Michael Mosley: When is it safe to reheat leftovers?

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Michael Mosley with a plate of his reheated curry

Reheating food seems like a good way of cutting down on waste and expenditure. But when is it dangerous, asks Michael Mosley.

After a generous meal we normally end up with leftover food which it seems a shame to throw away. Some leftover foods can even, potentially, be healthier second time round.

On the other hand, of course, none of us are that keen to risk a bout of food poisoning. If you've ever been poisoned by food (and I have, more than once) then you will know just how gruesome the whole experience can be. Typically it involves vomiting, diarrhoea and stomach cramps

About a million people every year in the UK get food poisoning - half from their own cooking (with summer barbecues and Christmas leftovers being the biggest culprits). Around 100 people each year (mainly the very young and the very old) get it so badly they die from it.

So what are the rules? How and when can you safely reheat food?

Food poisoning is usually caused by bacteria contaminating your food. The biggest culprit is not that well known but it is very widespread. It's called Campylobacter and according to the Food Standards Agency it is present in more than 65% of chickens on sale in the UK. A recent survey found it on 6% of the outside packaging of supermarket chickens.

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Campylobacter can survive up to a couple of hours on a kitchen surface, so it's easy to spread it around. It can also make you ill even in very low doses. Whereas you would need to consume tens of thousands of Salmonella bacteria to become sick, with Campylobacter it's only around 500.

Because it is so widespread in chickens and so easy to spread around, it is not a good idea to do what many people still do, which is to wash your chicken before cooking it. Far better to put the chicken straight in the oven after seasoning and then wash your hands thoroughly after handling.

The key to killing bacteria is to use heat - so thorough cooking, all the way through, is important, especially with chicken where the meat texture is loose enough that bacteria can be found throughout the bird.

But what do you do, once you have had your lovely meal and you have some left over?

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First of all you need to let it cool down, before putting it in the fridge. If you put hot food in a fridge all that does is raise the temperature inside the fridge and turn it into the perfect incubator for bugs - which is not great for the food that is already in the fridge.

I recently tried putting the remains of a cooked chicken in my fridge while it was still warm and it managed to raise the temperature by over five degrees.

Instead you should cover your left-over food, let it cool to room temperature (no more than four hours), and then put it straight into the fridge.

Once it has been cooked, how often can you reheat it? Well the Food Standards Agency recommends only reheating food once, but actually several times is fine as long as you do it properly. Though that is not likely to improve the taste.

When to be careful

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Foods which are commonly reheated and which the Food Standards Agency list as potentially hazardous include:

  • Cooked meat or cooked food containing meat, such as casseroles, curries and lasagne
  • Sauces containing cream or milk
  • Seafood including patties, fish balls, stews and sauces containing seafood and fish stock
  • Cooked rice and pasta
  • Foods containing eggs, beans, nuts or other protein-rich foods such as quiche, soy bean products and lentil burgers

The secret to reheating is to do it thoroughly. Most of us tend to use a microwave oven to reheat food, but there is a problem. A microwave oven will heat your food unevenly, leaving cool pockets where bacteria can thrive.

That is why it is important that you take the food out, part of the way through cooking, give it a good stir, and then zap it again. Your aim is to get every part piping hot.

One food that people are particularly cautious about reheating is rice. As I mentioned above, most cases of food poisoning are caused by bacteria, but in the case of rice it is a bit more complicated than that.

Rice can be contaminated by a bacterium called Bacillus cereus. The bacteria themselves are killed by heat, but they sometimes produce spores that are not only toxic but surprisingly heat resistant.

This can lead to "fried rice syndrome", so-called because it was once not uncommon for people to get ill after eating fried rice at a Chinese buffet (where fried rice dishes had been left sitting around at room temperature for many hours). These days hygiene standards in Chinese restaurants are much better than they were.

Despite widespread fears, it is safe to reheat rice, and I frequently use rice left-over from the night before as part of a stir fry, but not if you have left that rice out overnight in a warm kitchen. As with meat, once the rice has been cooked you should aim to cool it and keep it cool as quickly as possible.

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