One Major Side Effect of Reheating Leftovers, Experts Say

 If you've kept food in the deep freezer for a few weeks—or maybe even a few months—it might be time to give those leftovers a warm-up and enjoy them again. But if you're going from the freezer to the microwave, there's one common tip that still holds true for food safety: Thaw properly. 

The safest way is by putting the leftovers in the fridge, usually for at least 24 hours per pound of food, according to Janilyn Hutchings, certified food safety professional and food scientist at StateFoodSafety, a certification, and training program for the hospitality industry. 

You can also thaw in cold water, and this method takes about 20 to 30 minutes per pound of food, she says. The biggest advice here is to make sure the water stays below 41 degrees Fahrenheit the entire time.

In a hurry? Use the "defrost" setting on your microwave, she adds. Many microwaves allow you to defrost based on the type of food you're thawing, such as meat, and you can input the approximate weight. That's a much better option than trying to cook leftovers in the microwave directly from a frozen state. 

You can also thaw by cooking, but only if the food is in smaller pieces so it can heat quickly, says Hutchings. That includes ground or chopped meat. 

 

If you're going in the other direction of getting leftovers ready to store and reheat later, the biggest tip is to get it in the fridge or freezer, pronto. A common misconception is that any bacteria formed by letting food sit out at room temperature will be killed by freezing, according to Felicia Wu, Ph.D., professor in food safety, toxicology, and risk assessment at Michigan State University. That would be a relief if it were true, but unfortunately, it's not the case.

"Many foodborne microbes can withstand freezing and pose a problem later after thawing, and that gets even worse if it's not cooked well," says Wu.

In terms of reheating in the microwave, there's still controversy over whether you can use plastic. That's because research suggests many types of plastic will break down slightly when heated, causing chemicals to leach into your food. The biggest concern has been over bisphenol A (BPA), which is why many plastics these days are marked as BPA-free.

Still, some research indicates that even plastics without BPA may be a risk to put in the microwave, and that's why the FDA suggests avoiding the use of plastic in the microwave, with the exception of containers specifically labeled safe for microwave use.