A microwave oven, or microwave, is a kitchen appliance employing microwave radiation primarily to cook or heat food. Microwave ovens have revolutionized food preparation since their use became widespread in the 1970s.
History
Cooking food with microwaves was discovered by Percy Spencer while building magnetrons for radar sets at Raytheon. He was working on an active radar set when he noticed a strange sensation, and saw that a peanut candy bar he had in his pocket started to melt. Although he was not the first to notice this phenomenon, as the holder of 120 patents, Spencer was no stranger to discovery and experiment, and realized what was happening. The radar had melted his candy bar with microwaves. The first food to be deliberately cooked with microwaves was popcorn, and the second was an egg (which exploded in the face of one of the experimenters).
On 8 October 1945 Raytheon filed a patent for Spencer's microwave cooking process and in 1947, the company built the first microwave oven, the Radarange. It was almost 6 feet (1.8 m) tall and weighed 750 pounds (340 kg). It was water-cooled and produced 3000 watts, about three times the amount of radiation produced by microwave ovens today. An early commercial model introduced in 1954 generated 1600 watts and sold for $2,000 to $3,000. Raytheon licensed its technology to the Tappan Stove company in 1952. They tried to market a large, 220 volt, wall unit as a home microwave oven in 1955 for a price of $1,295, but it did not sell well. In 1965 Raytheon acquired Amana, which introduced the first popular home model, the countertop Radarange in 1967 at a price point of $495.
In the 1960s, Litton bought Studebaker's Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton then developed a new configuration of the microwave, the short, wide shape that is now common. The magnetron feed was also unique. This resulted in an oven that could survive a no-load condition indefinitely. The new oven was shown at a trade show in Chicago, and helped begin a rapid growth of the market for home microwave ovens. Sales figures of 40,000 units for the US industry in 1970 grew to one million by 1975. Market penetration in Japan, which had learned to build less expensive units by re-engineering a cheaper magnetron, was more rapid.
A number of other companies joined in the market, and for a time most systems were built by defense contractors, who were the most familiar with the magnetron. Litton was particularly well known in the restaurant business. By the late 1970s the technology had improved to the point where prices were falling rapidly. Formerly found only in large industrial applications, microwave ovens (often referred to informally as simply "microwaves") were increasingly becoming a standard fixture of most kitchens. The rapidly falling price of microprocessors also helped by adding electronic controls to make the ovens easier to use. By the late 1980s they were almost universal and had taken off in many other parts of the globe. Current estimates hold that nearly 95% of American households have a microwave.
Description
A microwave oven consists of:
A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes explained as a rotational resonance of water molecules, but this is incorrect: such resonance only occurs at much higher frequencies, in the tens of gigahertz. Moreover, large industrial/commercial microwave ovens operating at 915 MHz also heat water and food perfectly well. [1]
A common misconception is that microwave ovens cook food from the "inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry nonconductive substances at the surfaces of many common foods, and thus often deposit initial heat more deeply than other methods. Depending on water content the depth of initial heat deposition may be several centimeters or more with microwave ovens, in contrast to broiling (infrared) or convection heating, which deposit heat thinly at the food surface. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating.
Most microwave ovens allow the user to choose between several power levels, including one or more defrosting levels. In most ovens, however, there is no change in the intensity of the microwave radiation; instead, the magnetron is turned on and off in cycles of several seconds at a time. This can actually be observed when microwaving airy foods like Krembos (an Israeli confection): it blows up during heating phases, while it deflates when the magnetron is turned off.
The cooking chamber itself is a Faraday cage enclosure which prevents the microwaves from escaping into the environment. The oven door is usually a glass panel for easy viewing, but has a layer of conductive mesh to maintain the shielding. Because the size of the perforations in the mesh is much less than the wavelength of 12 cm, the microwave radiation can not pass through the door, while visible light (with a much shorter wavelength) can.
Microwave ovens are generally used for time efficiency in both industrial applications such as restaurants and at home, rather than for cooking quality, although some modern recipes using microwave ovens rival recipies using traditional ovens and stoves. Professional chefs generally find microwave ovens to be of limited usefulness. On the other hand, people who want fast cooking times can use microwave ovens to prepare food or to reheat stored food (including commercially available pre-cooked frozen dishes) in only a few minutes. Popcorn is a very popular item with microwave oven users but increasingly being made in domestic popcorn makers.
A variant of the conventional microwave is the convection microwave. A convection microwave is a combination of a standard microwave and a convection oven. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwaves are more expensive than a conventional microwave. They are not considered cost-effective if primarily used just to heat drinks or frozen food. They are usually used for cooking a prepared dish.
