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{{short description|Kitchen cooking appliance}}
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{{Infobox product
{{Listen| filename = Panasonic NN-E225M microwave.flac
| title = Microwave oven
|title = Panasonic NN-E225M |description = A cup of tea is heated for 20 seconds in a Panasonic NN-E225M microwave oven }}
| logo =
| logo_caption =
alt =
| image = Panasonic NN-SD69LS 20220410.jpg
| image_size =
| alt =
| caption = A modern microwave oven (2022)
| type = ]
| inception = {{start date and age|1947}}
| manufacturer = Various
| available = Globally
| current supplier =
| last production =
| models =
| slogan =
| website =
| notes =
}}

A '''microwave oven''' or simply '''microwave''' is an electric ] that heats and cooks food by exposing it to ] in the ] ] range.<ref>{{Cite web|url=https://www.britannica.com/technology/microwave-oven|title=Microwave Oven|date=26 October 2018|website=Encyclopedia Britannica|access-date=19 January 2019}}</ref> This induces ] in the food to vibrate<ref>{{Cite web|url=https://www.fda.gov/radiation-emitting-products/resources-you-radiation-emitting-products/microwave-ovens#Cooking_with_Microwaves|title=Microwave Ovens|date=12 October 2023|website=FDA|access-date=11 July 2024}}</ref> and produce ] in a process known as ]. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer {{nowrap|25–38 mm}} {{nowrap|(1–1.5 inches)}} of a ], high-water-content food item.

The development of the ] in the United Kingdom made possible the production of electromagnetic waves of a small enough wavelength (]) to efficiently heat up water molecules. American ] ] is generally credited with developing and patenting the world's first commercial microwave oven post ] from British ] technology developed before and during the war. Named the "RadaRange", it was first sold in 1947.


] later licensed its patents for a home-use microwave oven that was introduced by ] in 1955, but it was still too large and expensive for general home use. ] introduced the first microwave oven with a turntable between 1964 and 1966. The countertop microwave oven was introduced in 1967 by the ]. After microwave ovens became affordable for residential use in the late 1970s, their use spread into commercial and residential ] 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.
A '''microwave oven''', commonly referred to as a '''microwave''', is a ] that heats and cooks ] by exposing it to ] in the ]. This induces ] in the food to rotate and produce ] in a process known as ]. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer {{nowrap|25–38 mm}} {{nowrap|(1–1.5 inches)}} of a ] (high water content) food item; food is more evenly heated throughout (except in heterogenous, dense objects) than generally occurs in other cooking techniques.


Microwave ovens are a common ] and are popular for reheating previously cooked foods and cooking a variety of foods. They rapidly heat foods which can easily burn or turn lumpy if cooked in conventional pans, such as hot butter, fats, chocolate, or ]. Microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperature to produce ]s. 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.{{citation needed|date=July 2020}}
] invented the first microwave oven after ] from ] technology developed during the war. Named the "Radarange", it was first sold in 1946. ] later licensed its patents for a home-use microwave oven that was first introduced by ] in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the ], and their use has spread into commercial and residential ] around the world.


Microwave ovens have a limited role in professional cooking,<ref name="herve">{{cite book|first=Hervé |last=This|title= Révélations gastronomiques|language=fr|publisher= Éditions Belin|isbn=978-2-7011-1756-0|year=1995}}</ref> 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.<ref>{{Cite journal|last1=Datta|first1=A. K.|last2=Rakesh|first2=V.|date=2013|title=Principles of Microwave Combination Heating|journal=Comprehensive Reviews in Food Science and Food Safety|language=en|volume=12|issue=1|pages=24–39|doi=10.1111/j.1541-4337.2012.00211.x|issn=1541-4337|doi-access=}}</ref>
Microwave ovens are popular for reheating previously cooked foods and cooking a variety of foods. They are also useful for rapid heating of otherwise slowly prepared cooking items, such as hot ], ]s, and ]. Unlike conventional ]s, microwave ovens usually do not directly brown or caramelize food, since they rarely attain the necessary temperatures to produce ]. Exceptions occur in rare cases where the oven is used to heat frying-oil and other very oily items (such as bacon), which attain far higher temperatures than that of boiling water. The boiling-range temperatures produced in especially hydrous foods allow microwave ovens a limited role in professional cooking,<ref name="herve">Hervé This, Révélations gastronomiques, Éditions Belin. ISBN 2-7011-1756-9</ref> since it impedes flavors produced by the higher temperatures of frying, browning, or baking are needed. However, additional heat sources can be ], or into ], to produce these other heating effects, and microwave heating may cut the overall time needed to prepare such dishes. Some modern microwave ovens may be part of an over-the-range unit with built-in ]s.


== History == == History ==
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| width1 = 200 | width1 = 200
| image2 = Cooking with radio waves - Short Wave Craft Nov 1933 cover.jpg | image2 = Cooking with radio waves - Short Wave Craft Nov 1933 cover.jpg
| width2 = 177 | width2 = 200
| footer = Demonstration by Westinghouse of cooking sandwiches with a 60 MHz shortwave radio transmitter at the ] | footer = Demonstration by Westinghouse of cooking sandwiches with a 60 MHz shortwave radio transmitter at the ]
}} }}


The exploitation of high frequency ]s for heating substances was made possible by the development of ] ]s around 1920. By 1930 the application of ]s to heat human tissue had developed into the medical therapy of ]. At the ], Westinghouse demonstrated the cooking of foods between two metal plates attached to a 10 kW, 60 MHz ] ].<ref name="">{{cite journal The exploitation of high-frequency ]s for heating substances was made possible by the development of ] ]s around 1920. By 1930 the application of ]s to heat human tissue had developed into the medical therapy of ]. At the ], ] demonstrated the cooking of foods between two metal plates attached to a 10&nbsp;kW, 60&nbsp;MHz ] ].<ref>{{cite journal | title = Cooking with Short Waves | journal = Short Wave Craft | volume = 4| issue = 7 | page = 394 | date = November 1933 | url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-11.pdf | access-date = 23 March 2015}}</ref> The Westinghouse team, led by I. F. Mouromtseff, found that foods like steaks and potatoes could be cooked in minutes.<ref>{{Cite journal |last1=Lovelock |first1=J. E. |last2=Smith |first2=Audrey U. |name-list-style=vanc |year=1956 |title=Studies on Golden Hamsters during Cooling to and Rewarming from Body Temperatures below 0 degrees C. III. Biophysical Aspects and General Discussion |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=145 |issue=920 |pages=427–442 |bibcode=1956RSPSB.145..427L |doi=10.1098/rspb.1956.0054 |issn=0080-4649 |jstor=83008 |pmid=13359396 |s2cid=6474737}}</ref>
| title = Cooking with Short Waves
| journal = Short Wave Craft
| volume = 4
| issue = 7
| page = 394
| publisher = Popular Book Corp.
| location = New York
| date = November 1933
| url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-11.pdf
| issn =
| doi =
| id =
| accessdate = March 23, 2015}}</ref> 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 and also in Canada:<ref>{{US patent|2147689}} Chaffee, Joseph G., ''Method and apparatus for heating dielectric materials'', filed August 11, 1937; granted February 21, 1939</ref> The 1937 United States patent application by Bell Laboratories states:<ref>{{US patent|2147689}} Chaffee, Joseph G., ''Method and apparatus for heating dielectric materials'', filed 11 August 1937; granted 21 February 1939</ref>
{{Blockquote|This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field.}}


However, lower-frequency ], as described in the aforementioned patent, is (like ]) an ] heating effect, the result of the so-called ] effects that exist in an electromagnetic cavity that is small compared with the ] of the electromagnetic field. This patent proposed radio frequency heating, at 10 to 20 ] (wavelength 30 to 15 meters, respectively).<ref name= "patent">{{citation | url = http://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 | publisher = United States Patent and Trademark Office | title = 2,147,689: Method and Apparatus for Heating Dielectric Materials | first = Joseph G. | last = Chaffee | date = 21 February 1939 | access-date = 5 February 2015 | archive-date = 19 March 2022 | archive-url = https://web.archive.org/web/20220319201814/https://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 | url-status = dead }}</ref> 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 ] 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.
{{Quotation|"This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field."}}

However, lower-frequency ], as described in the aforementioned patent, is (like ]) an ] heating effect, the result of the so-called ] effects that exist in an electromagnetic cavity that is small compared with the ] of the electromagnetic field. This patent proposed radio frequency heating, at 10 to 20 ] (wavelength 15 to 30 meters).<ref>{{citation | url = http://pdfpiw.uspto.gov/.piw?PageNum=0&docid=02147689&IDKey=49D95666A76C&HomeUrl=http%3A%2F%2Fpatft.uspto.gov%2Fnetacgi%2Fnph-Parser%3FSect1%3DPTO2%2526Sect2%3DHITOFF%2526u%3D%25252Fnetahtml%25252FPTO%25252Fsearch-adv.htm%2526r%3D1%2526p%3D1%2526f%3DG%2526l%3D50%2526d%3DPALL%2526S1%3D2147689.PN.%2526OS%3Dpn%2F2147689%2526RS%3DPN%2F2147689 | publisher = United States Patent and Trademark Office | title = 2,147,689: Method and Apparatus for Heating Dielectric Materials | first = Joseph G. | last = Chaffee | date = 21 February 1939}}</ref> 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 ] 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 ] effect, as polarized molecules are affected by a rapidly alternating electric field.


