The danger of eating out-of-date hummus

It might not look like it’s gone off but there may be all sorts of nasties lurking in that two-day-old hummus in your fridge

We all hate wasting food but, faster than bananas can go brown or bread can turn mouldy, there will be a pot of dip in our fridge that is out of date before we get the chance to finish it. 
As we peek beneath the thin sheet of plastic – now that proper lids have been ditched at some shops – and see no mould despite an old use-by date and warning to “consume within two days” of opening, it’s tempting to get out the crudités. But doing so could be risking our health, warns Dr Primrose Freestone, a microbiologist at the University of Leicester. 
Two days is all it takes for dips to become unsafe to eat though this can happen faster, she warns. “The thing is, once you open the lid, airborne bacteria will start to settle on the top of the dip,” Dr Freestone says.
“It is then only a matter of time before contamination by germs happens and food poisoning becomes a possibility.” Bacteria, fungi and viruses can come from our hands as we’re eating a dip or from our mouths as we breathe, speak, sneeze or cough near it.
But, as well as germs getting into the dip, remember that dips won’t be sterile when you buy them. “They will already contain microorganisms and opening the lid provides a shot of oxygen and off they go – they will start growing rapidly,” Dr Freestone notes.
Temperature is also a factor. When bacteria have access to temperatures warmer than the fridge (above 5C), they can double in numbers in as little as 20 minutes, she says. 
These factors are why most dips come with an instruction to “consume within two days of opening”, even if a dip is still within its use-by date, because of how quickly these bugs can take over.
“As for what you can catch from a dip – food poisoning in short,” says Dr Freestone. There are around 2.4 million cases of food poisoning in the UK each year. 
While most sufferers recover within a few days without needing any treatment, it can be life-threatening for the very young, immunocompromised and elderly, as well as pregnant women and their unborn baby, she notes. “They should not mess around with dips or any other leftovers that have not been stored safely.”
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When it comes to dips, listeria is the most likely cause of food poisoning, as this bug grows in the cool temperatures of the fridge, Dr Freestone says. It’s impossible to know if your dip is contaminated but the risk increases if you eat after more than 48 hours of it being open, or past its use-by date. “If there are only a few listeria present when you eat the dip, you won’t get ill. If there are hundreds, you might and if there are thousands, you almost certainly will.”
For those who regularly eat dips after their use-by date or more than two days after opening them without becoming unwell, “you might have a really robust immune system,” she says.
“There are a huge number of dips with a huge number of preservative additives, so you can’t really generalise about what goes off fastest,” and is therefore the most risky, Dr Freestone says.
But dips that contain acidic ingredients – such as lemon juice, tomatoes or vinegar – may last slightly longer, because bacteria grows more slowly in an acidic environment. This means, in theory, that salsa, taramasalata and tzatziki may last longer than the likes of hummus and guacamole. 
Salt and sugar act as a preservative, so those high in these ingredients may also fare slightly better. Additionally, garlic and herbs are antimicrobial, which may mean bacterial growth is slower in aioli and sour cream and chive dip.
“Often there are no signs that a dip has gone off,” says Dr Freestone.
But one signal is swollen packaging. That’s a sign that bacteria is present, as they produce gas as they grow, which causes the packaging to bloat, she explains.
“Also, if it tastes a bit sour, has changed colour or it’s separated – chuck it,” she adds.
Once you’ve bought a dip, get it in the fridge quickly. “With the exception of listeria, the cold slows down the growth of bacteria such as E. coli, salmonella and bacillus,” Dr Freestone says.
Despite some supermarkets doing away with plastic lids for dips, there’s no need to decant it into a container, as this will only increase the risk of it becoming contaminated, she says. “Leave it the container it is already in, handle it as little as you can and get it in the fridge as fast as you can.”
Regardless of whether it’s shop-bought or homemade, “don’t leave it more than two days before finishing it,” Dr Freeston advises. 
If you’ve eaten some of it and want to keep the rest for later, “wrap it in a bit of cling film before putting it back in the fridge, as this keeps out air and in turn slows bacterial growth,” she says.
However, if you’ve left it at room temperature for a few hours or multiple people have been eating it, “don’t bother sticking it back in the fridge because the level of germs will have grown to a level where they can actually cause illness,” Dr Freestone says.
Stick to the use-by date, even if a dip looks perfectly fine, she urges. While best before dates flag when food quality – such as taste or texture – will start to worsen, use-by dates mark the end of when a food is safe to eat.
“Ultimately, dips aren’t expensive and is it worth getting food poisoning from a dip that’s potentially gone off? I don’t think it is. My strong advice is eat the dip as soon as you can.”