The kitchen is a place of mythmaking, where a watched pot never boils and soufflés will fall if you speak above a whisper. These are myths indeed—hokum passed down as gospel truths by well-meaning cooks over many years. Beyond those benign examples, however, a more pernicious myth would have you believe that putting hot food in your refrigerator will spoil the rest of the food within it. It may sound like common sense, but in fact the opposite is true: Waiting to put hot food in the fridge could actually put you in danger of foodborne illness.
It’s unclear exactly when this bad advice first emerged, or when the scientific tides changed in support of immediately cooling hot food. In 1942, the US government’s War Department Technical Manual specified that one should allow hot food “to obtain room temperature before placing it in the refrigerator.” A few years later, however, a guide published for US naval personnel working in ships’ commissaries noted that “hot leftover food should be placed in the refrigerator as soon as it is cool enough to handle.”
By the second half of the 20th century, the official guidance on storing hot food was clear. A 1975 edition of Consumer News, published by the US Department of Health, Education, and Welfare, declared: “FDA has always said that food should be refrigerated as soon as possible (but a boiling pot of soup does not have to be popped into the refrigerator immediately).” It added that “the modern refrigerator”—and this was nearly 50 years ago—is capable of rebounding from an increase in temperature from hot food.
Nevertheless, the myth persisted. A 1995 issue of Weekly World News suggested that letting food cool before refrigerating would lower your energy bill, since your fridge wouldn’t have to consume more electricity to cool hot leftovers. Even some refrigerator manufacturers have advised consumers to let hot foods cool to room temperature before storing in the fridge.
More recently, popular food sites have continued to offer conflicting advice, with one suggesting that large batches of hot foods can raise the temperature of the fridge to the so-called “danger zone,” while noting in the same breath that “there’s no harm in putting hot food directly into the fridge if you need to.”
You’d be forgiven if you’re still confused.
What is the “danger zone” for food?
Microbes, both good and bad, are all around us: in the air, and on our bodies, pets, surfaces, and food. Between 40° and 140° Fahrenheit, illness-causing bacteria like E. coli, Staphylococcus, Campylobacter, and Salmonella grow prodigiously. This is what is commonly referred to as the “danger zone” for food, in which colonies of malicious microbes can reach levels that put you at risk.
Luckily, food safety authorities, including the FDA and USDA, are unequivocal about the rules: Food must be refrigerated within two hours of cooking (or reheating, or being kept warm in an oven). Anything left out longer than that should be discarded, as should any food that’s been kept out of the fridge for an hour over 90º Fahrenheit—like potato salad at a picnic, for example. The same goes for perishable cold foods, like deli meats and cheeses. Chilling your food below 40º Fahrenheit as quickly as possible will greatly reduce your chances of breeding an overgrowth of bacteria.
As for chilling “large” quantities of food? It’s true that you wouldn’t want to stick a full pot of piping hot chicken stock into the fridge, as it will take a long time to cool down; during that time, it can stay in the danger zone long enough that bacteria will multiply. But rather than leaving the whole pot to cool to room temperature, you should instead divide its contents into small containers to cool it faster. You can also place the containers in an ice bath to chill leftovers more quickly before placing them in the fridge.
Of course, how you chill food in the fridge is a matter of preference and the particular food you’re refrigerating. Anything with a crispy top—like, say, a pie or baked mac and cheese—can be chilled uncovered or lightly covered with a clean cloth, away from potential overhead contamination; this will prevent condensation from forming and turning crusts soggy. After those foods have been chilled, they can be wrapped with plastic wrap or aluminum foil (or a more sustainable alternative if you prefer). Generally speaking, however, wrapped food lasts longer than unwrapped, which is one reason why vacuum-packed food has a comparably long shelf life.
If you want to freeze leftovers, don’t stick hot food directly in the freezer. Simply divide large batches of hot food into small containers, cool in the fridge or an ice bath, and then move to the freezer. As Ali Rosen notes in her cookbook Modern Freezer Meals, trying to freeze still-hot food can create large ice crystals in your food, producing some unpleasant textural irregularities. “The smaller the crystals, the less damage to the food because less of its cellular structure is changing,” Rosen writes. “The larger the crystals, the more damage can be to the cell membranes—and that damage is what causes changes in texture that you can taste once you defrost. The faster a food freezes, the smaller and more uniform the crystals.”
Remember: Even after food has been chilled, the spoilage clock keeps ticking. Food can still become unsafe at refrigerator temperatures given enough time. As a rule of thumb, the USDA advises that you keep refrigerated leftovers no longer than three or four days. Beyond that, you might find yourself back in the danger zone.
