Gone are the days when people thought the only salads that led to food poisoning were egg and potato salad. Leafy greens can harbour pathogens as well; green salads are now “the second most common source of outbreaks of foodborne illness.” The danger lies, however, not so much in fresh greens straight from the field but in packaged, pre-torn salad greens.
The source of the contamination is varied and includes “animal or insect contacts, soil, contaminated irrigation and wash waters, and nonhygienic equipment and human handling.”
As more farmers sell pre-cut packaged lettuce at their market stalls and in CSA boxes, hygienic salad preparation becomes a critical issue. Farmers can work to reduce exposure to pathogens by basic hygiene of farm workers, using clean wash water (pathogens can be spread by recycling wash water) and ensuring only well-composted manure is used when growing salad greens. After harvest, however, the culprit is “salad juice,” the liquid that comes from cut or damaged salad greens, including the drops of liquid that come from cut stems.
If disease-causing organisms are present on the greens, the salad juice makes the problem much worse. The researchers looked at salmonella on cut, packaged greens and found that salad juice helped the bacteria spread throughout the package and even stimulated the pathogen’s growth. They found that “salad juice exposure also helped the salmonella cells to attach to the salad leaves so strongly that washing could not remove them.” Another study found a similar effect with E. coli. The juice also helped the bacteria stick to the packaging.
To reduce the growth of pathogens, the re- searchers recommend “refrigeration after harvesting and rapid processing and packaging in a modified atmosphere containing reduced oxygen levels,” but add “despite all these efforts, salad-associated infections still occur.”
Farmers can find other solutions by avoiding the accumulation of salad juice. Selling heads of lettuce rather than pre-cut greens and minimizing damage and cutting of leaves can help reduce the risk.
Source: Salad leaf juices enhance Salmonella growth, fresh produce colonisation and virulence. Giannis Koukkidis, Richard Haigh, Natalie Allcock, Suzanne Jordan and Primrose Freestone. Applied and Environmental Microbiology. 2016. Volume 83,issue 1, e02416-16, pp: 1-13.
- Janet Wallace