Microwave oven
Overview
 
A microwave oven is a kitchen appliance that heats food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

 by dielectric heating
Dielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...

, using microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 radiation to heat polarized molecules
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

 within the food. Microwave ovens heat foods quickly and efficiently, and because excitation is fairly uniform in the outer 1 inches (2.5 cm) to 1.5 inches (3.8 cm) of a dense (high water content) food item, food is more evenly heated throughout (except in thick dense objects) than generally occurs in other cooking techniques.

The first commercial microwave oven was developed by Raytheon
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...

 after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 from radar technology developed during the war.
Encyclopedia
A microwave oven is a kitchen appliance that heats food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

 by dielectric heating
Dielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...

, using microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 radiation to heat polarized molecules
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

 within the food. Microwave ovens heat foods quickly and efficiently, and because excitation is fairly uniform in the outer 1 inches (2.5 cm) to 1.5 inches (3.8 cm) of a dense (high water content) food item, food is more evenly heated throughout (except in thick dense objects) than generally occurs in other cooking techniques.

The first commercial microwave oven was developed by Raytheon
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...

 after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 from radar technology developed during the war. Named the 'Radarange', it was first sold in 1947. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation
Amana Corporation
The Amana Corporation is an American brand of household appliances. It was founded in 1934 by George Foerstner as The Electrical Equipment Co. in Middle Amana, Iowa to manufacture commercial walk-in coolers. The business was later owned by the Amana Society and became known as Amana Refrigeration,...

, which had been acquired in 1965 by Raytheon.

Microwave ovens are popular for reheating previously-cooked foods and cooking vegetables. They are also useful for rapid heating of otherwise slowly-prepared cooking items, such as hot butter and fats, and melted chocolate. Unlike conventional oven
Oven
An oven is a thermally insulated chamber used for the heating, baking or drying of a substance. It is most commonly used for cooking. Kilns, and furnaces are special-purpose ovens...

s, microwave ovens usually do not directly brown or carmelize food, since they rarely attain the necessary temperatures to do so. Exceptions occur mostly in rare cases where the oven is used to heat frying-oil and other very oily items (such as bacon), which attain far higher temperatures than that of boiling water. The low boiling-range temperatures produced in high-water content foods give microwave ovens a limited role in professional cooking, since it usually makes them unsuitable for achievement of culinary effects where the flavors produced by frying, browning, or baking are needed. However, additional kinds of heat sources can be added to microwave
Convection microwave
A convection microwave is a combination of a standard microwave oven and a convection oven. It allows food cooked in the convection microwave to be cooked quickly, yet come out browned or crisped as in a convection oven...

 packaging, or into combination microwave ovens
Trivection oven
The Trivection oven is a convection microwave created by General Electric, which combines radiant heat, convection and microwaves for customized cooking...

, to produce these other heating effects, and microwave heating may cut the overall time to prepare such dishes.

History

The use of high-frequency electric fields for heating dielectric materials had been proposed in 1934, for example US patent 2,147,689 (application by Bell Telephone Laboratories, dated 1937) states "This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field."

However, lower frequency dielectric heating
Dielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...

 as is described in this patent, is (like induction heating
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...

) an electromagnetic
Electromagnetic
Electromagnetic may refer to:* Electromagnetism* Electromagnetic field* Electromagnetic force* Electromagnetic radiation* Electromagnetic induction* Electromagnetic spectrum...

 heating effect which is the result of the so-called near field
Near and far field
The near field and far field and the transition zone are regions of the electromagnetic radiation field that emanates from a transmitting antenna, or as a result of radiation scattering off an object...

 effects that exist in an electromagnetic cavity that is small compared with the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of the electromagnetic field. This patent proposed radiofrequency heating, at 10 to 20 megahertz (wavelength 15 to 30 meters). Heating from microwaves that have a wavelength that is small in relation to the cavity (as in a modern microwave oven) is due to "far field" effects that are due to classical electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...

 that describes freely-propagating light and microwaves suitably far from their source. Nevertheless, the primary heating effect of all types of electromagnetic fields at both radio and microwave frequencies, occurs via the dielectric heating
Dielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...

 effect, as polarized molecules are affected by a rapidly-alternating electric field.

The specific heating effect of a beam of high-power microwaves was discovered accidentally in 1945, shortly after high-powered microwave radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 transmitters were developed and widely disseminated by the Allies of World War II
Allies of World War II
The Allies of World War II were the countries that opposed the Axis powers during the Second World War . Former Axis states contributing to the Allied victory are not considered Allied states...

, using magnetron technology. Percy Spencer
Percy Spencer
Percy LeBaron Spencer was an American engineer and inventor. He became known as the inventor of the microwave oven....

