Microwave oven

File:NN-K125MBGPG Grill-Mikrowelle silber Panasonic.gif

A modern microwave oven

A wall-mounted combined microwave and fan-assisted oven, with the door open

A microwave oven (often referred to colloquially simply as a "microwave") is a kitchen appliance that heats food by dielectric heating. This is accomplished by using microwave radiation to heat polarized molecules within the food. This excitation is fairly uniform in the outer 1 inch (25 mm) to 1.5 inches (38 mm) inches of a dense (high water content) food item, leading to food being 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 after World War II 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, which had been acquired in 1965 by Raytheon.

Basic microwave ovens heat foods quickly and efficiently, but, unlike conventional ovens, do not brown or bake food. This makes them unsuitable for cooking certain foods, or to achieve certain culinary effects. Additional kinds of heat sources can be added to microwave packaging, or into combination microwave ovens, to produce these other heating effects.

History

Microwave ovens, several of which are from the 1980s

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 as is described in this patent, is (like induction heating) an electromagnetic heating effect which is the result of the so-called near field effects that exist in an electromagnetic cavity that is small compared with the wavelength 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 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 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 transmitters were developed and widely disseminated by the Allies of World War II, using magnetron technology. Percy Spencer, an American self-taught engineer from Howland, Maine, was working at the time building magnetrons for radar sets, with the American company Raytheon. He was working on an active radar set when he noticed that a Mr. Goodbar 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 Hayley Johnson traveled the world in search of the perfect microwave oven. 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 metres (5 ft 11 in) tall, weighed 340 kilograms (750 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. 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 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 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, 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, 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, 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 microprocessors 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

Further information: dielectric heating

An LG-brand microwave oven

A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (GHz)—a wavelength of 122 millimetres (4.80 in)—through the food. Microwave radiation is between common radio and infrared frequencies. Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. 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 and kinetic energy of atoms), is heat.

Microwave heating is more efficient on liquid water than on frozen water, where the molecules are not free to rotate. 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 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 runaways 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".

Design

A magnetron with section removed (magnet is not shown)

A microwave oven consists of:

• a high voltage power source, commonly a simple transformer or an electronic power converter, which passes energy to the magnetron
• a high voltage capacitor connected to the magnetron, transformer and via a diode to the case.
• a cavity magnetron, which converts high-voltage electric energy to microwave radiation
• a magnetron control circuit (usually with a microcontroller)
• a waveguide (to control the direction of the microwaves)
• a cooking chamber

Nearly all modern microwave ovens have a control panel with an LED, liquid crystal or vacuum fluorescent display (early models used an analog dial-type timer). 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 entrées, and popcorn, which when pressed cook the item for a preprogrammed time. In such cases a button for warming non-carbonated beverages (implying coffee) will also be present, along with another for heating and boiling water (including tea). 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.

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 bands 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 cycles 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 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. A convection microwave oven 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 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 halogen bulbs to their convection microwave models, marketing them under names such as "Speedcook", "Advantium" and "Optimawave" to emphasize their ability to cook food rapidly and with good browning. The bulbs heat the food's surface with infrared (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 in foods primarily made up of carbohydrates and Maillard reactions 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, sometimes an accessory browning tray is used, usually composed of glass or porcelain. It makes food crisp by oxidising the top layer until it turns brown. Ordinary plastic cookware is unsuitable for this purpose because it could melt.

Frozen dinners, pies, and microwave popcorn bags often contain a thin susceptor made from aluminium 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 (20 in) wide, 35 centimetres (14 in) deep and 30 centimetres (12 in) tall. These ovens are rated between 500 and 1000 watts and have less than 28 litres (0.99 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 (20 in) wide. Their interiors are typically between 30 and 45 litres (1.1 and 1.6 cu ft), 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 35 centimetres (9.8 by 13.8 in) 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 (20 in) wide, as much as 50 centimetres (20 in) deep, and at least 30 centimetres (12 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 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.