=== Cavity magnetron === === Cavity magnetron ===
{{Main|Cavity magnetron}}
]
] developed by ] and ] in 1940 at the ].]] ] developed by ] and ] in 1940 at the ], England]]


The invention of the ] made possible the production of ]s of a small enough ] (]s). The magnetron was originally a crucial component in the development of short wavelength ] during ].<ref>{{cite web | url = http://histru.bournemouth.ac.uk/Oral_History/Talking_About_Technology/radar_research/the_magnetron.html | title = The Magnetron | work = Radar Recollections - A Bournemouth University/CHiDE/HLF project | publisher = Defence Electronics History Society (formerly CHiDE)}}</ref> In 1937&ndash;1940, a multi-cavity magnetron was built by the British physicist ], together with a team of British coworkers, for the British and American military radar installations in World War II. A more high-powered microwave generator that worked at shorter ]s was needed, and in 1940, at the ], ] and ] produced a working prototype.<ref>{{cite journal|last=Willshaw|first=W. E.|author2=L. Rushforth |author3=A. G. Stainsby |author4=R. Latham |author5=A. W. Balls |author6=A. H. King |title=The high-power pulsed magnetron: development and design for radar applications|journal=The Journal of the Institution of Electrical Engineers&nbsp;— Part IIIA: Radiolocation|year=1946|volume=93|issue=5|doi=10.1049/ji-3a-1.1946.0188|url=http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5299321&tag=1|accessdate=22 June 2012}}</ref> The invention of the ] made possible the production of ]s of a small enough ] (]s). The cavity magnetron was a crucial component in the development of short wavelength ] during ].<ref>{{cite web | url = http://histru.bournemouth.ac.uk/Oral_History/Talking_About_Technology/radar_research/the_magnetron.html | title = The Magnetron | work = Radar Recollections - A Bournemouth University/CHiDE/HLF project | publisher = Defence Electronics History Society (formerly CHiDE)}}</ref> In 1937&ndash;1940, a multi-cavity magnetron was built by British physicist ] and coworkers, for the British and American military radar installations in World War II.<ref name="Magnetron"/> A higher-powered microwave generator that worked at shorter ]s was needed, and in 1940, at the ] in England, Randall and ] produced a working prototype.<ref>{{cite journal|last=Willshaw|first=W. E.|author2=L. Rushforth|author3=A. G. Stainsby|author4=R. Latham|author5=A. W. Balls|author6=A. H. King|title=The High-power Pulsed Magnetron: Development and Design for Radar Applications|journal=Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation|year=1946|volume=93|issue=5|doi=10.1049/ji-3a-1.1946.0188|url=https://ieeexplore.ieee.org/document/5299321|access-date=22 June 2012|pages=985–1005|archive-url=https://web.archive.org/web/20180505184050/https://ieeexplore.ieee.org/document/5299321/|archive-date=5 May 2018 |issn=2050-5485}}</ref> They invented a ] that could produce pulses of microwave radio energy at a wavelength of 10&nbsp;cm, an unprecedented discovery.<ref name="Magnetron">{{cite news|title=Briefcase 'that changed the world'|url=http://news.bbc.co.uk/1/hi/sci/tech/6331897.stm|publisher=BBC|date=20 October 2017}}</ref>


] travelled to the U.S. to offer them the magnetron in exchange for their financial and industrial help (see ]). An early 6 ] version, built in England by the ] Research Laboratories, ], ], was given to the ] in September 1940. Contracts were awarded to ] and other companies for mass production of the magnetron. ] 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 ]).<ref name="Magnetron"/> An early 6&nbsp;kW version, built in England by the ] Research Laboratories, ], London, was given to the ] in September 1940. The cavity magnetron was later described by American historian James Phinney Baxter III as "he most valuable cargo ever brought to our shores".<ref>{{cite book|first=James Phinney III|last= Baxter|title=Scientists Against Time|location= Boston|publisher=Little, Brown, and Co.|year= 1946|page= 142}}</ref> Contracts were awarded to ] and other companies for the mass production of the cavity magnetron.


=== Discovery === === Discovery ===
]
In 1945 the specific heating effect of a high-power microwave beam was accidentally discovered by ], an ] self-taught engineer from ]. Employed by ] at the time he noticed that microwaves from an active radar set he was working on started to melt a ] he had in his pocket. The first food deliberately cooked with Spencer's microwave was popcorn, and the second was an egg, which exploded in the face of one of the experimenters.<ref>{{cite web| url=http://www.gallawa.com/history.html |title=The history of the microwave oven |author=John Carlton Gallawa |year=1998}}</ref><ref>{{YouTube|4h1ESUz2H3E|Radar&nbsp;— Father of the Microwave Oven}}</ref> 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.
In 1945, the heating effect of a high-power microwave beam was independently and accidentally discovered by ], an American self-taught engineer from ]. Employed by ] at the time, he noticed that microwaves from an active radar set he was working on started to melt a ] 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.<ref>{{cite web| url=http://www.gallawa.com/microtech/history.html | url-status=dead | archive-url=https://web.archive.org/web/20110709081022/http://www.gallawa.com/microtech/history.html | archive-date=2011-07-09 |title=The History of the Microwave Oven |first=John Carlton|last= Gallawa |year=1998}}</ref><ref>{{YouTube|4h1ESUz2H3E|Radar — Father of the Microwave Oven}}</ref>


To verify his finding, Spencer created a high-density electromagnetic field by feeding microwave power from a magnetron into a metal box from which it had no way to escape. When food was placed in the box with the microwave energy, the temperature of the food rose rapidly. On 8 October 1945, Raytheon filed a United States patent application for Spencer's microwave cooking process, and an oven that heated food using microwave energy from a magnetron was soon placed in a Boston restaurant for testing.<ref>{{US patent reference |number=2495429 |y=1950 |m=January |d=24 |inventor=Spencer, Percy L. |title=Method of treating foodstuffs}}</ref>
On 8 October 1945,<ref>

{{US patent reference |number=2495429 |y=1950 |m=January |d=24 |inventor=Spencer, Percy L. |title=Method of treating foodstuffs}}
Another independent discovery of microwave oven technology was by British scientists, including ], who in the 1950s used it to reanimate ] frozen ].<ref>{{Cite journal|last1=Smith|first1=A. U.|last2=Lovelock|first2=J. E.|last3=Parkes|first3=A. S.|date=June 1954|title=Resuscitation of Hamsters after Supercooling or Partial Crystallization at Body Temperatures Below 0&nbsp;°C.|url=https://www.nature.com/articles/1731136a0|journal=Nature|language=en|volume=173|issue=4415|pages=1136–1137|doi=10.1038/1731136a0|pmid=13165726|bibcode=1954Natur.173.1136S|s2cid=4242031|issn=1476-4687}}</ref><ref>{{Cite journal|last1=Lovelock|first1=J. E.|last2=Smith|first2=Audrey U.|date=1959|title=Heat Transfer from and to Animals in Experimental Hypothermia and Freezing|url=https://nyaspubs.onlinelibrary.wiley.com/doi/abs/10.1111/j.1749-6632.1959.tb49226.x|journal=Annals of the New York Academy of Sciences|language=en|volume=80|issue=2|pages=487–499|doi=10.1111/j.1749-6632.1959.tb49226.x|pmid=14418500|bibcode=1959NYASA..80..487L|s2cid=38417606|issn=1749-6632}}</ref><ref>{{Cite journal|last1=Andjus|first1=R. K.|last2=Lovelock|first2=J. E.|date=1955|title=Reanimation of rats from body temperatures between 0 and 1&nbsp;°C by microwave diathermy|url= |journal=The Journal of Physiology|language=en|volume=128|issue=3|pages=541–546|doi=10.1113/jphysiol.1955.sp005323|issn=1469-7793|pmc=1365902|pmid=13243347}}</ref>
</ref> 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. The first time the public was able to use a microwave oven was in January 1947, when the Speedy Weeny vending machine was placed in ] to dispense "sizzling delicious" hot dogs. Among those on the development team was robotics pioneer ], who had spent the last part of the war developing ].
<!-- Deleted image removed: ] -->


=== Commercial availability === === Commercial availability ===
] nuclear-powered cargo ship, installed circa 1961]] ] nuclear-powered cargo ship, installed circa 1961]]
In 1947, Raytheon built the "Radarange", the first commercially available microwave oven.<ref>{{cite web| url=http://www.raytheon.com/ourcompany/history/leadership/ |archive-url=https://web.archive.org/web/20130322044917/http://www.raytheon.com/ourcompany/history/leadership/ |archive-date=2013-03-22 |title=Technology Leadership |publisher=Raytheon}}</ref> It was almost {{convert|1.8|m|ftin}} tall, weighed {{convert|340|kg|lb}} and cost about US$5,000 (${{formatnum:{{Inflation|US|5000|1947|r=-3}}}} in {{Inflation-year|US}} 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.<ref>{{cite book|last= Gallawa|first= J Carlton|chapter= A Brief History of the Microwave Oven|title= The complete microwave oven service handbook: operation, maintenance, troubleshooting, and repair|chapter-url= http://www.smecc.org/microwave_oven.htm|location= Englewood Cliffs, N.J.|publisher= Prentice Hall|date= 1989|access-date= 2017-10-11|isbn= 9780131620179|oclc= 18559256|url-access= registration|url= https://archive.org/details/completemicrowav00gall}} Chapter link is hosted at the Southwest Museum of Engineering, Communication and Computation; ].</ref> An early Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship ]. An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$2,000 to US$3,000 (${{formatnum:{{Inflation|US|2000|1954|r=-3}}}} to ${{formatnum:{{Inflation|US|3000|1954|r=-3}}}} in {{Inflation-year|US}} dollars). Raytheon licensed its technology to the ] Stove company of ] in 1952.<ref>{{cite web|url=https://ohiohistory.wordpress.com/2010/11/02/do-you-remember-your-familys-first-microwave/ |title=Do you remember your family's first microwave? |work=Ohio Historical Society |date=2 November 2010 |archive-url=https://web.archive.org/web/20160422025448/https://ohiohistory.wordpress.com/2010/11/02/do-you-remember-your-familys-first-microwave/ |archive-date=22 April 2016 }}</ref> 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$1,295 (${{formatnum:{{Inflation|US|1295|1955|r=-3}}}} in {{Inflation-year|US}} dollars)—sales were limited.<ref>{{cite web | url=https://spectrum.ieee.org/amp/a-brief-history-of-the-microwave-oven-2650274752 | title=A Brief History of the Microwave Oven }}</ref>

Japan's ] 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.<ref>{{cite web |title=History of Sharp |url=http://www.sharpusa.com/aboutsharp/companyprofile/sharpandtechnologyhistory.aspx |website=] |access-date=26 June 2018 |language=en}}</ref> In 1965, Raytheon, looking to expand their Radarange technology into the home market, acquired ] to provide more manufacturing capability. In 1967, they introduced the first popular home model, the countertop Radarange, at a price of US$495 (${{formatnum:{{Inflation|US|495|1967|r=-3}}}} in {{Inflation-year|US}} dollars). Unlike the Sharp models, a motor driven ] in the top of the oven cavity rotated allowing the food to remain stationary.

In the 1960s,{{specify|date=November 2013}} ] bought ]'s Franklin Manufacturing assets, which had been manufacturing magnetrons and building and selling microwave ovens similar to the Radarange. Litton developed a new configuration of the microwave oven: the short, wide shape that is now common. The magnetron feed was also unique. This resulted in an oven that could survive a no-load condition: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a ] in Chicago,{{citation needed|date=October 2013}} and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was even faster in Japan, due to a less expensive re-engineered magnetron.
Several other companies joined in the market, and for a time most systems were built by defence contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business.

===Residential use===
] 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.<ref>. ], 14 July 1972, p. 38.</ref> 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.

Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in ]. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971;<ref name="CPI_US">{{citation | url = http://www.bls.gov/cpi/cpimwo.htm | title = Hedonic Quality Adjustment Methods For Microwave Ovens In the U.S. CPI | first = Paul R. | last = Liegey | date = 16 October 2001 | access-date = 5 October 2013 | publisher = Bureau of Labor Statistics, United States Department of Labor}}</ref> the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997.<ref name="CPI_US"/><ref>{{citation | last1 = Cox | first1 = W. Michael | last2 = Alm | first2 = Richard | year = 1997 | title = Time Well Spent: The Declining Real Cost of Living in America | publisher = Federal Reserve Bank of Dallas | work = 1997 Annual Report | page = 22 (see Exhibit 8) | url = http://www.dallasfed.org/htm/pubs/pdfs/anreport/arpt97.pdf | access-date = 8 May 2016 | archive-date = 19 October 2004 | archive-url = https://web.archive.org/web/20041019184847/http://www.dallasfed.org/htm/pubs/pdfs/anreport/arpt97.pdf | df = dmy-all }}</ref> In Australia, a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily.<ref>{{citation | url = http://newsroom.electrolux.com/au/files/2010/01/Westinghouse-How-Australia-Cooks-Report1.pdf | title = The Westinghouse How Australia Cooks Report | date = October 2008 | publisher = Westinghouse | access-date = 5 February 2015 | archive-date = 5 February 2015 | archive-url = https://web.archive.org/web/20150205205537/http://newsroom.electrolux.com/au/files/2010/01/Westinghouse-How-Australia-Cooks-Report1.pdf | url-status = dead }}</ref> In Canada, fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998.<ref>{{cite journal | url = http://www.statcan.gc.ca/pub/11-008-x/11-008-x2000003-eng.pdf | title = Income and expenditures | first = Cara | last = Williams | date = Winter 2000 | number = 59 | journal = Canadian Social Trends — Catalogue No. 11-008 | pages = 7–12 | quote = Microwaves have been adopted even more avidly: in 1979, less than 5% of households had one, but by 1998 over 88% did.}}</ref> In France, 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004.<ref>{{citation | url = http://www.freedoniagroup.com/brochure/20xx/2015smwe.pdf | title = World Major Household Appliances: World Industry Study with Forecasts to 2009 & 2014 (Study #2015) | date = January 2006 | at = TABLE VI-5: FRANCE COOKING APPLIANCES SUPPLY & DEMAND (million dollars) | publisher = The Freedonia Group | location = Cleveland, Ohio}}</ref>

Adoption has been slower in ], as households with disposable income concentrate on more important household appliances like ] and ovens. In ], for example, only about 5% of households owned a microwave oven in 2013, well behind refrigerators at 31% ownership.<ref>{{cite web | title = Household penetration rate of home appliances in India in 2013 | url = http://www.statista.com/statistics/370635/household-penetration-home-appliances-india/ | access-date = 5 February 2015 | publisher = Statistica}}</ref> 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.<ref name=USDA-microwave/> Almost twice as many households in South Africa owned microwave ovens in 2008 (38.7%) as in 2002 (19.8%).<ref name=USDA-microwave/> Microwave oven ownership in Vietnam in 2008 was at 16% of households, versus 30% ownership of refrigerators; this rate was up significantly from 6.7% microwave oven ownership in 2002, with 14% ownership for refrigerators that year.<ref name=USDA-microwave>{{citation | url = http://www.ers.usda.gov/media/612721/householdamenities.xls | archive-url = https://web.archive.org/web/20130626225554/http://www.ers.usda.gov/media/612721/householdamenities.xls | archive-date = 26 June 2013 | format = XLS | title = Ownership of household amenities among selected countries | publisher = Economic Research Service, United States Department of Agriculture | year = 2009 | access-date = 5 February 2015 }}</ref>

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 ].

The size of household microwave ovens can vary, but usually have an internal volume of around {{convert|20|L|cuin cuft|sp=us|}}, and external dimensions of approximately {{convert|45|-|60|cm|ftin|abbr=on|}} wide, {{convert|35|-|40|cm|ftin|abbr=on|}} deep and {{convert|25|-|35|cm|ftin|abbr=on|}} tall.<ref>{{Cite web|last=Francis|first=Andrew|date=2017-04-12|title=Microwave Sizes Comparison|url=https://www.reviewho.com/microwave-sizes-and-dimensions-guide/|access-date=2021-03-10|website=reviewho.com|language=en}}</ref> Countertop microwaves vary in weight 23 – 45 lbs.<ref>{{Cite web |date=2020-11-17 |title=How Much Do Microwaves Weigh? Averages from 54 Models |url=https://applianceanalysts.com/microwave-weights/ |access-date=2024-12-17 |website=ApplianceAnalysts |language=en}}</ref>

Microwaves can be turntable or flatbed. Turntable ovens include a glass plate or tray. Flatbed ones do not include a plate, so they have a flat and wider cavity.<ref name=":1">{{Cite book|last=Williams|first=Alison|url=https://escholarship.org/content/qt3s29h7wd/qt3s29h7wd.pdf|title=Surveys of Microwave Ovens in U.S. Homes|publisher=Lawrence Berkeley National Laboratory|date=2012-12-05|pages=6, 18 and so on}}</ref><ref>{{Cite journal|last=ANNIS|first=PATTY J.|date=1980-08-01|title=Design and Use of Domestic Microwave Ovens|journal=Journal of Food Protection|volume=43|issue=8|pages=629–632|doi=10.4315/0362-028X-43.8.629|pmid=30822984|issn=0362-028X|doi-access=free}}</ref><ref name=":2">{{Cite journal|date=2016-07-01|title=Thawing in a microwave cavity: Comprehensive understanding of inverter and cycled heating|url=https://www.sciencedirect.com/science/article/abs/pii/S0260877416300395|journal=Journal of Food Engineering|language=en|volume=180|pages=87–100|doi=10.1016/j.jfoodeng.2016.02.007|issn=0260-8774|last1=Chen|first1=Fangyuan|last2=Warning|first2=Alexander D.|last3=Datta|first3=Ashim K.|last4=Chen|first4=Xing}}</ref>


By position and type, ] classifies them as (1) ] or (2) ] and built-in (wall oven for a ] or a ] model).<ref name=":1" />
In 1947, Raytheon built the "Radarange", the first commercially available microwave oven.<ref>{{cite web| url=http://www.raytheon.com/ourcompany/history/leadership/ |archiveurl=http://web.archive.org/web/20130322044917/http://www.raytheon.com/ourcompany/history/leadership/ |archivedate=2013-03-22 |title=Technology Leadership |publisher=Raytheon}}{{dead link|date=March 2015}}</ref> It was almost {{convert|1.8|m|ftin}} tall, weighed {{convert|340|kg|lb}} and cost about US$5,000 (${{formatnum:{{Inflation|US|5000|1947}}}} in today's dollars) each. It consumed 3&nbsp;]s, about three times as much as today's microwave ovens, and was water-cooled. An early Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship ]. An early commercial model introduced in 1954 consumed 1.6&nbsp;kilowatts and sold for US$2,000 to US$3,000 (${{formatnum:{{Inflation|US|2000|1954|r=-3}}}} to ${{formatnum:{{Inflation|US|3000|1954|r=-3}}}} in today's dollars). Raytheon licensed its technology to the ] Stove company of ] in 1952.<ref>{{cite web| url=http://ohiohistory.wordpress.com/2010/11/02/do-you-remember-your-familys-first-microwave |title=Do you remember your family's first microwave?| work=Ohio Historical Society| date=2 November 2010}}</ref> They tried to market a large 220&nbsp;volt wall unit as a home microwave oven in 1955 for a price of US$1,295 (${{formatnum:{{Inflation|US|1295|1955}}}} in today's dollars), but it did not sell well. In 1965, Raytheon acquired ]. In 1967, they introduced the first popular home model, the countertop Radarange, at a price of US$495 (${{formatnum:{{Inflation|US|495|1967}}}} in today's dollars).


A traditional microwave only has two power output levels, fully on and fully off. Intermediate heat settings are achieved using ] and switch between full power and off every few seconds, with more time on for higher settings.
In the 1960s,{{specify|date=November 2013}} ] bought ]'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: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a ] in ],{{citation needed|date=October 2013}} and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was faster in ], due to a re-engineered magnetron allowing for less expensive units.
Several other companies joined in the market, and for a time most systems were built by defense contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business.


An inverter type, however, can sustain lower temperatures for a lengthy duration without having to switch itself off and on repeatedly. Apart from offering superior cooking ability, these microwaves are generally more energy-efficient.<ref name=":3">{{Cite journal|last1=Kako|first1=H.|last2=Nakagawa|first2=T.|last3=Narita|first3=R.|date=August 1991|title=Development of compact inverter power supply for microwave oven|url=https://ieeexplore.ieee.org/document/85575|journal=IEEE Transactions on Consumer Electronics|volume=37|issue=3|pages=611–616|doi=10.1109/30.85575|s2cid=108870083 |issn=1558-4127}}</ref><ref name=":2" /><ref name=":4">{{Cite journal|last1=Lee|first1=Min-Ki|last2=Koh|first2=Kang-Hoon|last3=Lee|first3=Hyun--Woo|date=2004|title=A Study on Constant Power Control of Half Bridge Inverter for Microwave Oven|url=https://www.koreascience.or.kr/article/JAKO200411922336424.page|journal=KIEE International Transaction on Electrical Machinery and Energy Conversion Systems|volume=4B|issue=2|pages=73–79|issn=1598-2602}}</ref>
====Residential use====
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".


