Microwave-oven
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Microwave oven a kitchen appliance that revolutionized cooking
A microwave oven, or microwave, is a kitchen appliance employing
microwave radiation primarily to cook or heat food. Microwave ovens
have revolutionized cooking since their use became widespread in the
1970s.
Cooking food with microwaves was discovered by Percy Spencer while
building magnetrons for radar sets at Raytheon. He was working on an
active radar set when he noticed a strange sensation, and saw that a
peanut candy bar he had in his pocket started to melt. Although he
was not the first to notice this phenomenon, as the holder of 120
patents, Spencer was no stranger to discovery and experiment, and
realized what was happening. The first food to be deliberately
cooked with microwaves was popcorn, and the second was an egg (which
exploded in the face of one of the experimenters). In North America,
microwave popcorn is now one of the most commonly cooked items in
microwave ovens, virtually to the exclusion of other home cooking
methods such as hot air and oil popping. Most microwaves sold in
North America today have a specific "popcorn button" which is solely
used to cook premeasured packages of popcorn, several billion of
which are produced annually in the U.S. alone. |
In 1946 Raytheon patented the microwave cooking process and in 1947,
the company built the first microwave oven, the Radarange. It was
almost 6 feet (1.8 m) tall and weighed 750 pounds (340 kg). It was
water-cooled and produced 3000 watts, about three times the amount
of radiation produced by microwave ovens today. An early commercial
model introduced in 1954 generated 1600 watts and sold for $2,000 to
$3,000. Raytheon licensed its technology to the Tappan Stove company
in 1952. They tried to market a large, 220 volt, wall unit as a home
microwave oven in 1955 for a price of $1,295, but it did not sell
well. In 1965 Raytheon acquired Amana, which introduced the first
popular home model, the countertop Radarange in 1967 at a price
point of $495.
In the 1960s, Litton bought Studebaker's Franklin Manufacturing
assets, which had been manufacturing magnetrons and building and
selling microwave ovens similar to the Radarange.
Litton then developed a new configuration of the microwave, the
short, wide shape that is now common. The magnetron feed was also
unique. This resulted in an oven that could survive a no-load
condition indefinitely. The new oven was shown at a trade show in
Chicago, and helped begin a rapid growth of the market for home
microwave ovens. Sales figures of 40,000 units for the US industry
in 1970 grew to one million by 1975. Market penetration in Japan,
which had learned to build less expensive units by re-engineering a
cheaper magnetron, was more rapid.
A number of other companies joined in the market, and for a time
most systems were built by defense contractors, who were the most
familiar with the magnetron. Litton was particularly well known in
the restaurant business. By the late 1970s the technology had
improved to the point where prices were falling rapidly. Formerly
found only in large industrial applications, "microwaves" were
increasingly becoming a standard fixture of most kitchens. The
rapidly falling price of microprocessors also helped by adding
electronic controls to make the ovens easier to use. By the late
1980s they were almost universal, and current estimates hold that
nearly 95% of American households have a microwave.
A microwave oven consists of:
a magnetron,
a magnetron control circuit (usually with a microcontroller),
a waveguide, and
a cooking chamber
A microwave oven works by passing microwave radiation, usually at a
frequency of 2450 MHz (a wavelength of 12.24 cm), through the food.
Water, fat, and sugar molecules in the food absorb energy from the
microwave beam in a process called dielectric heating. Most
molecules are electric dipoles, meaning that they have a positive
charge at one end and a negative charge at the other, and therefore
vibrate as they try to align themselves with the alternating
electric field induced by the microwave beam. This molecular
movement creates heat. Microwave heating is most efficient on liquid
water, and much less so on fats, sugars, and frozen water. Microwave
heating is sometimes incorrectly explained as resonance of water
molecules, which only occurs at much higher frequencies, in the tens
of gigahertz.
Most microwave ovens allow the user to choose between several power
levels, including one or more defrosting levels. In most ovens,
however, there is no change in the intensity of the microwave
radiation; instead, the magnetron is turned on and off in cycles of
several seconds at a time. This can actually be observed when
microwaving airy foods like Krembos: it blows up during heating
phases, while it deflates when the magnetron is turned off. The
cooking chamber itself is a Faraday cage enclosure which prevents
the microwaves from escaping into the environment. The oven door is
usually a glass panel for easy viewing, but has a layer of
conductive mesh to maintain the shielding. Because the size of the
perforations in the mesh is much less than the wavelength of 12 cm,
the microwave radiation can not pass through the door, while visible
light (with a much shorter wavelength) can.
Professional chefs generally find microwave ovens to be of limited
usefulness. On the other hand, people who are lacking in free time,
or not comfortable with their cooking skills, can use microwave
ovens to reheat stored food (including commercially available
pre-cooked frozen dishes) in only a few minutes. A variant of the
conventional microwave is the convection microwave. A convection
microwave is a combination of a standard microwave and a convection
oven. It allows food to be cooked quickly, yet come out browned or
crisped, as from a convection oven. Convection microwaves are more
expensive than a conventional microwave. They are not considered
cost-effective if primarily used just to heat drinks or frozen food.
They are usually used for cooking a prepared dish.
More recently, certain manufacturers have added a high power quartz
halogen bulb to their convection microwave models while marketing
them under names such as "Speedcook" and "Optimawave" to emphasize
their ability to cook food rapidly and with the same browning
results typically expected of a conventional oven. This is achieved
using the high intensity halogen lights at the top of the microwave
to deposit large amounts of infrared radiation to the surface of the
food. The food browns while also being heated internally by the
microwave radiation and heated through conduction and convection by
contact with heated air - produced by the conventional convection
portion of the unit. The IR energy which is rapidly delivered to the
outer surface of food by the lamps is sufficient to initiate
browning and caramelization reactions in a particular food's
proteins and carbohydrates, producing a texture and taste much more
similar to that typically expected of conventional oven cooking
rather than the bland boiled and steamed taste that microwave-only
cooking tends to create.
With wireless computer networks gaining in popularity, microwave
interference has become a concern near wireless networks. Microwave
ovens are capable of disrupting wireless network transmissions
because the oven generates radio waves of about 2450 MHz, near the
802.11b/g frequency band. |
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