FEATURE ARTICLE
Safer Salads
Contaminated fruits and vegetables are more common than ever. Why? And what can consumers do to protect themselves?
Jorge M. Fonseca, Sadhana Ravishankar
Before the Harvest
The microbes found naturally on produce are very diverse, even between lots of the same crop. In most cases, the microflora that inhabit a fruit or vegetable are similar in the field and after harvest—a fact that highlights the importance of preharvest events on food safety. Fields that are used to contain animals are more likely than other places to harbor enteric pathogens in the soil; in rare cases, pathogenic bacteria can survive for months after the animals are gone. Thus, the U.S. Environmental Protection Agency mandates a minimum waiting period of almost a year after animal husbandry operations cease before growers can cultivate the same field for edible fresh crops. For the same reason, raw manure is a dangerous soil additive for croplands and should be adequately composted (with sufficiently high temperatures) before use as a fertilizer for food crops. (The use of fresh sheep manure caused the major outbreak of Listeria in cole slaw in the 1980s.) Flooded croplands are a particular concern because of their potential to carry bacteria from animal waste into the roots of some kinds of plants.
The type of irrigation system and the quality of the irrigation water directly affect the microbial quality of produce at harvest. Because bacteria contaminate a crop most easily through leaves or fruits (especially damaged areas), it is especially important to avoid irrigation with tainted water. Furthermore, the risk of contamination is higher in produce from fields with an overhead sprinkler system, or in circumstances when the crop is harvested immediately after irrigation or rainfall. In this case, the splashing can carry contaminants from the soil onto the leaves.
Given the risk of having animal feces in contact with food crops, one might think that organically grown crops—which use organic fertilizers such as composted manure instead of synthetic fertilizers—would be especially likely to be contaminated with enteric pathogens. However, this hypothesis appears to be untrue: No clear differences exist between organically grown and conventional produce in terms of microbial safety. It is important to note that such findings apply only to certified organic produce—several studies have shown that noncertified products claiming to be organic (or nature friendly) actually had more bacteria than conventional ones. New regulations say that growers of certified organic produce must carry a certificate that proves their products are pathogen-free.
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