Microwave oven

Early developments The exploitation of high-frequency radio waves for heating substances was made possible by the development of vacuum tube radio transmitters around 1920. By 1930 the application of short waves to heat human tissue had developed into the medical therapy of diathermy. At the 1933 Chicago World's Fair, Westinghouse demonstrated the cooking of foods between two metal plates attached to a 10 kW, 60 MHz shortwave transmitter. The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes. The 1937 United States patent application by Bell Laboratories states: However, lower-frequency dielectric heating, as described in the aforementioned patent, is (like induction heating) an electromagnetic heating effect, 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 radio frequency heating, at 10 to 20 megahertz (wavelength 30 to 15 meters, respectively). Heating from microwaves that have a wavelength that is small relative 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. Cavity magnetron developed by John Randall and Harry Boot in 1940 at the University of Birmingham, England The invention of the cavity magnetron made possible the production of electromagnetic waves of a small enough wavelength (microwaves). The cavity magnetron was a crucial component in the development of short wavelength radar during World War II. In 1937–1940, a multi-cavity magnetron was built by British physicist Sir John Turton Randall, FRSE and coworkers, for the British and American military radar installations in World War II. They invented a valve that could produce pulses of microwave radio energy at a wavelength of 10 cm, an unprecedented discovery. Sir Henry Tizard traveled to the US in late September 1940 to offer Britain's most valuable technical secrets including the cavity magnetron in exchange for US financial and industrial support (see Tizard Mission). Contracts were awarded to Raytheon and other companies for the mass production of the cavity magnetron. Discovery In 1945, the heating effect of a high-power microwave beam was independently and accidentally discovered by Percy Spencer, an American self-taught engineer from Howland, Maine. While employed at Raytheon, he noticed that microwaves from an active radar set he was working on started to melt a candy bar he had in his pocket. The first food deliberately cooked by Spencer was popcorn, and the second was an egg, which exploded in the face of one of the experimenters. To verify his finding, Spencer created a high-density electromagnetic field by feeding microwave power from a magnetron into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly. On October 8, 1945, Raytheon filed a 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. Commercial availability nuclear-powered cargo ship, installed In 1947, Raytheon built the "Radarange", the first commercially available microwave oven. It was almost tall, weighed and cost between $2,000 and $3,000 ($ to $ in 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. 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$3,000 ($ to $ in dollars). Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952. 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$1,295 ($ in dollars)—sales were limited. Japan's Sharp Corporation began manufacturing microwave ovens in 1961. Between 1964 and 1966, Sharp introduced the first microwave oven with a turntable, an alternative means to promote more even heating of food. In 1965, Raytheon, looking to expand their Radarange technology into the home market, acquired Amana to provide more manufacturing capability. In 1967, they introduced the first popular home model, the countertop Radarange, at a price of US$495 ($ in dollars). Unlike the Sharp models, a motor driven mode stirrer in the top of the oven cavity rotated allowing the food to remain stationary. Residential use microwave from 1965 While uncommon today, combination microwave-ranges were offered by major appliance manufacturers through much of the 1970s 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. 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. . By the late 1970s, technological advances led to rapidly falling prices. Often called "electronic ovens" in the 1960s, the name "microwave oven" later gained currency, and they are now informally called "microwaves". The late 1970s saw an explosion of low-cost countertop models from many major manufacturers. In addition to cooking food, microwave ovens are used for heating in many industrial processes. 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; the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997. In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily. In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998. In France, 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004. Adoption has been slower in less-developed countries, as households with disposable income concentrate on more important household appliances like refrigerators and ovens. In India, for example, only about 5% of households owned a microwave oven in 2013, well behind refrigerators at 31% ownership. However, microwave ovens are gaining popularity. In Russia, for example, the number of households with a microwave oven grew from almost 24% in 2002 to almost 40% in 2008. 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 , and external dimensions of approximately wide, deep and tall. Countertop microwaves vary in weight 23 – 45 lbs. Simply radiating microwaves into a chamber intrinsically results in very uneven energy distribution, over- and under-heating various parts of the food. Only the cheapest units lack mechanisms that mitigate this problem, requiring the user to more frequently stop the cooking to stir or move the food. For reasonably even heating, a microwave oven must have either multiple magnetrons, common in commercial units, a microwave stirrer, which is a fan-like mechanism in which microwave-reflective blades spin to vary the radiation pattern (usually at the top of the chamber, hidden above a microwave-transparent plastic cover), or a turntable, which effectively reduces the capacity of the oven to the space above the turntable, wasting at least nearly a quarter of the volume of a square chamber. Food is always placed above the bottom of the microwave-reflecting chamber, allowing microwave energy to reach the bottom of the food; in a flatbed unit the floor of the visible chamber is of a microwave-transparent material above the microwave-reflecting bottom, and in better units some of the microwave radiation enters through the transparent floor. In a non-flatbed unit there is either a glass tray shaped to fit the chamber, keeping the food some distance from the reflective bottom, or, in a unit with a turntable, the turntable itself is microwave-transparent and sits above the reflective bottom. 