Microwave oven
A microwave oven or simply microwave is an electric oven that heats and cooks food by exposing it to electromagnetic radiation in the microwave frequency range.[1] This induces polar molecules in the food to rotate and produce thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogeneous, high-water-content food item.
The development of the
Microwave ovens are a common
Microwave ovens have a limited role in professional cooking,[2] because the boiling-range temperatures of a microwave oven do not produce the flavorful chemical reactions that frying, browning, or baking at a higher temperature produces. However, such high heat sources can be added to microwave ovens in the form of a convection microwave oven.[3]
History
Early developments
The exploitation of high-frequency
The 1937 United States patent application by Bell Laboratories states:[6]
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
Cavity magnetron
The invention of the
Discovery
In 1945, the heating effect of a high-power microwave beam was accidentally discovered by Percy Spencer, an American self-taught engineer from Howland, Maine. Employed by Raytheon at the time, he noticed that microwaves from an active radar set he was working on started to melt a Mr. Goodbar candy bar he had in his pocket. The first food deliberately cooked with Spencer's microwave oven was popcorn, and the second was an egg, which exploded in the face of one of the experimenters.[12][13]
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 8 October 1945, Raytheon filed a United States patent application 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.[14]
Another early discovery of microwave oven technology was by British scientists, including
Commercial availability
In 1947, Raytheon built the "Radarange", the first commercially available microwave oven.[18] It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5,000 ($68,000 in 2023 dollars) each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. The name was the winning entry in an employee contest.[19] An early 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$2,000 to US$3,000 ($23,000 to $34,000 in 2023 dollars). Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952.[20] Under contract to Whirlpool, Westinghouse, and other major appliance manufacturers looking to add matching microwave ovens to their conventional oven line, Tappan produced several variations of their built-in model from roughly 1955 to 1960. Due to maintenance (some units were water-cooled), in-built requirement, and cost—US$1,295 ($15,000 in 2023 dollars)—sales were limited.[21]
Japan's
In the 1960s,[specify] Litton bought Studebaker's Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton developed a new configuration of the microwave oven: 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: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a trade show in Chicago,[citation needed] and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was even faster in Japan, due to a less expensive re-engineered magnetron. Several other companies joined in the market, and for a time most systems were built by defence contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business.
Residential use
While uncommon today, combination microwave-ranges were offered by major appliance manufacturers through much of the 1970's as a natural progression of the technology. Both Tappan and General Electric offered units that appeared to be conventional stove top/oven ranges, but included microwave capability in the conventional oven cavity. Such ranges were attractive to consumers since both microwave energy and conventional heating elements could be used simultaneously to speed cooking, and there was no loss of countertop space. The proposition was also attractive to manufacturers as the additional component cost could better be absorbed compared with countertop units where pricing was increasingly market-sensitive.
By 1972, Litton (Litton Atherton Division, Minneapolis) introduced two new microwave ovens, priced at $349 and $399, to tap into the market estimated at $750 million by 1976, according to Robert I Bruder, president of the division.[23] While prices remained high, new features continued to be added to home models. Amana introduced automatic defrost in 1974 on their RR-4D model, and was the first to offer a microprocessor controlled digital control panel in 1975 with their RR-6 model.
The late 1970s saw an explosion of low-cost countertop models from many major manufacturers.
Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in developed countries. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971;[24] the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997.[24][25] In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily.[26] In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998.[27] In France, 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004.[28]
Adoption has been slower in
Consumer household microwave ovens usually come with a cooking power of between 600 and 1200 watts. Microwave cooking power, also referred to as output wattage, is lower than its input wattage, which is the manufacturer's listed power rating.
The size of household microwave ovens can vary, but usually have an internal volume of around 20 liters (1,200 cu in; 0.71 cu ft), and external dimensions of approximately 45–60 cm (1 ft 6 in – 2 ft 0 in) wide, 35–40 cm (1 ft 2 in – 1 ft 4 in) deep and 25–35 cm (9.8 in – 1 ft 1.8 in) tall.[31]
Microwaves can be turntable or flatbed. Turntable ovens include a glass plate or tray. Flatbed ones do not include a plate, so they have a flat and wider cavity.[32][33][34]
By position and type, US DOE classifies them as (1) countertop or (2) over the range and built-in (wall oven for a cabinet or a drawer model).[32]
Traditional microwaves rely on internal high voltage power from a line/mains transformer, but many newer models are powered by an inverter. Inverter microwaves can be useful for achieving more even cooking results, as they offer a seamless stream of cooking power.[citation needed]
A traditional microwave only has two power output levels, fully on and fully off. Intermediate heat settings are achieved using duty-cycle modulation and switch between full power and off every few seconds, with more time on for higher settings.