, an American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 self-taught engineer from Howland, Maine
Howland, Maine
Howland is a city in Penobscot County, Maine, United States, which was settled in 1818. The city was named for John Howland, one of the passengers on the Mayflower.The population was 1,362 at the 2000 census.-Geography:...

, was working at the time building magnetrons for radar sets, with the American company Raytheon
Raytheon
Raytheon Company is a major American defense contractor and industrial corporation with core manufacturing concentrations in weapons and military and commercial electronics. It was previously involved in corporate and special-mission aircraft until early 2007...

. He was working on an active radar set when he noticed that a Mr. Goodbar
Mr. Goodbar
Mr. Goodbar is a chocolate-flavored candy bar containing peanuts, whose packaging can be easily identified by its distinctive yellow background and red text. It is manufactured by The Hershey Company and was introduced in 1925...

 he had in his pocket started to melt. The radar had melted his chocolate bar with microwaves. The first food to be deliberately cooked with Spencer's microwave was popcorn, and the second was an egg, which exploded in the face of one of the experimenters. To verify his finding, Spencer created a high density electromagnetic field by feeding microwave power from a magnetron into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly.

On October 8, 1945 Raytheon filed a US patent for Spencer's microwave cooking process, and an oven that heated food using microwave energy from a magnetron was soon placed in a Boston restaurant for testing. In 1947, the company built the "Radarange", the first commercial microwave oven. It was almost 1.8 metre tall, weighed 340 kilograms (749.6 lb) and cost about US$5000 each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. The first Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship NS Savannah
NS Savannah
NS Savannah, named for SS Savannah, was the first nuclear-powered cargo-passenger ship, built in the late 1950s at a cost of $46.9 million, including a $28.3 million nuclear reactor and fuel core, funded by United States government agencies as a demonstration project for the potential...

. An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$2000 to US$3000. Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio
Mansfield, Ohio
Mansfield is a city in the U.S. state of Ohio and the county seat of Richland County. The municipality is located in north-central Ohio in the western foothills of the Allegheny Plateau, approximately southwest of Cleveland and northeast of Columbus....

 in 1952. They tried to market a large, 220 volt, wall unit as a home microwave oven in 1955 for a price of US$1295, but it did not sell well. In 1965 Raytheon acquired Amana. In 1967 they introduced the first popular home model, the countertop Radarange, at a price of US$495.

In the 1960s, Litton
Litton Industries
Named after inventor Charles Litton, Sr., Litton Industries was a large defense contractor in the United States, bought by the Northrop Grumman Corporation in 2001.-History:...

 bought Studebaker
Studebaker
Studebaker Corporation was a United States wagon and automobile manufacturer based in South Bend, Indiana. Founded in 1852 and incorporated in 1868 under the name of the Studebaker Brothers Manufacturing Company, the company was originally a producer of wagons for farmers, miners, and the...

'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, or an empty microwave oven where there is no object to absorb the microwaves, indefinitely. The new oven was shown at a trade show in Chicago
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...

, and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the US industry in 1970 grew to one million by 1975. Market penetration was faster in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, due to a re-engineered magnetron allowing for less expensive units.
Several other companies joined in the market, and for a time most systems were built by defense contractors, who were 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. Often called "electronic ovens" in the 1960s, the name "microwave ovens" later became standardized, often now referred to informally as simply "microwaves." Formerly found only in large industrial applications, microwave ovens were increasingly becoming a standard fixture of most kitchens. The rapidly falling price of microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

s also helped by adding electronic controls to make the ovens easier to use. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971. Current estimates hold that over 90% of American households own a microwave oven.

Principles

A microwave oven works by passing non-ionizing
Non-ionizing radiation
Non-ionizing radiation refers to any type of electromagnetic radiation that does not carry enough energy per quantum to ionize atoms or molecules—that is, to completely remove an electron from an atom or molecule...

 microwave radiation, usually at a frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of 2.45 gigahertz (GHz)—a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of 122 millimetres (4.8 in)—through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat
Fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

, and other substances in the food absorb energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 from the microwaves in a process called dielectric heating
Dielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...

. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e., as both potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...

 and kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 of atoms), is heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

.

Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted. It is also less efficient on fats and sugars (which have a smaller molecular dipole moment) than on liquid water. Microwave heating is sometimes explained as a resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 of water molecules, but this is incorrect: such resonance only occurs in water vapor at much higher frequencies, at about 20 GHz. Moreover, large industrial/commercial microwave ovens operating at the common large industrial-oven microwave heating frequency of 915 MHz—wavelength 328 millimetres (12.9 in)—also heat water and food perfectly well.

