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. This induces polar molecules in the food to rotate and produce thermal energy (heat) in a process known as dielectric heating. Microwave ovens heat food quickly and efficiently; the heating effect is fairly uniform in the outer of a homogeneous, high-water-content food item.
The development of the cavity magnetron in the United Kingdom made possible the production of electromagnetic waves of a small enough wavelength (microwaves) to efficiently heat up water molecules. American electrical engineer Percy Spencer is generally credited with developing and patenting the world's first commercial microwave oven, the Raytheon "Radarange", which was first sold in 1947. He based it on British radar technology which had been developed before and during World War II.
Raytheon later licensed its patents for a home-use microwave oven that was first sold by Tappan in 1956, but it was still too large and expensive for general home use. Sharp Corporation introduced the first microwave oven with a turntable between 1964 and 1966, this feature created more even heating than the earlier microwave stirrer feature. The countertop microwave oven was introduced in 1967 by the Amana Corporation, which was, at the time, owned by Raytheon. After microwave ovens became affordable for residential use in the late 1970s, their use spread into residential kitchens around the world, and prices fell rapidly during the 1980s. In addition to cooking food, microwave ovens are used for heating in many industrial processes.
Microwave ovens are a common kitchen appliance in households and kitchens and are popular for reheating previously cooked foods and cooking a variety of different foods. They rapidly heat foods which can easily burn or turn lumpy if cooked rapidly in conventional pans, such as butter, fats, chocolate, or porridge. Microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperature to produce Maillard reactions. Exceptions occur in cases where the oven is used to heat frying-oil and other oily items (such as bacon), which attain far higher temperatures than that of boiling water.
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, there are hybrid appliances that combine infrared radiation, hot air, and microwaves, such as convection microwave ovens.
History 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: Chaffee, Joseph G., Method and apparatus for heating dielectric materials, filed August 11, 1937; granted February 21, 1939
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
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. A higher-powered microwave generator that worked at shorter wavelengths was needed, and in 1940, at the University of Birmingham in England, Randall and Harry Boot produced a working prototype. 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). An early 6 kW version, built in England by the General Electric Company Research Laboratories, Wembley, London, was given to the U.S. government in September 1940. The cavity magnetron was later described by American historian James Phinney Baxter III as "[t]he most valuable cargo ever brought to our shores". 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., Method of treating foodstuffs; application: October 8, 1945, issued: January 24, 1950, invented by Percy L Spencer.
Public demonstration, 1946
On 7 October 1946, Raytheon showed two "Radarange" units to the press at the Waldorf-Astoria in New York City.Pettigrew, Margaret. "Miracle Range Cuts Cooking Time; Radar Puts Pots and Pans on Fast Beam." Brooklyn Eagle, 8 October 1946, 9. Aimed initially for commercial use, one unit was designed for "for limited baking in sandwich shops," the other "for the preparation of frozen foods and designed for planes"Nickerson, Jane. "News of Food." New York Times, 8 October 1946, 26.—to enable serving a "complete meal on a single plate.""Electronic Cooking is Demonstrated." (Hagerstown, MD) Morning Herald, 8 October 1946, 1. Projected retail cost was then $1250.Yokley, Sara. "Art Of Cooking Will Be Obsolete With Radarange." Knoxville Journal, 23 October 1946, 2. On 30 December 1946, the Federal Communications Commission approved the 2450 megacycle (megahertz) frequency for Raytheon's new product.Oh Happy Day When Cooking Comes To This." Knoxville Journal, 7 January 1947, 1."No Gas? Tune In on Radio To Cook Dinner." (Adelaide) News, 31 December 1946, 1.
Commercial availability
In 1947, Raytheon built the "Radarange", the first commercially available microwave oven. It was almost tall, and weighed . 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. Chapter link is hosted at the Southwest Museum of Engineering, Communication and Computation; Glendale, Arizona. Because Raytheon was continuously refining the early models of its Radarange, early models were at first leased“Electronic Cooking Lures Hotel Men.” St. Louis Post-Dispatch, 14 November 1947, 11.—rather than sold—to hotels and restaurants. The cost was publicized as $5/day ($150/month).”Praise for Radar Quick Cooking.” Sydney (AU) Morning Herald, 17 July 1949, 17. By March 1948 about 100 units were in commercial use in the U.S., including some factory cafeterias.”Quick-as-Flash Cooking Next.” Pasadena Star-News, 24 March 1948, 18.
By early 1952 the Pennsylvania Railroad was advertising use of the Radarange on several of its eastern U.S. lines.”Enjoy New Travel Luxury Between The East’s Largest Cities: Gay New Coffee Shop Cars” (advertisement). Brooklyn Eagle, 31 March 1952, 10. The ocean liner SS United States was Radarange-equipped as well.”Color Preview of America’s Biggest Ship.” St. Louis Globe-Democrat, 6 April 1952, 115-16. On 23 March 1952, the Radarange was demonstrated on KTTV (Los Angeles).””Television Programs: Science Goes to Sea (Johns Hopkins Science Review).” Los Angeles Citizen News, 22 March 1952, 8. An early Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship NS Savannah.
In 1953-54, as actual home-version production grew nearer, demonstrator models were placed in select Raytheon TV-radio-appliance dealerships in several states.”Grand Opening Week” (Cleff Brothers advertisement). Lawrence County (IL) Evening News, 24 September 1953, 3. (“cooking demonstrations every 45 minutes”)”See World’s Most Advanced Method Of Cooking” (Moore’s Dream House advertisement). Record-Herald (Washington Court House, OH), 3 December 1953, 17.“First Showing: Raytheon Radarange” (R&D Sales advertisement). Progress-Bulletin (Pomona, CA), 5 April 1954, 21.“Radarange Preview Continues.” (Pomona, CA) Progress-Bulletin, 8 April 1954, 21.“See This Amazing New Way to Cook” (W. C. Teas Co. advertisement). Chattanooga Daily Times, 18 July 1954, 35.
In late 1954 a Raytheon VP disclosed $3500 pricing of its commercial units, while pledging that consumer models “to be manufactured by various stove companies in cooperation with Raytheon” would be marketed within a few months, priced at about $1000.”New radar (kitchen) range: Muffins in 2-1/2 seconds.” Los Angeles Daily News, 14 October 1954, 25.
The Tappan Stove Co. (Mansfield, OH ) had been licensed by Raytheon in 1952 to develop and market consumer microwave ovens. One was exhibited in New York on 27 October 1955;“Electronic Oven Does Things Fast.” The Franklin (Indiana) Evening Star, 27 October 1955, 2. Tappan had already had prototype models—freestanding and in-wall—in use by Mansfield housewives for the better part of a year.Secrest, Dave. “Housewives Test Microwave Ranges for Tappan Firm.” The Daily Times (New Philadelphia, OH), 26 October 1955, 7.