An urgent warning has been issued to anyone who stores hummus and other dips in their fridge after opening.

Many people find themselves peeling back the thin layer of plastic, which many shops have switched to in lieu of a proper lid, and popping their dips back in the fridge once done. But before you know it, your pot of hummus has been left to go past its use-by date.

A lot of dips also come with warnings saying “once open, consume within two days”. While you may still be tempted to dig in, an expert has revealed it could have consequences.

Speaking to The Telegraph, Dr Primrose Freestone, a microbiologist from the University of Leicester has warned that two days is enough for your dips to become unsafe. She said: “The thing is, once you open the lid, airborne bacteria will start to settle on top of the dip.

“It is then only a matter of time before contamination by germs happens and food poisoning becomes a possibility.”

Dr Freestone explains dips already contain microorganisms and that opening the lid provides them with a “shot of oxygen” that helps them “grow rapidly”. She also says bacteria, fungi, and viruses from our hands and mouths can spread to the dips as we eat them.

And leaving your dips out of the fridge is another problem. Bacteria can double at temperatures warmer than 5C – the average fridge – in as little as 20 minutes. These factors can all lead to a risk of food poisoning, she says.

There are around 2.4 million cases of food poisoning a year in the UK. For most people, they will recover within a couple of days, but it can pose more of a risk, even becoming life-threatening to people who are immuno-compromised, the elderly, or pregnant women and their unborn babies.

Dr Freestone says these groups of people should not “mess around” with dips that have not been “stored correctly”. As for why the dips could be linked to food poisoning, the academic says it is because of listeria.

The bacteria can grow in under 48 hours, so while it may be impossible to know if your dip is contaminated, it is best not to eat it after the two-day window. And if you regularly eat old dips and don’t get unwell, Dr Freestone says you may simply have a “robust immune system”.

It is thought that dips containing acidic ingredients such as lemon juice, tomatoes, or vinegar could last longer than others because bacteria grows more slowly in these environments. Which means hummus and salsa could in theory last longer than the likes of guacamole.

Meanwhile salt and sugar are preservatives, meaning it could help your dip stay fresher for longer. Garlic and herbs are also microbial, so a sour cream and chive dip could fare better.

The best way to preserve your dip is to get it in the fridge as soon as possible. Dr Freestone also warns not to attempt to decant the product, as it could encourage further contamination.

If you do want it to stay fresher, she says to wrap your dip in clingfilm, as bacteria grows slower without the air. Ultimately though, Dr Freestone says people eating dips should stick to the use-by dates to stay safe.

She says dips that have changed colour, separated or taste "a bit sour" should simply be thrown away.