Microwave heating can cause localized thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

s in some materials with low thermal conductivity, where dielectric constant increases with temperature. Under certain conditions, glass can exhibit thermal runaway in a microwave to the point of melting. Additionally, microwaves can melt certain types of rocks, producing small quantities of synthetic lava. Some ceramics can also be melted, and may even become clear upon cooling.

A common misconception is that microwave ovens cook food "from the inside out", meaning from the center of the entire mass of food outwards. In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry non-conductive substances at the surfaces of many common foods, and thus often induce initial heat more deeply than other methods. Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast to broiling (infrared) or convection heating, which deposit heat thinly at the food surface. Penetration depth of microwaves is dependent on food composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating further. Microwaves cook from the inside out only in the sense that each molecule is generating heat from "inside" and radiating it "outward".

Heating efficiency

A microwave oven converts only part of its electrical input into microwave energy. A typical consumer microwave oven consumes 1100 W of electricity in producing 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits. Such wasted heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents.

Design

A microwave oven consists of:
  • a high voltage power source, commonly a simple transformer
    Transformer
    A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

     or an electronic power converter
    Power converter
    A power converter is an electrical or electro-mechanical device for converting electrical energy. It may be converting AC to or from DC, or the voltage or frequency, or some combination of these.Amongst the many devices that are used for this purpose are;...

    , which passes energy to the magnetron
  • a high voltage capacitor
    Capacitor
    A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

     connected to the magnetron, transformer and via a diode
    Diode
    In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

     to the case.
  • a cavity magnetron
    Cavity magnetron
    The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field. The 'resonant' cavity magnetron variant of the earlier magnetron tube was invented by John Randall and Harry Boot in 1940 at the University of...

    , which converts high-voltage electric energy to microwave radiation
  • a magnetron control circuit (usually with a microcontroller
    Microcontroller
    A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...

    )
  • a waveguide
    Waveguide (electromagnetism)
    In electromagnetics and communications engineering, the term waveguide may refer to any linear structure that conveys electromagnetic waves between its endpoints. However, the original and most common meaning is a hollow metal pipe used to carry radio waves...

     (to control the direction of the microwaves)
  • a cooking chamber


Nearly all modern microwave ovens have a control panel
Control panel
Control panel may refer to:* Control panel , a flat, often vertical, area where control instrumentation is mounted.* Control panel , the tool in the operating system which allows most or all of the settings to be changed through a user interface** Control panel ** Control Panel ** Web hosting...

 with an LED
Light-emitting diode
A light-emitting diode is a semiconductor light source. LEDs are used as indicator lamps in many devices and are increasingly used for other lighting...

, liquid crystal or vacuum fluorescent display (early models and currently lower-cost models designed for smaller kitchenette
Kitchenette
A kitchenette is a small cooking area.In motel and hotel rooms, small apartments, college dormitories, or office buildings a kitchenette usually consists of a small refrigerator, a microwave oven or hotplate, and, less frequently, a sink...

s in college dorm and efficiency apartment settings use an analog dial-type timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

). The control panel keypad always contains a Start button and a Stop button (the latter sometimes also performing a Clear function), numeric buttons for entering the cook time, a button for selecting the power level (usually decrementing by 10 from 100 to 50, or using words such as High, Medium High and Medium; see more below), and a Defrost button. Other buttons may be present which name the type of food to be cooked, such as meat, fish, poultry, vegetables, frozen vegetables
Frozen vegetables
Frozen vegetables may be commercially packaged or frozen at home. A wide range of frozen vegetables are sold in supermarkets, usually packaged in either rectangular boxes or plastic bags....

, frozen entrées, and popcorn
Popcorn
Popcorn, or popping corn, is corn which expands from the kernel and puffs up when heated. Corn is able to pop because, like sorghum, quinoa and millet, its kernels have a hard moisture-sealed hull and a dense starchy interior. This allows pressure to build inside the kernel until an explosive...

, which when pressed cook the item for a preprogrammed time. In such cases a button for warming non-carbonated beverages (implying coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

) will also be present, along with another for heating and boiling water (including tea
Tea
Tea is an aromatic beverage prepared by adding cured leaves of the Camellia sinensis plant to hot water. The term also refers to the plant itself. After water, tea is the most widely consumed beverage in the world...

). Mid-priced and higher models generally feature a "sensor cook" button as well. The display can generally show the time of day, adjustment of which varies by model and is usually necessary after a loss of power or for seasonal time changes
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

.

The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the industrial, scientific, and medical (ISM) frequency band
ISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....

s set aside for non-communication purposes. Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8 GHz and 24.125 GHz, but are not used for microwave cooking because of the very high cost of power generation at these frequencies. The third, centered on 433.92 MHz, is a narrow band that would require expensive equipment to generate sufficient power without creating interference outside the band, and is only available in some countries. For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in the ITU Region 2 while 2.45 GHz is available worldwide.

