Food safety metrics for flooded leafy greens fields

What is it about?

The California Leafy Green Products Handler Marketing Agreement (LGMA) requires leafy green crops within 9 m of the edge of a flooded field not be harvested due to potential contamination. LGMA guidelines also state that flooded fields cannot be replanted with leafy crops until 60 days after the flooding event. The LGMA is a consortium of growers and handlers of leafy green commodities who subscribed to voluntary guidelines intended to decrease risks that may increase foodborne pathogens from contaminating these commodities. The research performed here was to model a “worst case scenario” flood to determine if the LGMA guidelines are scientifically valid. The upper end of a spinach bed (Beltsville, MD) established on a -5% grade was flooded with water containing 6 log CFU/mL Escherichia coli to model a worst-case scenario of bacterial movement through soil. Spinach plants and soil within the 9 m (30 ft) “no harvest” buffer zone from the edge of the flood were tested for the presence of E. coli. E. coli in soil was detected 9 m away from the flood after 1 day in the spring trial and after 3 days in the fall trial. No E. coli was recovered from spinach plants outside of the flood zone. Overall, E. coli populations in fall trial were higher than those in the spring trial. E. coli populations declined more quickly in the spring trial than in the fall trial. These results suggest LGMA-proposed metrics should be revised to include considerations of soil and ambient temperatures, possibly incident solar radiation, additional rainfall events, and field slope in determining intervals between flooding and planting, and determining buffer zones to prevent the harvest of contaminated leafy greens after a flooding event.

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