Why Are Some Dishes Not Microwave Safe?

Posted by Eric Troy on 02 Mar 2018 18:48

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I mentioned in a previous article that microwaves pass through many of materials we use in them. Yet, if you are an experienced microwave user, you know that not all containers or dishes are microwave safe.

First, certain plastics, like foam containers, simply melt easily from the heat of the food. You don't have to be a magnetron scientist to know that foam is not a good cooking vessel, though. You also know that you can't put metal in a microwave (the reason for that deserves a separate post).

Many plastics, however, do fine in the microwave. Many ceramics do as well. They are not actually heated up by the microwave energy itself because the microwaves do not interact with the molecules in these materials. Still, many ceramics will eventually get hot from the food.

Yet, some other ceramics seem to get scorching hot from the microwave itself! You've probably tried to heat water in a coffee mug, only to burn your hand on the red-hot handle! Yet, in your Pyrex measuring cup, the handle doesn't get hot, at least not at first. I personally have a couple of mugs that are so not microwave safe that the mug gets "red hot" while the water is barely warm. Not good. So, some mugs are not really microwave safe. It seems that those microwaves are not passing through the ceramic of that mug but instead are interacting with it and causing it to get hot.

If you've read the previous article I linked above, which concerns the myth that microwaves cook food from the inside out, you know that microwaves heat foods by interacting with the water molecules in the food. There certainly isn't any water in ceramics to absorb microwave energy. However, there are some minerals that do absorb microwaves. If a ceramic has enough of these minerals, they will get very hot and so the container will get very hot. Since the microwaves are essentially only reaching the first few centimeters into a highly absorptive material, it is possible, in some cases, for the container to become hot enough to cause a serious skin burn before the water or food inside gets hot at all. Basically, the microwaves aren't even making it to the contents, they are being held up by the material of the container which becomes very hot while doing a very poor job of transferring any of this heat energy to the inside contents.

Crystals may not be good in the microwave for this same reason, and they also might break from the heat stress. Also, if you have a glazed cup or another ceramic container with cracked glazing, you may want to avoid using it in the microwave, even if it had always been fine before. Water from washing could have gotten into the ceramic through the cracks and this water will heat and turn to steam, possibly causing the container to break.

Microwaves pass through plastics, but of course, as before, some plastics will melt from the heat of the food.

So, a truly microwave safe dish is one that does not interact with the microwave energy and which is not damaged by the heat of the food. It is also important, of course, that the container itself, after being heated by conduction, does not leach chemicals into the food. This is a big concern with microwave-ready convenience foods, which are microwaved in their packaging.

If you're like me, before you use an unfamiliar vessel in a microwave, you check the bottom to see if it says 'microwave safe.' This, however, is not something you should rely on wholeheartedly. Carefully consider the next section below.

Beware of Microwave Safe Dishes!

There is something you need to know about dishes which claim to be microwave safe. That is, the fact that a dish is microwave safe does not automatically mean you won't get burned from it. Microwave safe means that the microwaves themselves will not be absorbed by the material of the dish. They will pass right through it. So, for example, in a microwave-safe mug, you can heat up your water for instant coffee or hot cocoa and the handle will probably stay cool long enough for you to grab it safely (although you need to be careful not to slosh yourself, and put in the instant coffee or cocoa slowly). If you leave the mug in long enough, the handle will eventually heat through simple conduction from the material it is attached to. And, and the same goes for the handle of that Pyrex measuring cup, which I consider the champ of the microwave.

However, if you touch the sides of this microwave safe coffee mug, instead of the handle, it will be hot. Perhaps very hot. You know better than to grasp it by the sides because the hot water has heated the sides. Well, this same thing goes for any microwave safe dish. It can still get hot from the food inside. The better the material is at absorbing and retaining heat in general, the quicker it will heat up. Any large ceramic dish filled with food, even if it has handles and is microwave safe, will need to be handled carefully with pot holders, as by the time the food is cooked, the dish, and probably the handles as well will be hot (through heat conduction).

So, the basic warning is: Microwave-safe dishes can still burn you.

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