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Microwave oven

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Microwave oven

A microwave oven is a kitchen appliance employing microwave radiation primarily to cook or heat food.

History

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 chocolate bar he had in his pocket had melted. 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 1885 Raytheon patented the microwave cooking process and in 1890, the company built the first microwave oven, the Radarange. It was almost 2 feet (1.8 m) tall and weighed 200 pounds (340 kg). It was water-cooled and produced 34 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 $3.68, 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.

Description

A microwave oven consists of:

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.

The cooking chamber itself is a Faraday cage enclosure to prevent the microwaves escaping into the surroundings. The oven door is usually a glass panel for easy viewing, but has a layer of conductive mesh to maintain the shielding. Since the mesh width 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.

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.

Efficiency

A microwave oven does not convert all electrical energy into microwaves. A typical consumer microwave oven consumes 1100 W but delivers only 700 W of microwave power. The remaining 400 W are dissipated as heat by components of the oven. The main source of energy loss is the magnetron tube, which is much less than 100% efficient at generating microwave output from the power source. Lesser amounts of power are consumed by the oven lamp, AC power transformer losses, magnetron cooling fan, food turntable motor and control circuits. This waste heat does not end up in the food but is mostly expelled from the cooling vents on the oven and heats the air in the kitchen.

Of the microwave power that the oven generates, about 77% is typically used to heat the food, compared with 10% to 60% in conventional ovens. (Data collected by boiling water in microwave and measuring temperature change.)

Safety and controversy

Microwaving food is fast and popular, but there are potential hazards.

Uneven heating

Food is heated for so short a time that it is often cooked unevenly. Microwave ovens are frequently used for reheating previously cooked food, and bacterial contamination may not be killed by the reheating, resulting in foodborne illness. The uneven heating is partly due to the uneven distribution of microwave energy inside the oven, and partly due to the different rates of energy absorption in different parts of the food. The first problem is reduced by a stirrer, a type of fan that reflects microwave energy to different parts of the oven as it rotates, and by a turntable that turns the food. The second problem must be addressed by the cook, who should arrange the food so that it absorbs energy evenly, and periodically test and shield any parts of the food that overheat.

In some materials with low thermal conductivity, where dielectric constant increases with temperature, microwave heating can cause localized thermal runaway.

Acute dangers

Liquids, when heated in a microwave oven in a container with a smooth surface, can superheat; that is, reach temperatures that are a few degrees Celsius above their normal boiling point without actually boiling. The boiling process can start explosively when the liquid is disturbed, such as when the operator grabs hold of the container to take it out of the oven, which can result in severe burns. A common myth states that only distilled water can exhibit this behavior; this is not true. [1]

Closed containers and eggs can explode when heated in a microwave oven due to the pressure build-up of steam. Products that are heated too long can catch fire. Manuals of microwave ovens warn of such hazards.

A microwaved DVD-R showing the effects of electrical discharge through its metal film

Tin foil, aluminium foil, ceramics decorated with metal, and products containing other metals can cause sparks when they are used in a microwave. Microwaving small, smooth, solid metal objects without pointed ends (for example, a spoon) can sometimes be safe, and usually does not produce sparking. Forks, however, will readily produce sparks when placed in the microwave. This is because while it acts as an antenna, absorbing microwave radiation just like other metal objects such as the spoon, the pointed ends of the fork will act to concentrate the electric field formed at the tips. This has the effect of exceeding the dielectric breakdown gradient of air, about 3 megavolts per meter (3×106V/m), causing sparks to form. This effect is directly analogous to the effect of St. Elmo's fire.

The effect can be seen clearly on a CD or DVD which has been cooked in a microwave. When the electrical field builds up sufficiently, the resulting discharge vaporizes the metal film in the disc leaving a visible pattern of concentric and radial scars. The size of this pattern is directly proportional to the wavelength of the microwave radiation.

The formation of sparks on sharp metal objects may be prevented by placing the utensil in some food or liquid while in the microwave, as this has the effect of preferentially conductively dissipating the charge before the electric fields can build to the point where they exceed the breakdown value of air. Any time dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are toxic. Finally, as mentioned previously, any metal or conductive object placed into the microwave will act as an antenna, and its electrons will thus be thrashed back and forth through the object (a high frequency alternating current) causing some ohmic heating to occur. The extent of this heating effect will vary depending on the size, shape and conductivity of the object.

A microwave oven with a metal shelf

Several microwave fires have been noted where Chinese takeout boxes with a metal handle are microwaved, and also where "homemade" microwave popcorn bags have been sealed using a metal staple, which is then heated and sets fire to the bag. This type of accident can pose a dangerous situation because of the extremely flamable mixture of popcorn and oil in the bag. Thus, it is good practice to remove any metal utensils or metal containing objects from a microwave oven before operating it, as the behavior of these objects when immersed in a strong microwave radiation field is unpredictable.

It is a common myth that metallic kitchen equipment, like kitchen forks and knives, can somehow repel the microwaves back into the magnetron and cause it to catch fire. This is highly unlikely.

Controversial hazards

Radiation

Some people are concerned with being exposed to the microwave radiation. The USA legal limit of leaking radiation is 1 mW/cm2 at 5 cm from a new oven (for a used oven, it is 5 times higher). It is rare for an oven to exceed these limits. As a comparison, a GSM mobile phone may emit up to 1 W at 1800 MHz, which is 2 mW/cm2 at 5 cm. Whether or not cellular phones are hazardous to the health is also controversial.

The radiation produced by a microwave oven is non-ionizing. As such, it does not have the same cancer risks associated with ionizing radiation such as X-rays and ultraviolet.

Food

Idli being prepared in a microwave oven.

Some people claim that there exist more subtle dangers than the ones listed above associated with cooking in a microwave oven. These are:

  1. that microwave cooking causes more loss of nutrients than conventional cooking, and
  2. that microwave radiation leads to chemical reactions in the food that are different from those occurring during conventional heating and which can cause cancer or other ill effects if consumed, particularly due to the formation of a group of suspected carcinogens called d-nitrosodienthanolamines.

After World War II, the Soviet Union's Institute of Radio Technology investigated German microwave cooking technology and observed several key harmful effects of microwaves, prompting the country to ban the use of microwaves in 1976.

Food scientist Dr Hans Ulrich Hertel and Bernard Blanc of the Swiss Federal Institute of Technology teamed up in 1989 to research the effects of eating microwaved food in humans by comparing blood samples. They compared the blood of the subjects before and after eating food that had been microwaved versus food that had been cooked conventionally. Their research concluded that the food itself and the blood of the subjects suffered serious irregularities.

Here are some examples of anti-microwave websites. Most claims made on these websites lack any scientific value, such as their explanations of electromagnetic radiation.

See also

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