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I have for a long period cooked raw rice directly with hot water in a porcelain bowl for 15 minutes in a microwave, totally open without a lid. The water does boil a little bit but not that severely, maybe because of the rice rich in starch which holds the water calm, so that the water doesn’t leak. It works perfectly despite the side which becomes a little bit dry. I usually resolve it by adding some water at the last 2 minutes, or removing the side.

While water boils only a little bit and doesn’t leak, milk does quite a lot, so water and milk must be different in some way.

I am concerned about whether the material, plastic, is problematic. Although disposable microwave-safe lunch box can bear up to 130℃, and a microwave can heat things maximally up to 100℃, any way, 15 minutes is apparently a quite long duration, so I am not sure if it would be harmful, e.g. releasing some chemicals, especially if it releases "secretly" that I cannot taste or smell it.

added:

After use I will discard it. One box'd be used only once.

I've just come up with "instant soup" found in supermarkets, which should be heated as liquid for quite a few minutes in a microwave, and the plastic is disposable. I am not sure if normal disposable microwave-safe lunch boxes sold in dozens online could be the same type.

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    Plastics will deform easily with heat, either with few uses or over time, may not be an issue if you plan to discard it. There is also the concern of chemicals leaking into the food (getting that plastic-y taste)
    – Duarte Farrajota Ramos
    Apr 13, 2020 at 15:20
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    How hot does the ceramic bowl get? Some ceramics absorb the microwaves themselves & heat up more than the contents. On the other hand, I've been using disposable plastic containers, some for over a decade, without seeing any degradation. if they taste 'plasticy' you're using the wrong plastic.
    – Tetsujin
    Apr 13, 2020 at 15:22
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    If it is a microwave safe plastic container, it should be OK to use it.
    – Max
    Apr 13, 2020 at 15:22
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    After your edit - please don't buy single-use plastics. The planet doesn't need any more of them. If you buy a microwaveable meal from a supermarket, the container can be re-used for a decade. It will go in the microwave, in the dishwasher, it won't melt, it won't stain, it won't degrade.
    – Tetsujin
    Apr 13, 2020 at 15:39
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    There are plastic microwave rice cookers for a few bucks that you can reuse over years. Cooking rice for two servings needs only about 12 minutes (depending on microwave). See amazon.co.uk/Sistema-Microwave-Rice-Cooker-2-6/dp/B00BTIVNT4 as just one example.
    – Johannes_B
    Apr 15, 2020 at 8:30

2 Answers 2

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microwave can heat things maximally up to 100℃

WATER! Microwave can heat WATER to 100℃. And the temp is boiling point. Water can be heated to much higher temperatures in microwave if it's clear. It's called "superheat" (please see famous "hot coffe in the McDonalds case not the urban legend).
Adding food to water rise the boiling point as you add molecules to water.
Milk already have "additional" molecules. But boiling it in microwave is different as it depends on type of milk you use and how it reacts with rice (fat can separate from water and clinge to rice rising overall temperature making the separated "water" to boil faster and more violent)

For cooking in microwave it's best to use glass or ceramic containers with silicone or hightemp reistant plastic as cover. And never allow food you're cooking to touch the cover.

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It very much depends on the type of plastic,the thickness, and the manufacturing process. I highly recommend that if it's not . It's best to just learn about the company and see if they are FDA/ISO/ASTM approved or just some cheap Chinese company (nothing against Chinese things, however, there are good things made there too).

Here's a simple test that can be done: Put the box in the microwave, put a mug with water in the box, and microwave it for 30 seconds. If the box is warm, it probably isn't microwave safe (ESPECIALLY if the water is not). If the box is cool and the water is hot, you should be good. I can't provide a proof that this will work 100% of the time, however.

But may I also urge you to switch to glass? It'll save you money, it'll spread less plastic on our planet, and it feels better too.

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  • I wouldn't just test it - it could be melamine.
    – rumtscho ♦
    Apr 15, 2020 at 7:11

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