The source of contamination may come from the unwashed hands of field workers, from non-composted manure or from contaminated irrigation water.
It may be a cop out on the part of suppliers these days, but I have noticed that even those washed and packaged greens for salads often bear a warning on the container to always wash the produce before eating.
E. coli bacteria are easy to kill with heat. They can be washed off smooth surfaces quite readily, but bumpy and frilly vegetables present greater challenges. Sprouted seeds present real difficulties in ensuring non-contamination. Other ways to kill E. coli without heat include exposure to ultra-violet light and/or ozone.
Canadian Digestive Health Foundation
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