MOST people think nothing of chucking last night's leftovers into the microwave and digging into a stress-free dinner.
But doing so could actually put you at risk of several serious illnesses, which, in rare cases, can be deadly, a food safety specialist has warned.
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It turns out some meals should never be reheated, however convenient doing so might be, while others require special attention.
Dr Stacey Duvenage, from the Natural Resources Institute at the University of Greenwich in London, said: "In the hope to offset food wastage, leftover food is often kept for a second meal.
"However, care should be taken when preparing it and reheating it to prevent food poisoning and to maintain the nutritional properties."
Here, the expert lists nine products that you need to be extra careful with when scoffing them a day or two after cooking, and why.
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1. Rice
First up on the watch list is any dish involving rice - think curries, stir fries and risottos.
That's because uncooked rice contains a bacterium called Bacillus cereus.
Although cooking should kill the bug, it can live on in spore form, Dr Duvenage said.
"These spores can survive the cooking process and then begin to grow when rice is left at room temperature," she added.
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"This species of bacteria is able to produce a toxin which could lead to food poisoning."
It causes two types of gastrointestinal illness - vomiting and diarrhoea - which can also be accompanied by stomach pain.
While very rare, there have been several deaths attributed to Bacillus cereus.
To avoid complications, rice should be put in the fridge within two hours of cooking, Dr Duvenage said.
It should then be stored for no more than 24 hours, and heated to at least 74C throughout before eating again.
2. Eggs
Eggs shouldn't be reheated full stop, according to Dr Duvenage.
She said the protein-rich product should only be consumed cold, within 24 hours of cooking.
"They're enjoyed globally for their convenience, affordability and health benefits, but leftover eggs should not be reheated," she added.
"Doing so leads to the proteins being oxidised, which could lead to the presence of cancer-causing agents in the food."
3. Mushrooms
Reheating mushrooms not only makes them soggy, but can destroy proteins and nutrients, Dr Duvenage said.
They probably won't taste great or be particularly beneficial to your health, and they could also make you unwell.
If they've been left at room temperature for too long, you might experience a stomach ache or other digestive issues.
But in severe cases, sickness could develop into something far more sinister.
Dr Duvenage said: "Similarly to eggs, when proteins are oxidised, they can become carcinogenic [having the potential to cause cancer]."
4. Spinach
Spinach and cabbage are good for you, right? Well, not if you're reheating them, according to Dr Duvenage.
She said while the nitrate-rich vegetables should make up part of a healthy diet, warming them up the following day should be avoided.
"Nitrosamines, which are carcinogenic compounds, can form within these vegetables when heated at high temperatures or if reheated," the expert added.
Scientists have linked these compounds to an increased risk of oesophageal, stomach and nasopharynx cancer.
5. Potatoes
Potatoes are a staple in diets around the world; they're cheap and rich in vitamins and minerals.
But once cooked, cooled and reheated, our bodies find digesting them more challenging, Dr Duvenage said.
On top of that, Clostridium botulinum and other bacteria can be present if potatoes have been left out for several hours.
"The deadly toxins produced by Clostridium botulinum would not be destroyed during reheating," Dr Duvenage said.
Botulism, which is caused by these toxins and is not specific to potatoes, is a life-threatening condition that attacks the nerves, brain and spinal cord.
It initially causes nausea, vomiting, diarrhoea and constipation, but, without treatment, it can trigger paralysing muscle weakness.
This includes: drooping eyelids, blurred vision, difficulty swallowing, slurred speech and breathing problems, according to the NHS.
Botulism is serious and requires immediate medical attention.
It is, however, rare, with just 62 confirmed cases in the UK between 1922 and 2005.
6. Seafood
"Reheated seafood can be enjoyed safely, but caution must be taken in the preparation and immediate storage after cooking," Dr Duvenage said.
People should refrigerate it as soon as possible, and eat it within 24 hours.
This will preserve quality and flavour, and help protect you against food poisoning.
"If certain types of fish are not refrigerated, histamine can be produced which can lead to an unpleasant reaction in consumers," Dr Duvenage said.
People may experience nausea, vomiting, diarrhoea, a burning sensation in the mouth, a red rash, itching, and low blood pressure.
This is particularly prevalent in fish of the Scombridae and Scomberesocidae family - e.g. tuna, mackerel and bonito, scientists say.
7. Chicken
Generally, reheating chicken is fine - but you need to make sure it reaches the right temperature.
Dr Duvenage said: "To kill bacteria present, an overall temperature of 75C should be reached in the thickest part of the piece.
"It is a good idea to use a meat thermometer to check the temperature to ensure that thorough heating."
Failing to do so puts you at risk of food-borne illnesses.
The Food Standards Agency (FSA) says: "Remember, reheating means cooking again, not just warming up.
"Always reheat food until it is steaming hot all the way through (you should only do this once)."
8. Oil
Reheating food that has been cooked with olive, flaxseed or rapeseed oil can lead to the production of acrylamide in products like chips, Dr Duvenage said.
It's not deliberately added to foods, but is a natural by-product of the cooking or heating process.
Lab testing shows acrylamide in the diet causes cancer in animals, and has the potential to do the same in humans, the FSA says.
"We recommend that the amount of acrylamide we all consume is reduced, as a precaution," the agency adds.
The problem arises from the smoke that is produced.
"The more the oils that are cooked, the lower the temperature is at which smoke is produced," Dr Duvenage added.
9. Baby food and milk
Finally, baby food, breast milk and formula should only be heated once.
"If your baby hasn’t finished the whole warmed portion, it could contain bacteria from your baby’s mouth," Dr Duvenage said.
"When baby food is reheated, it may not reach the temperature
needed to kill these bacteria, and that could cause foodborne illness.
"Breast milk and formula should not be reheated because nutrients can be destroyed.
"Formula should only be reheated if it is stored in the fridge within two hours of preparation.
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"Breast milk can lose its flavour and nutrients when reheating, leading to babies sometimes rejecting the mother’s milk."
Dr Duvenage concluded: "Understanding how foods behave when reheated helps us make choices in the kitchen when storing and preparing leftovers."
