Can you refreeze thawed leftovers? A food safety specialist explains the complicated science

One cardinal rule of food prep and preservation I learned growing up was that once frozen food thaws, it cannot go back into the freezer. My mom taught me that, and her mom taught her.

For my mom, this rule holds fast across all food groups from meat and poultry to dairy and desserts. She never, ever would freeze, thaw, and then re-freeze raw chicken without cooking it. “That is a shanda,” she says, which is Yiddish for “scandal.”

So when I found my freezer door slightly ajar one day, it sent me into throes of anxiety: Yes, the extra electricity and wasted energy, but the worst part? ALL THE THAWED FOOD I WOULD NEED TO EAT IMMEDIATELY!

The belief that thawed food cannot be re-frozen proves widespread, considering how many articles address this question. So, is it true? As Meredith Carothers, a food safety specialist at the United States Department of Agriculture (U.S.D.A.), says, food safety and food quality are two different things here.

Is it safe to refreeze thawed food?

Yes — but there is a right way to do it.

“From a safety perspective, as long as the food product that has thawed has been stored below 40 degrees Fahrenheit [about 4.5 degrees Celsius], it's safe to refreeze it,” Carothers tells Inverse. By “below 40 degrees Fahrenheit,” she means “in the refrigerator.” (“Nobody that I know personally has their house at lower than 40 degrees,” she says.)

Forty degrees Fahrenheit is the magic number because that’s when cooler temperatures start to slow bacteria reproduction. Some common, harmful bacteria that grow quickly on food outside the fridge are E. coli, Salmonella, Listeria monocytogenes, and Campylobacter. Typically those infected with these bacteria exhibit vomiting, diarrhea, fever, and other symptoms.

There is a trick for thawing food at room temperature, as long as you pay attention to timing. “We do have a two-hour rule where really that bacteria won't have the time to multiply to dangerous levels within that two-hour timeframe,” Carothers says.

This is true for both raw and cooked food left at room temperature for more than two hours. Even if you plan to cook some raw meat in the oven, if the meat has been on the counter all day that could pose a problem. “When those foodborne illness bacteria have the opportunity to multiply to dangerous levels, they can actually produce some heat resistant toxins,” she says. “Ultimately they're heat resistant. They won't be killed later on by being exposed to those high temperatures.”

The same is true for food left on the counter in its original wrapping, like raw chicken or beef. The packaging doesn’t protect against temperature change, and that meat was never free of bacteria; it just didn’t possess dangerous levels of bacteria.

If you do re-freeze food (raw or cooked) left at room temperature longer than two hours, that’s also risky. “The freezer doesn't kill them,” Carothers says. “They're just going to be in that paused holding pattern until you start to thaw again.”

Bottom line: It’s safe to refreeze food that was thawed either in the fridge or at room temperature for less than two hours. The risk of food poisoning and foodborne illnesses comes when you refreeze food left on the counter defrosting all day.

Is it okay to refreeze thawed food?

While “safe” means the practice will not pose a risk to health, “okay” regards the food’s quality. Another tradition in my family is “secret” foods, which we call food — especially desserts — we froze and then forgot about. Sometimes a food is so secret, so utterly clandestine that by the time it’s recovered, it resembles a snowball. At that point, it’s usually beyond saving.

Carothers says that freezing draws some water out of the food, which can result in a duller flavor. When frozen, water forms ice crystals. These crystals move to food surfaces, which is why snacks left in the freezer too long can have a frosty coating. This moving process is called sublimation, in which a substance changes from a solid (not the food itself, but its water content turned to ice) to a gas. As water molecules exit, the frozen food dries up; oxygen causes it to further shrivel.

Freezing meals once can alter taste enough; freezing and refreezing just means even more lackluster leftovers. “There's that potential of the quality getting poorer each time you thaw or refreeze it.”

What’s the best way to thaw refrozen food?

As Carothers says, thawing refrozen foods in the fridge is the safest way to revive them, if a bit more time-consuming. If you plan to eat your preserves right away, zapping them in the microwave is ideal. Adding back a little water to some frozen foods, especially while heating them, can go a long way for texture.

Some foods freeze better than others in the first place. Soups, stews, curries, and some meat dishes fair pretty well, especially if you reheat them on the stove and add some fresh seasoning. Dairy, besides whole, fresh cheese, does decently in the cold box. Vegetables and fruits like berries, grapes, and bananas chill well, especially if you like banana “ice cream” or sucking on frozen grapes. Fresh fruits, however, can get mushy upon thawing completely.

With the science behind this maternal family secret, my mom and I can properly freeze, unfreeze, and refreeze all we want.

CHECK, PLEASE is an Inverse series that uses biology, chemistry, and physics to debunk the biggest food myths and assumptions.

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