Where should I keep potatoes and onions so they last? Can I eat a potato that has sprouted?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Potatoes and onions are staples of the Japanese kitchen, but the same failures come up again and again: "it had sprouted before I noticed", "it had turned green", "the onion had gone soft from the inside". The sprouts and green parts of a potato contain a natural toxin, and mass food poisoning has occurred in Japanese school gardens. Here we sort out where to store each vegetable and where the line between "edible" and "throw it away" lies.
Storage guide
| Vegetable | Where | Rough keeping time | Notes |
|---|---|---|---|
| Potatoes | Dark, cool place (around 10°C), in a paper bag or cardboard box to keep out light. In summer, wrap in newspaper and use the fridge's vegetable drawer | Room temperature (spring, autumn) 1–2 months; fridge vegetable drawer 2–3 weeks | Light turns them green and raises solanine levels. One apple in the box slows sprouting (ethylene) |
| Onions | Room temperature with good airflow. Hang them in a net or an old pair of tights, or lay them out so they do not touch | 1–2 months | Moisture makes them rot. Never leave them in the plastic bag. Wrap cut onion in cling film and refrigerate, 3–4 days |
| New-season onions (shin-tamanegi) | Fridge vegetable drawer, wrapped in newspaper | 1 week | High in water; spoil within days at room temperature |
| Sweet potatoes | 13–15°C, room temperature. Refrigeration causes chilling injury | 1 month | Sensitive to cold |
The potato's natural toxins (solanine and chaconine)
- Where they are found: in the sprouts and their bases, in skin that has turned green, and in small, immature potatoes. Exposure to light turns the skin green and increases the toxin. Damaged parts also contain more.
- Symptoms of poisoning: nausea, vomiting, stomach pain, diarrhoea and headache, a few minutes to a few hours after eating. Most cases are mild, but mass poisonings from potatoes grown and cooked in school gardens happen repeatedly in Japan (often from eating small, immature potatoes with the skin on).
- Cooking does not reduce it: solanine-type toxins break down very little when boiled or fried. "It is fine once cooked" does not apply.
- What to do: 1) gouge out sprouts deeply, down to the base; 2) peel green skin thickly; 3) do not eat small, immature potatoes or strongly green ones; 4) discard potatoes that have sprouted all over and gone soft.
- If it tastes bitter or acrid, do not eat it. Bitterness is linked to the amount of toxin.
Why potatoes do not belong in the fridge
- The Ministry of Agriculture, Forestry and Fisheries (MAFF) explains that at low temperatures (5°C or below) sugars (reducing sugars) increase, so potatoes burn more easily when fried or roasted, and more acrylamide (a harmful substance formed by heating) is produced. Bringing them back to room temperature for about a week reduces the sugars.
- In summer (room temperature above 25°C) the vegetable drawer is unavoidable. In that case, wrap in newspaper and loosely close a plastic bag, and use the potatoes for simmered or steamed dishes.
[Verify] Check the number of solanine-poisoning cases and the symptoms against MAFF and Ministry of Health, Labour and Welfare (MHLW) materials, and the acrylamide explanation against the relevant MAFF page.
Misunderstanding 1｜A green potato is fine once the sprouts are removed
The green skin also contains a lot of toxin and needs to be peeled thickly. The safe choice is to discard potatoes that are green all over, taste bitter, or are small and immature.
Misunderstanding 2｜Cooking destroys the toxin
Solanine-type toxins break down very little with heat. Removing them is the only countermeasure.
Misunderstanding 3｜Potatoes keep longest in the fridge
Cold increases the sugars, so fried and stir-fried potatoes burn more and produce more acrylamide. In spring and autumn, a dark place at room temperature; only in summer, the vegetable drawer, wrapped in newspaper.
Misunderstanding 4｜Onions can stay in the bag they came in
The moisture inside a plastic bag makes them rot. Take them out and hang them in a net or lay them in a basket. One rotten onion spreads to its neighbours.
Misunderstanding 5｜It is handy to keep potatoes and onions in one box
The moisture from the onions makes the potatoes spoil faster, and sprouting or rot on the potatoes spreads to the onions. Keep them in separate containers.
---
Sources
- Ministry of Agriculture, Forestry and Fisheries, "Preventing food poisoning from potatoes" [ジャガイモによる食中毒を予防するために]
https://www.maff.go.jp/j/syouan/seisaku/foodpoisoning/naturaltoxin/potato.html
- Ministry of Agriculture, Forestry and Fisheries, "Information on acrylamide in food: storing potatoes" [食品中のアクリルアミドに関する情報：ジャガイモの保存]
https://www.maff.go.jp/j/syouan/seisaku/acryl_amide/
- Ministry of Health, Labour and Welfare, "Risk profile of natural toxins: potatoes" [自然毒のリスクプロファイル：ジャガイモ]
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/shokuhin/syokuchu/poison/
⚠️ 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.
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.
Some ingredients need thorough cooking. Eat raw only foods that are sold for raw consumption. If you have allergies, check the ingredients.
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