The microwave oven is one of the great inventions of the 20th century - millions of homes in America have one. Microwave ovens are popular because they cook food incredibly quickly. They are also extremely efficient in their use of electricity because a microwave oven heats only the food - nothing else.
A microwave oven uses microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion - heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.
How Microwave Ovens Cook Food
You often hear that microwave ovens cook food "From the inside out." What does that mean? Here's an explanation to help make sense of microwave cooking.
Let's say you want to bake a cake in a conventional oven. Normally you would bake a cake at 350 degrees F or so,
but let's say you accidentally set the oven at 600 degrees
instead of 350. What is going to happen is that the outside of the cake will burn before
the inside even gets warm. In a conventional oven, the heat has to migrate (by conduction) from the
outside of the food toward the middle (See the HSW article entitled How a Thermos Works
for a good explanation of conduction and other heat transfer processes). You also have dry, hot air on the outside of the food
evaporating moisture. So the outside
can be crispy and brown (e.g. - bread forms a crust) while the inside is moist.
In microwave cooking, the radio waves penetrate the food and excite water and
fat molecules pretty much evenly throughout the food. There is no "heat
having to migrate toward the interior by conduction". There is heat
everywhere all at once because the molecules are all excited together. There
are limits of course. Radio waves 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 whole heating process is different because you are "exciting atoms" rather than "conducting heat".
In a microwave oven, the air in the oven is at room temperture, so there is no way to form a crust. That is why foods like
come with a little cardboard/foil sleeve. You put the food in the sleeve and then microwave it.
The sleeve reacts to microwave evergy by becoming very hot. This exterior heat lets the crust become crispy as it would
in a conventional oven.
Microwave ovens are described by several interesting links on the web. Try these:
- For an excellent discussion of the different parts of a microwave oven and how they work together, click here.
- For a great collection of in-depth questions and answers about microwaves, click here.