Are salad vegetables safe to eat?

Raw vegetable sprouts (including alfalfa, mung bean and radish)) have previously been implicated in numerous outbreaks of infection with Salmonella or E.coli O157.

Following this outbreak, in a recent edition of Gardeners’ Question Time on BBC Radio 4 the panel was asked about the safe production of salad vegetables. In answering this question there was no reference to safe use of farm manures, or to the advice from the Food Standards Agency on this subject. This article is written to draw attention to this advice and the reason for it.

The microbiological safety of ready-to-eat salad vegetables depends on preventing contamination with pathogenic microbes throughout production, distribution and service. Prevention of contamination of the growing crop is a major concern.

Two possible sources of contamination of salad vegetables with microbes that are pathogenic to humans are direct or indirect contact with animal faeces or manure and direct or indirect contact with sewage sludge. Indirect contact can occur if water contaminated from these sources is used for irrigation or washing vegetables or if the growing crop is affected by flood water.

Animal manure and sewage sludge are liable to contain microbes pathogenic to humans, and guidelines have been published relating to the application of these products to land.

In the UK sewage sludge is likely to be applied on large fields, and will probably not be relevant to gardeners and small-scale producers provided run-off does not spread to their land.

A study reported in 2004 showed that fresh manure from cattle, pigs and sheep was frequently contaminated with E.coli O157, Salmonella, Listeria, Campylobacter, Cryptosporidium and Giardia and fresh poultry manure was frequently contaminated with Salmonella, Listeria and Campylobacter. Horse manure is also liable to be contaminated with Salmonella, E.coli O157, Cryptosporidium and Giardia.

Many outbreaks of infection, particularly with Salmonella or E.coli O157, have been associated with consumption of fresh salad vegetables, including lettuce, fresh peppers, fresh basil and fresh spinach, and fruits.

Trace-back to determine the source of contamination is difficult, but the proximity of cattle and contaminated water sources to the growing area, and the presence of feral animals have been implicated. A few cases have been reported in which children have been infected with E.coli O157:H7 following application of raw cow manure to garden soil and faecally contaminated mud has resulted in outbreaks.

The Food Standards Agency has published guidelines, on “Managing farm manures for food safety” at www.food.gov.uk.

The advice states that composting of solid manures is a particularly effective method of controlling microbial pathogens, but for best results, the process needs to be actively managed.

Composting of solid manures.

The manure should be treated as a batch (i.e. no additions of fresh manure are made to the store during this period) and turned regularly (at least twice within the first seven days).

This should generate high temperatures over a period of time (e.g. above 55° C for 3 days) which are effective in killing pathogens, and this temperature should be monitored.

Allow the compost to mature as part of the treatment process - the whole process should last at least 3 months Batch-stored manure.

If the manure is stored rather than composted, again it should be stored as a batch and it should be stored for at least six months. There are also ways to actively treat manure, but these may not be so habitually employed by gardeners.

Manure that is treated or batch-stored as described can be applied any time before drilling and planting.

Fresh manure or slurry should not be spread within 12 months of harvest of a ready-to-eat crop and there should also be a minimum period of six months between the manure application and drilling/planting of the crop.