{{As of|2020}}, the majority of countertop microwave ovens (regardless of brand) sold in the United States were manufactured by the ].<ref>{{cite news |last1=McCabe |first1=Liam |last2=Sullivan |first2=Michael |title=The Best Microwave |url=https://thewirecutter.com/reviews/best-microwave/ |access-date=21 May 2020 |work=Wirecutter |publisher=The New York Times |date=20 May 2020}}</ref>
Formerly found only in large industrial applications, microwave ovens increasingly became a standard fixture of residential kitchens in ]. By 1986, roughly 25% of households in the U.S. owned a microwave oven, up from only about 1% in 1971;<ref name="CPI_US">{{citation | url = http://www.bls.gov/cpi/cpimwo.htm | title = Hedonic Quality Adjustment Methods For Microwave Ovens In the U.S. CPI | first = Paul R. | last = Liegey | date = 16 October 2001 | accessdate = 5 October 2013 | publisher = Bureau of Labor Statistics, United States Department of Labor}}</ref> the U.S. Bureau of Labor Statistics reported that over 90% of American households owned a microwave oven in 1997.<ref name="CPI_US"/><ref>{{citation | last = Cox | first = W. Michael | last2= Alm | first2 = Richard | year = 1997 | title = Time Well Spent: The Declining Real Cost of Living in America | publisher = Federal Reserve Bank of Dallas | work = 1997 Annual Report | page = 22 (see Exhibit 8) | url = http://www.dallasfed.org/htm/pubs/pdfs/anreport/arpt97.pdf | accessdate = 12 August 1999 | archivedate = 13 August 2006 | archiveurl = http://www.dallasfed.org/assets/documents/fed/annual/1999/ar97.pdf}}</ref> In ], a 2008 market research study found that 95% of kitchens contained a microwave oven and that 83% of them were used daily.<ref>{{citation | url = http://newsroom.electrolux.com/au/files/2010/01/Westinghouse-How-Australia-Cooks-Report1.pdf | title = The Westinghouse How Australia Cooks Report | date = October 2008 | publisher = Westinghouse | accessdate = 5 February 2015}}</ref> In ], fewer than 5% of households had a microwave oven in 1979, but more than 88% of households owned one by 1998.<ref>{{cite journal | url = http://www.statcan.gc.ca/pub/11-008-x/11-008-x2000003-eng.pdf | title = Income and expenditures | first = Cara | last = Williams | publisher = Statistics Canada | date = Winter 2000 | number = 59 | journal = Canadian Social Trends — Catalogue No. 11-008 | pages = 7–12 | quote = Microwaves have been adopted even more avidly: in 1979, less than 5% of households had one, but by 1998 over 88% did.}}</ref> In ], 40% of households owned a microwave oven in 1994, but that number had increased to 65% by 2004.<ref>{{citation | url = http://www.freedoniagroup.com/brochure/20xx/2015smwe.pdf | title = World Major Household Appliances: World Industry Study with Forecasts to 2009 & 2014 (Study #2015) | date = January 2006 | at = TABLE VI-5: FRANCE COOKING APPLIANCES SUPPLY & DEMAND (million dollars) | publisher = The Freedonia Group | location = Cleveland, Ohio}}</ref>


=== Categories ===
Adoption has been slower in ], as households with disposable income concentrate on more important household appliances like ] and ]s. In ] in 2013, for example, only about 5% of households owned a microwave, well behind refrigerators at 31% ownership.<ref>{{cite web | title = Household penetration rate of home appliances in India in 2013 | url = http://www.statista.com/statistics/370635/household-penetration-home-appliances-india/ | accessdate = 5 February 2015 | publisher = Statistica}}</ref> Microwave ovens are gaining popularity, however. In ], the number of households with a microwave grew from almost 24% in 2002 to almost 40% in 2008.<ref name=USDA-microwave/> Almost twice as many households in ] owned microwaves in 2008 (38.7%) than in 2002 (19.8%).<ref name=USDA-microwave/> Microwave ownership in Vietnam was at 16% of households in 2008—versus 30% ownership of refrigerators—but this rate was up significantly from 6.7% microwave ownership in 2002—and 14% for refrigerators.<ref name=USDA-microwave>{{citation | url = http://www.ers.usda.gov/media/612721/householdamenities.xls | format = XLS | title = Ownership of household amenities among selected countries | publisher = Economic Research Service, United States Department of Agriculture | year = 2009 | accessdate = 5 February 2015}}</ref>
]
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).<ref>{{Cite web |title=Microwave Fact Sheets |url=https://www.microwaveassociation.org.uk/factsheets/index.php |access-date=2023-03-12 |website=www.microwaveassociation.org.uk}}</ref>


== Principles == == Principles ==
{{details|dielectric heating}} {{further|Dielectric heating}}
] ]
]
A microwave oven heats food by passing ] through it. Microwaves are a form of ] ] with a ] higher than ordinary ]s but lower than ]. Microwave ovens use frequencies in one of the ], which are reserved for this use, so they don't interfere with other vital radio services. Consumer ovens usually use 2.45 ] (GHz)—a ] of {{convert|12.2|cm|sigfig=3}}—while large industrial/commercial ovens often use 915 ] (MHz)—{{convert|32.8|cm|sigfig=3}}.<ref>{{cite web|title=Litton&nbsp;— For Heat, Tune to 915 or 2450 Megacycles|url=http://www.smecc.org/litton_-_for_heat,_tune_to_915_or_2450_megacycles.htm|work=], 1965|publisher=]|accessdate=12 December 2006|year=2007}}</ref> ], ], and other substances in the food absorb ] from the microwaves in a process called ]. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e. as both ] and ] of atoms), is ]. Sometimes, microwave heating is explained as a ] of water molecules, but this is incorrect;<ref>{{cite web|last=Bloomfield|first=Louis A.|title=Question 1456: My science book said that a microwave oven uses a laser resonating at the natural frequency of water. Does such a laser exist or was that a major typo?|url=http://www.howeverythingworks.org/page1.php?QNum=1456|publisher=HowEverythingWorks.org|accessdate=28 March 2009}} "It's a common misconception that the microwaves in a microwave oven excite a natural resonance in water. ... In fact, using a frequency that water molecules responded to strongly (as in a resonance) would be a serious mistake -- the microwaves would all be absorbed by water molecules at the surface of the food and the center of the food would remain raw."</ref> such resonances occur only at above 1 ] (THz).<ref>{{cite book |last=Schmitt |first=Ron |title=Electromagnetics Explained: a handbook for wireless/RF, EMC, and high-speed electronics |url=http://books.google.fi/books?id=7gJ4RocvEskC |accessdate=3 December 2012 |year=2002 |publisher=Elsevier |location=Burlington, Mass., USA |isbn=978-0-7506-7403-4 |page=343}}</ref>


A microwave oven heats food by passing ] through it. Microwaves are a form of ] ] with a ] in the so-called ] (300&nbsp;MHz to 300&nbsp;GHz). Microwave ovens use frequencies in one of the ], which are otherwise used for communication amongst devices that do not need a license to operate, so they do not interfere with other vital radio services.
Microwave heating is more efficient on liquid water than on frozen water, where the movement of molecules is more restricted. Dielectric heating of liquid water is also temperature-dependent: At 0&nbsp;°C, ] is greatest at a field frequency of about 10&nbsp;GHz, and for higher water temperatures at higher field frequencies.<ref name=Martin>{{cite web|last=Chaplin|first=Martin|title=Water and Microwaves|url=http://www.lsbu.ac.uk/water/microwave.html|work=Water Structure and Science|publisher=]|accessdate=4 December 2012|date=28 May 2012}}</ref>


It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. Instead, microwave ovens heat by causing molecules to spin under the influence of a constantly changing electric field, usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times. Typically, consumer ovens work around a nominal 2.45&nbsp;] (GHz) – a ] of {{convert|12.2|cm|sigfig=3}} in the 2.4&nbsp;GHz to 2.5&nbsp;GHz ISM band – while large industrial / commercial ovens often use 915&nbsp;] (MHz) – {{convert|32.8|cm|sigfig=3}}.<ref>{{cite web |title=For heat, tune to 915 or 2450&nbsp;Megacycles |year=2007 |publisher=] |orig-date=1965 |via=Southwest Museum of Engineering, Communications, and Computation |url=http://www.smecc.org/litton_-_for_heat,_tune_to_915_or_2450_megacycles.htm |access-date=12 December 2006}}</ref> 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.<ref name=depth>{{cite web |title=Cooking with Microwave Ovens|website=Food Safety and Inspection Service |publisher=] |url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#3}}</ref>
Compared to liquid water, microwave heating is less efficient on fats and sugars (which have a smaller molecular ]).<ref>"Efficient" here meaning more energy is deposited, not necessarily that the temperature rises more, because the latter also is a function of the ], which is often less than water for most substances. For a practical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have less heat capacity than the water they replace.{{Citation needed|date=January 2009}}</ref> Sugars and triglycerides (fats and oils) absorb microwaves due to the dipole moments of their ] or ]. However, due to the lower ] of fats and oils and their higher vaporization temperature, they often attain much higher temperatures inside microwave ovens.<ref name=Martin/> This can induce temperatures in oil or very fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional ] or deep fat frying. Foods high in water content and with little oil rarely exceed the boiling temperature of water.


A microwave oven takes advantage of the electric ] structure of ] ]s, fats, and many other substances in the food, using a process known as ]. 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 constantly spin around as they continually try to align themselves with the electric field. This can happen over a wide range of frequencies.<ref name="Soltysiak">{{cite conference |last1=Soltysiak |first1=Michal |last2=Celuch |first2=Malgorzata |last3=Erle |first3=Ulrich |title=Measured and simulated frequency spectra of the household microwave oven |conference=2011 IEEE MTT-S International Microwave Symposium |date=June 2011 |pages=1–4 |doi=10.1109/MWSYM.2011.5972844 |isbn=978-1-61284-754-2 |s2cid=41526758}}</ref><ref name=microwave>{{cite web |author=Bloomfield, Louis |title=Question 1456 |website=How Everything Works |url=http://www.howeverythingworks.org/page1.php?QNum=1456 |access-date=9 February 2012 |archive-url=https://web.archive.org/web/20131017005928/http://www.howeverythingworks.org/page1.php?QNum=1456 |archive-date=17 October 2013}}</ref><ref>{{cite web |first=Christopher S. |last=Baird |date=15 October 2014 |title=Why are the microwaves in a microwave oven tuned to water |website=Science Questions with Surprising Answers |publisher=] |place=Canyon, TX |url=http://wtamu.edu/~cbaird/sq/2014/10/15/why-are-the-microwaves-in-a-microwave-oven-tuned-to-water/}}</ref> The electric field's energy is absorbed by the dipole molecules as rotational energy. Then they hit non-dipole molecules, making them move faster as well. This energy is shared deeper into the substance as molecular rotations, vibrations or other movement signifying an increase in the ] of the food. Once the electrical field's energy is initially absorbed, heat will gradually spread through the object similarly to any other heat transfer by contact with a hotter body.<ref>{{cite book |last=Zitzewitz |first=Paul W. |date=February 2011 |title=The Handy Physics Answer Book |publisher=Visible Ink Press |isbn=9781578593576 |language=en |url=https://books.google.com/books?id=qGTkgFZBJZQC&q=microwave+oven+principle&pg=PA153 |via=Google Books}}</ref>
Microwave heating can cause localized ]s in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave to the point of melting if preheated.<ref>{{dead link|date=March 2015}}</ref> Additionally, microwaves can melt certain types of rocks, producing small quantities of synthetic lava.{{Citation needed|date=November 2012}} Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.


