Leftover Rice and Pasta: How Bacillus cereus Works and How to Store Them Safely

Leftover rice has a strange reputation: some people bin it on principle, others keep it in the pan for two days and fry it. The risk is real but narrow. As the UK Food Standards Agency puts it, the problem is not reheating; it is how the rice was stored before it was reheated (FSA). Rice that was cooled quickly, kept in the fridge for no more than a day and reheated once until steaming hot is fine, fried rice included.

What is happening

The spores come with the grain. Uncooked rice can carry spores of Bacillus cereus, and those spores survive cooking (FSA, FDA). Boiling kills most competing microbes and leaves the spores in warm, moist, starchy food, which suits them very well.

The growth window is the kitchen. B. cereus grows between about 4°C and 48°C, fastest at 28–35°C (FDA). A pan on the stove, a switched-off rice cooker or a bowl on the counter sit right in that range for hours.

Two different illnesses.

• The vomiting type. The toxin, cereulide, is made in the food before you eat it. It is linked mostly to rice and other starchy foods — pasta, potatoes, cheese products (FDA). Nausea and vomiting start 30 minutes to 6 hours after eating (FDA).
• The diarrhoeal type. The toxin is made in the gut after eating. Many foods can cause it; symptoms start 6 to 15 hours later (FDA).

Both usually settle within about a day (FDA). Rarely, the vomiting toxin has been linked to liver failure and death, even in previously healthy people (FDA).

Why reheating does not help. Cereulide is still active after 30 minutes at 121°C (FDA). A microwave or frying pan will kill the bacteria and leave the toxin untouched. Once it has formed, no amount of heat takes it out, which is why the rules for rice are about time, not temperature.

What to do

The UK Food Standards Agency publishes the most specific rules for rice; they are the ones below.

• Cool it within an hour. Spread it on a tray or divide it into small portions; standing the container in cold water helps. Do not leave it to cool in the rice cooker, steamer or pan (FSA).
• Fridge, then eat within 24 hours. Keep rice in the fridge for no more than one day before reheating (FSA).
• Cold is fine if it was cooled quickly. Rice salad and cold rice are safe when the rice went into the fridge fast (FSA).
• Reheat once, steaming hot all the way through (FSA). Take out only the portion you will eat.
• For longer, freeze it within an hour of cooking. Defrost in the fridge, reheat until steaming hot and eat straight away (FSA).
• Takeaway rice: eat it soon. Businesses may cook rice in advance and reheat it before it reaches you, so another day in your fridge is a bigger gamble (FSA).
• Fried rice is fine when done right. Day-old rice works because it is cold and dry, provided it spent the night in the fridge and not on the stove: see fried rice from almost anything.
• Pasta and potatoes follow the same logic. The FDA names them alongside rice; the FSA's numbers are for rice only. Cool them fast and use the general limits in how to reheat leftovers.

When it is different

• Rice left out overnight has to go, whatever it smells like: see food left out overnight.
• If you have already eaten it and are vomiting, most B. cereus illness passes within about a day (FDA). Sip fluids little and often: dehydration and oral rehydration. Vomiting within hours of a meal is not always rice — Staphylococcus aureus toxins give a similar picture 1–7 hours after eating (FDA).
• See a doctor (CDC, US) with blood in the stool, diarrhoea lasting more than three days, a fever above 39°C (102°F), vomiting so frequent that you cannot keep fluids down, or signs of dehydration: passing little urine, a dry mouth and throat, dizziness on standing.
• Babies and young children, older people and anyone with a weakened immune system: serious consequences are more likely with a weak immune system (FDA), so seek help earlier.
• Pregnant and feverish? Fever with flu-like symptoms such as tiredness and muscle aches is a reason to contact a doctor even without vomiting or diarrhoea: it can be listeriosis, which is dangerous for the baby even when the mother feels only mildly ill (CDC).

Sources

• Food Standards Agency — Home food fact checker (rice)
• FDA — Bad Bug Book, 2nd edition (Bacillus cereus)
• CDC — Symptoms of food poisoning
• CDC — Symptoms of Listeria infection

Checked: 23 September 2026