Purpose: Evaluate the ability of in-line water disinfection techniques to mitigate the risk of Shiga Toxigenic E. coli (STEC) and Salmonella contamination on drip-irrigated tomatoes.
Methods: Tomatoes were cultivated on bare ground or plastic mulch raised beds. Plots were drip irrigated with water from 1) a municipal water supply (negative control) or 2) a pond harboring STEC and Salmonella (positive control). The surface water was also treated with 3) ultraviolet light to achieve a dose of 10,000 µW s cm-2, 4) 20 ppm free chlorine, or 5) 20 ppm peroxyacetic acid (PAA). Irrigation water was enumerated for Salmonella and STEC. Tomatoes were harvested, rinsed in BPW containing 0.2% Tween 80 and plated onto Chromagar STEC and XLT4. The remaining rinsate was used in selective enrichments of Salmonella in Tetrathionate Broth and STEC in mTSB with 8 mg/l sodium novobiocin, and pathogen presence was confirmed using confirmation testing.
Results: Over seven weeks, populations of STEC and Salmonella in untreated source irrigation water were found to fluctuate between 0.6 to 2.3 log CFU/100 ml and 0.07 to 2.5 log CFU/100 ml, respectively. Chlorine and ultraviolet water treatment methods were successful at reducing levels of both Salmonella and STEC to below detectable levels before application. None of the sampled tomatoes were contaminated with STEC post-harvest, and one sample of tomatoes from chlorine-treated irrigation water was positive for Salmonella post-harvest.
Significance: Economically feasible irrigation water disinfection systems may provide a way to lower the food safety risk associated with using a surface water irrigation source.