Most microwave ovens allow users to choose between several power 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 duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....

s of several seconds at a time. This can actually be heard (a change in the humming sound from the oven), or observed when microwaving airy foods which may inflate during heating phases and deflate when the magnetron is turned off. For such an oven, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. Newer models have inverter power supplies which use pulse width modulation to provide effectively-continuous heating at reduced power so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating.

The cooking chamber itself is a Faraday cage
Faraday cage
A Faraday cage or Faraday shield is an enclosure formed by conducting material or by a mesh of such material. Such an enclosure blocks out external static and non-static electric fields...

 which prevents the microwaves from escaping. The oven door usually has a window for easy viewing, but the window has a layer of conductive mesh some distance from the outer panel to maintain the shielding. Because the size of the perforations in the mesh is much less than the microwaves' wavelength, most of the microwave radiation cannot pass through the door, while visible light (with a much shorter wavelength) can.

Variants and accessories


A variant of the conventional microwave is the convection microwave
Convection microwave
A convection microwave is a combination of a standard microwave oven and a convection oven. It allows food cooked in the convection microwave to be cooked quickly, yet come out browned or crisped as in a convection oven...

. A convection microwave oven is a combination of a standard microwave and a convection oven
Convection oven
Although the word convection is usually used to describe the natural circulation of gas or liquid caused by temperature differences, the convection in "convection oven" has a more general definition: the transfer of heat via movement of gas or liquid...

. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwaves are more expensive than conventional microwave ovens. Some convection microwaves—those with exposed heating elements—can produce smoke and burning odors as food spatter from earlier microwave-only use is burned off the heating elements.

More recently, some manufacturers have added high power quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 halogen bulbs to their convection microwave models, marketing them under names such as "Speedcook", "Advantium
Advantium
Advantium is a line of fast-cooking electric ovens for household use sold by General Electric. They use both halogen lamps and microwave energy, either separately or together....

" and "Optimawave" to emphasize their ability to cook food rapidly and with good browning. The bulbs heat the food's surface with infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 (IR) radiation, browning surfaces as in a conventional oven. The food browns while also being heated by the microwave radiation and heated through conduction through contact with heated air. The IR energy which is delivered to the outer surface of food by the lamps is sufficient to initiate browning caramelization
Caramelization
Caramelization is the browning of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color. As the process occurs, volatile chemicals are released, producing the characteristic caramel flavor....

 in foods primarily made up of carbohydrates and Maillard reaction
Maillard reaction
The Maillard reaction is a form of nonenzymatic browning similar to caramelization. It results from a chemical reaction between an amino acid and a reducing sugar, usually requiring heat....

s in foods primarily made up of protein. These reactions in food produce a texture and taste similar to that typically expected of conventional oven cooking rather than the bland boiled and steamed taste that microwave-only cooking tends to create.

In order to aid browning
Browning
-Places:* Browning, Illinois, USA* Browning, Montana, USA* Browning, Wisconsin, USA* Browning, Missouri, USA* Browning Hill, in Brown County, Indiana; sometimes called Browning Mountain* Browning, Saskatchewan, Canada* Browning No...

, sometimes an accessory browning tray is used, usually composed of glass or porcelain
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

. It makes food crisp by oxidising
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 the top layer until it turns brown. Ordinary plastic
Plastic
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

 cookware
Cookware and bakeware
Cookware and bakeware are types of food preparation containers commonly found in the kitchen. Cookware comprises cooking vessels, such as saucepans and frying pans, intended for use on a stove or range cooktop. Bakeware comprises cooking vessels intended for use inside an oven...

 is unsuitable for this purpose because it could melt.

Frozen dinners, pies, and microwave popcorn bags often contain a thin susceptor
Susceptor
A susceptor is a material used for its ability to absorb electromagnetic energy and convert it to heat . This energy is typically radiofrequency or microwave radiation used in industrial heating processes, and also occasionally in microwave cooking...

 made from aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 film in the packaging or included on a small paper tray. The metal film absorbs microwave energy efficiently and consequently becomes extremely hot and radiates in the infrared, concentrating the heating of oil for popcorn or even browning surfaces of frozen foods. Heating packages or trays containing susceptors are designed for single use and are discarded as waste.

