Value Village ／ Food & living
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Seaweed is an everyday part of eating in Japan, with a strong healthy image. But "eat lots of kombu every day and you will be healthier" is not true: too much iodine is a real issue, and the inorganic arsenic in hijiki has become widely known. Here are the basics of storage, and a guide to sensible amounts.
| Seaweed | Unopened | Opened | Notes |
|---|---|---|---|
| Dried wakame (cut wakame) | Room temperature, dark, about 1 year | Airtight container with desiccant, dark, room temperature. 3–6 months | Damp wakame loses flavour and can grow mould. Swells 8–12 times when rehydrated (one serving is 2–3 g dried) |
| Salted wakame (enzō wakame) | Refrigerated, several months | Refrigerate; rinse off the salt before use | Desalt in water for about 5 minutes; too long and it goes limp |
| Fresh wakame | Refrigerated, 2–3 days | Blanch (it turns bright green), then refrigerate 3–4 days or freeze 1 month | In season in spring (February–May) |
| Kombu (dried, for stock) | Room temperature, dark, 1 year or more (the flavour matures) | Airtight, away from moisture | The white powder on the surface is an umami compound (mannitol); do not wash it off |
| Hijiki (dried) | Room temperature, dark, 1 year | Airtight, away from moisture | Swells 8–10 times when rehydrated |
| Toasted nori (yaki-nori) | Room temperature, dark; goes soft within days of opening | Seal and freeze (let it return to room temperature before opening each time), or seal with a desiccant | Softened nori recovers if you pass it over a flame or warm it briefly in a frying pan |
| Item | Details |
|---|---|
| Recommended iodine intake | Adults 130 μg a day, tolerable upper limit 3,000 μg a day (Dietary Reference Intakes for Japanese, 2025 edition). For pregnant and breastfeeding women the upper limit is 2,000 μg |
| Iodine in kombu | Dried kombu: about 200,000 μg per 100 g. One tablespoon of kombu tsukudani (about 15 g) can contain several thousand μg. Even one cup (200 ml) of kombu stock: several hundred to around 1,000 μg |
| Wakame, nori, hijiki | One to two orders of magnitude less than kombu. Dried wakame: about 10,000 μg per 100 g; one sheet of toasted nori (3 g): about 60 μg |
| Effects of too much | Prolonged excess iodine can cause underactive thyroid or goitre. Japanese intakes are high to begin with (estimated average 1,000–3,000 μg a day), though populations used to seaweed are also reported to be less affected |
| Inorganic arsenic in hijiki | Hijiki contains more inorganic arsenic than other seaweeds. The UK Food Standards Agency (2004) advised against eating it. Japan's Food Safety Commission and MHLW assess that "at normal intakes (a few times a week, one small bowl at a time) the health concern is low". Soaking or boiling reduces inorganic arsenic by about 50–90% (MAFF survey); discard the soaking water |
[Verify] Check the recommended and upper-limit iodine figures against the "Dietary Reference Intakes for Japanese (2025 edition)", the iodine content of kombu and wakame against the Standard Tables of Food Composition, and the reduction rate of inorganic arsenic in hijiki against the MAFF survey material.
Kombu is very high in iodine; eaten in quantity every day it exceeds the upper limit. Keep to stock and a small side dish.
Soaking or boiling greatly reduces inorganic arsenic, and at normal Japanese intakes the concern is assessed as low. Discarding the soaking water and not eating it excessively often is enough.
It is an umami compound (mannitol). Washing it off weakens the stock. Mould shows as green or black spots and smells different.
Condensation each time it goes in and out of the fridge makes it soft. Seal it, freeze it, and let it return to room temperature before opening: that works.
This is a classic habit that leads to excess iodine, and effects on the thyroid have been pointed out. People with thyroid conditions and pregnant women in particular should avoid it.
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https://www.mhlw.go.jp/stf/newpage_44138.html
https://www.maff.go.jp/j/syouan/nouan/kome/k_as/
https://www.mext.go.jp/a_menu/syokuhinseibun/mext_01110.html
⚠️ 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.
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