Here’s What Happens When You Eat Leftover Rice—and Why Reheating It Doesn’t Help

Updated: Nov. 03, 2023

If it's handled and stored improperly, leftover rice can make you sick. Here's why—and how to avoid so-called fried rice syndrome.

Every year, a whopping one in six Americans gets sick from a foodborne illness. Some of the causes are familiar: salmonella from raw or undercooked meat or poultry, E. coli from contaminated produce and listeria from unpasteurized milk or cheese. But one source of food poisoning may surprise you. It comes from the bacteria Bacillus cereus, and it’s an illness so distinct it has earned the name fried rice syndrome.

B. cereus is a spore-former, a small, specialized group of bacteria that’s very hard to kill,” explains Keith R. Schneider, PhD, a food safety professor and researcher at the University of Florida. “I can cook it, but it’s just going to get mad, and as soon as it gets into the ideal temperature zone, it’s going to become active again and start multiplying and become problematic.”

Worried? Don’t be. With Schneider’s help, we’re going to teach you about the cause of fried rice syndrome, plus the steps you can take to prevent it. You’ll learn how to store food in the fridge, how long rice lasts in the fridge (by the way, here’s what happens when you don’t wash rice), how long leftovers last and more. You’ll also discover what other foods may be contaminated with B. cereus and whether or not non-perishable foods are on that list. So read on to find out.

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What is fried rice syndrome?

In short, fried rice syndrome is food poisoning from B. cereus. The bacteria is commonly present in uncooked rice, though it is usually harmless if the rice is prepared and handled properly. Unlike illnesses caused by other common bacteria, the issues with B. cereus come from the spores.

“With spores, what happens is when the bacteria gets into an unfavorable environment [like a pot of boiling water], it goes into defense mode and sheds about 50% of its mass and envelops itself in an endospore. These are very resistant to heat,” explains Schneider. “Then there’s a danger zone between 40 and 140 degrees.”

Let’s say you heated one of these spores through the cooking process. As it’s cooling, you could run into trouble. “Once it gets below 140 [degrees] and until it gets to 70 [degrees], it will start to germinate and actively grow,” he says. “At that point, if it produces toxins, you can get sick.”

It’s because of this toxin-forming danger zone that illness caused by B. cereus typically happens from eating fried rice, though there are a few different foods susceptible to the bacteria that you should be aware of (more on those below).

“Fried rice is typically made from leftover rice that sits out and then cooks really quickly in a wok,” says Schneider. “This can mean hours out or back and forth from the fridge.” During this time, the rice can stay in that danger zone for hours, giving the bacteria the opportunity to produce the toxins that can make you sick.

What can happen if you eat leftover rice?

If the rice is handled and stored properly during preparation at the restaurant and once you bring it home, it’s as safe to eat as other reheated leftovers. The same goes for rice you make at home. But if you don’t pay attention to proper food storage and handling—that includes avoiding aluminum foil for leftovers—you set yourself up for potential foodborne illness.

“If you take the takeout box and leave it in your car while you go shopping, then put it in the fridge and eat it a few times, taking it out and putting it back in each time, you can have proliferation [of bacteria],” notes Schneider. And that can lead to some unpleasant symptoms.

There are two main types of illnesses that fried rice syndrome can cause: the emetic form and the diarrheal form. While these are by far the most common, the bacteria can also lead to rare but serious sickness.

Emetic illness (aka vomiting)

Fried rice syndrome can cause vomiting when your body notices the bacteria within the first one to six hours of consumption. “Once you consume that rice, your body will immediately realize and vomit it out,” says Schneider.

Diarrheal illness

When it comes to sure signs of food poisoning from B. cereus, diarrhea caused by fried rice syndrome is common. “You can ingest the microorganism, and if it can survive and set up camp in your GI tract, it can start producing those toxins inside your body,” Schneider says. At that point, when your body does expel the toxins, they are going to come out the other end. This will typically begin six to 15 hours after you eat the contaminated food and can last for up to 24 hours.

More serious illness

Fried rice syndrome rarely causes more serious illness beyond vomiting and diarrhea. In one 1993 outbreak reported in Morbidity and Mortality Weekly Report, the most common symptom was nausea, followed by abdominal pain and diarrhea.

But in very rare cases, if there is enough bacteria present, it can be deadly: As the Journal of Clinical Microbiology reported, in 2008, a young man died after eating cooked pasta that had been left at room temperature for five days and was later found to contain massive amounts of B. cereus.

Can reheating kill the bacteria that causes problems?

Unfortunately, no. The spores that B. cereus produces are extremely heat resistant, so no amount of reheating you do at home is going to get rid of them. If your fried rice has been left out at room temperature for a few hours, or if you’re not sure how long it’s been out, you shouldn’t reheat it in the microwave. It’s safest to toss it in the trash.

How common is fried rice syndrome?

According to the Centers for Disease Control and Prevention (CDC), there are an estimated 63,400 cases of fried rice syndrome per year in the United States. For comparison, there are about 265,000 E. coli infections every year. The vast majority of B. cereus infections are minor illnesses; of those 63,400 cases, the CDC estimates that B. cereus is responsible for only 20 hospitalizations each year.

What other foods can cause fried rice syndrome?

While rice is the food most commonly contaminated with high levels of B. cereus, there are a few other foods that can cause fried rice syndrome. These include other grains, pastas, beans, potatoes and some vegetables. Schneider says to be wary of foods that come from the earth and that we typically cook at a high temperature (using methods such as boiling) and then cool before eating.

Interestingly, one food you don’t need to worry about is sushi, despite the fact that sushi rice is typically kept out at room temperature all day. “With sushi rice, they add rice wine vinegar to reduce the pH of the rice and make it acidic, thus preventing B. cereus from growing,” Schneider explains.

How can you prevent fried rice syndrome?

Taking steps to cool and handle food properly will prevent fried rice syndrome. Here’s what you need to do.

  • Don’t leave leftovers out. “Most leftovers shouldn’t be left out for more than two hours,” says Schneider. “If it’s a really hot day, like a picnic, it’s one hour.” That said, while this is a good general practice, Schneider acknowledges that it’s highly unlikely that B. cereus will produce enough toxins to make you sick in just two hours. It needs at least six, and that’s only if there is a high concentration of B. cereus in the rice to begin with. But it’s always best to bring your leftovers from the car to the fridge as quickly as possible.
  • Refrigerate in shallow containers. This is especially key if you’ve cooked a large pot of rice or ordered enough from the restaurant to serve 10 or more people. “If we put rice in the fridge but not in a shallow container, it’s going to stay warm for hours,” Schneider says. “The fridge is not good at heat removal. The rice can stay warm overnight. So these toxins can potentially build up and germinate.” To avoid this, spread rice or other leftovers in food storage containers that are no deeper than 4 inches to help them cool down and leave the danger zone faster.
  • When in doubt, throw it out. If you’re not sure how long a container of food has been sitting out on the counter and it contains rice or one of the other foods mentioned above, it’s best to throw it away. This is especially true for people who may have a compromised immune system, such as older folks and pregnant people.

Now that you’re an expert on avoiding fried rice syndrome, here are the foods you should toss after the expiration date passes.

About the expert

  • Keith R. Schneider, PhD, is a food safety professor and researcher at the University of Florida. The primary focus of his research is food safety and, in particular, produce safety and food defense.