=== Defrosting ===
A common 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 homogenous 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 to ] (infrared) or convection heating—methods which deposit heat thinly at the food surface. Penetration depth of microwaves is dependent on ] and the frequency, with lower microwave frequencies (longer wavelengths) penetrating further.
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&nbsp;°C, ] is greatest at a field frequency of about 10&nbsp;GHz, and for higher water temperatures at higher field frequencies.<ref name=Martin>{{cite web |last=Chaplin |first=Martin |title=Water and microwaves |url=http://www.lsbu.ac.uk/water/microwave.html |series=Water Structure and Science |publisher=] |access-date=4 December 2012 |date=28 May 2012}}</ref>


===Fats and sugar===
== Heating efficiency ==
]s and ]s (fats and oils) absorb microwaves due to the dipole moments of their ] or ]. Microwave heating is less efficient on fats and sugars than on water because they have a smaller ].{{efn|Here "efficient" means that more energy is deposited and temperature rises faster, not necessarily that the temperature rises to a higher maximum. The maximum temperature is also a function of the material's ], which for most substances is lower than water. For a practical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have less heat capacity than the water they replace.{{Citation needed|date=January 2009}}}}
A microwave oven converts only part of its electrical input into microwave energy. An average consumer microwave oven consumes 1100 W of electricity in producing 700 W of microwave power, an efficiency of 64%. The other 400 W are dissipated as heat, mostly in the magnetron tube. Additional power is used to operate the lamps, AC power transformer, magnetron cooling fan, food turntable motor and the control circuits. Such wasted heat, along with heat from the product being microwaved, is exhausted as warm air through cooling vents.


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&nbsp;°C than does water (they have lower ]) and they begin cooling off by "boiling" only after reaching a higher temperature than water (the temperature they require to ] is higher), so inside microwave ovens they normally reach higher temperatures – sometimes ''much'' higher.<ref name=Martin/> This can induce temperatures in oil or fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional ], braising, or deep fat frying.
For cooking or reheating small amounts of food, the microwave oven may use less energy than a cook stove. Although microwave ovens are touted as the most efficient appliance,<ref> Retrieved 16 March 2012</ref> the energy savings are largely due to the reduced heat mass of the food's container.<ref> Retrieved 18 June 2012</ref> The amount of energy used to heat food is generally small compared to total energy usage in typical residences in the United States.{{citation needed|date=September 2013}}


The effect is most often noticed by consumers from unexpected damage to plastic containers when microwaving foods high in sugar, starch, or fat generates higher temperatures. Foods high in water content and with little oil rarely exceed the boiling temperature of water and do not damage plastic.
== Design ==

{{unreferenced section|date=January 2010}}
===Cookware===
] must be transparent to microwaves. Conductive cookware, such as metal pots, reflects microwaves, and prevents the microwaves from reaching the food. Cookware made of materials with high ] will absorb microwaves, resulting in the cookware heating rather than the food. Cookware made of ] is a common type of cookware that will heat in a microwave oven, reducing the effectiveness of the microwave oven and creating a hazard from burns or shattered cookware.

=== Thermal runaway ===
Microwave heating can cause localized ]s in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave oven to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of molten rock. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.<ref>{{cite conference |last1=Jerby |first1=Eli |last2=Meir |first2=Yehuda |last3=Faran |first3=Mubarak |title=Basalt melting by localized-microwave thermal-runaway instability |doi=10.13140/2.1.4346.1126 |url=https://www.eng.tau.ac.il/~jerby/Jerby_Basalt_Ampere-2013_Proc_PDF.pdf |conference=14th International Conference on Microwave and High Frequency Heating, AMPERE-2013 |location=Nottingham, UK |date=September 2013}}</ref>

=== 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 ] (infrared) or convection heating methods which thinly deposit heat at the food surface. Penetration depth of microwaves depends on ] and the frequency, with lower microwave frequencies (longer wavelengths) penetrating deeper.<ref name="depth"/>

=== Energy consumption ===
In use, microwave ovens can be as low as 50% efficient at converting electricity into microwaves,<ref>{{cite web |last1=Wirfs-Brock |first1=Jordan |last2=Jacobson |first2=Rebecca |date=23 February 2016 |title=A watched pot: What is the most energy-efficient way to boil water? |website=Inside Energy |url=http://insideenergy.org/2016/02/23/boiling-water-ieq/}}</ref> but energy-efficient models can exceed 64% efficiency.<ref>{{cite web |title=Energy Label- Criteria |website=www.energylabel.org.tw |publisher=Bureau of Energy, Ministry of Economic Affairs, Taiwan |url=https://www.energylabel.org.tw/englishlabel/application_en/efficiency/upt.aspx?Cid=48 |access-date=7 March 2022 |language=zh-Hant-TW}}</ref> Stovetop cooking is 40–90% efficient, depending on the type of appliance used.<ref>{{cite web |title=Level-up your cooking game with an energy-efficient stovetop |date=2020-08-24 |url=https://leap-va.org/energy-news/level-up-your-cooking-game-with-an-energy-efficient-stovetop/ |access-date=2022-12-23 |archive-date=23 December 2022 |archive-url=https://web.archive.org/web/20221223161204/https://leap-va.org/energy-news/level-up-your-cooking-game-with-an-energy-efficient-stovetop/ |url-status=dead }}</ref>

Because they are used fairly infrequently, the average residential microwave oven consumes only 72&nbsp;kWh per year.<ref>{{cite journal |last1=Gallego-Schmid |first1=Alejandro |last2=Mendoza |first2=Joan Manuel F. |last3=Azapagic |first3=Adisa |title=Environmental assessment of microwaves and the effect of European energy efficiency and waste management legislation |journal=Science of the Total Environment |date=March 2018 |volume=618 |pages=487–499 |doi=10.1016/j.scitotenv.2017.11.064 |pmid=29145100 |bibcode=2018ScTEn.618..487G |url=https://www.research.manchester.ac.uk/portal/en/publications/environmental-assessment-of-microwaves-and-the-effect-of-european-energy-efficiency-and-waste-management-legislation(ac7e65c2-c53e-4d0f-9bc8-b03ceb65541f).html |access-date=}}</ref> Globally, microwave ovens used an estimated 77&nbsp;TWh per year in 2018, or 0.3% of global electricity generation.<ref>{{cite journal |last1=Detz |first1=Remko J. |last2=van der Zwaan |first2=Bob |title=Surfing the microwave oven learning curve |journal=Journal of Cleaner Production |date=20 October 2020 |volume=271 |pages=122278 |doi=10.1016/j.jclepro.2020.122278 |s2cid=225872878 |access-date=|doi-access=free }}</ref>

A 2000 study by ] found that the average microwave drew almost 3 watts of ] when not being used,<ref>{{cite book |last1=Ross |first1=J.P. |last2=Meier |first2=Alan |title=Energy Efficiency in Household Appliances and Lighting |chapter=Whole-House Measurements of Standby Power Consumption |year=2001 |pages=278–285 |doi=10.1007/978-3-642-56531-1_33 |isbn=978-3-540-41482-7 |chapter-url=https://digital.library.unt.edu/ark:/67531/metadc743210/ }}</ref> which would total approximately 26&nbsp;kWh per year. New efficiency standards imposed in 2016 by the ] require less than 1&nbsp;watt, or approximately 9&nbsp;kWh per year, of standby power for most types of microwave ovens.<ref>{{cite web |last=de&nbsp;Laski |first=Andrew |date=3 June 2013 |title=New standards cut "vampire" energy waste |website=Appliance Standards Awareness Project (ASAP) |url=https://appliance-standards.org/blog/new-standards-cut-vampire-energy-waste |access-date=4 October 2021}}</ref>

== Components ==
] ]
]
A microwave oven consists of:
A microwave oven generally consists of:


* a high voltage power source, commonly a simple ] or an electronic ], which passes energy to the magnetron * a high-voltage DC power source, either:
* a high voltage ] connected to the magnetron, transformer and via a ] to the chassis **a large high voltage ] with a ] (a high-voltage ] and a ])
**an electronic ] usually based around an inverter.
* a ], which converts high-voltage electric energy to microwave radiation
* a ], which converts the high-voltage DC electric energy to microwave radiation
* a magnetron control circuit (usually with a ]) * a magnetron control circuit (usually with a ])
* a short ] (to couple microwave power from the magnetron into the cooking chamber) * a short ] (to couple microwave power from the magnetron into the cooking chamber)
* a turntable and/or ]
* a metal cooking chamber
* a control panel
* a turntable or metal fan


Modern microwave ovens use either an analog dial-type ] or a digital ] for operation. Control panels feature an ], liquid crystal or vacuum fluorescent display, numeric buttons for entering the cook time, a power level selection feature and other possible functions such as a defrost setting and pre-programmed settings for different food types, such as meat, fish, poultry, vegetables, ], ], and ]. In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off. As such, the choice of power level does not affect the intensity of the microwave radiation; instead, the magnetron is cycled on and off every few seconds. Newer models have inverter power supplies that use ] to provide effectively continuous heating at reduced power, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating. 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 ]. Newer models use ''inverter'' power supplies that use ] 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.<ref>{{cite web|url=http://www.thetelegram.com/opinion/columnists/a-notable-advance-in-microwave-technology-135137/|title=A notable advance in microwave technology|work=]|date=22 September 2013|access-date=10 May 2018|archive-url=https://web.archive.org/web/20180502233757if_/http://www.thetelegram.com/opinion/columnists/a-notable-advance-in-microwave-technology-135137/|archive-date=2018-05-02|url-status=dead}}</ref><ref name=":3" /><ref name=":2" /><ref name=":4" />


The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the ]s set aside for non-communication purposes. For household purposes, 2.45&nbsp;GHz has the advantage over 915&nbsp;MHz in that 915&nbsp;MHz is only an ISM band in the ] 2 while 2.45&nbsp;GHz is available worldwide.{{citation needed|date=January 2013}}{{vague|date=January 2013}} Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8&nbsp;GHz and 24.125&nbsp;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&nbsp;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 microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the ]s set aside for unlicensed purposes. For household purposes, 2.45&nbsp;GHz has the advantage over 915&nbsp;MHz in that 915&nbsp;MHz is only an ISM band in some countries (] 2) while 2.45&nbsp;GHz is available worldwide.{{citation needed|date=January 2013}} Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking. Two of them are centered on 5.8&nbsp;GHz and 24.125&nbsp;GHz, but are not used for microwave cooking because of the very high cost of power generation at these frequencies.{{citation needed|date=May 2019}} The third, centered on 433.92&nbsp;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.{{citation needed|date=May 2019}}


The cooking chamber is similar to a ] (but there is no continuous metal-to-metal contact around the rim of the door), and prevents the waves from coming out of the oven. 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&nbsp;cm for the usual 2.45&nbsp;GHz), most of the microwave radiation cannot pass through the door, while ] (with its much shorter wavelength) can. The cooking chamber is similar to a ] 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, ] 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&nbsp;cm for the usual 2.45&nbsp;GHz), microwave radiation cannot pass through the door, while ] (with its much shorter wavelength) can.<ref>{{Cite web|last=Staff|first=Straight Dope|date=2003-11-04|title=What keeps microwave radiation from leaking out the oven door?|url=https://www.straightdope.com/21343269/what-keeps-microwave-radiation-from-leaking-out-the-oven-door|access-date=2021-03-01|website=The Straight Dope|language=en}}</ref>


=== Variants and accessories === === Control panel ===
Modern microwave ovens use either an analog dial-type ] or a digital ] for operation. Control panels feature an ], ] 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.
] feature]]
A quantitative, model-based understanding of heat exchange in infrared and combined infrared-microwave heating of food inside an oven is developed.<ref>{{cite web|last= Almeida |first= Marialuci |title= Modeling infrared and combination infrared-microwave heating of foods in an oven|url= http://dspace.library.cornell.edu/bitstream/1813/198/1/almeida_ch0LA1.pdf |date=January 2005|accessdate=6 January 2015}} "A quantitative, model-based understanding of heat exchange in infrared and combined infrared-microwave heating of food inside an oven is developed."</ref> A variant of the conventional microwave is the ]. A convection microwave oven is a combination of a standard microwave and a ]. It allows food to be cooked quickly, yet come out browned or crisped, as from a convection oven. Convection microwaves are more expensive than conventional microwave ovens. Some convection microwaves—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.


