The green mulchy water that collects at the bottom of salad bags is the perfect environment for growing salmonella, scientists have found.
Experiments showed that it could greatly increase the growth of the dangerous bacterium. The researchers said it meant that in some cases salad could even be the real source of food poisoning incidents that people wrongly blamed on meat.
“The idea you could also get food poisoning from the salad garnish and not from the chicken main course is a new one,” Dr Primrose Freestone, from the University of Leicester, said. “The public needs to be aware that there is potentially a food poisoning risk from the salad.”
The study, published in the journal Applied and Environmental Microbiology, did not look at salmonella prevalence in salad bags. Instead the microbiologists investigated how it promoted growth once the bacteria were present. Cos, baby green oak and red romaine lettuce, spinach and red chard taken from salad bag mixes were used in the tests, in which they ground the leaves in a pestle and mortar and added them to water or petri dishes containing salmonella.
When added to water, juice from broken leaves increased salmonella prevalence by 110 per cent. When added to a nutrient medium, it increased prevalence 2,400-fold.
As well as greatly helping its multiplication they showed that the salad juice, which would be familiar to anyone who has kept a bag of salad for longer than a few days, also made the bacteria harder to remove, helping them stick to the leaves and the side of the bag.
The scientists behind the study said that they were not telling people to avoid salad. Rather, they wanted people to be more vigilant. “The researchers involved in this study still eat salads,” they said. “However, we have all become more careful about buying salads, and using them quickly. Avoid bags of salad with mushed up leaves, avoid any bags or salad containers that look swollen. Store in the fridge and use the salad as quickly as possible to minimise the growth of any pathogens.”
Martin Adams, from the University of Surrey, said one aspect of particular concern from the study was that the salmonella grew perfectly happily at the temperature of the fridge.
“If salmonella did manage to gain access to a prepared salad then the food would obviously be a risk to the health of consumers but if it were able to grow at chill temperatures then that risk would increase over time.” He said we should wash pre-washed salads.
“Although prepared bagged salads have already been washed, another washing before use would give an added level of reassurance.”