Why Reheating Rice Cannot Undo Unsafe Cooling

Rice that sat out for hours and was then reheated until steaming can still make people sick. Heat kills germs, so many people expect a thorough reheat to make old rice safe again.

The risk is set before reheating starts. Raw rice carries Bacillus cereus, a soil bacterium whose dormant spores (tough, resting cells) survive cooking. If the cooked rice then stays warm, the spores grow into active bacteria that release a toxin. Heat does not break that toxin down. Reheating kills the living bacteria but leaves the toxin in the food, so the way the rice was cooled sets the risk.

Spores Survive the Cooking Pot

A spore is the bacterium’s dormant form.

Photo by Suki Lee on Pexels

It has a protective coat and almost no active metabolism, which lets it outlast conditions that kill growing cells. A 1974 study of boiled rice found that B. cereus spores survived cooking and could later germinate and grow in the rice1. A 2023 study also observed growth from spores that had survived heating.

Correctly cooked rice is therefore not sterile. Cooking kills the active bacteria but leaves a small population of spores. Those spores do no harm until conditions let them wake up.

The Danger Zone Clock

Cooked rice is moist and full of starch, which suits the spores once they germinate. What follows depends on temperature and time.

1. Germination. As the rice cools into a warm range, spores turn back into active cells. The 2023 study saw growth after a minimum of 6 hours at 25 to 30 degrees Celsius2.
2. Multiplication. B. cereus grows across a reported range of 4 to 48 degrees Celsius, fastest at 28 to 35 degrees Celsius3. The 1974 rice study put the optimum at 30 to 37 degrees Celsius.
3. Toxin production. The bacteria make cereulide, the toxin behind the vomiting form of the illness. Production is reported between 10 and 40 degrees Celsius, peaking at 20 to 25 degrees Celsius. That peak sits close to ordinary room temperature.

The numbers climb fast. In one cooked-rice study, counts rose from 3.4 to more than 7 log CFU/g within 12 hours at 25 to 45 degrees Celsius4. A log CFU/g is a base-10 count of live colonies per gram, so a jump of about 4 logs means roughly ten thousand times more bacteria. At 15 degrees Celsius, counts still went above 6 log CFU/g within 24 hours. A separate experiment found spore growth in rice held at 25 degrees Celsius for 12 hours5.

A deep pot also cools slowly in the centre. The middle can sit in these ranges long after the surface feels cool.

Toxins Outlast the Reheating

Bacteria and toxin are separate hazards. Heat kills the organism. Cereulide is a chemical the bacteria have already made, and it does not need them alive to cause harm.

Cereulide is heat-stable. A study in FEMS Microbiology Letters describes it as forming during growth in contaminated food and surviving cooking6. A Food Safety Authority of Ireland factsheet reports that it can survive 126 degrees Celsius for 90 minutes7, which is hotter and longer than any home reheat. New Zealand’s Ministry for Primary Industries states the point directly:

“Further cooking or reheating doesn’t destroy the toxin.” (Ministry for Primary Industries, New Zealand)

Steaming-hot rice shows only that the bacteria are dead now. It says nothing about toxin made during the hours before.

What the Record Shows

The evidence supports three links:

• Spores survive cooking: laboratory studies of boiled rice, from 1974 and 2023.

• Warm storage drives growth: controlled cooked-rice experiments measuring counts against time and temperature.

• The toxin resists heat: a peer-reviewed study and two government food-safety agencies.

The limits matter. These are laboratory measurements, mostly of inoculated or sampled rice, not tracked household meals. The collected sources do not state how much cereulide causes illness, and growth rates varied between studies. The temperature ranges are reported ranges, not exact cutoffs for every batch of rice.

Time Spent Warm Sets the Risk

Growth and toxin production both depend on time spent warm. The studies show slower growth at lower temperatures: counts passed 7 log CFU/g within 12 hours at 25 to 45 degrees Celsius, while at 15 degrees Celsius they passed 6 log CFU/g within 24 hours4. Toxin that has already formed is a separate matter, because the sources show it persists through heating.

So the risk in leftover rice depends on its history: how many hours it spent between roughly 10 and 40 degrees Celsius. Reheating cannot rewrite that history. Spores survive the cooking pot, grow quickly in warm rice, and leave behind cereulide, a toxin that forms in the food and survives cooking. Its toxin can survive 126 degrees Celsius for 90 minutes, far beyond what a microwave or pan reaches.

Sources

1. Journal of Hygiene (1974), Bacillus cereus spore survival and growth in boiled rice
2. PMC (2023), study of growth from surviving Bacillus cereus spores
3. Ministry for Primary Industries, New Zealand, Bacillus cereus risk profile document
4. ScienceDirect, cooked-rice study of Bacillus cereus growth at different storage temperatures
5. Food Science and Technology (SciELO), cooked-rice storage experiment
6. FEMS Microbiology Letters (2005), study on cereulide production and heat stability
7. Food Safety Authority of Ireland, Bacillus cereus factsheet (2016)
8. Ministry for Primary Industries, New Zealand, Bacillus cereus bacteria in rice and starchy food