A microwave oven , and microwave oven or microwave (just microwave ), a device for rapid heating, cooking and cooking food, liquids and other suitable materials. Its effect is based on the conversion of electromagnetic field energy into thermal energy during the absorption of microwaves .
The American engineer Percy Spencer (1894–1970) discovered that food can be heated by microwave radiation when he built magnetrons for radar systems at Raytheon (a manufacturer of high-frequency technology). While working on a radar, he got a strange feeling and noticed a candy bar in his pocket began to melt. He wasn't the first to notice this phenomenon, but as the holder of 120 patents , he was familiar with discoveries and experiments and understood what had happened: the radar had melted the chocolate using the microwave radiation. Popcorn was the first food specifically prepared in this way, the second an egg (which exploded in front of the experimenters). In North America, microwave popcorn is one of the most common microwave cooked foods. Other methods of private preparation (for example with hot air) have been almost completely replaced.
In 1946, Spencer built the first copy of a "radar cooker '(microwave), called Raydarange (1947 Radarange called); he was almost six feet tall and weighed 340 pounds. It was water-cooled and had an output of 3000 W, about three times that of today's household appliances. In 1954, one of the first commercial microwave ovens was 1,600 watts and cost between $ 2,000 and $ 3,000 . Microwave ovens were developed and sold by companies that had experience with magnetrons from developing radars; they were initially mainly manufactured for passenger aircraft before the devices found their way into canteen kitchens and private households. As a joint development with Raytheon , the US company Tappan launched the first microwave oven for home use with dimensions suitable for living rooms in 1955. By 1964 the RL-1 had been sold 1396 times, despite the high price of 1,300 USD. A model stands in the National Museum of American History today . In 1965 the first device with widespread use came on the market for 495 USD (purchasing power 2015, depending on the calculation method, at least 3700 USD).
As the price of microwave ovens fell rapidly in the 1970s, sales increased significantly. In 1970 40,000 devices were sold in the USA, in 1975 it was already a million. In 1997, 95% of American households owned a microwave oven, in Germany it was 74% in 2016.
Mode of action
Microwaves heat certain materials differently than for example, by absorption of infrared radiation or heat conduction occurs. Although the heating is also carried out by absorption, the depth of penetration is much greater in the microwave area - there is a certain transparency , so to speak - so that absorption also takes place in the depths of the food. Usual microwave dishes are practically completely transparent to microwaves (exception: browning dishes). In the case of conventional heating using radiation (e.g. fire, grill, oven, glass ceramic hob), on the other hand, absorption takes place in the top layers, since thermal radiation in the relevant wavelength range only has a very low penetration depth - at least with the usual materials that food is made of and cookware are made; these materials are, as it were, opaque. The heat is distributed from the surface through thermal conduction and possibly convection .
More precisely, microwave radiation stimulates a rotational movement of the molecules; The prerequisite is that the molecules have an electrical dipole moment . A completely different mechanism works when infrared radiation (i.e. the main contribution of thermal radiation ) hits molecules : It stimulates molecular vibrations , whereby the atomic nuclei perform trembling movements around their position of rest. An external dipole moment is not required for this excitation. Another prerequisite for the effect of microwave radiation is that the dipole molecules can rotate. This mobility is not given in a crystal structure , such as in frozen water (ice). In liquid water, on the other hand, the movement-inhibiting hydrogen bonds are very short-lived, on the order of 200 fs ( femtoseconds ; for comparison: the period of the usual 2.455 GHz oscillation is more than two thousand times longer), so that the water molecules can be set in motion with sufficient excitation .
When irradiated with microwaves, the electric field strength component of the electromagnetic wave (E-field component) now effects a force on the water molecule (force is equal to field strength times charge), so that a torque acts on the molecule and it rotates. Neighboring molecules also experience a torque and rotate. The rotation increases the kinetic energy of the water and thus the temperature .
A quantitative model of this dielectric heating was developed by Peter Debye and named after him. According to Debye relaxation, there is no resonance frequency in the strict sense, but there is a broad frequency range in which the dielectric loss factor (that is the imaginary part of the complex permittivity of a material at a given frequency) is particularly large. For water, it is in the order of 30 GHz, depending on the temperature and salinity.
If the frequencies are too high, the molecules can no longer follow the change in the electromagnetic field due to their inertia , and there is no rotational excitation and thus no increase in temperature. In technical practice, however, this is largely irrelevant, since significantly lower frequencies are used anyway for other reasons.
