In Summary
- Experiments have found that washing of contaminated crops, postharvest, can reduce bacterial numbers, however does not remove the contamination entirely. And when this wash water was used to clean uncontaminated crops, the crops became contaminated themselves.
- Abrasive washing, such as washing in an abrasive brush-lined drum, has been shown to remove contamination from specific produce such as carrots.
Cross contamination between crops during washing
A 2016 study funded by Food Standards Scotland and the Food Standards Agency reviewed pre- and postharvest preventive measures and intervention strategies to control microbial food safety hazards in fresh vegetables. Bovine slurry that was naturally contaminated with non-toxigenic Escherichia coli O145 (a surrogate for toxin-producing E. coli O157) was applied without dilution or diluted 1/10 in borehole water to experimental plots growing potato, leek, or carrot. The application of manure was one week prior to harvest to simulate a worst case, near-harvest contamination event by manure deposition or an application of contaminated irrigation water. The fate of the bacteria was followed as the vegetables were harvested, processed through a crop-specific standard commercial wash treatment and distribution through either a retail chain at 4oC or a wholesale chain at ambient temperatures. At harvest, crops were contaminated with around 100 cfu/g of E. coli O145. Washing transferred E. coli O145 into the wash water of a flotation tank used to wash potatoes and did not completely remove all traces of contamination from the crop. A second batch of uncontaminated potatoes washed immediately after a contaminated batch acquired small amounts of contamination from the wash water. There was also cross contamination as a consequence of washing for leeks, which were misted with a fine water spray. Washing in an abrasive, brush-lined drum, removed the outer surface of the carrots and resulted in effective decontamination. For potato, E. coli O145 was isolated after simulated retail but not wholesale distribution. Leeks were contaminated with a small number of cells after simulated distribution down both chains. There were no post-distribution isolations from carrots.
References
Hutchison, M. L., D. Harrison, J. F. Heath, and J. M. Monaghan. (2017) Fate of Escherichia coli O145 present naturally in bovine slurry applied to vegetables before harvest, after washing and simulated wholesale and retail distribution. Journal of Applied Microbiology. 123:1597-1606.