How long you can keep leftover food and which are most 'high risk'

You could be putting your health at risk by snacking on some go-to leftovers, an expert has warned.

Speaking to The Conversation, Dr Primrose Freestone, a microbiologist at the University of Leicester, revealed that some foods are 'high risk' and could give you food poisoning if you're not extremely careful.

"Food poisoning is caused by eating food that has become contaminated with pathogenic bacteria, fungi or viruses," she said.

"Although most people know that food poisoning can be caused by poorly cooked foods or risky food preparation habits, improperly stored leftovers are also a key cause.

"It's therefore extremely important you take care when storing leftovers to avoid harming your health."

These are four of the worst offenders:

Cold pizza

The toppings on pizza can pose a risk of food poisoning (Nitas/ Getty Images)

"Surprisingly, the dried herbs and spices that people often sprinkle on their pizzas (such as basil, pepper and oregano) can be susceptible to microbial contamination," Dr Freestone explained.

Contamination can occur during harvesting, and some foodborne pathogens can survive on dried herbs, leading to food poisoning later.

"Even if these dried herbs have been sterilised by the heat of a freshly baked pizza, if left at room temperature for too long after cooking, these or any of the other pizza toppings can provide the perfect snack for potentially harmful germs," she said.

To avoid this, pizza should be refrigerated within two hours of delivery, as germs can grow quickly.

You then need to cover it in the fridge and eat it within two days.

Rice

Rice can also pose a risk to human health (Cris Cantón/ Getty Images)

Whether it's fried rice, risotto or burritos, leftover grains can harbor spores of Bacillus cereus, a common food-poisoning bacterium that prefers starchy foods, the expert said.

"Although Bacillus cells are killed by the heat of cooking, their spores are heat-resistant and can survive."

Therefore, once again, rice should be refrigerated within two hours of being left out; otherwise, the spores have time to develop and multiply.

"These spores are also able to release toxins into the cooked rice, which can potentially cause severe vomiting and diarrhoea lasting up to 24 hours," she warned.

Once refrigerated, you have just 24 hours to eat cooked rice safely.

Cooked chicken

Cooked chicken can be dangerous (Getty Stock Image)

Cooked chicken also poses a high risk since it's highly perishable once cooled.

"To stay safe, once you've removed your cooked chicken from the oven or rotisserie packaging, any that you aren't planning to immediately eat should be covered and refrigerated as soon as possible after cooling," she recommended.

It should be kept at room temperature for no more than a couple of hours, then stored in the fridge for three days, she said.

Canned foods

Tinned food can cause problems (Getty Stock Image)

Last but not least, canned foods should also be treated with caution.

While fine when still inside the can, once opened, they need to be sealed to avoid contamination from airborne germs.

Their storage time depends on the food. Tomatoes can last five to seven days, while meats, fish, fruit, vegetables and pasta can only be stored for up to three days.