More recently, certain manufacturers have added a high power quartz halogen bulb to their convection microwave models while marketing them under names such as "Speedcook", "Advantium" and "Optimawave" to emphasize their ability to cook food rapidly and with the same browning results typically expected of a conventional oven. This is achieved using the high intensity halogen lights at the top of the microwave to deposit large amounts of infrared radiation to the surface of the food. The food browns while also being heated internally by the microwave radiation and heated through conduction and convection by contact with heated air - produced by the conventional convection portion of the unit. The IR energy which is rapidly delivered to the outer surface of food by the lamps is sufficient to initiate browning and caramelization reactions in a particular food's proteins and carbohydrates, producing a texture and taste much more similar to that typically expected of conventional oven cooking rather than the bland boiled and steamed taste that microwave-only cooking tends to create.
With wireless computer networks gaining in popularity, microwave interference has become a concern near wireless networks. Microwave ovens are capable of disrupting wireless network transmissions because the ovens generate radio waves of about 2450 MHz, near the 802.11b/g frequency band.
Efficiency
A microwave oven does not convert all electrical energy into microwaves. A typical consumer microwave oven consumes 1100 W but delivers only 700 W of microwave power, yielding 64% efficiency. The lost 400 W are dissipated as heat by components of the oven. The main source of energy loss is the magnetron tube, which is much less than 100% efficient at generating microwave output from the power source. Lesser amounts of power are consumed by the oven lamp, AC power transformer losses, magnetron cooling fan, food turntable motor and control circuits. This waste heat does not end up in the food but is mostly expelled from the cooling vents on the oven and heats the air in the kitchen.
Conversely, conventional ovens can use fuel such as natural gas which may or may not be cheaper than the same amount of electrical energy.
Safety and controversy
Microwaving food is fast and popular, but there are potential problems.
Uneven heating, deliberate and not
In a microwave oven, food may be heated for so short a time that it is cooked unevenly, since heat requires time to diffuse through food, and microwaves only penetrate to a limited depth. Microwave ovens are frequently used for reheating previously cooked food, and bacterial contamination may not be killed by the reheating, resulting in foodborne illness.
Uneven heating in microwaved food is 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, and by a turntable or carousel that turns the food. It is also important not to place food or a container in the center of a microwave's turntable. That actually defeats its purpose. Rather, it should be placed a bit off-center so that the item travels all around the area of oven's cooking cavity, thus assuring even heating.
The second problem is due to food composition and geometry, and must be addressed by the cook, who should arrange the food so that it absorbs energy evenly, and periodically test and shield 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. As an example, uneven heating in frozen foods is a particular problem, since ice absorbs microwave energy much less well than liquid water, leading to defrosted sections of food warming faster due to more rapid heat deposition there. Due to this phenomenon, microwave ovens set at too-high power levels may even start to cook the edges of the frozen food, while the inside of the food remains frozen. The low power levels which mark the "defrost" oven setting are designed to allow time for ice in a food to melt by conduction from food volumes where melting has occurred, without temperatures of the ice-free volumes rising too high.
Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may contain ceramic or aluminum-flake containing materials which are designed to absorb microwaves and re-radiate them as infrared heat (similar to broiling radiation), which is not as penetrating and which aids in baking or crust preparation. Such ceramic patches affixed to cardboard are positioned next to the food, and are typically smokey blue or gray in color, usually making them easily identifiable. Microwavable cardboard packaging may also contain overhead ceramic patches which function in the same way. The technical term for such a microwave-absorbing patch is a susceptor.
Dangers
Liquids, when heated in a microwave oven in a container with a smooth surface, can superheat; that is, reach temperatures that are a few degrees Celsius above their normal boiling point without actually boiling. The boiling process can start explosively when the liquid is disturbed, such as when the operator grabs hold of the container to take it out of the oven, which can result in severe burns. A common myth states that only distilled water can exhibit this behavior; this is not true. [2]
Closed containers and eggs can explode when heated in a microwave oven due to the pressure build-up of steam. Products that are heated too long can catch fire. Manuals of microwave ovens warn of such hazards.
A microwaved DVD-R showing the effects of electrical discharge through its metal film
Tin foil, aluminium foil, ceramics decorated with metal, and products containing other metals can cause sparks when they are used in a microwave. Microwaving small, smooth, solid metal objects without pointed ends (for example, a spoon) can sometimes be safe, and usually does not produce sparking (putting a spoon into a liquid also helps prevent superheating). Forks, however, will readily produce sparks when placed in the microwave. This is because while it acts as an antenna, absorbing microwave radiation just like other metal objects such as the spoon, tines of the fork will act to concentrate the electric field formed at the tips. This has the effect of exceeding the dielectric breakdown gradient of air, about 3 megavolts per meter (3×106V/m), causing sparks to form. This effect is somewhat analogous to the effect of St. Elmo's fire.
