The Most Dangerous Leftover in Your Kitchen

It’s a breeding ground for bacteria.

Believe me when I say we’re the last people to shame leftovers. Food preservation exists for a reason, and incorporating leftovers into a new meal (Or just eating them as-is, food safety permitting) makes way more sense than chucking them into 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.

Why Leftover Rice Can Be Dangerous

Rice is starchy and traps moisture—in other words, it’s a straight-up haven for bacteria, specifically Bacillus Cereus, which causes food poisoning. Bacillus Cereus is particularly resilient because it creates spores that allow it to withstand higher temperatures.

Leftovers enter the danger zone for bacterial growth between 41 and 135 degrees F (5 and 57 degrees C); a good way to think of this is above fridge temperatures but below cooking temperatures. This is also why it’s best to let leftovers cool off before placing them in the fridge—otherwise, the warm leftovers will mess with the fridge’s internal temperature.

Generally, when you heat food up, any bacteria that developed while the food was in the danger zone will be killed, provided the food is reheated adequately. However, heat-resistant bacteria like Bacillus Cereus won’t go away, so warming it up won’t make a difference. 

“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. It left untended to, that will just exhibit the above-rampant growth.” 

Safety Tips for Handling Cooked Rice

• Make only what you need. The more rice you make (and eventually reheat), the more space that bacteria gets for breeding. “The higher the concentration, the worse off you’ll be,” Casarella says. “Cook only as much as you need up front.” When you are reheating rice, only take what’s necessary—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.