At a time when reducing food waste is a hot topic and batch cooking is proof that you’ve got your life together, reheating leftovers may seem like a sensible move, and budget-friendly, too. But just how many times we can get away with reviving that leftover curry, stew or stir-fry without putting ourselves at risk of food poisoning is a question that science journalist and television presenter Michael Mosley tackles in Trust Me I’m a Doctor.
Leftovers on repeat
Mosley’s verdict is clear cut: “You can actually reheat your leftovers as many times as you like, as long as you make sure every morsel is piping hot all the way through,” he says. SBS Food put the matter to Lydia Buchtmann, spokesperson for the Food Safety Information Council, who agreed. “You can reheat food as many times as you like, but you need to reheat it to 75 degrees celsius and the only way you can work that out is by using a cooking thermometer,” she adds. The trick is to measure the temperature in the centre of the dish (making sure you’re not touching the thermometer to the base of a hot baking tray, for example) to ensure an accurate reading.
Reheating within reason
While you can technically reheat food as many times as you like, there’s still a time frame to be aware of. If you’re still eyeing off that leftover pad Thai a week on, it’s time to toss it. “You can keep leftovers for two to three days in your fridge, as long as that fridge is at 5 degrees celsius,” advises Buchtmann. Freezing leftovers will buy you more time, as bacteria don’t form at freezer temperatures. “I’d still advise people to label leftover food so they know how long it’s been in the freezer, because not everything keeps in the freezer forever,” says Dorothy Richmond, accredited practising dietitian and food safety trainer.
The exception to the two to three day reheating rule is pregnant women, the elderly and people with compromised immune systems (for instance cancer patients or people taking certain medications). These people need to consume refrigerated leftovers within 24 hours, advises Buchtmann, to minimise the risk of exposure to listeria, a foodborne illness.
Mastering reheating methods
Microwaves are undoubtedly the most convenient way to reheat leftovers, but according to Mosley, they don’t always heat food evenly. “Microwaves create hot spots and dangerous cool spots in your food…and in the cool spots, bacteria can thrive,” he says. “So if you use a microwave, be sure to heat and stir and heat and stir, until you’re satisfied it’s really hot all the way through.”
It’s also a good idea to make sure the turntable in your microwave is working properly to ensure even heating, and to follow the instructions in the manual, which may have specific guidelines about different voltages and times required.
As for reheating food in the oven or on the stove? “Preheat the oven, and if you’re reheating food on the stove make sure the saucepan isn’t too small, that you stir the food and don’t just bring the temperature up to simmering point and serve it; it should be sitting on the stove for 10 to 20 minutes bubbling away,” advises Richmond.
When it’s risky to reheat
Reheating leftovers to 75 degrees celsius will kill most bacteria that cause food poisoning – except for several sinister types that don’t play by the rules. One of these is bacillus cereus, a pathogen which lives in rice. “The thing about bacillus cereus is that it produces a toxin, and unfortunately the toxin is heat stable,” says Mosley. Heat resistant spores and toxins form when rice is allowed to cool slowly (for instance, leaving it in the rice cooker after it’s been switched off), and once they have formed, no amount of reheating will destroy them.
“It’s not just rice that’s the problem with bacillus cereus; it’s anything starchy really,” cautions Richmond. So, leftover pasta, noodles and starchy vegetables like potatoes can all be affected, if they are allowed to cool slowly. Similarly, clostridium botulinum andclostridium perfringens are nasties that germinate when food cools slowly, that aren’t killed by reheating.
Thankfully, there’s an easy preventative solution. “All those things that produce toxins will be fixed by dividing leftovers up into small containers so they cool quickly, as soon as the food has stopped steaming,” advises Buchtmann. “Then, pop them straight in the fridge or freezer.”
Dodging food poisoning comes down to this golden rule: every time you’re handling leftovers (even if you’re onto the third zap in the microwave), stick to the ‘divide and cool’ method and always reheat to 75 degrees celsius.
Watch Michael Mosley’s guide to reheating leftovers in Trust Me I’m a Doctor: