were not infective to mice; however, sporulated oocysts irradiated at doses greater than 0.4 kGy were able to encyst. Sporozoites were infective but not capable of inducing a viable infection in mice. T. gondii was detected in histological sections of mice up to 5 days, but not 7 days after feeding oocysts irradiated at 0.5 kGy. Raspberries inoculated with sporulated T. gondii oocysts were rendered nonviable after irradiation at 0.4 kGy. An irradiation of 0.5 kGy was recommended in this study to inactivate coccidian parasites on fruits and vegetables.
Against viruses the effectiveness of irradiation is dependent on the size of the virus, the suspension medium and/or type of food product, and the temperature of exposure [50,51]. Because of their smaller size and genetic makeup (often single-stranded RNA), most viruses are more resistant to irradiation than bacteria, parasites, or fungi . According to Bidawid et al. , only a few studies have examined the efficiency of irradiation on viruses in or on food products, including work on poliovirus in fish fillets , coxsackievirus B in ground beef , and rotavirus and HAV in clams and oysters . Lettuce and strawberries were inoculated with HAV and irradiated with doses ranging between 1 and 10kGy at ambient temperature . Plaque assays of HAV after irradiation showed a linear pattern of inactivation, i.e., a linear decrease in virus titer occurred when the irradiation dose was increased. Data analysis by a linear model indicated that D values were 2.72 ± 0.05 and 2.97 ± 0.18 kGy for HAV in lettuce and strawberries, respectively. These data were similar to those reported of 2.0 kGy for HAV in both clams and oysters . No noticeable deterioration was observed in the texture and appearance of lettuces and strawberries, even at the highest dose of 10 kGy.
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