Cold Leftovers: Safe to Eat?

There are few things better than a cold slice of pizza for breakfast. But as delicious as scarfing down cold pizza is, there's also a risk of food poisoning if you aren't careful.

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

Here's my advice as a microbiologist for staying safe when eating your favourite cold leftovers.

Leftover pizza

You can get food poisoning from cooked pizza in a number of ways. Whether that's because some of the ingredients are raw, undercooked or spoiled, or if the pizza has touched a surface with germs on it (including being handled by someone who hasn't washed their hands).

Surprisingly, the dried herbs and spices that people often sprinkle on their pizzas (such as basil, pepper and oregano) can also be susceptible to microbial contamination. This contamination can occur during the harvesting and production phases, or due to improper storage by consumers. Some of the foodborne pathogens that can potentially survive on dried herbs include bacteria that can cause food poisoning, including Salmonella, Bacillus cereus and Clostridium perfringens.

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.

So if you're a cold pizza lover, the best way to reduce your risk of food poisoning is to ensure any leftovers are refrigerated within two hours of being delivered or cooked. This should mean the pizza is safe to have cold for breakfast.

Once in the fridge, the leftover pizza needs to be stored covered (to avoid contamination from airborne germs) and eaten within two days . Note that putting leftover food in the fridge only slows bacterial growth , which is why leftovers should be eaten within two days maximum.

If the pizza is left at room temperature for more than a few hours, germs will grow quickly. This can make the pizza unsafe to eat the next day - no matter how tasty it might still look or smell.

Leftover chicken

Cooked chicken is highly perishable once cooled. Its high water and nutrient content and low acidity favours the growth of food poisoning bacteria, especially if it isn't stored correctly after cooking.

It's also important you only save chicken for leftovers if it has been cooked properly. If there's any trace of blood in the cooked chicken's juices, do not eat it - and certainly don't save it for later.

This is because raw chicken may be contaminated with the food poisoning germs Campylobacter, Salmonella or Clostridium perfringens, so thoroughly cooking your chicken is essential.

If even a tiny amount of the chicken under-cooked, food poisoning germs still present within the tissues can start growing even when the meat is stored in the fridge. These germs may not be detectable by smell or sight.

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. Ideally, it should spend no more than two hours at room temperature.

Cooked chicken can be stored for up to three days in the fridge. But again, if you notice blood in any part of the chicken, you absolutely should not eat it - whether cold or reheated - as this indicates it has been under-cooked and may be contaminated with germs.

Leftover rice dishes

Leftover rice dishes of any kind - whether that's fried rice, burritos or risotto - have a major food poisoning risk. This is because uncooked rice can contain spores of Bacillus cereus , a common food poisoning bacteria that prefers starchy foods.

Although Bacillus cells are killed by the heat of cooking, their spores are heat resistant and can survive . If a cooked rice dish is then left at room temperature for more than two hours, the Bacillus spores have time to develop into bacteria 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.

If cooked rice needs to be saved, it should be covered once cooked, cooled quickly, then refrigerated for no more than 24 hours .

Cooked rice can be eaten cold , but only if has been cooled quickly after cooking and stored as quickly as possible in the fridge. It's also best to consume cold cooked rice within 24 hours as B cereus spores can germinate during longer storage periods.

Left over canned foods

To safely store canned leftovers, it's essential they're covered and refrigerated to avoid contamination from airborne germs .

It's considered safe to store the food in the original can as this has been sterilised in processing. But for flavour reasons, you might want to transfer it to a covered plastic or glass container.

Highly acidic foods, such as canned tomatoes, can be stored refrigerated for five to seven days . Low acidity canned foods, such as meat, fish, fruit, vegetables and pasta, can only be stored for up to three days. Acidic foods last longer because the acid inhibits the growth of food poisoning bacteria.

Leftovers can be safe to eat cold. Just make sure you refrigerate them as quickly as possible after cooking and consume within a day or two.

Primrose Freestone does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.