MORE ON MICROWAVE OVENS - HOW THEY WORK

by Pam North

The microwave oven is one of the great inventions of the 20th century;
over 90% of American homes have one.  Their popularity comes from the
fact that they can cook food incredibly quickly, and are extremely
efficient in their use of electricity.  They have revolutionized
the cooking process.

Microwave ovens use various combinations of electrical circuits and
mechanical devices to produce and control the output of microwave
energy for heating and cooking.  Microwaves are radio waves, and the
ovens use a radio wave frequency of roughly 2,500 megahertz (2.5
gigahertz).  Radio waves of that frequency are absorbed by water, fats
and sugars, and when they are absorbed they are converted directly
into heat.  The microwaves also are not absorbed by most plastics,
ceramics and glass, so the containers used for cooking remain cool.
Metal reflects microwaves, which is why metal pans do not work well in
a microwave oven.

When the microwave oven is turned on, the electricity passes though
the magnetron (the tube that produces microwaves).  The microwaves are
then channeled down a metal tube (waveguide) and through a slowly
rotating fan (stirrer), which scatters them into the part of the oven
where the food is placed.  The oven's walls are made of metal, which
reflects microwaves the way that a mirror reflects light, so when the
microwaves hit the stirrer and are scattered into the food chamber,
they bounce off the metal walls and penetrate the food from every
direction.

Microwave ovens and conventional ovens cook differently.  In a
conventional oven, the heat has to migrate by conduction from the
outside of the food toward the middle.  The dry, hot air on the
outside of the food evaporates moisture, forming a crispy, brown
crust, while the inside remains moist.  In microwave cooking, the
radio waves penetrate about an inch into the food and excite the water
and fat molecules pretty evenly throughout the food (thick or
irregular pieces of food may heat unevenly, since the radio waves
don't make it all the way into the middle, and hot spots, caused by
wave interference, may also occur).  The air in the microwave oven is
at room temperature, so no crust can form (that's why some food
products come with cardboard/foil sleeves to allow the crust to become
crispy).  The higher temperatures (as high as 550 degrees) of
conventional ovens are needed to caramelize sugars and break down
proteins, carbohydrates and other substances and combine them into
more complex flavors; the microwave oven, which rarely cooks at
temperatures higher than 212 degrees, can't do this, nor can it bake.

You may have heard that microwave ovens are dangerous; at one time they
were, because the microwaves could leak out.  Federal standards were
set up in 1971, which resolved this problem, and all microwaves sold
in the United States have to be manufactured to meet these safety
requirements.

When cooking in microwave ovens, here are a couple of quick hints.
Microwave your foods in a round container; it seems to allow heating
better than a square container (although no one seems to be able to
explain why).  Cut up food into smaller, more uniform pieces so that
heat can distribute evenly.

As for Dr. Percy Spencer, the self-taught scientist and originator of
the microwave, he remained a senior consultant at Raytheon until his
death at age 76, at which time he held 150 patents and was considered
one of the world's leading experts in the field of microwave energy --
not bad for an individual whose education had never extended past
grammar school.