At ten in the morning, the cook at an alpine chalet tasted a teaspoon of fried rice balls and vomited almost at once. He left the kitchen, lay down for two hours and recovered quickly. He did not connect the episode with the food, so at seven that evening he served the same rice balls to himself and the whole chalet team. About thirty minutes later everyone was vomiting, then came diarrhea. Eight hours after supper the 25-year-old cook went to a local hospital. From there he was moved to a university intensive care unit with acute liver failure.
Doctors at the Medical University of Graz described the case in 2021. The rice had been boiled three days earlier. Nobody forgot it on the counter or left it in a warm pan. It sat in a fridge that was running at 46-48°F (8-9°C).
Why rice specifically
The bacterium in question is Bacillus cereus. The UK Food Standards Agency puts it plainly: “Uncooked rice can contain spores of a bacterium called Bacillus cereus.” That means ordinary dry rice from the packet, with nothing visibly wrong with it.
A spore is a survival form, and boiling does not bother it. Gilbert and colleagues showed this back in 1974 while working through a series of food poisoning outbreaks linked to cooked rice, mostly from Chinese restaurants and takeout places. The spores survived cooking, then germinated and grew.
After that, temperature decides the outcome. The researchers stored boiled rice between 39 and 131°F (4 and 55°C) and found the fastest growth between 86 and 99°F (30 and 37°C). Growth also occurred at 59°F (15°C) and at 109°F (43°C). A pan cooling on the stove passes through that entire band, and the more rice it holds, the longer it stays there.
A toxin that shrugs off the frying pan
This is where intuition lets people down. We reheat food expecting to kill bacteria, and with live Bacillus cereus cells that works. The vomiting form of the illness, though, comes from a substance the cells have already made.
While the rice sat warm, the bacteria were building up cereulide, a small ring-shaped molecule. The Graz authors call it the most critical of the Bacillus cereus toxins and note that it is “notably resistant to heat and acid during cooking”. Reheating kills the living cells and leaves the finished toxin on the plate.
That is the core of the whole topic. The danger is created in the hours between the pan and the fridge, and reheating cannot undo them.
Two illnesses from one bacterium
Much of the confusion comes from the fact that Bacillus cereus causes two quite different kinds of food poisoning. Three protein toxins, known as Nhe, Hbl and CytK, are linked to the diarrheal form. Cereulide is responsible for the vomiting form, and that one is fast: at the chalet, everyone fell ill roughly half an hour after supper.
Some outbreaks involve both. In 2018, diners fell ill after a multi-course dinner at a restaurant in Canberra, and the patients split into two groups by incubation period and symptoms. Retained food samples showed 19,000 CFU/g of Bacillus cereus in the beef, rated unsatisfactory, and 50 CFU/g in the arancini, which are rice balls, rated satisfactory. Genome sequencing found diarrheal toxin genes. When questioned, the restaurant could not verify its heating and cooling procedures. The problem dish there was the beef, so the rule about hours spent warm applies to any cooked food.
There is also some reassurance. Emetic strains are uncommon: Belgian researchers point out that their prevalence in the environment has been shown to be very low. A Korean group profiled 1,082 Bacillus cereus strains from dried red peppers, rice and sunsik, a mix of powdered raw grains. Their conclusion: these foods were free of the emetic toxin, though not of enterotoxins. That is why most plates of yesterday’s rice end without incident.
How much toxin turns up in practice
Belgium’s Scientific Institute of Public Health measured cereulide in rice dishes collected from a range of restaurants and compared them with an outbreak. Put the Graz figure alongside and you get a scale.
- Ordinary restaurant rice dishes: low concentrations, around 4 ng/g.
- Share of samples containing cereulide on the day of sampling: 7.4%. After being held at 77°F (25°C), that rose to 12.9%.
- Rice leftovers from a family outbreak after takeout from an Asian restaurant: up to 13,200 ng/g.
- Rice balls from the alpine chalet: 37 µg/g, or 37,000 ng/g, among the highest concentrations recorded.
