Value Village ／ Food & living
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"Curry tastes better on day two" is common knowledge in Japanese homes, but the habit of "leaving the pot on the hob overnight" draws food-safety warnings almost every year. The culprit is Clostridium perfringens (ウェルシュ菌, Welshu-kin in Japanese). It is not killed by cooking, and it multiplies while a big pot cools slowly. Here we set out a storage routine that gives you both the flavour and the safety.
| Item | Details |
|---|---|
| What it is | Clostridium perfringens (Welshu-kin). Widespread in soil, water and the gut of humans and animals; arrives on meat, fish and vegetables |
| Why cooking does not kill it | It forms spores (heat-resistant shells) that survive several hours at 100°C. As the pot cools after cooking, the spores germinate and multiply |
| Growth temperature | 12–50°C. Optimum 43–47°C, dividing about once every 10 minutes (millions-fold overnight) |
| Oxygen | Dislikes oxygen (anaerobic). Oxygen does not reach the bottom or centre of a thick, large pot: ideal conditions |
| Symptoms | Stomach pain and diarrhoea 6–18 hours after eating. Fever and vomiting are uncommon; most people recover in 1–2 days, but watch for dehydration in older people and children |
| Statistics | Around 20–30 outbreaks a year in Japan, with 1,000–2,000 patients. Many patients per outbreak (large-pot cooking in school meals, catering and events). Nicknamed the "school-lunch disease" (kyūshoku-byō) |
1. Within 2 hours of cooking, take the heat off and refrigerate. "Taking the heat off" is not leaving it out; it is rapid chilling: - Divide into small portions (spread thinly in shallow containers or freezer bags) → stand the pot in iced water, or put ice packs around it → stir to let air in as it cools. - Target: below 20°C within 2 hours, below 10°C within 4 hours. 2. Fridge (10°C or below): 2–3 days. Freezer (−18°C or below): 1 month. 3. Reheat before eating: transfer to a pan, stir well and bring to the boil right through (75°C or above for at least 1 minute). Microwaves heat unevenly, so stir partway through and reheat again. 4. Heat only what you will eat. The more times it is reheated, the more chance the bacteria get.
[Verify] Check the division rate of C. perfringens and the food-poisoning statistics against MHLW and Tokyo Metropolitan Government materials, and the cooling targets (below 20°C within 2 hours) against the MHLW "Hygiene Management Manual for Large-Scale Cooking Facilities".
The spores of C. perfringens survive 100°C. Other bacteria die with heat; this one multiplies while the pot cools. How you cool it is what counts.
The longer the pot spends between 12 and 50°C, the more the bacteria multiply. Small portions, iced water and stirring: below 20°C within 2 hours.
Heat reduces the bacterial count, but bacteria that have multiplied in bulk, and their spores, remain. Rapid chilling the night before is the precondition; reheating means "stir and bring to the boil".
The migration of umami and the change in thickness happen in the fridge too. There is no need at all to rest it at room temperature.
Even refrigerated, 2–3 days is the guide. Beyond that, freeze. Bacteria multiply slowly in the fridge too.
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https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/shokuhin/syokuchu/
https://www.hokeniryo.metro.tokyo.lg.jp/shokuhin/micro/
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.
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.
This is general information. Results vary from person to person and depend on your own conditions and surroundings.
Please decide for yourself, using your own judgement, whether any of this suits your situation. We cannot accept responsibility for any damage, loss or injury arising from following this article.
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