An inverter type, however, can sustain lower temperatures for a lengthy duration without having to switch itself off and on repeatedly. Apart from offering superior cooking ability, these microwaves are generally more energy-efficient.[35][34][36]
As of 2020[update], the majority of countertop microwave ovens (regardless of brand) sold in the United States were manufactured by the Midea Group.[37]
Categories
Domestic microwave ovens are typically marked with the microwave-safe symbol, next to the device's approximate IEC 60705 output power rating, in watts (typically either: 600W, 700W, 800W, 900W, 1000W), and a voluntary Heating Category (A-E).[38]
Principles
A microwave oven heats food by passing
It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. Instead, microwave ovens heat by causing molecules to spin under the influence of a constantly changing electric field, usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times. Typically, consumer ovens work around a nominal 2.45
A microwave oven takes advantage of the electric
Defrosting
Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted. Defrosting is done at a low power setting, allowing time for conduction to carry heat to still frozen parts of food. Dielectric heating of liquid water is also temperature-dependent: At 0 °C, dielectric loss is greatest at a field frequency of about 10 GHz, and for higher water temperatures at higher field frequencies.[45]
Fats and sugar
Although fats and sugar typically absorb energy less efficiently than water, paradoxically their temperatures rise faster and higher than water when cooking: Fats and oils require less energy delivered per gram of material to raise their temperature by 1 °C than does water (they have lower specific heat capacity) and they begin cooling off by "boiling" only after reaching a higher temperature than water (the temperature they require to vaporize is higher), so inside microwave ovens they normally reach higher temperatures – sometimes much higher.[45] This can induce temperatures in oil or fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional broiling (UK: grilling), braising, or deep fat frying.
The effect is most often noticed by consumers from unexpected damage to plastic containers when microwaving foods high in sugar, starch, or fat generates higher temperatures. Foods high in water content and with little oil rarely exceed the boiling temperature of water and do not damage plastic.
Cookware
Cookware must be transparent to microwaves. Conductive cookware, such as metal pots, reflects microwaves, and prevents the microwaves from reaching the food. Cookware made of materials with high electrical permittivity will absorb microwaves, resulting in the cookware heating rather than the food. Cookware made of melamine resin is a common type of cookware that will heat in a microwave oven, reducing the effectiveness of the microwave oven and creating a hazard from burns or shattered cookware.
Thermal runaway
Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave oven to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of molten rock. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.[46]
Penetration
Another misconception is that microwave ovens cook food "from the inside out", meaning from the center of the entire mass of food outwards. This idea arises from heating behavior seen if an absorbent layer of water lies beneath a less absorbent drier layer at the surface of a food; in this case, the deposition of heat energy inside a food can exceed that on its surface. This can also occur if the inner layer has a lower heat capacity than the outer layer causing it to reach a higher temperature, or even if the inner layer is more thermally conductive than the outer layer making it feel hotter despite having a lower temperature. In most cases, however, with uniformly structured or reasonably homogeneous food item, microwaves are absorbed in the outer layers of the item at a similar level to that of the inner layers.
Depending on water content, the depth of initial heat deposition may be several centimetres or more with microwave ovens, in contrast with
Energy consumption
In use, microwave ovens can be as low as 50% efficient at converting electricity into microwaves,[47] but energy efficient models can exceed 64% efficiency.[48] Stovetop cooking is 40-90% efficient depending on the type of appliance used.[49]
Because they are used fairly infrequently, the average residential microwave oven consumes only 72 kWh per year.[50] Globally, microwave ovens used an estimated 77 TWh per year in 2018, or 0.3% of global electricity generation.[51]
A 2000 study by Lawrence Berkeley National Laboratory found that the average microwave drew almost 3 watts of standby power when not being used,[52] which would total approximately 26 kWh per year. New efficiency standards imposed in 2016 by the United States Department of Energy require less than 1 watt, or approximately 9 kWh per year, of standby power for most types of microwave ovens.[53]
Components
A microwave oven generally consists of:
- a high-voltage DC power source, either:
- a large high voltage transformer with a voltage doubler (a high-voltage capacitor and a diode)
- an electronic power converterusually based around an inverter.
