There’s nothing wrong with letting food cool in the kitchen for a half hour or so before putting it in the fridge if you want to conserve energy. (Refrigerating warmer food means your icebox has to work harder to maintain its chill.) But the longer food is left out at room temperature, “the greater the chance of bacteria multiplying and growing,” says Dr. Carl Winter, a food toxicologist and director of the food safety program at the University of California–Davis. Sinister bugs such as salmonella, campylobacter, and E. coli all grow much faster at room temperature than in a cool environment.
The USDA advises chilling as promptly as possible and says you should never leave food out for more than two hours (or, when the temperature is above 90 degrees Fahrenheit, just one hour).
If you’re dealing with a serious quantity of food—say, a large roast or a vat of chili—you could try stirring it in an ice bath or dividing it into smaller portions before you refrigerate to cool it more quickly.
Finally, to cover or not to cover? Covered food cools more slowly. But uncovered food is susceptible to cross contamination, like condensation dripping into it from other items, or from the fridge itself. So if it’s a large amount of food that might take a long time to cool—for instance, a stockpot full of soup—cover it loosely until it chills a bit, then seal it.