Sizes

Portable or Desktop: This is the smallest size of microwave oven in the market. The common models measure around 28 centimetres (11 in) tall, 38 centimetres (15 in) wide and 25 centimetres (9.8 in) deep. Some of the experimental models on trial are as small as 19 centimetres (7.5 in) tall, 6 centimetres (2.4 in) wide and 15 centimetres (5.9 in) deep. Some of these use 12 V DC power supplies.
Compact: A compact microwave oven, also called small, is the smallest type typically available. Compacts are the most popular size of microwave oven, dominating the market. A typical model is no more than 50 centimetres (19.7 in) wide, 35 centimetres (13.8 in) deep and 30 centimetres (11.8 in) tall. These ovens are rated between 500 and 1000 watts and have less than 28 litre (0.988810653954517 cu ft) in capacity. These ovens are primarily used for reheating food and making microwave meals and popcorn. The largest models can accommodate 2 litres (1.8 imp qt) round casserole dishes and are suitable for light cooking. These ovens are not made to cook large amounts of food. Typically these models cost less than US$100 (around £50).
Medium-capacity: These models' heights and depths are only marginally larger than compacts, but they are typically more than 50 centimetres (19.7 in) wide. Their interiors are typically between 30 and 45 l (1.1 and 1.6 cuft), and power ratings are 1000–1500 W. These are the common "family-sized" microwave ovens. They tend to have a few more "auto-cook" features, and some incorporate grills or even conventional-oven heating elements.
Large-capacity: These are designed for cooking large meals. Large-capacity ovens can handle 25 by casserole dishes and cook tall items like roasts or turkey breasts, with a large number of "auto-cook" and precise temperature control measures. Large-capacity ovens normally use over 2000 W and have over 60 litres (2.1 cu ft) of capacity. These ovens are normally well over 50 centimetres (19.7 in) wide, as much as 50 centimetres (19.7 in) deep, and at least 30 centimetres (11.8 in) high.
Built-in: These are built into cabinetry and are typically more expensive than similar sized countertop models. Some models include exhaust fans to allow installation above cooktops.

Microwave-safe plastics

Many current plastic containers and food wraps
Plastic wrap
Plastic wrap, cling film , cling wrap or food wrap, is a thin plastic film typically used for sealing food items in containers to keep them fresh over a longer period of time...

 are specially designed to withstand microwave radiation. Some products may use the term "microwave safe", may carry a microwave symbol (three lines of waves, one above the other) or simply provide instructions for proper microwave use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.

Benefits and safety features

Commercial microwave ovens all use a timer in their standard operating mode; when the timer runs out, the oven turns itself off.

Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, with the exception of an induction cooktop, leaves a potentially dangerous heating element or trivet
Trivet
A trivet is an object placed between a serving dish or bowl, and a dining table, usually to protect the table from heat damage.Trivet also refers to a tripod used to elevate pots from the coals of an open fire...

 that will stay hot for some time. Likewise, when taking a casserole
Casserole
A casserole, from the French for "saucepan", is a large, deep dish used both in the oven and as a serving vessel. The word casserole is also used for the food cooked and served in such a vessel, with the cookware itself called a casserole dish or casserole pan...

 out of a conventional oven, one's arms are exposed to the very hot walls of the oven. A microwave oven does not pose this problem.

Food and cookware taken out of a microwave oven are rarely much hotter than 100 °C (212 °F). Cookware used in a microwave oven is often much cooler than the food because the cookware is transparent to microwaves; the microwaves heat the food directly and the cookware is indirectly heated by the food. Food and cookware from a conventional oven, on the other hand, are the same temperature as the rest of the oven; a typical cooking temperature is 180 °C (356 °F). That means that conventional stoves and ovens can cause more serious burns.

The lower temperature of cooking (the boiling point of water) is a significant safety benefit compared to baking in the oven or frying, because it eliminates the formation of tars and char
Char
Char is the solid material that remains after light gases and tar coal tar have been driven out or released from a carbonaceous material during the initial stage of combustion, which is known as carbonization, charring, devolatilization or pyrolysis.Further stages of efficient combustion are...

, which are carcinogenic. Microwave radiation also penetrates deeper than direct heat, so that the food is heated by its own internal water content. In contrast, direct heat can fry the surface while the inside is still cold. Pre-heating the food in a microwave oven before putting it into the grill or pan reduces the time needed to heat up the food and reduces the formation of carcinogenic char. Unlike frying and baking, microwaving does not produce acrylamide
Acrylamide
Acrylamide is a chemical compound with the chemical formula C3H5NO. Its IUPAC name is prop-2-enamide. It is a white odourless crystalline solid, soluble in water, ethanol, ether, and chloroform. Acrylamide is incompatible with acids, bases, oxidizing agents, iron, and iron salts...

 in potatoes, however unlike deep-frying, it is of only limited effectiveness in reducing glycoalkaloid (i.e. Solanine
Solanine
Solanine is a glycoalkaloid poison found in species of the nightshade family , such as the potato . It can occur naturally in any part of the plant, including the leaves, fruit, and tubers. Solanine has fungicidal and pesticidal properties, and it is one of the plant's natural defenses...

