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A microwave oven
Various microwave ovens; many from the 1980s


A microwave oven, or a microwave, is a kitchen appliance that cook or heats food by dielectric heating. This is accomplished by using microwave radiation to heat water and other polarized molecules within the food. This excitation is fairly uniform, leading to food being adequately heated throughout (except in thick objects), a feature not seen in any other heating technique.

Basic microwave ovens heat food quickly and efficiently, but do not brown or bake food in the way conventional ovens do. This makes them unsuitable for cooking certain foods, or to achieve certain effects. Additional kinds of heat sources can be added to microwave packaging, or into combination microwave ovens, to add these additional effects.

Microwaving food raises several safety issues, but also reduces certain risks, such as that of fire from high temperature heat sources.

History

Cooking food with microwaves was discovered accidentally in the 1940s. Percy Spencer, an Americamarker self-taught engineer from Howland, Mainemarker, was building magnetrons for radar sets with the American company Raytheon. He was working on an active radar set when he noticed that a peanut chocolate bar 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 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 U.S. patent for Spencer's microwave cooking process and an oven that heated food using microwave energy was placed in a Boston restaurant for testing. In 1947, the company built the Radarange, the first microwave oven in the world. It was almost tall, weighed and cost about 5000US dollars each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. 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 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, which introduced the first popular home model, the countertop Radarange, in 1967 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 indefinitely. The new oven was shown at a trade show in Chicagomarker, 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 in Japanmarker, which had learned to build less expensive units by re-engineering a cheaper magnetron, was faster.

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 have a microwave oven.

Principles

A microwave oven works by passing non-ionizing microwave radiation, usually at a frequency of 2.45 gigahertz (a wavelength of ), 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 positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. This molecular movement represents heat which is then dispersed as the rotating molecules hit other molecules and put them into motion.

Microwave heating is more efficient on liquid water than on fats and sugars (which have a smaller molecular dipole moment), and also more efficient than on frozen water (where the molecules are not free to rotate). 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 , also heat water and food perfectly well.

A common misconception is that microwave ovens cook food from the "inside out". 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 better.

Design

A magnetron with section removed (magnet is not shown)


A microwave oven consists of:



The frequencies used in microwave ovens were chosen based on two constraints. The first is that they should be in one of the ISM 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 the user to choose between several power levels, including one or more defrosting levels. In most ovens, however, there is no change in the intensity of the microwave radiation; instead, the magnetron is turned on and off in 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 ovens, 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 truly continuous low-power microwave heating.

The cooking chamber itself is a Faraday cage enclosure which prevents the microwaves from escaping into the environment. The oven door is usually a glass panel for easy viewing, but has a layer of conductive mesh to maintain the shielding. Because the size of the perforations in the mesh is much less than the 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 is a combination of a standard microwave and a convection oven. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwaves are more expensive than a conventional microwave and are not considered cost-effective if primarily used just to heat drinks or frozen food. They are usually used for cooking prepared dishes. Convection microwaves also suffer from smoke and burning odors when microwaved foods spatter grease and food particles. This spatter collects on the heating elements and does not do anything when used solely for microwaving, but it all burns off when later used for convection.

More recently, certain manufacturers have added a high power quartz halogen bulb to their convection microwave models while marketing them under names such as "Speedcook", "Advantium" and "Optimawave" to emphasize their ability to cook food rapidly and with the same browning results typically expected of a conventional oven. This is achieved using the high intensity halogen lights at the top of the microwave to deposit large amounts of infrared radiation to the surface of the food. The food browns while also being heated internally by the microwave radiation and heated through conduction and convection by contact with heated air - produced by the conventional convection portion of the unit. The IR energy which is rapidly delivered to the outer surface of food by the lamps is sufficient to initiate browning caramelization in foods primarily made up of sugars (carbohydrates), and Maillard reactions of those foods primarily made up of protein. These reactions in food produce a texture and taste much more similar to that typically expected of conventional oven cooking rather than the bland boiled and steamed taste that microwave-only cooking tends to create.

