They’re convenient, ubiquitous and easy to use. But how do these things work, exactly? What do people mean when they say a microwave cooks food inside out?
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Love ‘em or hate ‘em, there’s no denying: Microwave ovens are super convenient. They can heat food much more quickly than a conventional oven – although not always with the same results.
But how do they… you know. Work?
Yeah, yeah. Microwaves. Sure. But what the heck are those? Good question. Microwaves are a type of wave on the electromagnetic spectrum, sandwiched between radio waves and infrared radiation. In the case of microwave ovens, the most common wave frequency is roughly 2,450 megahertz (2.45 gigahertz).
Waves in this frequency range have an interesting property: They're absorbed by water, fats and sugars. Once absorbed, they're converted directly into atomic motion -- heat.
These waves have another convenient property: They're not absorbed by most plastics, glass or ceramics.
Metal, however, does reflect microwaves, which is why it’s a bad idea to leave a spoon in your cheese dip when the oven’s on.
You’ll often hear people say microwave ovens cook “from the inside out”, right? That’s the key to the speed. Think about it like this: Let’s say you’re baking a cake in a conventional oven. Normally you would bake it at 350 degrees F (177 degrees C) or so, but this time you accidentally set the oven at 600 degrees F (316 degrees C). The outside of the cake will burn before the inside even gets warm, and you’ll have ruined somebody’s birthday.
In a conventional oven, the heat has to migrate by conduction from the outside of the food toward the middle. Hot, dry air on the outside evaporates moisture, so the outside can be crispy and brown – like the crust on bread - while the inside is moist.
In microwave cooking, the radio waves penetrate the food and excite water and fat molecules more or less evenly throughout. No heat has to migrate toward the interior by conduction. There's heat everywhere, all at once, because the molecules are all excited together.
There are limits, though. Microwaves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The heating process is different because you are "exciting atoms" rather than "conducting heat."
Inside a microwave oven, the air is at room temperature, so there's no way to form a crust. That’s why microwavable pastries or hot pockets sometimes come with a little sleeve made out of foil and cardboard. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.
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