The widespread assumption that the frequency of the microwave oven corresponds to a particular resonance frequency of water is wrong. The lowest resonance frequency of the free water molecule is 22.23508 GHz. As soon as its immediate neighbors influence the molecule - which is definitely the case with liquid or solid water - the corresponding spectral line is widened beyond recognition , so that even frequencies that are relatively far away lead to an excitation. Due to the other technical and practical boundary conditions, a significantly lower frequency is used:
- The power oscillator (magnetron) with at least 500 W must be easy and cheap to manufacture and have a high degree of efficiency.
- The wavelength must be significantly smaller than the dimensions of the cooking space.
- The wavelength must remain constant because the resonance seal of the door gap is matched to it and becomes ineffective if the deviations are too large.
- The frequency range must be freely available for private and industrial applications ( ISM band ).
- The penetration depth of the electromagnetic waves in food should be a few centimeters. The lower the frequency, the greater the penetration depth, but the lower the absorption. If the frequency is too high, the penetration depth is small, so that only the surface is heated.
In practice, the frequency of 2.455 GHz has largely prevailed, corresponding to a wavelength of 12.22 cm. A magnetron for generating electromagnetic fields with this frequency can be manufactured inexpensively and compactly; its frequency cannot be changed due to the internal mechanical dimensions. The depth of penetration into organic substances and also water is then in the range of a few centimeters. In some countries, such as the United States , the frequency around 915 MHz is also used for industrial microwave ovens. There, the range between 902 and 928 MHz can be freely used as an ISM frequency band.
A microwave oven converts only 65 percent of the electrical energy it consumes into microwave radiation (the rest is waste heat), but the microwaves generated only heat the food and not the oven itself or its surroundings. Therefore, the microwave oven is more energetically efficient with smaller portions. A guideline is 250 ml of liquid substance. It is better to heat 250 ml of liquid (or 250 grams of a water-based food) in a microwave oven than in a saucepan on an electric stove.
In addition, food can be heated directly on the plate and drinks (such as milk) in the cup or glass in the microwave oven. There is no need to clean the cookware after using a normal cooker. If you include their resource consumption (energy, water, cleaning agents) in the analysis, the efficiency of the entire preparation on a conventional stove deteriorates further.
The microwaves are generated with the help of a magnetron and guided into the cooking chamber by means of a waveguide . The cooking space is metallic and high-frequency tight, which prevents microwaves from spreading out of the device.
To supply the magnetron, a high anode voltage is required (approx. 5 kV ), which is generated in the device with the help of a high-voltage transformer and a voltage doubler circuit. The high voltage applied to the cathode changes periodically between 0 and about −5 kV at a frequency of 50 Hz. The threshold voltage of the magnetron has the effect that short current pulses occur only when the supply voltage is greater than the threshold voltage. The anode of the magnetron is connected to the housing of the microwave oven, i.e. to earth potential, so that the transmitting antenna is not at high voltage potential. The transformer also supplies power to the magnetron's hot cathode . A fan cools the magnetron and transformer and blows their heat loss through the oven to keep it dry.
The radiation field of the built-in microwave antenna fills the oven cavity unevenly. In order to achieve even heating of the food, so-called wobblers or stirrers - rotating metallic impellers - are used, which constantly change the vibration modes of the cooking space. These wheels are usually located under an artificial mica or plastic cover in the ceiling of the cooking space and are driven by the flow of cooling air. In many appliances, the food also rotates on a turntable.
The door is security-relevant in several ways. It shields the microwaves from the outside and prevents the device from working when the door is open. It also provides a view of the cooking space through a pane in front of which there is a perforated sheet. The openings in the perforated plate are much smaller than the wavelength of the microwaves of around 12 cm, so that the area beyond the built-in pane remains shielded from the electromagnetic field in the cooking space .
The door frame forms a circumferential gap with the edge of the cooking space. It is a so-called resonance seal. The width of the door gap is a quarter of the wavelength (λ / 4), i.e. approx. 3 cm, the thickness of the gap (distance door / cooking space) is not critical. Without electrical contact, the gap acts as a frequency-selective seal for the electromagnetic fields in the furnace. The function is based on the skillful combination of pieces that have a length of λ / 4. The same is used for waveguide connections .
The power regulation of a microwave oven is done in most devices by intermittent operation using an electromechanical timer or a microcontroller. The magnetron and its heating voltage are switched on and off via the transformer every few seconds to achieve the power set by the operator. The average power is controlled by the ratio of on and off time. A 1200 watt device that the operator has set to 600 watts of power will therefore alternately apply 1200 watts of radiation power to the food for 5 seconds, for example, and then be idle for 5 seconds. The power specification via the control panel is the mean value. The switching period cannot be shortened further due to the mechanical switches (relay, timer) and the life-reducing switching on of the magnetron without cathode preheating and can lead to different results depending on the maximum output even with the same output preselection; for sensitive foods, such as B. fish, even short-term use of maximum power can lead to local burns. There are therefore devices with a switched-mode power supply (so-called inverter technology) that continuously heat the cathode and can electronically control the average anode current. The desired power is then delivered by the magnetron almost continuously.