From an ordinary plate to one that ends in intensive care, the concentration climbs almost ten thousandfold. The temperature history of the dish accounts for that gap. The chalet rice balls also held 8 × 10⁶ CFU/g, eight million cells in every gram. The Belgian team adds a caution of its own: little is known yet about the possible chronic toxicity of cereulide, so vigilance matters even at low levels.
Why the cook fared worst
Five healthy adults ended up in the hospital after that supper, and only one of them came close to a liver transplant. The authors put the difference down to dose. The cook was the only person who ate the rice balls twice, a spoonful to taste in the morning and a full portion at night. His four colleagues, young women aged about 22, ate them only at supper. They had mild gastrointestinal symptoms and went home without lasting effects after two or three days.
The cook spent five days in intensive care. His doctors considered a liver transplant as a rescue option, but he did not fully meet the criteria, and his liver began to recover quickly. He was discharged three days after leaving the unit. The vomiting after that morning spoonful was a warning, and it came nine hours before supper.
What the 1974 paper already said
Gilbert’s advice reads as if it were written last week. After boiling, rice should either be kept hot, above 145°F (63°C), or cooled quickly and put in a refrigerator within two hours of cooking. Boiled or fried rice must not be stored warm, especially in the 59-122°F (15-50°C) range.
The paper had one more suggestion for restaurants: boil rice in smaller batches several times a day, so it waits less time before frying. At home that translates simply. Cook what you will eat, and cool the rest straight away. A shallow container with a thin layer of rice gives off heat faster than a deep bowl.
The fridge that was not cold enough
Back to the chalet. The rice there did go into the fridge, so on paper everything was done right. The real storage temperature was 46-48°F (8-9°C), and over three days that was enough for a record dose of toxin.
A mountain lodge is nothing special in this respect. Food Standards Scotland put temperature loggers into the fridges of 361 households. The mean came out at 41.3°F (5.17°C), and 55.4% of fridges ran above the recommended 32-41°F (0-5°C). Household averages ranged from 31.3°F (-0.39°C) to 51.7°F (10.94°C), and top shelves were warmer than bottom ones, 42.6°F (5.89°C) against 40.0°F (4.45°C).
Two practical points follow. Put a thermometer in your fridge and see what it actually reads. Keep the rice container inside the main compartment, away from the door: according to an EFSA review, the door shelf is usually the warmest spot in a fridge and the middle shelf the coldest. The same thermometer helps when you are deciding about refreezing.
The rules that actually work
The Food Standards Agency’s home guidance on rice runs as follows.
- “The longer cooked rice is left at room temperature, the more likely it is that the bacteria or toxins will make the rice unsafe to eat.”
- “If you have leftover rice you should chill it as quickly as possible, ideally within one hour.”
- “Keep rice in the fridge for no more than one day until reheating.”
- “When you reheat any rice always check the dish is steaming hot all the way through.”
- “You should never reheat rice more than once.”
The last rule looks like superstition until you view it through cereulide. Each cooling and reheating cycle is another pass through the warm range, and whatever toxin has formed stays put through the next reheat.
In practice: spread hot rice in a thin layer in a shallow container, let it cool, cover it and put it in the fridge, not in the door, within the hour. The next day, reheat a single portion until it steams and eat it.
Summary
You can reheat rice. What matters is the rice you are reheating.
Rice that went into the cold straight after cooking and has been there less than a day can be reheated until steaming and eaten. That is what the UK rules allow. Rice that stood overnight on the stove, or spent three days in a fridge at 48°F (9°C), is beyond what a frying pan can fix. The toxin is already there, and heat does not remove it. And if a dish made you vomit when you tasted it, do not serve it to anyone.
How few cells it takes to make you ill is covered in our piece on infectious dose. Why contact time with a surface matters less than people think is shown through the five-second rule. How to avoid spreading bacteria around the kitchen on boards and hands is in our piece on kitchen cross-contamination. How to read the dates on a pack, and what you can still do before a use-by date, is covered in our guide to use-by, best-before and sell-by dates. Another toxin that kitchen heat cannot break down is covered in our piece on green potatoes and solanine. The wider picture is in our food safety hub. The same spore logic, running over six months instead of overnight, is taken apart in our piece on botulism in home canning.