- a cavity magnetron, which converts the high-voltage DC electric energy to microwave radiation
- a magnetron control circuit (usually with a microcontroller)
- a short waveguide(to couple microwave power from the magnetron into the cooking chamber)
- a turntable and/or metal wave guide stirring fan
- a control panel
In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. (One variant of the GE Spacemaker had two taps on the transformer primary, for high and low power modes.) Usually choice of power level does not affect intensity of the microwave radiation; instead, the magnetron is cycled on and off every few seconds, thus altering the large scale duty cycle. Newer models use inverter power supplies that use pulse-width modulation to provide effectively continuous heating at reduced power settings, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating.[54][35][34][36]
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
The cooking chamber is similar to a
Control panel
Modern microwave ovens use either an analog dial-type
Power settings are commonly implemented not by actually varying the power output, but by switching the emission of microwave energy off and on at intervals. The highest setting thus represents continuous power. Defrost might represent power for two seconds followed by no power for five seconds. To indicate cooking has completed, an audible warning such as a bell or a beeper is usually present, and/or "End" usually appears on the display of a digital microwave.
Microwave control panels are often considered awkward to use and are frequently employed as examples for user interface design.[56]
Variants and accessories
A variant of the conventional microwave oven is the convection microwave oven. A convection microwave oven is a combination of a standard microwave oven and a convection oven. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwave ovens are more expensive than conventional microwave ovens. Some convection microwave ovens—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. Some ovens use high speed air; these are known as impingement ovens and are designed to cook food quickly in restaurants, but cost more and consume more power.
In 2000, some manufacturers began offering high power
In order to aid
Heating characteristics
Microwave ovens produce heat directly within the food, but despite the common misconception that microwaved food cooks from the inside out, 2.45 GHz microwaves can only penetrate approximately 1 centimeter (0.39 in) into most foods. The inside portions of thicker foods are mainly heated by heat conducted from the outer 1 centimeter (0.39 in).[58][59]
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 oven 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 darkened which can 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.[60]
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
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 either use low power levels or turn the power off and on repeatedly - 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 can take place by placing food off-center on the turntable tray instead of exactly in the center, as this results in more even heating of the food throughout.[62]
There are microwave ovens on the market that allow full-power defrosting. They do this by exploiting the properties of the electromagnetic radiation LSM modes. LSM full-power defrosting may actually achieve more even results than slow defrosting.[63]
Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may include materials that contain ceramic or aluminium flakes, which are designed to absorb microwaves and heat up, 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.[64]
Effects on food and nutrients
Any form of cooking diminishes overall nutrient content in food, particularly
Spinach retains nearly all its
Safety benefits and features
All microwave ovens use a timer to switch off the oven at the end of the cooking time.
Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, unless it is an
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 with baking in the oven or frying, because it eliminates the formation of tars and
Use in cleaning kitchen sponges
Studies have investigated the use of the microwave oven to clean non-metallic
A 2017 study was less affirmative: about 60% of the germs were killed but the remaining ones quickly re-colonized the sponge.[73]
Issues
High temperatures
Closed containers
Closed containers, such as
Fires
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 the use of microwave ovens results in additional hazard.
Superheating
In rare cases, water and other homogeneous liquids can superheat[74][75] 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 cause severe scalding.[76]
Metal objects
Contrary to popular assumptions, metal objects can be safely used in a microwave oven, but with some restrictions.
Any object containing pointed metal can create an
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, porcelain with thin metal films can also be destroyed or damaged by microwaving. Aluminium 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, aluminium foil is generally unsafe in microwaves, as manipulation of the foil causes sharp bends and gaps that invite sparking. The USDA recommends that aluminium foil used as a partial food shield in microwave oven cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.[80]
Another hazard is the resonance of the magnetron tube itself. If the microwave oven is run without an object to absorb the radiation, a
Certain foods such as grapes, if properly arranged, can produce an electric arc.[81] Prolonged arcing from food carries similar risks to arcing from other sources as noted above.