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 since 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. Instructions are included to leave the item in the box while cooking it. The metal film absorbs microwave energy efficiently and consequently becomes extremely hot and radiates in the infrared, concentrating the heating of the popcorn oil, or even browning surfaces of frozen foods. Heating packages or trays containing susceptors are designed for a single use and are then discarded with the other food packaging waste.

Sizes

Consumer microwaves typically come in two types in three sizes:

Compact: A compact microwave, also called small, portable, or countertop, is the smallest type of typically available consumer microwave. Compacts are the most popular size of microwave oven, dominating the market. A typical model is no more than wide, or less deep and or less tall. These ovens are rated between 500 and 1000 watts of power and measure less than in capacity. These ovens are primarily used for reheating food and making microwave meals and popcorn. The largest models will accommodate a round casserole dish and are suitable for light cooking. These ovens are not made to cook large amounts of food. Typically these models cost less than USD$100 (around £50).
Medium-capacity: These microwaves are larger than compact microwaves. Their heights and depths are only marginally larger than compacts, but they are typically wide or more. Their interiors are typically between and power runs from 1000–1500 watts. These are the standard "family" sized microwave. They tend to have a few more "auto-cook" features, and some incorporate grills or even conventional oven heating elements.
Large-capacity: These are big microwaves designed for cooking large meals. Large-capacity ovens can handle casserole dishes, and cook tall items like roasts or turkey breast, with a large number of "auto-cook" and precise temperature control measures. Large capacity ovens normally use over 2000 watts and have over of capacity. These ovens are normally well over wide, as much as deep, and or more high.
Built-in: Built-in microwaves are ovens that are built into the cabinetry similar to traditional ovens. These ovens are typically more expensive than similar sized models. Some built in microwaves are combined with an exhaust fan for installation above a cooktop.


Increasingly, microwaves are sold with additional features including combining them with convection cooking, "top browning" elements that will brown food (similar to the broiling function on an oven) and even rotisseries in the oven. Most microwaves have white enamel interiors but high end models are often stainless steel, like the original Radarange.


Portable / Desktop Mini-Microwave Ovens: These are the smallest kinds of microwaves in the market. The common ones measure around tall, wide and deep. Some of the experimental models on trial are as small as tall, wide and deep. These are ultra small microwave made to increase portability.


These are tailored to be powered from common sources in such places too. Some have DC power supply cords to connect to the charger sockets in the car or clips to derive power directly from the car battery.


Other manufacturers are experimenting with USB power ports from desktop computers for use at work. However, given their size and potential power sources, these microwaves have relatively lower wattages (around 300 W), leading to longer cooking times. Typical microwaves use anywhere between 750 W and 1100 W.


Uses

Microwave ovens are generally used for time efficiency in both industrial applications such as restaurants and at home, rather than for cooking quality, although some modern recipes using microwave ovens rival recipes using traditional ovens and stoves. 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. On the other hand, people who want fast cooking times can use microwave ovens to prepare food or to reheat stored food (including commercially available pre-cooked frozen dishes) in only a few minutes. Microwave Ovens can also be used to defrost items that will later be cooked by traditional methods, cutting the time it takes to defrost foods naturally. Microwave ovens are also useful for the ease in which they can perform some traditionally cumbersome kitchen tasks, such as softening butter or melting chocolate. Popcorn is one example of a very popular item with microwave oven users.

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. This waste heat, along with heat from the food, is exhausted as warm air through cooling vents.

A consideration for rating the efficiency of a microwave oven is to assess how much energy is wasted by using other forms of cooking. For example, when heating water for a coffee, a microwave oven heats just the mugful of water itself. When using a kettle, an element heats the kettle itself plus the water plus any extra water which is then left unused in the kettle, although electric kettles automatically shut off as soon as the water is boiled. Depending upon the size of the kettle and the amount of excess water, the efficiency of microwave ovens can be comparable, though if only the required amount of water is used electric kettles are generally more efficient.