A microwave oven should never be switched on with an empty oven, as the power output of the magnetron must always be sufficiently absorbed. Otherwise it will be reflected back into the magnetron, which could damage it.
Due to the reflections, a three-dimensional pattern of interference maxima , known as hot spots , is created in the cooking space , where more energy is transferred to the food. The location of hotspots can be detected by inserting thermal paper . Despite countermeasures such as turntables or rotating reflecting mirrors (stirrer, English for “stirrer”), individual areas in the food can be overheated. Due to the different water content of different dishes, inhomogeneous heating can occur despite the stirrer and turntable . Bones heat up only slightly compared to meat. Salty food warms up more than fatty food. To ensure that the food is cooked through, it is therefore advisable to cover it and, if necessary, to cook it for longer at lower power or to stir it after a short warm-up phase. Stirring the baby food before administration is particularly useful in order to mix hot areas.
In dry food, hot spots can cause charring and pollutants.
The effect of overheating water (see delayed boiling ) in smooth vessels is a possible source of danger. It can happen that water is heated above the boiling point without boiling - this risk arises particularly with repeated heating in the microwave due to the decreasing proportion of dissolved gases. The overheated water can suddenly evaporate when moving (for example when dispensing); part of the water evaporates explosively and boiling water is hurled out of the vessel. A glass rod or spoon placed in the vessel and scratching the bottom of the vessel with these devices will help to avoid delayed boiling, as the contact points on the bottom act as a germ for the formation of vapor bubbles.
Harness porcelain , glass or thermoplastic plastic material does not absorb microwaves, and thus will not be heated by the radiation, but only indirectly as the food by conduction. However, plastics can soften or melt at a certain temperature. This is why some containers are labeled with the “microwave-safe” mark. On the other hand, special so-called tanning dishes and other lossy dielectric or moderately electrically conductive substances and ferromagnetic ceramics are also heated directly.
Unglazed earthenware dishes or those with cracks in the glaze can contain water, which then also heats it up directly. Earthenware ceramics can also contain iron oxide, which also absorbs microwaves. This can lead to the vessels becoming very hot, while the contents are barely heated.
The problem with thawing is that the water molecules in the ice are not very mobile. For this reason, areas of the frozen food that have already thawed absorb microwaves better than those that are still frozen, so that the already thawed areas quickly become warmer than frozen areas. The thawing of frozen food is partially carried out by heat transfer from water that is already liquid. Defrosting is also accelerated if liquid water adheres to the frozen food as a result of previous work steps. The defrosting function of a microwave oven therefore works with low power, so that there is enough time for the heat to pass from the liquid areas to the frozen areas.
Microwave radiation also has a warming effect on human tissue . At the standard 2.45 GHz, the penetration depth is given as a few centimeters. Local heating can damage the tissue (burns, denaturation ). In particular, tissue with poor blood circulation, which cannot quickly dissipate heat via the bloodstream and which is also located near the surface (such as the eyes), is at risk. Also, changes in the structure and function of the eyes induced by microwaves in the range of low average power without a relevant temperature increase have been demonstrated in animal experiments.
In the case of an intact microwave oven, the leakage radiation is relatively low due to the shielding of the cooking space. According to the Federal Office for Radiation Protection , an "emission limit value of five milliwatts per square centimeter (corresponds to 50 watts per square meter) at a distance of five centimeters from the device surface" is specified in the applicable safety standards (radiation density or power flux density in W / m 2 ). The limit values were always observed during controls. Nevertheless, the BfS recommends "in principle to avoid unnecessary exposure to high-frequency fields". Therefore, "particular care should be taken that children do not stand in front of or next to the appliance while the food is being prepared". The BfS also names pregnant women as particularly vulnerable. At a distance of 30 centimeters, only “about five to ten percent of the microwave intensity measured on the surface of the device” is available.
Even in the event of a fault, microwave ovens are well protected against operation with the door open by multiple fuses. Devices with a damaged or bent housing or doors should not be used any longer, since in this case relatively strong high-frequency electromagnetic fields can occur outside the device, which can cause injury. The occasional opinion that the use of microwaves causes cancer is scientifically not tenable. A mutagenic effect of electromagnetic radiation occurs only with ionizing radiation . The range of ionizing radiation begins at wavelengths shorter than about 250 nm, corresponding to a frequency greater than 1,200,000 GHz; that is in the ultraviolet range (transition from UV-A to UV-B). However, the frequency of microwaves is almost six orders of magnitude below the limit for ionizing radiation.