Uses

Microwave ovens are generally used for time efficiency in both commercial applications, such as restaurants, in the office and at home, rather than for cooking quality. Professional chefs generally find microwave ovens to be of limited usefulness because browning, caramelization, and other flavour-enhancing reactions cannot occur due to the temperature range, and their experience with the necessary techniques particular to the microwave is typically very limited; different reactions can occur even within the same temperature ranges, effectively rendering a master chef worse than inexperienced when handling a microwave oven, due to a wealth of irrelevant knowledge about how conventional heating affects food. People who want fast cooking times, however, can use microwave ovens to prepare food or to reheat stored food (including commercially available pre-cooked frozen dishes) in only a few minutes, and many have developed their own personal techniques to optimize flavor to their own liking. Microwave ovens can also be used to defrost items that will later be cooked by traditional methods, reducing the time it takes to defrost foods naturally. Microwave ovens are also useful for the ease with which they can perform some traditionally cumbersome kitchen tasks, such as softening butter or melting chocolate.

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.

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 that will stay hot for some time. Likewise, when taking a casserole 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, 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 in potatoes, however unlike deep-frying, it is of only limited effectiveness in reducing glycoalkaloid (i.e. Solanine) 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, and bacterial contamination may not be repressed if the safe temperature is not reached, resulting in foodborne illness, 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 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 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. Video demonstration 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, where dielectric constant increases with temperature, microwave heating can cause localized thermal runaway. Under certain conditions, glass can exhibit thermal runaway in a microwave to the point of melting. Video

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 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, 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.

Effects on food and nutrients

Raisins when cooked in a microwave produce considerable smoke

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 from the active to inactive form, making approximately 30-40% of the B₁₂ contained in foods unusable by mammals. Microwaving broccoli loses 74% or more of phenolic compounds (97% of flavonoids), 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 the water-soluble vitamins folic acid, thiamin and riboflavin, with the exception of ascorbic acid, of which 28.8% which is lost (vs. 16% with boiled water blanching).

Hazards

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

Liquids can superheat 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 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) which may be violent enough to eject the boiling liquid from the container and produce severe scalding. It is commonly, but wrongly, thought that only distilled water exhibits this behaviour.

Closed containers and eggs can explode when heated in a microwave oven due to the increased pressure from steam. 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 (beryllia) added. The beryllium 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; therefore, broken ceramic insulators or magnetrons should not be handled. This is obviously only a danger 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 to some degree, resulting in an electric current. 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 (sparks) when microwaved. This includes cutlery, 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: This is because 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 megavolts per meter (3×10⁶ V/m). The air forms a conductive plasma, 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 and nitrogen oxides are formed, both of which are unhealthy in large quantities.

A microwave oven with a metal shelf

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 or DVD (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 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 grapes, if carefully arranged, can produce electric arc. 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 or neon glow-bulb inside the cooking chamber, creating glowing plasma inside the low-pressure bulb of the device.

Direct microwave exposure

Further information: Microwave burn and Microwave § Health effects

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 5 cm (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 risks associated with ionizing radiation such as X-rays and high-energy particles. Long-term rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from 2.45 GHz 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 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 burns.

See also

• Induction cooker
• Microwave chemistry
• Robert V. Decareau
• IEEE 802.11 Wi-Fi operates using the same frequency bands.
• Thelma Pressman

External links

• Ask a Scientist Chemistry Archives, Argonne National Laboratory
• How a microwave oven works Description with circuit diagrams
• How microwaves and microwave ovens work Java animation suitable for young people
• Further Reading On The History Of Microwaves and Microwave Ovens
• Microwave oven history from American Heritage magazine
• Microwave Oven Radiation, USFDA Center for Devices and Radiological Health
• Superheating and microwave ovens
• Superheating and Microwave Ovens, University of New South Wales (includes video)
• MicrowaveCam.com Videos of the inside of the microwave oven compartment
• "The Microwave Oven" Short explanation of microwave oven in terms of microwave cavities and waveguides, intended for use in a class in Electrical Engineering
• U.S. patent 2,147,689 - Method and apparatus for heating dielectric materials