) levels.
Acrylamide has been found in other microwaved products like popcorn.

Heating characteristics

In a microwave oven, food may be heated for so short a time that it is cooked unevenly, because heat requires time to diffuse through food, and microwaves only penetrate to a limited depth. Microwave ovens are frequently used for reheating previously cooked food
Leftovers
Leftovers are the uneaten edible remains of a meal after the meal is over, and everyone has finished eating. Food scraps that are not considered edible are not regarded as leftovers, but rather as waste material; any remaining edible portions constitute the leftovers.The ultimate fate of leftovers...

, and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l contamination may not be repressed if the safe temperature is not reached, resulting in foodborne illness
Foodborne illness
Foodborne illness is any illness resulting from the consumption of contaminated food, pathogenic bacteria, viruses, or parasites that contaminate food, as well as chemical or natural toxins such as poisonous mushrooms.-Causes:Foodborne illness usually arises from improper handling, preparation, or...

, as with all inadequate reheating methods.

Uneven heating in microwaved food can be 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
Reflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

 microwave energy to different parts of the oven as it rotates, or by a turntable or carousel that turns the food; turntables, however, may still leave spots, such as the center of the oven, which receive uneven energy distribution. The location of dead spots and hot spots in a microwave can be mapped out by placing a damp piece of thermal paper
Thermal paper
Thermal paper is a special fine paper that is coated with a chemical that changes color when exposed to heat. It is used in thermal printers and particularly in inexpensive or lightweight devices such as adding machines, cash registers, and credit card terminals.The surface of the paper is coated...

 in the oven. When the water saturated paper is subjected to the microwave radiation it becomes hot enough to cause the dye to be released which will provide a visual representation of the microwaves. If multiple layers of paper are constructed in the oven with a sufficient distance between them a three dimensional map can be created. Many store receipts are printed on thermal paper which allows this to be easily done at home. See Video of thermal paper technique

The second problem is due to food composition and geometry, and must be addressed by the cook, by arranging the food so that it absorbs energy evenly, and periodically testing and shielding any parts of the food that overheat. In some materials with low thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

, where dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

 increases with temperature, microwave heating can cause localized thermal runaway
Thermal runaway
Thermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...

. Under certain conditions, glass can exhibit thermal runaway in a microwave to the point of melting.

Due to this phenomenon, microwave ovens set at too-high power levels may even start to cook the edges of frozen food while the inside of the food remains frozen. Another case of uneven heating can be observed in baked goods containing berries. In these items, the berries absorb more energy than the drier surrounding bread and cannot dissipate the heat due to the low thermal conductivity of the bread. Often this results in overheating the berries relative to the rest of the food. "Defrost" oven settings use low power levels designed to allow time for heat to be conducted within frozen foods from areas that absorb heat more readily to those which heat more slowly. In turntable-equipped ovens, more even heating will take place by placing food off-centre on the turntable tray instead of exactly in the centre.

Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may include materials that contain ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 or aluminum flakes, which are designed to absorb microwaves and heat up, thereby converting microwaves to less penetrating infrared, which aids in baking or crust preparation by depositing more energy shallowly in these areas. Such ceramic patches affixed to cardboard are positioned next to the food, and are typically smokey blue or gray in colour, usually making them easily identifiable; the cardboard sleeves included with Hot Pockets
Hot Pockets
Hot Pockets are microwaveable turnovers usually containing a combination of cheese, meat, and vegetables. Hot Pockets are currently produced by Hylan Steez.- Varieties :...

, which have a silver surface on the inside, are a good example of such packaging. Microwavable cardboard packaging may also contain overhead ceramic patches which function in the same way. The technical term for such a microwave-absorbing patch is a susceptor
Susceptor
A susceptor is a material used for its ability to absorb electromagnetic energy and convert it to heat . This energy is typically radiofrequency or microwave radiation used in industrial heating processes, and also occasionally in microwave cooking...

.

Effects on food and nutrients

Several studies have shown that if properly used, microwave cooking does not change the nutrient content of foods to a larger extent than conventional heating, and that there is a tendency towards greater retention of many micronutrients with microwaving, probably due to the shorter preparation time. Microwaving human milk at high temperatures is contraindicated, due to a marked decrease in activity of antiinfective factors.

