Those Mysterious Microwaves
Microwave Ovens
Microwave ovens were first conceived of in the 1940s. In 1940, Percy Spencer was walking by
an experimental high frequency radio wave communications device when the chocolate bar in
his pocket melted. Ten years later, microwave ovens came onto the market after a United
States military project using extremely short-length radio waves for communication found that
the ‘microwaves’ had other potential applications.
Microwave ovens operate at a frequency of 2.54 GHz (Giga= 1X109 ). This was chosen for
several reasons. First of all, it gives them a wavelength of about 12 cm, which is long enough
that they can easily be contained within the oven without fear of radiation into the surrounding
environment (i.e. a kitchen). Secondly, it resonates mildly with water, but not enough that the
water in the outer shell of food would absorb all the energy and leave the inside raw: it is
absorbed enough that it heats polar molecules quickly, but penetrates enough to heat foods
evenly.
Once the operating frequency was determined, it was necessary to find a way to distribute the
microwaves evenly in the operating chamber. Otherwise only certain areas will be exposed and
cooked by the microwaves. Two main ways were devised to heat food evenly – one by moving
the microwaves, one by moving the food. The first case involved the placement of metal fan
blades between the magnetron (which emits the microwaves) and the chamber, scattering the
radiation and makes the interference migrate throughout the chamber. The second involved the
placement of a turntable in the microwave, to rotate the food though the areas of high and low
concentration of microwave radiation. Very rarely is a turntable placed in a microwave with a
fan as well.
Frequently asked questions about microwaves:
How do microwaves heat?
Some of the molecules in food-particularly water molecules-behave like tiny electric magnets.
(the molecules are electric dipoles or, in other words, they are polar.) They tent to lineup with
the direction of an electric field, just as the magnet in a compass tends to line up with Earth’s
magnetic field. The microwaves in your oven are producing an electric field that reverses its
direction 4.9 billion times a second. The water molecules keep flipping their orientations back
and forth 4.9 billion times a second.
In their flipping the microwave-energized molecules bag up against neighboring
molecules and knock them around, sort of like the way in which an exploding kernel of
popcorn scatters its neighbors. Once knocked, a formerly stationary molecule becomes a fast
moving molecule, and a fast molecule is by definition a hot molecule. Remember when matter
moves fast enough it literally turns into energy.
Why don’t frozen foods microwave well?
Water molecules in ice are tied tightly together into a rigid framework so they can’t flip back
and forth under the influence of the microwaves’ oscillation as much. When you defrost frozen
food in your microwave you’re heating mostly the other, non-ice parts of the food, and the
resulting heat then flows into the ice crystals and melts them.
Why does microwaved food sometimes have to stand for a while after it’s been heated?
Unlike their electromagnetic cousins the X rays, which are of much higher frequency and
energy, microwaves can’t penetrate food more than an inch or so; their energy is completely
absorbed and turned into heat within that region. It takes time f or the outer heat to work its
way into the food’s interior. This is why recipes will often tell you to stop and stir the food
before continuing the heating.
Second, much of the water has actually been turned into steam, which then diffuses through
the food, giving up its heat along the way. That’s why most microwave heating is done in
loosely covered containers; you want to keep that hot steam in, but you don’t want it to build
up pressure and pop the lid. Both of these heat-transfer processes are slow, so if the heat
isn’t given enough time to distribute itself uniformly, you’ll wind up with hot and cold spots in
the food.
Virtually all food contains water, so virtually all food can be heated by microwaves. Molecules
of certain foods other than water, like fats and sugars, are also heated by microwaves. That’s
why bacon cooks so well in the microwave oven, and why the sweet raisins inside a
microwave-heated raisin bran muffin can get dangerously hot, even though the cake part is
merely warm.
So be careful with fatty and sugary foods coming out of the microwave. Very hot water
molecules can boil off as steam, but very hot fat and sugar molecules stay in place as
unexpected hazards. That’s another reason why it is always wise to wait for a while for the
steam to calm down and the hot spots to even out before removing and eating microwaved
food.
Why mustn’t one put metal into a microwave oven?
Light bounces off mirrors; microwaves bounce off metal. Radar is a kind of microwave that
bounces off your speeding car and cooks your goose! If what you put in the oven reflects too
many microwaves back instead of absorbing them, the magnetron tube can be damaged. There
must always be something in the oven to absorb microwaves. That’s why you shouldn’t ever
run it empty.
Microwaves set up electrical currents in metals, and if the metal object is too thin it may not
be able to support the current and will turn red hot and melt, as in the blowing of an
overloaded fuse. And if it has sharp points, it may even act like a lightning rod and
concentrate so much microwave energy at the points that it will send off lightning-like sparks.
On the other hand, the engineers who designed microwave oven can devise safe sizes and
shapes of metal that won’t cause trouble, and some ovens actually do contain metal trays or
racks.
Glass and paper are always safe in a microwave; they don’t absorb microwaves at all. Crystal
is glass with a high lead content. Crystal does absorb microwaves to some extent and may
therefore get warm. Plastics don’t absorb microwaves either. But microwaved food can get
pretty hot, and any container, no matter what it’s made of, will then get hot from the food.
Some wimpy plastics may even melt from the food’s heat. Ceramic cups and dishes are
usually okay, unless they contain minerals that absorb microwave energy and get hot.