"Bagged salad can fuel the growth of food-poisoning bugs like salmonella and make them more dangerous," BBC News reports.
They grew salmonella in salad juice and leaves at different temperatures to see what happened, and found salad leaf juice – released from the leaves when they're damaged or broken – supports the growth of salmonella, even at fridge temperature.
They also found that if leaves are contaminated, the bacteria aren't removed by washing in water.
However, the chances of a salad bag being contaminated by salmonella or other bacteria in the first place are thought to be low.
An independent expert commented: "The rates of produce that have been found to be contaminated are between 0-3%."
That said, it is important not to be complacent. An E. coli outbreak in July this year, thought to be linked to contaminated salad, killed two people and hospitalised 62 others.
You should wash your hands thoroughly with soap and water, then dry them carefully, after using the toilet and before eating or preparing food.
You should also wash fruit and vegetables before eating them – although washing did not remove salmonella in this study – and pay attention to use-by dates.
Where did the story come from?
The study was carried out by researchers from the University of Leicester. No sources of financial support are reported.
The UK media's reporting was accurate, but some of the headlines could imply that salad bags have suddenly been found to be contaminated with salmonella: they haven't.
The potential for contamination is nothing new and something that the food industry tries to guard against.
What the study shows is that if salmonella is present, it will quickly grow to levels that could trigger food poisoning, even if the salad bag is placed in a fridge.
What kind of research was this?
This laboratory study aimed to examine the "behaviour" of salmonella bacteria if present in a bag of salad leaves.
Consumption of salad leaves like lettuce and spinach has increased considerably in recent years.
But they are highly perishable and require rapid processing and special packaging to keep them fresh.
They are particularly at risk of colonisation from the gut microbes E. coli, salmonella and listeria.
These may be present in contaminated soil or be transferred during the various processes of trimming, washing, packaging and transport, or through contaminated water or poor hygiene among people involved in food production.
The potentially contaminated leaves are usually eaten raw, which doesn't give any further opportunities to eradicate the bacteria through cooking, for example.
Salad leaves are said to be ranked as the second most common source of outbreaks of foodborne illness.
This study aimed to look at the factors that could enhance the growth of bacteria present in salad – for example, the effects of opening the bag or storing it at room temperature.
What did the research involve?
The laboratory tests involved a variety of salad juices, which were prepared by crushing and mixing cos lettuce, baby green oak lettuce, red romaine, spinach and red chard – each obtained from a selection of pre-prepared bagged salads.
The researchers conducted a series of tests. In one experiment, they put salmonella cultures into a fluid medium and incubated salad leaf juice in this fluid for 18 hours at 37C (98.6F).
To model what may happen in a salad bag, they also cultured salmonella in sterile water mixed with 2% leaf juices and refrigerated this at 4C (39.2F) for five days.
The researchers also looked at the growth on salad leaves by mixing salmonella with sterile water, 2% leaf juice and three spinach leaf pieces.
These were incubated at room temperature for 30 minutes, after which the leaves were washed in sterile water.
The researchers similarly tested growth on plastic salad bags and looked at the effect of washing.
What were the basic results?
As would be expected, the researchers found that when they incubated the mix of salmonella, water and leaf juice at the high 37C temperature, salmonella grew on all varieties of leaf juice.
Even though the 4C temperature restricted growth, the bacteria still grew in number over the course of five days in the fridge.
Higher concentrations of leaf juice fluid further increased growth, suggesting the bugs may be able to use the leaf nutrients leached into the bag to support their growth.
This happened with increased concentration of any of the leaf juices, but spinach seemed to have the strongest effect.
The researchers also found salmonella attached to both the plastic bag and the salad leaves. The presence of salad juice increased the ability of salmonella to colonise both of these surfaces.
Bacteria better colonised cut leaf surfaces because the leaked leaf juice supported their growth.
The bacteria attachment to salad leaves was resistant to several washings in water.
How did the researchers interpret the results?
The researchers say their results show that, "exposure to salad leaf juice may contribute to the persistence of salmonella on salad leaves, and strongly emphasises the importance of ensuring the microbiological safety of fresh produce".
This laboratory study principally demonstrates that salad leaf juice – released from salad leaves when they are damaged or broken – supports the growth of salmonella bacteria, even at fridge temperature. If leaves are contaminated with salmonella, this isn't removed by washing in water.
The results don't show that all packaged salad leaves are contaminated with gut bacteria like salmonella.
What they do show is that if the bags have been contaminated with gut bacteria, these bacteria will replicate, even in the fridge, and there's little you can do to remove them.
The best thing to do is to throw the bag out, although there's no way of knowing whether a particular bag is contaminated or not.
The study also cannot tell us whether we may be safer buying packaged salads unwashed, washed in spring water, or washed in chlorinated water.
And neither can it tell us whether we may be safer buying non-packaged lettuce – it's still possible that an unpacked lettuce may have been contaminated at some point along the line.
But any risk of food poisoning is far outweighed by the health benefits of eating fresh veg, such as reducing the risk of heart disease, stroke and some cancers.
You should be reassured that the contamination levels in the food chain are in reality very low, with only 0-3% of raw food products found to be contaminated.
Commonsense precautions will also reduce the risk:
- Hands should be thoroughly washed with soap and water, and dried, after using the toilet and before eating or preparing food.
- Keep salad in the fridge as, although it will not be prevented, growth of salmonella can be reduced.
- Discard leaves that look damaged or "mushy".
- Always wash salad before eating it – while this may have a limited effect on salmonella, washing can remove soil and debris.
- Follow use-by dates and use salad within a few days of opening the packet.
Get more advice on food safety.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 18 November 2016
Mail Online, 18 November 2016
The Guardian, 18 November 2016
ITV News, 18 November 2016
The Daily Telegraph, 18 November 2016
Daily Mirror, 18 November 2016
Links to the science
Applied and Environmental Microbiology. Published online November 18 2016