Any form of cooking will destroy some nutrients in food, but the key variables are how much water is used in the cooking, how long the food is cooked, and at what temperature. Nutrients are primarily lost by leaching into cooking water, which tends to make microwave cooking healthier, given the shorter cooking times it required. Microwave ovens do convert vitamin B12
Vitamin B12
Vitamin B12, vitamin B12 or vitamin B-12, also called cobalamin, is a water-soluble vitamin with a key role in the normal functioning of the brain and nervous system, and for the formation of blood. It is one of the eight B vitamins...

 from the active to inactive form, making approximately 30-40% of the B12 contained in foods unusable by mammals. A single study indicated that microwaving broccoli loses 74% or more of phenolic compounds
Phenols
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group...

 (97% of flavonoid
Flavonoid
Flavonoids , are a class of plant secondary metabolites....

s), while boiling loses 66% of flavonoids, and high-pressure boiling loses 47%, though the study has been contradicted by other studies. To minimize phenolic losses in potatoes, microwaving should be done at 500W.

Spinach retains nearly all its folate when cooked in a microwave; in comparison, it loses about 77% when cooked on a stove, because food on a stove is typically boiled, leaching out nutrients. Bacon cooked by microwave has significantly lower levels of carcinogenic nitrosamines than conventionally cooked bacon. Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop. Microwave blanching is 3-4 times more effective than boiled water blanching in the retaining of the water-soluble vitamins folic acid, thiamin and riboflavin, with the exception of ascorbic acid, of which 28.8% is lost (vs. 16% with boiled water blanching).

Hazards

Liquids can superheat
Superheating
In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling...

 when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start explosively
Steam explosion
A steam explosion is a violent boiling or flashing of water into steam, occurring when water is either superheated, rapidly heated by fine hot debris produced within it, or the interaction of molten metals A steam explosion (also called a littoral explosion, or fuel-coolant interaction, FCI) is a...

 when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven or while adding solid ingredients such as powdered creamer or sugar. This can result in spontaneous boiling (nucleation
Nucleation
Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...

) which may be violent enough to eject the boiling liquid from the container and produce severe scalding.

Closed containers, such as eggs
Egg (food)
Eggs are laid by females of many different species, including birds, reptiles, amphibians, and fish, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell, albumen , and vitellus , contained within various thin membranes...

, can explode when heated in a microwave oven due to the increased pressure from steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

. Insulating plastic foams of all types generally contain closed air pockets, and are usually microwave-unsafe, as the air pockets explode and the foam (which can be toxic if consumed) may melt. Not all plastics are microwave-safe, and some plastics absorb microwaves to the point that they become dangerously hot.

Products that are heated for too long can catch fire. Though this is inherent to any form of cooking, the rapid cooking and unattended nature of microwave oven use results in additional hazard. Because the microwave oven's cavity is enclosed and metal, fires are generally well contained. Switching off the oven and allowing the fire to consume the available oxygen with the door closed will typically contain and quickly extinguish the fire and limit damage to the oven itself.

Some magnetrons have ceramic insulators with beryllium oxide
Beryllium oxide
Beryllium oxide , also known as beryllia, is an inorganic compound with the formula BeO. This colourless solid is a notable electrical insulator with a higher thermal conductivity than any other non-metal except diamond, and actually exceeds that of some metals. As an amorphous solid, beryllium...

 (beryllia) added. The beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...

 in such oxides is a serious chemical hazard if crushed and ingested (for example, by inhaling dust). In addition, beryllia is listed as a confirmed human carcinogen by the IARC
International Agency for Research on Cancer
The International Agency for Research on Cancer is an intergovernmental agency forming part of the World Health Organisation of the United Nations....

; therefore, broken ceramic insulators or magnetrons should not be handled. This is obviously a danger only if the microwave oven becomes physically damaged, such as if the insulator cracks, or when the magnetron is opened and handled directly, and as such should not be a concern during normal usage.

Metal objects

Any metal or conductive object placed into the microwave will act as an antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

 to some degree, resulting in an electric current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

. This causes the object to act as a heating element. This effect varies with the object's shape and composition, and is sometimes utilized for cooking.

Any object containing pointed metal can create an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 (sparks) when microwaved. This includes cutlery
Cutlery
Cutlery refers to any hand implement used in preparing, serving, and especially eating food in the Western world. It is more usually known as silverware or flatware in the United States, where cutlery can have the more specific meaning of knives and other cutting instruments. This is probably the...

, crumpled aluminum foil (though not all foil use in microwaves is unsafe, see below), twist-ties containing metal wire, the metal wire carry-handles in paper Chinese take-out food containers, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape. Forks are a good example: the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...

s per meter (3×106 V/m). The air forms a conductive plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

, which is visible as a spark. The plasma and the tines may then form a conductive loop, which may be a more effective antenna, resulting in a longer lived spark. When dielectric breakdown occurs in air, some ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 and nitrogen oxide
Nitrogen oxide
Nitrogen oxide can refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds:* Nitric oxide, also known as nitrogen monoxide, , nitrogen oxide* Nitrogen dioxide , nitrogen oxide...

s are formed, both of which are unhealthy in large quantities.

