Purpose: Evaluate the ability of in-line water disinfection to reduce the transfer of Salmonella and Shiga-toxigenic Escherichia coli (STEC) to drip-irrigated cabbage.
Methods: A nearby pond containing populations of STEC and Salmonella was used as the surface water irrigation source for this study. Cabbage was grown in bare ground or plastic mulch covered plots that were drip irrigated with municipal water (negative control) or sand-filtered pond water (positive control). The sand-filtered pond water was also treated with ultraviolet light (47,000 µW s cm-2; UV), 20 ppm chlorine dioxide (ClO2), or 20 ppm peroxyacetic acid (PAA). Irrigation water samples were enumerated weekly for Salmonella and STEC. Harvested cabbage samples were enumerated on Chromagar STEC and XLT4, and selectively enriched. Positive results were validated using standard confirmation methods.
Results: Over 12 weeks, populations of Salmonella and STEC in untreated irrigation water fluctuated between not detected to 1.60 log CFU/100 ml and not detected to 2.88 log CFU/100 ml, respectively. UV, ClO2, and PAA treatment methods were able to reduce Salmonella and STEC populations to below the detection limit (1 CFU/100 ml) on a regular basis. However, a single positive was observed with UV (4 CFU/100 ml Salmonella and 1 CFU/100 ml STEC) and ClO2 (2 CFU/100 ml STEC). There were no Salmonella-positive cabbage samples in plots covered with plastic mulch and irrigated with UV or PAA treated water. Furthermore, there were no STEC-positive samples attributed to PAA/bare ground, chlorine dioxide/plastic mulch, or UV/plastic mulch plots.
Significance: Our findings suggest that in-line irrigation disinfection systems, when properly applied and monitored, can reduce the food safety risk associated with using surface water irrigation sources.