Packets of supermarket salad are breeding grounds for the type of bacteria that causes food poisoning, researchers have said.
A new study found that when salad leaves are broken, they leak fluid into the bags which can increase the growth of salmonella 2,400-fold.
Researchers warned shoppers to avoid pre-prepared salad if possible.
For those who do buy it, they advised rinsing the salad thoroughly before using and highlighted the importance of keeping it chilled.
For the study, published in the journal Applied and Environmental Microbiology, scientists from the University of Leicester measured levels of salmonella in salad bags and analysed the way the bacteria grew on broken leaves as well as how it attached itself to plastic bag surfaces.
They discovered that juice from broken leaves increased salmonella growth in water by 110%.
This growth increased more than 2,400-fold when the juice was added to a nutrient medium supporting salmonella.
Salad leaves from cos, baby green oak, and red romaine lettuce, as well as spinach and red chard, were analysed for the study.
“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,” said lead author Dr Primrose Freestone, from the University of Leicester’s Department of Infection, Immunity and Inflammation, according to The Telegraph.
“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.”
She said the study emphasised 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 a bag of salad leaves reaches its use by date” – even when those bags are kept in the fridge.
Off the back of the study, researchers advise consumers to eat bagged salad as soon as possible after opening it.
The Food Standards Agency (FSA) told The Huffington Post UK: “Salmonella is not a problem regularly linked with bagged salads – there has only been one incident reported to the FSA in the past 5 years.”
Salmonella causes food poisoning, which results in symptoms such as diarrhoea, stomach cramps, nausea, vomiting and fever. These unpleasant symptoms usually develop between 12 and 72 hours after becoming infected, and illness usually lasts between four and seven days.
As well as bagged salad, foods such as eggs, chicken, pork and dairy produce can carry the bacteria, says Public Health England. Other fruit and vegetables may also become contaminated if they have been in contact with livestock, manure or untreated water.