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.
In 2000,<ref>{{cite news|last= Fabricant |first= Florence |title= Son of Microwave: Fast and Crisp |url= http://www.nytimes.com/2000/09/27/dining/test-kitchen-son-of-microwave-fast-and-crisp.html |date=27 September 2000|accessdate=6 January 2015}} "a new generation of ovens now arriving on the market"</ref> some manufacturers began offering high power ] ]s to their convection microwave models, marketing them under names such as "Speedcook", "]" , "Lightwave" and "Optimawave" to emphasize their ability to cook food rapidly and with good browning. The bulbs heat the food's surface with ] (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 ] in foods primarily made up of carbohydrates and ]s 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.


Microwave control panels are often considered awkward to use and are frequently employed as examples for user interface design.<ref>{{cite web|url=https://www.theguardian.com/technology/2015/jul/13/microwave-oven-user-interface-digital-complicated |title=User Interfaces: Why are Microwave Ovens All So Difficult to Use?|work=]|date= 13 July 2015|access-date= 4 January 2019}}</ref>
In order to aid ], sometimes an accessory browning tray is used, usually composed of glass or ]. It makes food crisp by ] the top layer until it turns ]. Ordinary ] ] is unsuitable for this purpose because it could melt.


== Variants and accessories ==
]s, pies, and ] bags often contain a ] made from thin ] 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 single use and are discarded as waste.
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 ]. 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 ] ]s to their convection microwave oven models,<ref>{{cite news|last= Fabricant |first= Florence |title= Son of Microwave: Fast and Crisp |url= https://www.nytimes.com/2000/09/27/dining/test-kitchen-son-of-microwave-fast-and-crisp.html |date=27 September 2000|access-date=6 January 2015|work=]}}</ref> marketing them under names such as "Speedcook", "]", "Lightwave" and "Optimawave" to emphasize their ability to cook food rapidly and with good browning. The bulbs heat the food's surface with ] (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 ] in foods primarily made up of carbohydrates and ]s 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.
== Microwave-safe plastics ==
Some current plastic containers and food ] are specifically designed to resist radiation from microwaves. Products may use the term "microwave safe", may carry a microwave symbol (three lines of waves, one above the other) or simply provide instructions for proper microwave use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.<ref>{{cite web| url=http://www.plasticsinfo.org/s_plasticsinfo/sec_level2_faq.asp?CID=703&DID=2837 |title=FAQs: Using Plastics in the Microwave |publisher=American Chemistry Council}}</ref>


In order to aid ], sometimes an accessory browning tray is used, usually composed of glass or ]. It makes food crisp by ] the top layer until it turns ].{{citation needed|date=July 2020}} Ordinary plastic ] is unsuitable for this purpose because it could melt.
== Benefits and safety features ==
Commercial microwave ovens all use a timer in their standard operating mode; when the timer runs out, the oven turns itself off.


]s, pies, and ] bags often contain a ] made from thin ] 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.
Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, unless it is an ], leaves a potentially dangerous heating element or ] that will stay hot for some time. Likewise, when taking a ] 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.


== Heating characteristics ==
Food and cookware taken out of a microwave oven are rarely much hotter than {{convert|100|C}}. 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 {{convert|180|C}}. That means that conventional stoves and ovens can cause more serious burns.
]


Microwave ovens produce heat directly within the food, but despite the common misconception that microwaved food cooks from the inside out, 2.45&nbsp;GHz microwaves can only penetrate approximately {{convert|1|cm|sp=us}} into most foods. The inside portions of thicker foods are mainly heated by heat conducted from the outer {{convert|1|cm|sp=us}}.<ref name="Püschner">{{cite web |title=Microwave penetration depths |website=pueschner.com |publisher=Püschner GMBH + CO KG MicrowavePowerSystems |url=http://www.pueschner.com/en/microwave-technology/penetration-depths |access-date=1 June 2018 |lang=en}}</ref><ref name="FDA">{{cite web |title=Microwave oven radiation |date=12 December 2017 |department=Radiation-emitting products / Resources for you |website=fda.gov |series=Center for Devices and Radiological Health |publisher=U.S. ] |url=https://www.fda.gov/Radiation-EmittingProducts/ResourcesforYouRadiationEmittingProducts/ucm252762.htm |access-date=1 June 2018 |lang=en }}</ref>
The lower temperature of cooking (the boiling point of water) is a significant safety benefit compared to baking in the oven or frying, because it eliminates the formation of tars and ], which are ].<ref>{{cite web | url = http://web.archive.org/web/20101230043041/http://www.pcrm.org/health/reports/worst_grill.html | title = The Five Worst Foods to Grill | publisher = Physicians Committee for Responsible Medicine | year = 2005 }}</ref> 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 fry 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 ] in ]es,<ref>{{cite web | title = Acrylamide: Information on Diet, Food Storage, and Food Preparation | url = http://www.fda.gov/Food/FoodborneIllnessContaminants/ChemicalContaminants/ucm151000.htm | date = 22 May 2008 | publisher = U.S. Food and Drug Administration | work = Food | quote = Boiling potatoes and microwaving whole potatoes with skin on to make “microwaved baked potatoes” does not produce acrylamide.1 (Footnote1: Based on FDA studies.)}}</ref> however unlike deep-frying, it is of only limited effectiveness in reducing glycoalkaloid (i.e. ]) levels.<ref>{{citation | title = 3-Picoline : Review of Toxicological Literature | first = Raymond | last = Tice | first2 = Brigette | last2 = Brevard | date = February 1999 | url = http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPdf/Picoline_508.pdf | publisher = Integrated Laboratory Systems | location = Research Triangle Park, North Carolina }}</ref> Acrylamide has been found in other microwaved products like popcorn.


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 ] 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.
=== Heating characteristics ===
Microwave ovens are frequently used for reheating ], and ]l contamination may not be repressed if the ] is not reached, resulting in ], as with all inadequate reheating methods.


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 ] 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 can be mapped out by placing a damp piece of ] in the oven. When the water saturated paper is subjected to the microwave radiation it becomes hot enough to cause the dye to be released which will 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.<ref>{{cite web | url = http://maartenrutgers.org/fun/microwave/microwave.html#fax | title = Physics inside a Microwave Oven | at = Finding the hot spots in your microwave with fax paper | first = Maarten | last = Rutgers | date = 1999 | archiveurl = http://web.archive.org/web/20030720183703/http://maartenrutgers.org/fun/microwave/microwave.html | archivedate = 20 July 2003}}</ref> : The location of dead spots and hot spots in a microwave oven can be mapped out by placing a damp piece of ] 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.<ref>{{cite web | first = Maarten | last = Rutgers | year = 1999 | title = Finding the hot spots in your microwave with fax paper | department = Physics inside a microwave oven |website = maartenrutgers.org | url = http://maartenrutgers.org/fun/microwave/microwave.html#fax | archive-url = https://web.archive.org/web/20030720183703/http://maartenrutgers.org/fun/microwave/microwave.html | archive-date = 20 July 2003}}</ref>


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 ] any parts of the food that overheat. In some materials with low ], where ] increases with temperature, microwave heating can cause localized ]. Under certain conditions, glass can exhibit thermal runaway in a microwave to the point of melting.<ref>{{YouTube|id=cskB5c0mJ58#t=98s|title=Video of microwave effects}}</ref> 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 ] any parts of the food that overheat. In some materials with low ], where ] increases with temperature, microwave heating can cause localized ]. Under certain conditions, glass can exhibit thermal runaway in a microwave oven to the point of melting.<ref>{{YouTube |title=Video of microwave effects |id=cskB5c0mJ58#t=98s }}</ref>


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 use low power levels designed to allow time for heat to be conducted within frozen foods from areas that absorb heat more readily to those which heat more slowly. In turntable-equipped ovens, more even heating will take place by placing food off-centre on the turntable tray instead of exactly in the centre. 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.<ref>{{cite thesis |degree=Masters |last=Pitchai |first=K. |year=2011 |title=Electromagnetic and Heat Transfer Modeling of Microwave Heating in Domestic Ovens |publisher=] |place=Lincoln, NB |url=https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1038&context=foodscidiss |access-date=28 August 2020 }}</ref>


There are microwave ovens on the market that allow full-power defrosting. They do this by exploiting the properties of the electromagnetic radiation ]s. LSM full-power defrosting may actually achieve more even results than slow defrosting.<ref>{{cite book |first=P. |last=Risman |year=2009 |section=Advanced topics in microwave heating uniformity |pages=76–77 |editor1-first=M.W. |editor1-last=Lorence |editor2-first=P.S. |editor2-last=Pesheck |title=Development of Packaging and Products for Use in Microwave Ovens |publisher=Elsevier |isbn=978-1845696573 }}</ref>
Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may include materials that contain ] or aluminium flakes, which are designed to absorb microwaves and heat up, thereby converting microwaves to less penetrating infrared, which aids in baking or crust preparation by depositing more energy shallowly in these areas. Such ceramic patches affixed to cardboard are positioned next to the food, and are typically smokey blue or gray in colour, usually making them easily identifiable; the cardboard sleeves included with ], which have a silver surface on the inside, are a good example of such packaging. Microwavable cardboard packaging may also contain overhead ceramic patches which function in the same way. The technical term for such a microwave-absorbing patch is a ].<ref>{{Cite journal | last = Labuza | first = T | authorlink = |author2=Meister | title = An Alternate Method for Measuring the Heating Potential of Microwave Susceptor Films | journal = J. International Microwave Power and Electromagnetic Energy | volume = 27 | issue = 4 | pages = 205–208 | publisher = | location = | year = 1992 | language = | url = http://www.jmpee.org/JMPEE_PDFs/27-4_bl/JMPEE-Vol27-Pg205-Labuza.pdf | jstor = | doi = | id = | mr = | zbl = | jfm = | accessdate = 23 September 2011 }}</ref>


Microwave heating can be deliberately uneven by design. Some microwavable packages (notably pies) may include materials that contain ] 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 '']''. 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 ], 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.<ref>{{cite journal | last1 = Labuza | first1 = T. | last2 = Meister | first2 = J. | year = 1992 | title = An alternate method for measuring the heating potential of microwave susceptor films | journal = Journal of Microwave Power and Electromagnetic Energy | volume = 27 | issue = 4 | pages = 205–208 | doi = 10.1080/08327823.1992.11688192 | bibcode = 1992JMPEE..27..205L | url = http://www.jmpee.org/JMPEE_PDFs/27-4_bl/JMPEE-Vol27-Pg205-Labuza.pdf | access-date = 23 September 2011 | archive-url = https://web.archive.org/web/20111104020217/http://www.jmpee.org/JMPEE_PDFs/27-4_bl/JMPEE-Vol27-Pg205-Labuza.pdf | archive-date = 4 November 2011 | df = dmy-all }}</ref>
== Effects on food and nutrients ==
]s when overcooked in a microwave produce considerable smoke.]]


