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Would the cooking time change if I microwave some food in a porcelain or stoneware container rather than plastic?

More details (if you have time to waste)

I generally do not like to put plastic in the microwave. I had some accidents in the past where the plastic softened or even melted. In those cases I had to throw the food away because honestly the idea of accidentally ingesting plastic scares me. And this happened even when plastic containers were claimed to be microwave safe.

Therefore, recently I decided to move food from plastic "microwaveable" packages to my own microwaveable porcelain and stoneware containers (dishes or mugs with lids).

Would this mean that I have to cook the food for longer time or shorter time, compared to the cooking time suggested on the package?

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    It depends entirely on the structure of the alternative - if the bowl heats up before the contents, then you are dealing with shielding & most 'simple' calculations go right out of the window. Instead invest in higher temperature plastics… These are given away free with instant microwaveable meals & if looked after will last for decades. – Tetsujin Dec 11 '19 at 20:29
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  • @Tetsujin thanks and two questions: (1) would the cooking time still change if I use these alternative plastics, since it's still not the original package? (2) Is there a way to know for sure that these higher temperature plastics never release invisible toxic substances (like dioxin)? The packages you find at the supermarket also claim to be safe for the microwave, but they still soften and melt, so I don't know how much I can trust these claims in general. – Kubuntuer82 Dec 17 '19 at 13:59
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    The things they sell microwave meals in are to all intents & purposes 'microwave invisible'. They don't slow the process at all. They're made of HPET if you want to do some health research, but practically I've had some of these things maybe 20 years. They don't really soften with heat & even hot fat doesn't scar them. I don't buy much microwave food, so I don't pick them up often & then reuse them until I manage to eventually break them. They're great for a can of beans, or leftovers re-heated for tomorrow's lunch etc, or as plate covers if someone gets in late & has to mike their dinner... – Tetsujin Dec 17 '19 at 14:10
  • PS I think that overall the info you are providing can be considered as an answer :) You may consider copying and pasting it in an actual answer... – Kubuntuer82 Dec 17 '19 at 14:52
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Yes, it's likely to change the time. There is no way to predict how it will change it, though, since it is a combination of the material, mass and shape of the vessel. So you'll have to test it for each vessel you use.

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For a properly microwave-suitable dish, you're likely to need to add a small but almost constant time. This is because in such a dish the food heats up, but transfers some of its heat to the dish (much more than to a plastic dish). I would expect to need up to about a minute extra on reheating a dish of 1-2 servings, to take into account the thermal mass of the container, but this is a guess because I haven't tested your dishes.

Some ceramics don't work well in the microwave: they absorb the energy and heat up, before transferring the heat to the food. It's not a good idea to use these; they can get extremely hot while failing to cook your food well. They're not very common IME.

You can tell the difference by placing the dish you want to test in the microwave next to another dish of cold water, and heating for a minute or two. The dish being tested should stay cold while the water warms up.

In all cases, and even if you use plastic containers, you need to check the food is actually hot through (or develop your own reliable methods to ensure it does get hot right through). The distribution of heating is very variable.

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    & rather irritatingly, even crockery from the same set isn't the same. We have soup-, side- & dinner-plates & cereal bowls from the same set, all ostensibly 'porcelain'. All the plates are microwave invisible, the cereal bowls get red hot without the contents heating much at all. – Tetsujin Dec 17 '19 at 14:15
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    @Tetsujin more irritating than surprising. I've had supposedly identical sets of crockery bought from the same shop at the same time that didn't even look or weigh the same (both were matched the photos on the box well enough, but the difference was much more than different shades of glaze). Mass supply will do that – Chris H Dec 17 '19 at 14:34
  • I like your answer, especially the part about the test to see how much heat is absorbed by the container rather than passing through... – Kubuntuer82 Dec 17 '19 at 14:56
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Posting comments as answer…

It depends entirely on the structure of the alternative - if the bowl heats up before the contents, then you are dealing with shielding & most 'simple' calculations go right out of the window. Instead invest in higher temperature plastics… These are given away free with supermarket instant microwaveable meals & if looked after will last for decades.

The things they sell microwave meals in are to all intents & purposes 'microwave invisible'. They don't slow the process at all. They're made of HPET[1] if you want to do some health research, but practically I've had some of these things maybe 20 years. They don't really soften with heat & even hot fat doesn't scar them[2].
I don't buy much microwave food, so I don't pick them up often & then reuse them until I manage to eventually break them. They're great for a can of beans, or leftovers re-heated for tomorrow's lunch etc, or as plate covers if someone gets in late & has to mike their dinner…

& rather irritatingly, even crockery from the same set isn't the same. We have soup-, side- & dinner-plates and cereal bowls from the same set, all ostensibly 'porcelain'. All the plates are microwave invisible, the cereal bowls get red hot without the contents heating much at all.

[1]I once did a fair bit of research on this, when I was trying to find replacements for old, cracked plastic-ware without having to buy the bloody awful meals they contain, but my research was a dead-end. I had to eat the damn stuff to get the dishes, or buy them by the pallet in 10,000s.
Here's a beginner's guide to plastic types, by 'embossed symbol'. My newer ones class themselves as 'PET PP 05' [& no, I don't know what that really means.]
Hunker - What Are the Numbers That Say That Plastic Is Safe to Use in the Microwave Oven?

[2]Conversely, some of my expensive 'tupperware' [not by brand, just by type of 'permanent long term freezer to microwave or keep food in the fridge] type dishes & bowls are permanently scarred & discoloured if there is any kind of oil or fat on the food being microwaved.

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