The Most Dangerous Leftover in Your Kitchen

Credit: Getty ImagesKey PointsLeftover rice can quickly become a breeding ground for bacteria if you don't store or reheat it properly—keep it out of the "danger zone" temperature range to prevent harmful bacterial growth.

Cooling rice quickly or reheating only what you need can help keep your leftovers safe to eat.

Believe me when I say we're the last people to shame leftovers. Food preservation exists for a reason, and turning leftovers into a new meal (or eating them as-is, food safety permitting) makes way more sense than tossing them in the trash and wasting money, ingredients, and, above all, time.

Still, not all foods are created equally—and some leftovers can make you sicker than others. The most dangerous among them is leftover rice (yes, even if you warm it up). Read on to learn more.

Leftovers enter the danger zone for bacterial growth between 41 and 135 degrees F (5 and 57 degrees C). Think of it as the range above refrigerator temperatures but below cooking temperatures. While very large amounts of hot food can temporarily raise a refrigerator's temperature, it's safer to divide leftovers into shallow containers and refrigerate them promptly rather than letting them cool on the counter.

Generally, when you heat food, any bacteria that developed while the food was in the danger zone will be killed, provided you reheat it adequately. However, heat-resistant bacteria like Bacillus cereus won't go away, so warming it up won't help.

"If rice stays in the temperature danger zone, you can very easily double the amount of bacteria in as little as 15 to 20 minutes," says Clark Casarella, a scientist, chef, and food writer based in Sioux Falls, South Dakota. "After an hour, you're looking at 8 times the bacteria since it'll just keep doubling." What is it about rice that makes it such a hotbed for bacterial growth? Well, it contains lots of starch and doesn't contain much protein, which means it's an ideal breeding ground. Warm it up, and it's a straight-up paradise for heat-resistant bacteria. This applies to other grains like couscous and farro, potatoes, and even pasta.

"Basically any warm, wet, starchy environment is a breeding ground for that bacteria strain to cause emetic food poisoning," Casarella says. "You don't usually see that bacterial growth in other foods. Rice is just wet starch. If left [unattended], that will just exhibit the above-[mentioned] rampant growth."

Make only what you need. The more rice you make (and eventually reheat), the more opportunity bacteria have to multiply. "The higher the concentration, the worse off you'll be," Casarella says. "Cook only as much as you need up front." When reheating rice, only reheat the amount you plan to eat—your immune system will thank you.

Cool it down quickly, or keep it warm. The less time your rice spends in the temperature danger zone, the better. Try spreading it in a shallow dish to help it cool quickly. "I tend to spread any leftover rice on a sheet tray and pop it into the freezer to cool it even quicker," Casarella says. You can also use the "keep warm" setting on your rice cooker to keep rice out of the danger zone.