P3-184 Efficacy of Conventional Curing Practices to Reduce Generic E. coli and Salmonella on Dry Bulb Onions

Tuesday, July 28, 2015
Hall B (Oregon Convention Center)
Alex Emch, Oregon State University, Corvallis, OR
Chris Letchworth, Oregon State University, Corvallis, OR
Courtney Mick, Oregon State University, Corvallis, OR
Joy Waite-Cusic, Oregon State University, Corvallis, OR
Introduction:  Implementation of the Food Safety Modernization Act (FSMA) has placed increased emphasis on microbial risks of irrigation water.  The Treasure Valley area of eastern Oregon and western Idaho has the highest yield of dry bulb onions in the country. Unfortunately, their irrigation water is often of poor microbiological quality.   

Purpose:  Determine efficacy of conventional curing practices on reducing generic E. coli and Salmonella in dry bulb onions and soil to assist with mitigation strategies to comply with FSMA regulations.

Methods:  Spanish yellow dry bulb onions (Allium cepa var. Ovation) were grown in two soil types from onion farms: silt loam (Treasure Valley) and muck (Willamette Valley).  Onions were grown in the greenhouse and irrigated with contaminated water (200 ml) containing generic E. coli and Salmonella spp. (3.47 log CFU/ml) every 2 - 3 days. At maturity, irrigation was stopped for 2 weeks followed by two weeks of curing.  Onion samples were collected, rinsed, and massaged with 0.1% peptone water (1:1) while soil samples (100 g) were mixed with 0.1% peptone water. Serial dilutions were plated onto Hektoen Enteric (HE) Agar and selectively enumerated following incubation (37°C, 24 - 48 hours) and as microbial levels decreased, a most-probable-number (MPN) was used in lieu of plating.

Results:  The irrigation period resulted in a final contamination level of 3.69 ± 0.34 log CFU/g onion of both Salmonella and generic E. coli. On day 124, 13 days after irrigation, generic E. coli and Salmonella spp. were reduced to < 1 CFU/g.  E. coli and Salmonella spp. levels were stable throughout the remainder of the curing process. 

Significance:  Reduction of generic E. coli and Salmonella levels in onions during conventional curing demonstrates the low risk of contaminated irrigation water used in onion production.  This data supports extended irrigation-to-harvest intervals as an effective strategy to mitigate risk associated with irrigation water.