Bags of ready to eat salads that contain broken leaves could be harbouring dangerously high levels of the food poisoning bug, a new study warned.
Prepared mixed or spinach salads have become increasingly popular but the very way the fresh leaves are prepared and stored could be to blame for a growth in Salmonella.
Juices leach out from just a small amount of damaged or cut leaves into the moist plastic bag which massively stimulate Salmonella growth, it virulence and its colonisation even if kept in the fridge.
And more worryingly, the pathogen clings to the leaves in a “biofilm” surviving washing under running water, University of Leicester microbiologists warn.
By the time the salads reach their best before date the juices increase Salmonella pathogen growth 2400-fold over a control group, the study found.
In the course of a typical five day refrigeration storage time around 100 Salmonella bacteria multiplied to approximately 100,000 individual bacteria.
The study found once the bag has been opened, the food bug begins to multiple.
Other fresh fruit and vegetables can carry Salmonella such as bean sprouts, and peppers,
But because salad leaves are much more perishable, contained within moist plastic containers retain leached juices, they do seem to feature more in terms of recognised outbreaks.
And Salmonella grows especially well on spinach with Associate Professor in Clinical Microbiology Dr Primrose Freestone said: “It seems the pathogen prefers spinach.”
She recommends you 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.
She added she no longer stores salad bags beyond one day in the fridge.
She said: “Salad leaves are cut during harvesting and we found that even microliters 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 a bag of salad leaves 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.
“It also serves as a reminder to consume a bagged salad as soon as possible after it is opened.
“We found that once opened, the bacteria naturally present on the leaves also grew much faster even when kept cold in the fridge.
“This research did not look for evidence of Salmonella in bagged salads. Instead, it examined how Salmonella grows on salad leaves when they are damaged.”
The study intention was to highlight the importance of careful handling of leafy salads during production, harvesting, packing and distribution to help minimise food safety risks from food poisoning bacteria such as Salmonella.
Over the years a number of outbreaks have been traced back to ready to eat salads and efforts have been made to risk the contamination risk.
But very few studies have investigated the behaviour of Salmonella once the leaves have been bagged.
It found the normal microbial flora on salad leaves did not respond to leaf juices, suggesting that the leaf juices give Salmonella a marked advantage in colonising salad leaves as compared to competing bacteria.
The study also found recycled wash water could spread Salmonella.
Earlier studies also found the bacteria are so powerfully attracted to salad leaf and root juices that they can find their way into the plant vasculature during the salad plant’s germination, and once inside, there is no way to wash them out.
PhD student Giannis Koukkidis added: “Preventing enteric pathogen contamination of fresh salad produce would not only reassure consumers but will also benefit the economy due to fewer days lost through food poisoning.
“We are now working hard to find ways of preventing salad-based infections.”
The Food Standards Agency said there are more than 500,000 cases of food poisoning in the UK each year with poultry meat was the most common source of infection.
But some 48,000 of food poisoning cases were from vegetables, fruit, nuts and sprouting seeds
The study was published in Applied and Environmental Microbiology.