Some other objects that may conduct sparks are plastic/holographic print Thermos flasks and other heat-retaining containers (such as Starbucks novelty cups) or cups with metal lining. If any bit of the metal is exposed, all the outer shell can burst off the object or melt.[citation needed]
The high electrical fields generated inside a microwave oven 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
Direct microwave exposure is not generally possible, 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. Furthermore, ovens are equipped with redundant safety interlocks, which remove power from the magnetron if the door is opened. This safety mechanism is required by United States federal regulations.
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 life span) far larger than humans are likely to encounter from any leaking ovens.[86][87] However, with the oven door open, the radiation may cause damage by heating. Microwave ovens are sold with a protective interlock so that it cannot be run when the door is open or improperly latched.
Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off, any more than light from an electric lamp remains in the walls and furnishings of a room when the lamp is turned off. They do not make the food or the oven radioactive. In contrast with conventional cooking, the nutritional content of some foods may be altered differently, but generally in a positive way by preserving more micronutrients – see above. There is no indication of detrimental health issues associated with microwaved food.[88]
There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action.[89][90] This exposure generally results in physical burns to the body, as human tissue, particularly the outer fat and muscle layers, has a similar composition to some foods that are typically cooked in microwave ovens and so experiences similar dielectric heating effects when exposed to microwave electromagnetic radiation.
Chemical exposure
The use of unmarked plastics for microwave cooking raises the issue of plasticizers leaching into the food,[91] or the plastics chemically reacting to microwave energy, with by-products leaching into the food,[92] suggesting that even plastic containers marked "microwavable" may still leach plastic by-products into the food.[citation needed]
The plasticizers which received the most attention are bisphenol A (BPA) and phthalates,[91][93] although it is unclear whether other plastic components present a toxicity risk. Other issues include melting and flammability. An alleged issue of release of dioxins into food has been dismissed[91] as an intentional red herring distraction from actual safety issues.
Some current plastic containers and food wraps are specifically designed to resist radiation from microwaves. 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 oven use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.[94]
Plastic containers can release microplastics into food when heated in microwave ovens.[95]
Uneven heating
Microwave ovens are frequently used for reheating leftover food, and bacterial contamination may not be repressed if the microwave oven is used improperly. If safe temperature is not reached, this can result in foodborne illness, as with other reheating methods. While microwave ovens can destroy bacteria as well as conventional ovens can, they cook rapidly and may not cook as evenly, similar to frying or grilling, leading to a risk of some food regions failing to reach recommended temperatures. Therefore, a standing period after cooking to allow temperatures in the food to equalize is recommended, as well as the use of a food thermometer to verify internal temperatures.[96]
Interference
Microwave ovens, although shielded for safety purposes, still emit low levels of microwave radiation. This is not harmful to humans, but can sometimes cause interference to Wi-Fi and Bluetooth and other devices that communicate on the 2.45 GHz wavebands; particularly at close range.[97] Conventional transformer ovens do not operate continuously over the mains cycle, but can cause significant slowdowns for many metres around the oven, whereas inverter based ovens can stop nearby networking entirely while operating.[98]
See also
- Countertop
- Electromagnetic reverberation chamber
- Induction cooker
- List of cooking appliances
- List of home appliances
- Microwave chemistry
- Peryton (astronomy)
- Robert V. Decareau
- Thelma Pressman
- Wall oven
Notes
- ^ Here "efficient" means that more energy is deposited and temperature rises faster, not necessarily that the temperature rises to a higher maximum. The maximum temperature is also a function of the material's specific heat capacity, which for most substances is lower than water. For a practical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have less heat capacity than the water they replace.[citation needed]
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External links
- U.S. patent 2,495,429: Percy Spencer's original patent
- Ask a Scientist Chemistry Archives Archived 26 February 2015 at the Wayback Machine, Argonne National Laboratory
- Further Reading On The History Of Microwaves and Microwave Ovens
- Microwave oven history from American Heritage magazine
- Superheating and Microwave Ovens, University of New South Wales (includes video)
- "The Microwave Oven": Short explanation of microwave oven in terms of waveguides, intended for use in a class in electrical engineering
- How Things Work: Microwave Ovens, David Ruzic, University of Illinois