Having leftovers in the fridge is a source of security…and for the more neurotic among us, stress. On the one hand, it’s nice to know that a meal is ready and waiting for you when hunger strikes. But at the same time, eating it before it spoils can feel like an impossible race against the clock.
Food poisoning is no joke, so it’s no wonder why a days-old casserole might be more off-putting than alluring. The hungrier you are, though, the more tempting it may be to believe there could be a way to reset that clock—especially when trusted sources tout the idea that a spin in the microwave is all it takes to add an extra three to four days to your food’s lifespan.
Basically, the thinking behind this theory goes that as long as you reheat a dish before it’s had time to contract illness-causing pathogens, you’ll continuously delay it ever contracting them at all. While that would be great if true—how convenient would it be to be able to eat the same food on an infinite loop or, more likely, ’til there’s none left?—the food safety specialists we spoke with informed us that there’s little more to the notion than wishful thinking.
Here’s why they say you can never truly give your old food a fresh start, how to err on the side of caution with that pasta in your fridge, and what you can do to get the most out of your leftovers before their time is up.
First: What makes leftovers dangerous?
Food poisoning is caused by the presence of pathogens—i.e. disease-causing bacteria, viruses, and parasites—in what we eat, but what’s generally less well understood is when they form and where they come from.
Harmful strains of bacteria find their way into our food through various channels: cross-contamination during prep, improperly washing ingredients, and even simple contact between what you’re cooking and any germs on your hands from touching things like a dirty light switch or faucet, Bill Sullivan, PhD, Showalter professor in microbiology and immunology at the Indiana University School of Medicine, tells SELF.
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Working in a clean environment, performing regular handwashing, avoiding recalled food, and practicing caution with potential cross-contaminants—like washing tongs that touched raw chicken—will all limit the presence of these kinds of bacteria. And if harmful pathogens are never introduced, it's that much more likely they won’t appear at all. (Your leftovers will still eventually spoil, but the responsible bacteria won’t necessarily be those that make you sick.)
The real risk arrives at the stage between cooking and cooling, when food enters the temperature “danger” zone between 40 and 140 degrees Fahrenheit, says Dr. Sullivan. “As heated food is cooling, any bacteria that gets into it are going to replicate quickly—some bacteria can reproduce in just 20 minutes,” he explains. “The longer food stays in this danger zone, the greater the bacterial contamination and the higher the risk that heat-resistant toxins and spores are present.”
What reheating can and can’t do—and when it’s a problem
Reheating leftovers zaps certain types of bacteria—and, of course, makes a dish more appetizing—but it’s not a magic reset button, Darin Detwiler, PhD, a food safety expert and associate teaching professor at Northeastern University’s College of Professional Studies, tells SELF. “Heating helps...but it’s not absolute protection,” he explains, adding that that’s especially true in the case of leftovers that have been sitting around for more than three to four days—the recommended window for safe consumption, per the FDA.
When food is reheated to 165 degrees Fahrenheit (the proper reheating temperature), most active bacteria are killed. But some toxins and spores are heat-resistant—like Bacillus cereus in rice and Clostridium perfringens in meat-based casseroles—and those will remain intact, he explains. “So reheating reduces risk, but it doesn’t erase it.” Dr. Detwiler
The other issue is that repeated reheating creates more opportunities for your leftovers to reenter the temperature danger zone as they cool. “Repeating that cycle increases cumulative risk,” Dr. Detwiler explains. That’s why he recommends only reheating the portion you’ll be eating, and to get your food into the fridge as soon as possible after the original cooking. (No, that hot food won’t wreak havoc on whatever’s nearby.)
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You can’t really extend the lifespan of refrigerated leftovers, but here’s what you can do.
Dr. Sullivan says it’s possible to get an extra day or two out of your leftovers by following proper safety procedures during and after cooking, as well as cooling them down as fast as possible—but you definitely won’t get a full reset. Even transferring food directly to the fridge will only shorten the time it spends in the danger zone, not eliminate it entirely, so you can’t ever be 100% sure—especially since the types of bacteria that cause foodborne illness aren’t visible to the naked eye.
So what are your other options? Start by tossing any food you’re second guessing, especially if it smells bad and is slimy or discolored, says Dr. Detwiler. From there, employ all the best safety practices during the original cooking, ensure you’re reheating leftovers to 165 degrees Fahrenheit, and check that the temperature is consistent throughout the food with a thermometer before you dig in. And finally, remember that freezing is always a great way to get a longer lifespan—so you can have your infinite leftovers, without the fear of getting sick.
Originally published on SELF.