=== Effects on food and nutrients ===
Comparative cooking method studies generally find that, if properly used, microwave cooking does not affect the nutrient content of foods to a larger extent than conventional heating, and that there is a tendency towards greater retention of many micronutrients with microwaving, probably due to the reduced preparation time.<ref>{{cite journal|last=Lassen|first=Anne|author2=Ovesen, Lars|title=Nutritional effects of microwave cooking|journal=Nutrition & Food Science|date=1 January 1995|volume=95|issue=4|pages=8–10|doi=10.1108/00346659510088654}}</ref> Microwaving human milk at high temperatures is contraindicated, due to a marked decrease in activity of anti-infective factors.<ref>{{cite journal |author=Quan R, Yang C, Rubinstein S, et al. |title=Effects of microwave radiation on anti-infective factors in human milk |journal=Pediatrics |volume=89 |issue=4 Pt 1 |pages=667–9 |date=April 1992 |pmid=1557249 }}</ref>


Any form of cooking will destroy some nutrients in food, but the key variables are how much water is used in the cooking, how long the food is cooked, and at what temperature.<ref name="NYTimes">{{cite news| url=http://www.nytimes.com/2006/10/17/health/17real.html | work=The New York Times | title=The Claim: Microwave Ovens Kill Nutrients in Food | first=Anahad | last=O'Connor | date=17 October 2006}}</ref> Nutrients are primarily lost by leaching into cooking water, which tends to make microwave cooking healthier, given the shorter cooking times it requires.<ref>{{cite web|title=Microwave cooking and nutrition|url=http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml|work=Family Health Guide|publisher=Harvard Medical School|accessdate=23 July 2011}}</ref> Like other heating methods, microwaving converts ] from an active to inactive form. The amount inactivated depends on the temperature reached, as well as the cooking time. Boiled food reaches a maximum of {{convert|100|Celsius}} (the boiling point of water), whereas microwaved food can get locally hotter than this, leading to faster breakdown of vitamin B<sub>12</sub>. The higher rate of loss is partially offset by the shorter cooking times required.<ref name="pmid10554220">{{cite journal |author=Fumio Watanabe, Katsuo Abe, Tomoyuki Fujita, Mashahiro Goto, Miki Hiemori, Yoshihisa Nakano |title=Effects of Microwave Heating on the Loss of Vitamin B(12) in Foods |journal=Journal of Agricultural and Food Chemistry |volume=46 |issue=1 |pages=206–210 |date=January 1998 |pmid=10554220|doi=10.1021/jf970670x|url=http://pubs.acs.org/cgi-bin/abstract.cgi/jafcau/1998/46/i01/abs/jf970670x.html}}</ref> A single study indicated that microwaving broccoli loses 74% or more of ] (97% of ]s), while boiling loses 66% of flavonoids, and high-pressure boiling loses 47%,<ref>{{cite journal |doi= 10.1002/jsfa.1585 |author= Vallejo F, Tomás-Barberán FA, García-Viguera C |title=Phenolic compound contents in edible parts of broccoli inflorescences after domestic cooking |journal=J Sci Food Agric |volume=83 |issue=14 |pages=1511–6 |year=2003 }}</ref> though the study has been contradicted by other studies.<ref>{{cite web|last=Greene|first=Moss|title=Healthy Microwave Cooking of Vegetables|url=http://www.bellaonline.com/articles/art52758.asp|accessdate=23 July 2011}}</ref> To minimize phenolic losses in potatoes, microwaving should be done at 500W.<ref>{{cite journal|last=Barba|first=Anna Angela|coauthors=Antonella Calabrettia, Matteo d'Amorea, Anna Lisa Piccinellia and Luca Rastrelli|title=Phenolic constituents levels in cv. Agria potato under microwave processing|journal=LWT&nbsp;— Food Science and Technology|date=16 January 2008|volume=41|issue=10|pages=1919–1926|doi=10.1016/j.lwt.2008.02.004|url=http://www.sciencedirect.com/science/article/pii/S0023643808000509|accessdate=23 July 2011}}</ref> Any form of cooking diminishes overall nutrient content in food, particularly ] ]s 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.<ref name=harvard-med>{{cite web |title=Microwave cooking and nutrition |date=6 February 2019 |series=Family Health Guide |publisher=] |website=health.harvard.edu |url=http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml |access-date=13 April 2021 |archive-url=https://web.archive.org/web/20110717050842/http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml |archive-date=17 July 2011 |df=dmy-all}}</ref><ref name=NYTimes>{{cite news |first=Anahad |last=O'Connor |date=17 October 2006 |newspaper=] |title=The claim: Microwave ovens kill nutrients in food |url=https://www.nytimes.com/2006/10/17/health/17real.html|access-date=13 April 2021}}</ref> 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.<ref name="harvard">{{cite web|title=Microwave cooking and nutrition|url=http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml|work=Family Health Guide|publisher=Harvard Medical School|date=6 February 2019|access-date=13 April 2021|archive-url=https://web.archive.org/web/20110717050842/http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml|archive-date=17 July 2011|df=dmy-all}}</ref> Like other heating methods, microwaving converts ] 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 {{convert|100|Celsius}} (the boiling point of water), whereas microwaved food can get internally hotter than this, leading to faster breakdown of vitamin B{{sub|12}}.{{citation needed|date=April 2021}} The higher rate of loss is partially offset by the shorter cooking times required.<ref name=pmid10554220>{{cite journal |first1=Fumio |last1=Watanabe |first2=Katsuo |last2=Abe |first3=Tomoyuki |last3=Fujita |first4=Mashahiro |last4=Goto |first5=Miki |last5=Hiemori |first6=Yoshihisa |last6=Nakano |date=January 1998|title=Effects of microwave heating on the loss of vitamin B(12) in foods |journal=Journal of Agricultural and Food Chemistry |volume=46 |issue=1 |pages=206–210 |pmid=10554220 |doi=10.1021/jf970670x |s2cid=23096987 }}</ref>


Spinach retains nearly all its ] when cooked in a microwave; in comparison, it loses about 77% when boiled, leaching out nutrients. Bacon cooked by microwave has significantly lower levels of carcinogenic ]s than conventionally cooked bacon.<ref name=NYTimes/> Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.<ref name="NYTimes"/> Microwave ] is 3-4 times more effective than boiled water blanching in the retaining of the water-soluble vitamins folic acid, thiamin and riboflavin, with the exception of ascorbic acid, of which 28.8% is lost (vs. 16% with boiled water blanching).<ref>{{cite web|title=Effects of microwave blanching vs. boiling water blanching on retention of selected water-soluble vitamins in turnips, foods, and greens using HPLC|url=http://www.uga.edu/nchfp/papers/2003/03iftturnipgreensposter.html|publisher=National Center for Home Food Preservation, University of Georgia|accessdate=23 July 2011|author=M. A. OSINBOYEJO, L. T. Walker, S. Ogutu, and M. Verghese }}</ref> Spinach retains nearly all its ] when cooked in a microwave oven; when boiled, it loses about 77%, leaching nutrients into the cooking water.<ref name=harvard/> Bacon cooked by microwave oven has significantly lower levels of ]s than conventionally cooked bacon.<ref name=NYTimes/> Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.<ref name="NYTimes"/> Microwave ] is 3–4&nbsp;times more effective than boiled-water blanching for retaining of the water-soluble vitamins, folate, ] and ], with the exception of {{nobr|],}} of which 29% is lost (compared with a 16% loss with boiled-water blanching).<ref>{{cite web |first1=M.A. |last1=Osinboyejo |first2=L.T. |last2= Walker |first3=S. |last3=Ogutu |first4=M. |last4=Verghese |title=Effects of microwave blanching vs. boiling water blanching on retention of selected water-soluble vitamins in turnips, foods, and greens using HPLC |series=National Center for Home Food Preservation |publisher=] |url=http://www.uga.edu/nchfp/papers/2003/03iftturnipgreensposter.html |date=15 July 2003 |access-date=23 July 2011 }}</ref>


=== Safety benefits and features ===
==Use in cleaning kitchen sponges==
All microwave ovens use a timer to switch off the oven at the end of the cooking time.
Studies have investigated the use of the microwave to clean non-metallic ] which have been thoroughly wetted. A 2006 study found that microwaving wet sponges for two minutes (at 1000 watt power) removed 99% of ]s, ] and ]s, but ] spores were killed at 4 minutes of microwaving. A 2009 study showed that the microwave and the ] were both effective ways to clean domestic sponges.<ref>http://www.sciencedirect.com/science/article/pii/S0956713513003745</ref>


Microwave ovens heat food without getting hot themselves. Taking a pot off a stove, unless it is an ], leaves a potentially dangerous heating element or ] that remains hot for some time. Likewise, when taking a ] 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.
== Hazards ==
] disc showing the effects of electrical discharge through its metal film]]