Effects with electrically conductive substances (including metals)
Due to the high transmission power of the magnetron, currents of more than 20 amperes can flow in all conductive materials, including metal parts in the oven . Thin metal layers, for example aluminum foil or the gold rim of plates, can melt or evaporate due to arcing . Thicker metal objects such as cutlery can get hot.
If one metal pieces with improper geometry, such as forks, or introducing a small distance to the wall in the cooking chamber can sparkovers occur if the generated electric field strength is sufficient (= 10 6 V · m -1 ).
The heating of food that is completely or partially wrapped in aluminum foil harbors the risk of sparks and arcs at overlaps and edges, and the aluminum also reflects the microwaves.
Completely metallic encapsulated food is not heated and the magnetron is thermally overloaded due to mismatch / back reflection of the microwaves.
There are risks associated with introducing metal into the microwave oven.
Certain foods and other substances can heat up so much in a microwave that they smolder or burn. Microwaves not only heat water molecules, but all molecules with a dipole moment and electrically conductive substances. The high field strengths can also lead to the formation of arcs in food. Strong induced currents can also lead to charring. These are often not visible - at least from the outside.
The resulting charring and also flames absorb microwave radiation and intensify the effect. Many microwave ovens therefore have a thermal switch above the cooking space in the area of the air outlet , which switches off the device if it overheats. Fires that take place inside are isolated from the environment by the double-walled housing, but can develop harmful gases from the fire, which are also led to the outside by the fan, which usually continues to run after the heating period.
Effect on nutrients
Microwaves themselves have too little energy to break chemical bonds. However, as it is heated in other cooking methods, hydrogen bonds in water and in biomolecules are disrupted and this causes denaturation .
The view that microwave ovens reduce the nutritional content of food by destroying vitamins and phytochemicals more than other heating processes is largely unsubstantiated. One study found that, for example , antioxidants in broccoli were more destroyed by microwave heating than other heating methods. Comparisons with other heating methods can hardly be made because conventional cooking results in a more homogeneous temperature distribution, while roasting and baking a much more inhomogeneous one.
The generation of pollutants through the microwave heating process is discussed repeatedly. As with other cooking methods, a possible source of pollutants can be local burns on food. Overheating of food represents a real danger to nutrients, as many nutrients are destroyed at high temperatures. However, this also applies to other preparation methods.
Hygiene of food
As mentioned above under power regulation , the heating takes place unevenly. In addition to the hot spots mentioned there, there are also cold spots . They pose a hygienic hazard when food is heated with microwave radiation, since salmonella or listeria can survive there.
In an experiment, American researchers had freshly purchased, listeria-infested chickens cooked in the microwave. In more than half of the samples, Listeria were still detectable, regardless of the power, the size of the cooking space or whether a turntable was present. For this reason, the German Nutrition Society recommends, for example, not to cook chicken or minced meat in microwave ovens.
No tanning reaction
During normal cooking (apart from the local combustion processes already mentioned) in the microwave, temperatures that are necessary for the development of browning reactions ( Maillard reaction , e.g. for roasts or fried eggs) are not reached. The associated with the Maillard reaction formation of melanoidins ( roasting -Aromastoffen) remains out then. For this reason, microwave ovens with grill or hot air function (combination devices) are also offered. There are also special browning dishes for smaller portions of meat, where metal oxides are embedded in the surface coating, which are heated by the microwave energy and can thus achieve browning effects.
Large microwave devices are used industrially as an alternative to autoclaves for the production of fiber composite materials; their possible uses are being explored. The energy savings compared to other production methods for fiber composite materials are interesting. This is based on the fact that only the workpiece itself is heated (see mode of operation and efficiency ). Further applications are the drying of food such as pasta , the heat stabilization of grain seedlings or the drying of other materials. Furthermore, microwave devices are used for synthesis in the chemical laboratory.
Microwaves with up to several hundred watts are also used therapeutically to warm tissue in the medical procedure of diathermy . As with microwave ovens, the heat input is controlled via pulsed switching on and off.
Microwaves with an output of many kilowatts are used for industrial drying and heating, for plasma generation and in particle accelerators . As in the microwave, they are generated with magnetrons or klystrons .
Destruction of RFID chips
After the introduction of the ePassport , the Chaos Computer Club and opponents of increasing surveillance measures called for an act of civil disobedience to use a microwave oven to destroy the chip contained in the document, on which the holder's personal data is stored. It is also pointed out that the passport remains valid, as it still enables the person to be identified.
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