Damaged leaves could be harbouring salmonella in bagged salads, according to research at the University of Leicester.
The study, published in Applied and Environmental Microbiology, observed that salad juices in water doubled the motility of individual salmonella bacteria.
Over five days in a refrigerated environment, 100 salmonella bacteria multiplied to about 100,000 under conditions similar to those found in bagged salads.
Salad juices were also found to make the formation of biofilms on salad leaves more likely. Microbial biofilms are resistant to washing, and provide an environment in which bacteria can grow.
The researchers were surprised to find that the normal microbial flora on salad leaves did not respond to leaf juices, suggesting that the juices specifically give salmonella an advantage.
The study’s co-author, Primrose Freestone, said: ‘Salad leaf crops are grown in open fields where they can be exposed to salmonella, via insects, bird poop, and manure, among other sources. While outbreaks of salmonellosis due to such contamination are uncommon, they are nonetheless a public health problem. Such outbreaks may occur despite practices used to mitigate the problem, such as irrigation with clean water, good hygiene, leaf washing, and the like.
‘Moreover, earlier studies have shown that salmonella 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.’
It is estimated that the bagged salad market is worth over £1 billion in the UK and we buy nearly 500 million bags each year. With recent crazes for clean eating and juicing we are consuming more than ever. They are generally viewed as being as healthy as whole lettuces but are less hassle, since you don’t have to wash and trim them yourself.
This study suggests they may not be quite as healthy as we thought. It was concluded that, even in refrigerated bags of lettuce leaves, 100 salmonella bacteria multiplied to more than 100,000 — this is an infectious dose.
This phenomenon is in part due to ‘salad juice’, the liquid released from broken or damaged leaves, which contains all the nutrients needed for bacteria to thrive, such as sugars, proteins and minerals.
Such levels of salmonella can cause food poisoning, leading to diarrhoea or vomiting, and in severe cases can be fatal.
The researchers used common leaves including cos lettuce, baby green oak lettuce, red romaine lettuce, spinach and red chard, all from commercially available bags. They didn’t include rocket or watercress and so the results cannot be applied to those leaves. The salad juice came from the leaves after they had been blended and was kept at the standard fridge temperature of 4°C.
This is an important study: it is scientifically sound, with real-world validity and with significant implications for public health.