Broken leaves in bags of prepared salad may dramatically increase the risk of salmonella, a study has shown.
Juice from damaged leaves can boost growth of the food poisoning bug more than 2,400-fold, scientists discovered.
It also has the effect of increasing the bacteria’s virulence, making it more likely to cause an infection.
Experts warned consumers to avoid ready-cut salad if possible, to rinse bagged salad thoroughly, and not to let it get warm.
The scientists did not measure levels of salmonella in bought salad but investigated the way the bacteria grew on damaged leaves and attached itself to plastic bag surfaces.
Cos, baby green oak, and red romaine lettuce, spinach, and red chard obtained from commercially available bag mixes were all used in the tests.
The experiments showed that juice from broken leaves increased salmonella growth in water by 110 per cent. When the juice was added to a nutrient medium supporting salmonella, the bacteria’s growth was boosted more than 2,400-fold.
Lead scientist Dr Primrose Freestone, from the University of Leicester, said: “Salad leaves are cut during harvesting and we found that even microlitres of the juices (less than 1/200th of a teaspoon) which leach from the cut ends of the leaves enabled salmonella to grow in water, even when it was refrigerated.
“These juices also helped the salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.
“This strongly emphasises the need for salad leaf growers to maintain high food safety standards as even a few salmonella cells in a salad bag at the time of purchase could become many thousands by the time the bag reaches its use by date, even if kept refrigerated. Even small traces of juices released from damaged leaves can make the pathogen grow better and become more able to cause disease.”
She said the research, published in the journal Applied and Environmental Microbiology, also served as a reminder to consume bagged salad as soon as possible after opening.
“We found that, once opened, the bacteria naturally present on the leaves also grew much faster even when kept cold in the fridge,” Dr Freestone added.
More than 500,000 cases of food poisoning a year are recorded in the UK according to the Food Standards Agency.