Graduate Thesis Or Dissertation

Mitigation of Microbiological Water Quality by Last Irrigation-to-Harvest Intervals

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  • Foodborne outbreaks involving fresh produce have been on the rise since the late 1990's. Pathogens such as Salmonella spp. and Escherichia coli are prevalent in agricultural environments and commonly travel between farms via irrigation water. The Food Safety Modernization Act (FSMA) has placed an increased emphasis on microbiological standards of irrigation water and fresh produce. The Produce Rule is a defined portion of FSMA that labels all ready-to-eat produce as covered under the rule and requires producers to comply with FDA's proposed water standards for a relative level of public safety. If producers can demonstrate a post-irrigation mitigation strategy to reduce microbial concentrations and meet FDA water standards, they will be able use of their water source even if it initially does not meet the microbiological criteria. The purpose of this study was to determine if the conventional finishing process for dry bulb onions is able to effectively reduce generic E. coli and Salmonella spp. to the FDA's standards. A secondary objective was to evaluate the contribution of soil type on the hypothetical irrigation-to-harvest intervals for root crops. The first study investigated the effectiveness of the conventional curing process to reduce microbial levels on dry bulb onions grown with contaminated irrigation water. Spanish yellow dry bulb onions were grown in Owyhee silt loam and Semiahmoo muck soils in containers in two greenhouses and irrigated with contaminated water containing a cocktail of rifampicin-resistant generic E. coli and Salmonella spp. (4.80 log CFU/ml) every 2-3 days from day 40 to day 111. At maturity, irrigation was stopped for 12 days, lifted, and then followed by 16 days of curing. Onion and soil samples were collected, rinsed, and massaged with 0.1% peptone water (1:1). Serial dilutions of the rinsate were plated onto Hektoen Enteric (HE) Agar and selectively enumerated following incubation (37°C, 24 hours). As microbial levels decreased, a most-probable-number (MPN) was used in lieu of plating. The irrigation period resulted in a final contamination level of 3.69±0.34 log CFU/g onion of both Salmonella spp. and generic E. coli. 12 days after ceasing irrigation, generic E. coli and Salmonella spp. were reduced to <10 CFU/g. E. coli and Salmonella spp. levels were stable throughout the remainder of the curing process. The second study analyzed differences in the survival of generic E. coli and Salmonella spp. in different agricultural soils around Oregon. Six soil types, Quatama loam, Latourell loam, Willamette loam, Adkins loam, Madras loam, and Cullius loam, were transported from OSU experiment stations to OSU campus greenhouses. Soils were irrigated with well water containing generic E. coli and Salmonella spp. (4.76 log CFU/ml) in the greenhouses used for the previous study. Sample collection and analysis matched those of the previous study. Generic E. coli and Salmonella spp. had the weakest survival and persistence in Quatama, Latourell, and Willamette loam. Madras, Adkins, and Cullius loams harbored generic E. coli and Salmonella spp. for extended periods (>80 d). The first study verified that conventional curing practices mitigate the risk posed by contaminated irrigation water during onion production. This greenhouse study was designed to provide an opportunity to compare the survival of an indicator (generic E. coli) to the target pathogen (Salmonella spp.) from contaminated irrigation water to soil and dry bulb onions using industry-relevant finishing practices. Based on similar survival curves, use of generic E. coli in future field trials would serve as a suitable predictor for the behavior of Salmonella spp. in these systems. The second study found a significant difference between the six soils in the microbial die-off of generic E. coli and Salmonella spp. We also observed that the rates of die-off for generic E. coli and Salmonella spp. were quite similar; however, the persistence between the two was significantly different in Quatama and Willamette loam soils.
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