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). , the majority of countertop microwave ovens (regardless of brand) sold in the United States were manufactured by the Midea Group. 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). == Principles ==

A microwave oven heats food by passing microwave radiation through it, a process known as dielectric heating. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency between 300 MHz and 300 GHz. Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, so they do not interfere with other vital radio services. Dielectric heating takes advantage of the electric dipole structure of water molecules, fats, and many other substances in the food. These molecules have a partial positive charge at one end and a partial negative charge at the other. In an alternating electric field, they will rotate as they continually try to align themselves to the field. Once the electrical field's energy is initially absorbed, heat will gradually spread through the object similarly to any other contact with a hotter body. It is a common misconception that microwave ovens heat food at a special resonance of water molecules in the food. Instead, all polar molecules participate, and dielectric heating for each molecule can happen over a wide range of frequencies. Typically, consumer ovens work around a nominal 2.45 gigahertz (GHz) – a wavelength of in the 2.4 GHz to 2.5 GHz ISM band – while large industrial / commercial ovens often use 915 megahertz (MHz) – . Among other differences, the longer wavelength of a commercial microwave oven allows the initial heating effects to begin deeper within the food or liquid, and therefore become evenly spread within its bulk sooner, as well as raising the temperature deep within the food more quickly. 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. Fats and sugar Sugars and triglycerides (fats and oils) absorb microwaves due to the dipole moments of their hydroxyl groups or ester groups. Due to their molecular dipole moment, however, they are heated less efficiently. 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. 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 broiling / grilling (infrared) or convection heating methods which thinly deposit heat at the food surface. Penetration depth of microwaves depends on food composition and the frequency, with lower microwave frequencies (longer wavelengths) penetrating deeper. but energy-efficient models can exceed 64% efficiency. Stovetop cooking is 40–90% efficient, depending on the type of appliance used. Because they are used fairly infrequently, the average residential microwave oven consumes only 72 kWh per year. Globally, microwave ovens used an estimated 77 TWh per year in 2018, or 0.3% of global electricity generation. A 2000 study by Lawrence Berkeley National Laboratory found that the average microwave drew almost 3 watts of standby power when not being used, 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. == 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 converter usually 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. 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. The cooking chamber is similar to a Faraday cage to prevent the waves from coming out of the oven. Even though there is no continuous metal-to-metal contact around the rim of the door, choke connections on the door edges act like metal-to-metal contact, at the frequency of the microwaves, to prevent leakage. The oven door usually has a window for easy viewing, with 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 (12.2 cm for the usual 2.45 GHz), microwave radiation cannot pass through the door, while visible light (with its much shorter wavelength) can. Control panel Modern microwave ovens use either an analog dial-type timer or a digital control panel for operation. Control panels feature an LED, LCD or vacuum fluorescent display, buttons for entering the cook time and a power level selection feature. A defrost option is typically offered, as either a power level or a separate function. Some models include pre-programmed settings for different food types, typically taking weight as input. In the 1990s, brands such as Panasonic and GE began offering models with a scrolling-text display showing cooking instructions. 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. == 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 quartz halogen bulbs to their convection microwave oven models, marketing them under names such as "Speedcook", "Advantium", "Lightwave" 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 oxidizing 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 susceptor made from thin 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 a single use and are then discarded as waste. == Heating characteristics ==

