Final Examination for the Degree of MSc Food Science - HONGRAN WANG

Examining Committee
Dr. Gisèle LaPointe, Chair
Dr. Keith Warriner, Advisor
Dr. Ryan Prosser, Co-Advisor
Dr. Lawrence Goodridge, Department Member

TITLE: Application of Advanced Oxidation Process as a Pre-Oxidation Step for Spent Lettuce Wash Water and for Decontaminating Fruit and Sliced Mushrooms

ABSTRACT: Hydroxyl- and chloride-radicals exhibit high oxidant capacity that have found utility in treating wastewater in a process termed Advanced Oxidation Process(AOP). The following thesis developed, validated and verified radical treatments for treating spent wash water, in addition to gas phase hydroxyl- or chloride processes for decontaminating fruit and mushrooms. With regards the latter, the inactivation of Listeria monocytogenesinoculated on mushrooms wassupported by a gas phase hydroxyl-radical treatment based on the UV-C mediated degradation of hydrogen peroxide and ozone. It was found the that the generation of hydroxylradicals (as assessed by log reduction of L. monocytogenes) was inversely dependent on the hydrogen peroxide concentration and UV-C dose. A chloride-radical process based on the photolytic degradation of electrolyzed water was also developed. Both treatments resulted in a >2 log CFU reduction of L. monocytogenes without negatively affecting mushroom quality or shelf-life. A gas phase-chloride-radical treatment was also developed for citrus and soft-fruit(lemons, strawberries, blueberries, grapes and tomatoes). The efficacy of the chloride-radical process was found to be independent of the UV-C dose, ozone concentration and pH but was dependent on the chloride source (NaCl vs hypochlorite). The optimized process could effectively inactivateSalmonella, Escherichia coliO157:H7 and L. monocytogenes along with spoilagemoulds. The chloride-radical treatment was superior to chlorine-washes and extended the shelf-life of fruit. Finally, an AOP based process for reducing the chlorine demand of spentlettuce wash water was developed. The process was based on the addition of hydrogen peroxide and ferric-catalyst to spent wash water that was subsequently passed through a UV-C reactor. The pre-oxidation treatment reduced the chlorine demand of water by 20 ppm and enhanced the stability of hypochlorite within the wash water. It was demonstrated that by applying the pre-oxidation step prior to chlorination prevented crosscontamination of pathogen between lettuce batches although resulted in the generation of disinfection byproducts of unknown toxicity. Collectively, the research has demonstrated the potential of hydroxyl- and chloride-radical processes to enhance the food safety and shelf-life of fresh produce and mushrooms.