Cooking in conventional ovens entails heating the internal structure of the oven and the air it contains to cooking temperature and, additionally, it involves maintaining that temperature against convective and radiative losses of heat for a longer time than is usual with a microwave oven. The efficiencies of conventional cooking methods can be difficult to quantify but tend to be lower.

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 is rarely much hotter than . 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 . 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.

Heating characteristics

In a microwave oven, food may be heated for so short a time that it is cooked unevenly, since 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 bacteria contamination may not be killed 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.

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 the 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 also cannot dissipate the heat due to the low thermal conductivity of the bread. The result is frequently the overheating of the berries relative to the rest of the food. The low power levels which mark the "defrost" oven setting are designed to allow time for heat to be conducted from areas which absorb heat more readily to those which heat more slowly. 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 contain ceramic or aluminum-flake containing materials 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. 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

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. 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.

Spinach retains nearly all its folate when cooked in a microwave; in comparison, it loses about 77 percent when cooked on a stove, because food on a stove is typically boiled, leaching out nutrients. Steamed vegetables tend to maintain more nutrients when cooked on a stovetop than in a microwave. Bacon cooked by microwave has significantly lower levels of carcinogenic nitrosamines than conventionally cooked bacon.

Hazards



Liquids, when heated in a microwave oven in a container with a smooth surface, can superheat, that is, reach temperatures that are a few degrees in temperature above their normal boiling point, without actually boiling. The boiling process can start explosively when the liquid is disturbed, such as when the operator takes hold of the container to remove it from the oven or while adding impurities such as powdered creamer or sugar, and can then result in a violent burst of water and vapor resulting in liquid and steam burns. A common myth states that only distilled water can exhibit this behaviour; but this is not true.

Closed containers and eggs can explode when heated in a microwave oven due to the increasing pressure of steam. Products that are heated 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. Microwave oven manuals frequently warn of such hazards. Because the microwave oven's cavity is enclosed and metal, fires are generally well contained. Simply switching off the oven and allowing the fire to consume available oxygen with the door closed will typically contain damage to the oven itself.

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, aluminium foil, ceramics decorated with metal, 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 tine of the fork resonate with the microwave radiation and produce high voltage at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolt 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 CD 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. It can also be illustrated by placing a radiometer inside the cooking chamber, creating plasma inside the vacuum chamber.

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 'cook' itself and burn out. Thus dehydrated food, or food wrapped in metal which does not arc, is problematic without being an obvious fire hazard.

Some magnetrons have ceramic insulators with a piece of beryllium oxide (beryllia) added—these ceramics often appear somewhat pink or purple-colored. The beryllium in such oxides is a serious chemical hazard if crushed and ingested (eg, 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 (ie, cracked ceramics) or upon opening and handling the magnetron directly, and as such should not occur during normal usage.

Certain foods, if carefully arranged, can also produce arcing, such as grapes. A naked flame, being made of conductive plasma, will do the same, so burning candles, matches, paper, etc should not be put in a microwave oven.

Microwave radiation

The microwaves emitted by the source in a microwave oven are confined in the oven by the material out of which microwave 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 US 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 inches 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, ultraviolet light, 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 (i.e., large fraction of life span) exposure levels, 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. Nearly every microwave sold has a protective interlock so that it cannot be run when the door is open or improperly latched.

Cultural references

Perhaps the earliest fictional references to what would now be called microwave ovens were in Robert Heinlein's novels. In his 1950 Farmer in the Sky, which takes place sometime in the 21st century, the narrator describes how dinners are made in the future:

I grabbed two Syntho-Steaks out of the freezer and slapped them in quickthaw, added a big Idaho baked potato for Dad...then stepped up the gain on the quickthaw so that the spuds would be ready when the steaks were.


A Heinlein novel from two years earlier, Space Cadet, also mentions the use of "high-frequency heating" to prepare frozen, pre-cooked foods.

See also



References

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