It is possible for metal objects to be microwave-oven compatible, although experimentation by users is not encouraged. Microwaving an individual smooth metal object without pointed ends, for example, a spoon or shallow metal pan, usually does not produce sparking. Thick metal wire racks can be part of the interior design in microwave ovens (see illustration). In a similar way, the interior wall plates with perforating holes which allow light and air into the oven, and allow interior-viewing through the oven door, are all made of conductive metal formed in a safe shape.

The effect of microwaving thin metal films can be seen clearly on a Compact Disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

 or DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

 (particularly the factory pressed type). The microwaves induce electric currents in the metal film, which heats up, melting the plastic in the disc and leaving a visible pattern of concentric and radial scars. Similarly, china with thin metal films can also be destroyed or damaged by microwaving. Aluminum foil is thick enough to be used in microwave ovens as a shield against heating parts of food items, if the foil is not badly warped. When wrinkled, aluminum foil is generally unsafe in microwaves, as manipulation of the foil causes sharp bends and gaps that invite sparking. The USDA recommends that aluminum foil used as a partial food shield in microwave cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.

Another hazard is the resonance of the magnetron tube itself. If the microwave is run without an object to absorb the radiation, a standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...

 will form. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to overload and burn out. For the same reason, dehydrated food, or food wrapped in metal which does not arc, is problematic for overload reasons, without necessarily being a fire hazard.

Certain foods such as grape
Grape
A grape is a non-climacteric fruit, specifically a berry, that grows on the perennial and deciduous woody vines of the genus Vitis. Grapes can be eaten raw or they can be used for making jam, juice, jelly, vinegar, wine, grape seed extracts, raisins, molasses and grape seed oil. Grapes are also...

s, if carefully arranged, can produce electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

. A naked flame which comprises conductive plasma, will do the same. Therefore, burning candles or other burning objects should not be put into a microwave oven, unless this is the desired effect.

The high electrical fields generated inside a microwave often can be illustrated by placing a radiometer
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....

 or neon glow-bulb inside the cooking chamber, creating glowing plasma inside the low-pressure bulb of the device.

Direct microwave exposure

Direct microwave exposure is not generally of any hazard, as microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which the oven is constructed. Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary. According to the United States Food and Drug Administration's Center for Devices and Radiological Health, a U.S. Federal Standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts of microwave radiation per square centimeter at approximately (2 in) from the surface of the oven. This is far below the exposure level currently considered to be harmful to human health.

The radiation produced by a microwave oven is non-ionizing. It therefore does not have the cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...

 risks associated with ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...

 such as X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

s and high-energy particles
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

. Long-term rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from microwave radiation even with chronic exposure levels, i.e., large fraction of one's life span, far larger than humans are likely to encounter from any leaking ovens. However, with the oven door open, the radiation may cause damage by heating; as with any cooking device. Every microwave oven sold has a protective interlock
Interlock (engineering)
Interlocking is a method of preventing undesired states in a state machine, which in a general sense can include any electrical, electronic, or mechanical device or system....

 so that it cannot be run when the door is open or improperly latched.

There are, however, a few cases where people have been exposed to direct microwave exposure from malfunctioning microwave ovens, or where infants have been placed inside them, resulting in microwave burn
Microwave burn
Microwave burns are burn injuries caused by thermal effects of microwave radiation absorbed in a living organism.In comparison with radiation burns caused by ionizing radiation, where the dominant mechanism of tissue damage is internal cell damage caused by free radicals, the primary damage...

s.

See also

  • Induction cooker
    Induction cooker
    An induction cooker uses induction heating for cooking. Unlike other forms of cooking, heat is generated directly in the pot or pan , as opposed to being generated in the stovetop by electrical coils or burning gas...

  • Microwave chemistry
    Microwave chemistry
    Microwave chemistry is the science of applying microwave irradiation to chemical reactions. Microwaves act as high frequency electric fields and will generally heat any material containing mobile electric charges, such as polar molecules in a solvent or conducting ions in a solid...

  • Robert V. Decareau
    Robert V. Decareau
    Robert Vincent Decareau was an American food scientist who was involved in the development of microwave applications in food technology, specifically technology that would lead to the development of the microwave oven. He also served in the United States Navy during World War II and in the United...

  • Thelma Pressman
    Thelma Pressman
    Thelma Pressman was a pioneering microwave cooking consultant, product development consultant, and cookbook author. In 1969 she opened the first microwave cooking school in the United States. She was the author of several microwave cookbooks and was a regular columnist for Bon Appétit magazine...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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