What are the common misunderstandings about choosing food storage containers?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
With the spread of batch cooking (tsukurioki, 作り置き), storage containers have become a fixture of the Japanese kitchen. "Is plastic dangerous?" and "Can glass take anything?" are questions asked in every country. Here we set out how Japan regulates food utensils and containers, and what each material is good and bad at.
The rules, and how the materials compare
- In June 2020, an amendment to the Food Sanitation Act introduced a positive-list system for synthetic-resin utensils and food packaging. The state defines which substances may be used, and nothing else is allowed (the transition period ended at the end of May 2025).
- Containers carry labels for heat tolerance, cold tolerance, microwave use and dishwasher use (some fall under the Household Goods Quality Labelling Act). The basic rule is do not exceed the temperature on the label.
| Material | Good at | Bad at | Guide |
|---|---|---|---|
| Polypropylene (PP) | Light, does not break, cheap | Picks up grease, colours and smells; scratches | Heat tolerance about 120–140°C; usually labelled microwave-safe |
| Heat-resistant glass | No smell or colour transfer, oven-safe, you can see the contents | Heavy, breaks, sensitive to sudden cooling or heating | Tolerates a temperature difference of about 120°C |
| Enamel (hōrō, ホーロー) | Stands up to acid and salt, can go on a direct flame, cools quickly | Not for the microwave; rusts if chipped | — |
| Stainless steel | Tough, cools quickly, no smell transfer | Not for the microwave; you cannot see the contents | — |
| Silicone | Folds flat, freezer- and microwave-safe | Holds on to smells | Heat tolerance about 200°C |
[Verify] Check the start date of the positive-list system and the end of the transition period with the Ministry of Health, Labour and Welfare and the Consumer Affairs Agency; check heat tolerances on each product's label.
Misunderstanding 1｜Plastic containers are dangerous in the microwave
The starting point is a container labelled "microwave-safe," used within the temperature on the label. Oily foods can exceed 140°C, which can warp even a PP container. Removing or loosening the lid prevents steam from warping or bursting it.
Misunderstanding 2｜Glass never breaks, whatever you do
Even heat-resistant glass can break from sudden cooling or heating (freezer straight into the oven; a hot container set on a wet worktop). The basic sequence is freezer → let it come partly back to room temperature → heat.
Misunderstanding 3｜The lid has the same heat tolerance as the container
Lids are often polyethylene or another material with a lower heat tolerance. For most products the body is microwave-safe but the lid is not. Check the two labels separately.
Misunderstanding 4｜A scratched container is fine once washed
Fine scratches are where dirt and bacteria linger. If colours or smells will not come out, scratches are obvious, or the lid no longer closes, it is time to replace it.
Misunderstanding 5｜If it is sealed, it keeps for any number of days
A seal only prevents drying out and smell transfer; what stops food going off is temperature. The usual guide for batch-cooked dishes is 2–3 days in the fridge, 2–3 weeks in the freezer (see the batch-cooking article). Writing the date on the container is standard practice among people living in Japan.
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Sources
- Ministry of Health, Labour and Welfare, "Positive-list system for food utensils and containers/packaging" [食品用器具・容器包装のポジティブリスト制度]
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/kenkou_iryou/shokuhin/kigu/index_00001.html
- Consumer Affairs Agency, "Household Goods Quality Labelling Act — Quality labelling rules for miscellaneous industrial goods (synthetic-resin products)" [家庭用品品質表示法 雑貨工業品品質表示規程（合成樹脂加工品）]
https://www.caa.go.jp/policies/policy/representation/household_goods/guide/zakka/
- Tokyo Metropolitan Bureau of Public Health, "Food Hygiene Window — Utensils and containers/packaging" [食品衛生の窓 器具・容器包装]
https://www.hokeniryo.metro.tokyo.lg.jp/shokuhin/
⚠️ 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.
Sources
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