I would like to know why it is not recommended to put an egg in the microwave. Where does this "legend" comes from ?
This probably refers to whole eggs mostly...
A microwave can boil water very rapidly, and a tight but fragile container like a whole egg will violently rupture if such rapid boiling happens inside it, because the overpressure inside it is already significant when the shell finally breaks.
Here is a video showing an experiment with ca. 180 eggs in a microwave oven, which after a minute or two rupture simultaneously and with enough force to blow the oven open, with debris being propelled several meters.
As someone who tried this out at a young age, I thoroughly DO recommend it! The microwave makes a huge bang and jumps 2 inches off the bench. Good times! (However, you need to do some heavy duty cleaning before mum gets home.)
However, by breaking another rule, you can boil an egg in a microwave.
- 2/3 fill a coffee cup with boiling water.
- Wrap an egg in aluminium foil.
- Place wrapped egg in cup. Ensure the water covers all the aluminium foil.
- Microwave for 4 - 5 minutes.
There may be some overflow from boiled water in the microwave if your coffee cup isn't big enough. A paper towel under the cup will take care of that.
The foil prevents the microwaves exploding the egg, however the water surrounding the foil prevents the arcing that normally happens with metal in the microwave.
A great snack for office workers.
A simple undamaged raw egg in a microwave at full power will explode. Unless you have a deep desire to thoroughly scrub your microwave I can not recommend that approach.
The problem is that the egg will be heated to more than 100C, that means the water will start to vaporize and steam has the nasty habit of needing way more room than liquid, and you get a pressurized egg or an "egg-bomb". An egg's shell is a quite robust construction, especially where pressure from the outer side is concerned. Nevertheless, at one point the shell will burst and the egg "explodes".
So how to prevent this?
Avoid steam. If you heat the egg very, very gently, i.e. in short bursts witl low energy, you might keep the interior pressure low enough.
Frankly, if I have a pot and a stove, I'd go that route any time, especially as I can limit the inner temperature better. As boiling water doesn't exceed 100C, the egg will always stay below that, meaning no steam is created.
Avoid the build-up of pressure. If you crack the shell and transfer the egg to a bowl, you can easily boil it or make microwave scrambeled eggs etc.
Delia Smith recommends using a pin to prick a small hole in the wide end of the egg, where the air bubble is. That way the steam escapes from the shell avoiding a pressure build up. This has always worked for me when water boiling an egg and the shell never cracks.
This might work in the microwave too. It would be an interesting experiment to match the hole size to the rate of steam production, to the power output of the oven. Is that over sciencing the egg making process?
In addition to the excellent answers you already got, there are a few more issues:
Microwaving cooks inside out. As a result, the yolk will cook first (or at least at the same time as the white).
Some late-night-TV gadgets promise to let you cook omlettes in the microwave. In reality, the eggs will still explode, even after scrambling, but the mess is contained in the gadget. The other issue is that you end up with bits of egg superglued to the plastic.
There is a way to cook eggs in a microwave that has worked safely for me: poaching. Fill a bowl with water, and crack a couple eggs into the water. Cook for three to five minutes. Remove with a slotted spoon. The eggs will not be as nice as stove-poached eggs, because the yolk will be hard-boiled, but other than that, the eggs always turn out nice.
The real problem with the egg occurs after it is already hard-boiled.
When you buy a package of hard-boiled eggs from the refrigerated section in some grocery stores, those come with a warning "do not microwave". That is a mstake you only make once...
When you heat a hard-boild egg in the microwave, it will turn out nice and hot - and then explode in your face and burn your lip when you bite into it.
Cutting the egg in half does not help - the yolk will explode and jump right out of the half egg and evenly distribute in the microwave.
I don't know exactly why all this happens. My best guess is that there are two separate processes at work. First, the yolk gets heated faster than the white. It tries to expand, but instead builds up pressure. Secondly, even in a hard-boiled egg, there probably is enough residual water to create quite a bit of steam. Worse: the boiled egg white, as well as the egg yolk are fairly gas tight, so the steam is trapped in lots of pockets in the yolk.