What are the common misunderstandings about food poisoning from retained-heat cooking and stews?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
"Retained-heat cooking" (保温調理, hoon chōri), turning off the flame and letting the pot finish cooking in its own heat, spread in Japan as an energy-saving idea. Leaving curry or stew in the pot at room temperature and eating it the next day, however, is the classic route to Clostridium perfringens food poisoning, and the Ministry of Health, Labour and Welfare (MHLW) warns about it repeatedly.
The numbers
| Item | Details | Source |
|---|---|---|
| Clostridium perfringens food poisoning | 20–30 outbreaks a year, 1,000–2,000 patients. Many patients per outbreak (catering, mass cooking) | MHLW food poisoning statistics |
| Foods involved | Curry, stew, simmered dishes, soup: made in bulk and left standing | Same |
| Growth temperature | About 12–50°C, fastest at 43–47°C. Multiplies at the bottom of the pot, where there is little oxygen | Same |
| Spores | Survive several hours at 100°C. Germinate and multiply as the food cools | Same |
| Countermeasures | Divide into small portions and cool quickly (to 20°C or below within 2 hours), then refrigerate. Reheat to 75°C or above, stirring the whole pot | Same |
- Retained-heat cooking (turning off the flame and leaving the lid on, or using a thermal cooker) works as an energy-saving method if the food is eaten within a few hours. The problem is "leaving it as it is until tomorrow".
- A thermal cooker keeps the food warm, which means it stays for a long time in the temperature range where bacteria multiply. If it will be hours before you eat, cool it and refrigerate.
[Verify] Check the outbreak numbers and temperatures against the MHLW's material on Clostridium perfringens and the latest year of the food poisoning statistics.
Misunderstanding 1｜A stew can be eaten the next day without refrigeration
Curry left at room temperature is the classic case of Clostridium perfringens poisoning. Finish it the same day, or divide it into portions and refrigerate.
Misunderstanding 2｜Boiling it the next day makes it safe
Spores survive heating, and some of the bacteria that have multiplied can survive reheating. Reheating is necessary, but not leaving it standing comes first.
Misunderstanding 3｜Putting the whole pot in the fridge is enough
The centre of a large pot takes hours to cool, and the bacteria multiply in the meantime. Divide it into shallow containers, or stand the pot in water to cool it.
Misunderstanding 4｜Retained-heat cooking is safe for any number of hours
Inside a thermal cooker, the food stays for a long time in the bacteria's growth range. Eat within a few hours, or refrigerate. Making it in the morning and keeping it warm until evening is not recommended.
Misunderstanding 5｜Clostridium perfringens does not happen at home
Catering and restaurants dominate the statistics because each outbreak affects many people. At home, "stomach ache after eating yesterday's curry" is a common case; severe illness is rare but does occur.
---
Sources
- Ministry of Health, Labour and Welfare, "About Clostridium perfringens food poisoning" [ウェルシュ菌食中毒について]
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/shokuhin/syokuchu/
- Ministry of Health, Labour and Welfare, "Food poisoning statistics" [食中毒統計資料]
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/shokuhin/syokuchu/04.html
- Ministry of Agriculture, Forestry and Fisheries, "Take care with leftover curry (Clostridium perfringens)" [カレーの作り置きに注意（ウェルシュ菌）]
https://www.maff.go.jp/j/syouan/seisaku/foodpoisoning/
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
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