Survival of generic E. coli and Salmonella during the growth, curing, and storage of dry bulb onions produced with contaminated irrigation water.

Onion growers and handlers have a strong historical record of producing a crop safe for human consumption; however, newly proposed regulations would require frequent testing of irrigation water for generic E. coli levels as a predictor of risk associated with irrigated crops. A majority of dry bulb onion production occurs in areas where water is a scarce resource and reclamation for numerous reapplications is required. This reuse leads to unpredictably high levels of generic E. coli that would prohibit the use of this water for irrigation. Evidence is required to demonstrate that potentially high levels of generic E. coli and pathogenic Salmonella in irrigation water would not lead to production of an unsafe onion crop. In the proposed study, late;season onions will be transplanted from commercial growing fields into a greenhouse and irrigated with contaminated water for the remainder of the growing season. The onions will be finished following standard commercial practices. Onion and soil samples will be analyzed throughout irrigation, drying, curing, and storage for the survival of generic E. coli and Salmonella. The results of the study would provide initial evidence for the onion industry to request an exemption from water testing requirements of the proposed rule.

The Treasure Valley area of eastern Oregon and western Idaho is famous for the highest yield of dry bulb onions in the country; however, due to its high desert climate, water is a scarce resource. Water is supplied through irrigation canal systems that were originally constructed in the late 1920s and assisted in transforming the area into the agricultural powerhouse of today. This network of canals distributes and reclaims water multiple times throughout the valley to efficiently utilize the minimal water that is available. This continual reuse leads to high microbiological loads in the water, specifically generic E. coli. The recently proposed produce safety rules have indicated a maximum genetic E. coli level (235 MPN/100 ml) that cannot be consistently met throughout the growing season and places a huge financial burden on onion growers in the region. Due to the relatively large time frame between the last irrigation and harvest, storage, and distribution of onions, it is likely that survival of generic E. coli and foodborne pathogens, such as Salmonella, would be very minimal in this crop. The primary aim of the proposed research is to quantify the survival of generic E. coli and Salmonella associated with dry bulb onions through the late stages of growth, water cessation, curing, and storage when inoculated through contaminated irrigation water at realistically high levels (5,000;10,000 CFU/100 ml). Field grown onions and associated soil will be transferred to individual containers to finish the growing season in a greenhouse at Oregon State University. These onions will be irrigated with contaminated water for the final two weeks of the growing season. Onions will be finished and cured following conventional or green;top practices. After finishing, onions will be harvested and stored at ambient temperature with adequate ventilation for the remainder of the study. Survival of generic E. coli and Salmonella will be evaluated on the onion as well as in the soil throughout the various stages of production and storage. Data collected from this project will provide a conservative estimate of the survival of generic E. coli and Salmonella in dry bulb onions. This evidence will provide an initial understanding of the potential risk of contaminated irrigation water in this crop. The outcomes of this study may be submitted to FDA to support an exemption or variance from the water testing requirements of the proposed produce rule for dry bulb onion production.

1.) Quantify generic E. coli and Salmonella transfer from late season contaminated irrigation water to dry bulb onions through the soil 

2.) Determine the impact of water cessation, curing, and storage on the survival of generic E. coli and Salmonella associated with dry bulb onions and soil 

3.) Compare survival of generic E. coli and Salmonella standard water cessation and curing practices with abbreviated practices for early season “greentop” dry bulb onions

Generic E. coli and Salmonella spp. levels on onions and in soil were significantly reduced by conventional finishing process used in this study. During the first two weeks of water cessation, generic E. coli and Salmonella spp. counts were reduced by 3.22 to 3.66 log CFU/g in muck soil and onion samples and by 3.69 to 4.19 log CFU/g in silt loam soil and onion samples. Generic E. coli and Salmonella spp. levels remained stable at low levels (<1 MPN/g) throughout the 2-week curing and storage periods. The greentop storage practices tested in this study did not demonstrate any differences in potential risk; however, abbreviated water cessation practices for greentop harvest should be evaluated. While levels of generic E. coli and Salmonella spp. were not completely eliminated through curing and storage, the >3.2 log CFU/g reduction in combination with the use of “indirect” irrigation practices (furrow or drip) should efficiently mitigate any risk associated with lower levels of natural E. coli in irrigation water. Survival of generic E. coli and Salmonella were comparable at all time points in all samples, indicating that these generic E. coli strains would provide an accurate prediction of the survival of Salmonella spp. in soils and on onions for future field trials. There was no difference in the survival of these organisms in the two different soil types (muck and silt loam).