The effect can be seen clearly on a CD or DVD which has been cooked in a microwave. When the electrical field builds up sufficiently, the resulting electric current vaporizes the metal film and melts the plastic in the disc, leaving a visible pattern of concentric and radial scars.
The formation of sparks on sharp metal objects may be prevented by placing the utensil in some food or liquid while in the microwave, as this has the effect of preferentially conductively dissipating the charge before the electric fields can build to the point where they exceed the breakdown value of air. Any time dielectric breakdown occurs in air, some ozone and nitrogen oxides are formed, both of which are toxic. Finally, as mentioned previously, any metal or conductive object placed into the microwave will act as an antenna, and its electrons will thus be thrashed back and forth through the object (a high frequency alternating current) causing some ohmic heating to occur. The extent of this heating effect will vary depending on the size, shape and conductivity of the object.
Several microwave fires have been noted where Chinese takeout boxes with a metal handle are microwaved, and also where "homemade" microwave popcorn bags have been sealed using a metal staple, which is then heated and sets fire to the bag. This type of accident can pose a dangerous situation because of the extremely flammable mixture of popcorn and oil in the bag. Metal wire-containing twist-ties are notorious for microwave sparking. Thus, it is good practice to remove any metal utensils or metal-containing objects from a microwave oven before operating it, as the behavior of these objects when immersed in a strong microwave radiation field is unpredictable.
It is a common myth that metallic kitchen equipment, like kitchen forks and knives, can somehow repel the microwaves back into the magnetron and cause it to catch fire. This concern is unfounded as there are apparently no cases of such an incident occurring, and is implausible considering the entire cooking box where microwaves are injected is made of metal.
Controversial hazards
Radiation
Some people are concerned with being exposed to the microwave radiation. The U.S. legal limit of leaking radiation is 1 mW/cm² at 5 cm (about 2 inches) from a new oven — for a used oven the allowed radiation is five times higher. It is rare for an oven to exceed these limits. As a comparison, a GSM mobile phone may emit up to 1 W at 1800 MHz, which is 3.2 mW/cm², at 5 cm, with a human being staying nearby (such as within 5 cm for far longer periods of time). Whether or not cellular phones are hazardous to the health is also controversial (see mobile phone radiation and health).
Microwave ovens produced after 1971 must meet the Food and Drug Administration safety requirements for radiation leakage; less than 5 mW/cm² at 5 cm from the surface of the oven. This is far below the exposure level that is currently considered to be harmful to human health.
The radiation produced by a microwave oven is non-ionizing. As such, it does not have the cancer risks associated with ionizing radiation such as X-rays, ultraviolet light, and nuclear radioactive decay. Any cancer risk from microwave ovens would necessarily occur by an unknown mechanism. Such a mechanism, if existent, would also need to be far weaker than that for ionizing radiation, since long-term rodent studies of a type which easily identify cancer risks of ionizing radiation, have so far failed to clearly identify any carcinogenicity from 2450 MHz microwave radiation at chronic (large fraction of life span) exposure levels, far larger than humans are likely to encounter even from leaking ovens. PMID 9806599 PMID 9453703
Food
Some people claim there exist more subtle dangers than the ones listed above. Claimed dangers associated with microwave cooking include:
- Microwave cooking causes more nutrient loss than conventional cooking. This is not substantiated by available studies of nutrient loss in microwaved foods, which conclude the opposite--- that nutrient loss is more severe in conventional cooking. PMID 2775405
- microwave radiation leads to chemical reactions in the food that are different from those occurring during conventional heating [citation needed] — and consuming that food causes cancer[citation needed], particularly due to the formation of suspected carcinogens called d-nitrosodiethanolamines[citation needed].
There are a limited number of studies on the effects of microwave cooking.
- 1992, Pediatrics, Volume: 89, Issue 4,pp. 667-669, Effects of microwave radiation on anti-infective factors in human milk by R Quan, C Yang, S Rubinstein, NJ Lewiston, P Sunshine, DK Stevenson and JA Kerner
- 1994, Journal of Nutrition and Food Science, Volume: 95 Issue: 4 Page: 8 - 10, Nutritional effects of microwave cooking by Anne Lassen, Lars Ovesen
- 1998, Journal of Agricultural and Food Chemistry, Effects of microwave heating on the loss of vitamin B12 in foods by Fumio Watanabe,* Katsuo Abe, Tomoyuki Fujita, Mashahiro Goto, Miki Hiemori, and Yoshihisa Nakano
- 2003, Journal of the Science of Food and Agriculture, Volume 83, Issue 14 , Pages 1511 - 1516, Phenolic compound contents in edible parts of broccoli inflorescences after domestic cooking by F Vallejo, FA Tomás-Barberán, C García-Viguera.
See also
External links and references