Microwave ovens have a limited role in professional cooking, 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. 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 into most foods. The inside portions of thicker foods are mainly heated by heat conducted from the outer 1 cm. 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. 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 oven to the point of melting. Due to this phenomenon, microwave ovens set at too-high power levels may even start to cook the edges of frozen food while the inside of the food remains frozen. Another case of uneven heating can be observed in baked goods containing berries. In these items, the berries absorb more energy than the drier surrounding bread and cannot dissipate the heat due to the low thermal conductivity of the bread. Often this results in overheating the berries relative to the rest of the food. "Defrost" oven settings either use low power levels or repeatedly turn the power off and on – intended 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. 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. 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. The technical term for such a microwave-absorbing patch is a susceptor. 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. Effects on food and nutrients Any form of cooking diminishes overall nutrient content in food, particularly water-soluble vitamins common in vegetables, 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 effective, given the shorter cooking times it requires and that the water heated is in the food. Like other heating methods, microwaving converts vitamin B from an active to inactive form; the amount of conversion depends on the temperature reached, as well as the cooking time. Boiled food reaches a maximum of (the boiling point of water), whereas microwaved food can get internally hotter than this, leading to faster breakdown of vitamin B. The higher rate of loss is partially offset by the shorter cooking times required. Spinach retains nearly all its folate when cooked in a microwave oven; when boiled, it loses about 77%, leaching nutrients into the cooking water. 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 induction cooktop, leaves a potentially dangerous heating element or trivet that remains 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 . 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 with 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 burn 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 at high-temperatures, it is of only limited effectiveness in reducing glycoalkaloid (i.e., solanine) levels. Acrylamide has been found in other microwaved products like popcorn. Use in cleaning kitchen sponges Studies have investigated the use of the microwave oven to clean non-metallic domestic sponges which have been thoroughly wetted. A 2006 study found that microwaving wet sponges for 2 minutes (at 1000-watt power) removed 99% of coliforms, E. coli, and MS2 phages. Bacillus cereus spores were killed at 4 minutes of microwaving. A 2017 study was less affirmative: About 60% of the germs were killed but the remaining ones quickly re-colonized the sponge. == Issues ==

High temperatures Closed containers Closed containers, such as eggs, can explode when heated in a microwave oven due to the increased pressure from steam. Intact fresh egg yolks outside the shell also explode as a result of superheating. Insulating plastic foams of all types generally contain closed air pockets, and are generally not recommended for use in a microwave oven, 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 may become dangerously hot. 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 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. Metal objects Contrary to popular assumptions, metal objects can be safely used in a microwave oven, but with some restrictions. Any metal or conductive object placed into the microwave oven acts 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 aluminium foil (though some foil used in microwave ovens is safe, see below), twist-ties containing metal wire, the metal wire carry-handles in oyster pails, or almost any metal formed into a poorly conductive foil or thin wire, or into a pointed shape. Forks are a good example: the tines of the fork respond to the electric field by producing high concentrations of electric charge at the tips. This has the effect of exceeding the dielectric breakdown of air, about 3 megavolts per meter. 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. 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. disc showing the effects of electrical discharge through its metal film 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. Another hazard is the resonance of the magnetron tube itself. If the microwave oven is run without an object to absorb the radiation, a standing wave forms. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to overload and burn out. High reflected power may also cause magnetron arcing, possibly resulting in primary power fuse failure, though such a causal relationship is not easily established. Thus, 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 properly arranged, can produce an electric arc. 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. 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. Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary. According to the United States Food and Drug Administration's Center for Devices and Radiological Health, a U.S. Federal Standard limits the amount of microwaves that can leak from an oven throughout its lifetime to 5 milliwatts of microwave radiation per square centimeter at approximately (2 in) from the surface of the oven. This is far below the exposure level currently considered to be harmful to human health. The radiation produced by a microwave oven is non-ionizing. It therefore does not have the cancer 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 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. 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 dissipate thoroughly 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. There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action. 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. The plasticizers which received the most attention are bisphenol A (BPA) and phthalates, 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 Plastic containers can release microplastics into food when heated in microwave ovens. 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. 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. 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. == See also ==