Food and cookware taken out of a microwave oven are rarely much hotter than {{convert|100|C}}. 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 {{convert|180|C}}. 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 ], which are ].<ref>{{cite web |title=The five worst foods to grill |publisher=Physicians Committee for Responsible Medicine |year=2005 |url=http://www.pcrm.org/health/reports/worst_grill.html |archive-url=https://web.archive.org/web/20101230043041/http://www.pcrm.org/health/reports/worst_grill.html |archive-date=30 December 2010 }}</ref> 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 ] in potatoes,<ref>{{cite web | title = Acrylamide: Information on diet, food storage, and food preparation | date = 22 May 2008 | publisher = U.S. ] | website = fda.gov | url = https://www.fda.gov/Food/FoodborneIllnessContaminants/ChemicalContaminants/ucm151000.htm | quote = Boiling potatoes and microwaving whole potatoes with skin on, to make "microwaved baked potatoes", does not produce acrylamide.⁽¹⁾ (Footnote&nbsp;(1): Based on FDA studies.)}}</ref> however unlike deep-frying at high-temperatures, it is of only limited effectiveness in reducing glycoalkaloid (i.e., ]) levels.<ref>{{cite report | first1 = Raymond | last1 = Tice | first2 = Brigette | last2 = Brevard | date = February 1999 | title = 3-Picoline : Review of toxicological literature | publisher = Integrated Laboratory Systems | place = Research Triangle Park, NC | url = http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPdf/Picoline_508.pdf }}</ref> 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 ] which have been thoroughly wetted. A 2006 study found that microwaving wet sponges for 2&nbsp;minutes (at 1000-watt power) removed 99% of ]s, '']'', and ]s. '']'' spores were killed at 4&nbsp;minutes of microwaving.<ref>{{cite journal | last1 = Taché | first1 = J. | last2 = Carpentier | first2 = B. | date = January 2014 | title=Hygiene in the home kitchen: Changes in behaviour and impact of key microbiological hazard control measures | journal=Food Control | volume=35 | issue = 1 | pages=392–400 | doi = 10.1016/j.foodcont.2013.07.026 }}</ref>

A 2017 study was less affirmative: About 60% of the germs were killed but the remaining ones quickly re-colonized the sponge.<ref>{{cite journal | last1 = Egert | first1 = Markus | last2 = Schnell | first2 = Sylvia | last3 = Lueders | first3 = Tillmann | last4 = Kaiser | first4 = Dominik | last5 = Cardinale | first5 = Massimiliano | date = 2017-07-19 | df=dmy-all | title=Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by ''Acinetobacter'', ''Moraxella'', and ''Chryseobacterium'' species | journal=Scientific Reports | volume = 7 | issue = 1 | pages = 5791 | doi = 10.1038/s41598-017-06055-9 | pmid = 28725026 | pmc = 5517580 | bibcode = 2017NatSR...7.5791C }}</ref>

== Issues ==
=== High temperatures === === High temperatures ===
====Closed containers====
] liquids can ]<ref>{{cite web|title=Superheated Water|url=http://www.newton.dep.anl.gov/askasci/chem00/chem00636.htm|work=]|publisher=]|accessdate=28 March 2009|author=Mike P.|coauthors=Alcir Grohmann, Darin Wagner, Richard E. Barrans Jr., Ph.D., Vince Calder|year=2001–2002}} (from the U.S. Dept. of Energy "Ask A Scientist" series's "Chemistry Archive" 2001315)</ref><ref>{{cite web|title=Superheating and microwave ovens|url=http://www.phys.unsw.edu.au/~jw/superheating.html|work=School of Physics|publisher=]|accessdate=25 October 2010}}</ref> 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 ] 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 (]) which may be violent enough to eject the boiling liquid from the container and cause severe ].<ref>{{cite web|last=Beaty|first=William J.|title=High Voltage in your Kitchen: Unwise Microwave Oven Experiments|url=http://amasci.com/weird/microwave/voltage3.html#dirt|publisher=Amasci.com|accessdate=21 January 2006}}</ref>
Closed containers, such as ]s, can explode when heated in a microwave oven due to the increased pressure from ]. 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.<ref>{{Cite web |date=2024-01-17 |title=Microwave safe plastics - How safe are they? {{!}} CAG |url=https://www.cag.org.in/blogs/microwave-safe-plastics-how-safe-are-they |access-date=2024-05-29 |website=www.cag.org.in |language=en}}</ref>


====Fires====
Closed containers, such as ]s, can explode when heated in a microwave oven due to the increased pressure from ]. Insulating plastic foams of all types generally contain closed air pockets, and are generally not recommended for use in a microwave, 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.
]
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====
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.
In rare cases, water and other ] liquids can ]<ref>{{cite web |author=Mike P. |author2=Alcir Grohmann |author3=Darin Wagner |author4=Richard E. Barrans Jr |author5=Vince Calder |year=2001–2002 |title=Superheated Water |url=http://www.newton.dep.anl.gov/askasci/chem00/chem00636.htm |archive-url=https://web.archive.org/web/20090322201613/http://newton.dep.anl.gov/askasci/chem00/chem00636.htm |archive-date=2009-03-22 |access-date=28 March 2009 |work=] |publisher=]}} (from the U.S. Dept. of Energy "Ask A Scientist" series's "Chemistry Archive" 2001315)</ref><ref>{{cite web |title=Superheating and microwave ovens |url=http://www.phys.unsw.edu.au/~jw/superheating.html |access-date=25 October 2010 |work=School of Physics |publisher=]}}</ref> 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 ] 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 (]) which may be violent enough to eject the boiling liquid from the container and cause severe ].<ref>{{cite web |last=Beaty |first=William J. |title=High Voltage in your Kitchen: Unwise Microwave Oven Experiments |url=http://amasci.com/weird/microwave/voltage3.html#dirt |access-date=21 January 2006 |publisher=Amasci.com}}</ref>


=== Metal objects === === Metal objects ===
Any metal or conductive object placed into the microwave will act as an ] to some degree, resulting in an electric ]. This causes the object to act as a ] element. This effect varies with the object's shape and composition, and is sometimes utilized for cooking.


Contrary to popular assumptions, metal objects can be safely used in a microwave oven, but with some restrictions.<ref>{{Cite web|url=https://www.goodhousekeeping.com/food-recipes/cooking/tips/a19797/microwave-metal/|title = Yes, You Can Microwave Metal|date = 31 January 2014}}</ref><ref>{{cite web |url=https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/cooking-microwave-ovens#11 |title=Cooking with Microwave Ovens: What containers and wraps are safe to use in the microwave oven? |website=]}}</ref> Any metal or conductive object placed into the microwave oven acts as an ] to some degree, resulting in an electric ]. This causes the object to act as a ] 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 ] (sparks) when microwaved. This includes ], crumpled ] (though some foil used in microwaves are safe, see below), twist-ties containing metal wire, the metal wire carry-handles in paper Chinese take-out food containers, or almost any metal formed into a poorly conductive foil or thin wire; or into a pointed shape.<ref> Accessed 25 October 2009.</ref> Forks are a good example: the ]s 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 ] of air, about 3 ]s per ] (3×10<sup>6</sup> V/m). The air forms a conductive ], 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 ] and ]s are formed, both of which are unhealthy in large quantities.

Any object containing pointed metal can create an ] (sparks) when microwaved. This includes ], crumpled ] (though some foil used in microwave ovens is safe, see below), twist-ties containing metal wire, the metal wire carry-handles in ]s, or almost any metal formed into a poorly conductive foil or thin wire, or into a pointed shape.<ref>{{cite web|url=http://www.conagrafoods.com/utilities/mwbasics.jsp?cookietest=true|title=Microwave cooking|at=sec. "Q: What is a microwave-safe plate or container?"|website=ConagraFoods.com|archive-url=https://web.archive.org/web/20120330102812/http://www.conagrafoods.com/utilities/mwbasics.jsp?cookietest=true|archive-date=30 March 2012|access-date=25 October 2009}}</ref> Forks are a good example: the ]s 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 ] of air, about 3 ]s per meter (3×10<sup>6</sup> V/m). The air forms a conductive ], 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 ] and ]s are formed, both of which are unhealthy in large quantities.


] ]


It is possible for metal objects to be microwave-oven compatible, although experimentation by users is not encouraged. 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. 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 ] or ] (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 cooking cover no more than one quarter of a food object, and be carefully smoothed to eliminate sparking hazards.<ref>{{cite web|title=Microwave Ovens and Food Safety|url=http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf|work=]|publisher=]|accessdate=10 August 2011|date=October 2011}}{{dead link|date=March 2015}}</ref>
The effect of microwaving thin metal films can be seen clearly on a ] or ] (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, ] 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 ] 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.<ref>{{cite web|title=Microwave Ovens and Food Safety |url=http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf |work=] |publisher=] |access-date=10 August 2011 |date=October 2011 |archive-url=https://web.archive.org/web/20110108064919/http://www.fsis.usda.gov/PDF/Microwave_Ovens_and_Food_Safety.pdf |archive-date=8 January 2011 }}</ref>


Another hazard is the resonance of the magnetron tube itself. If the microwave is run without an object to absorb the radiation, a ] will form. The energy is reflected back and forth between the tube and the cooking chamber. This may cause the tube to overload and burn out. For the same reason, ], or food wrapped in metal which does not arc, is problematic for overload reasons, without necessarily being a fire hazard. Another hazard is the resonance of the magnetron tube itself. If the microwave oven is run without an object to absorb the radiation, a ] 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, ], or food wrapped in metal which does not arc, is problematic for overload reasons, without necessarily being a fire hazard.


Certain foods such as ]s, if properly arranged, can produce an ].<ref>{{cite web|last=Popa|first=Adrian|title=Re: Why do grapes spark in the microwave?|url=http://madsci.org/posts/archives/dec97/882909591.Ph.r.html|publisher=]|accessdate=23 February 2006|date=23 December 1997}}</ref> Prolonged arcing from food carries similar risks to arcing from other sources as noted above. Certain foods such as grapes, if properly arranged, can produce an ].<ref>{{cite web|last=Popa|first=Adrian|title=Re: Why do grapes spark in the microwave?|url=http://madsci.org/posts/archives/dec97/882909591.Ph.r.html|publisher=]|access-date=23 February 2006|date=23 December 1997}}{{Dead link|date=October 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> 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 thermoses (such as ] novelty cups) or cups with metal lining. If any bit of the metal is exposed, all the outer shell will burst off the object or melt.{{citation needed|date=March 2013}} Some other objects that may conduct sparks are plastic/holographic print ] and other heat-retaining containers (such as ] novelty cups) or cups with metal lining. If any bit of the metal is exposed, all the outer shell can burst off the object or melt.{{citation needed|date=March 2013}}


The high electrical fields generated inside a microwave often can be illustrated by placing a ] or neon glow-bulb inside the cooking chamber, creating glowing plasma inside the low-pressure bulb of the device. The high electrical fields generated inside a microwave oven often can be illustrated by placing a ] 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 ===
{{Further|Microwave burn|Microwave#Effects on health}} {{Further|Microwave burn|Microwave#Effects on health}}
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.<ref> Retrieved 12 Aug 2014.</ref> Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary.<ref>{{cite web|url=http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#6|title=Radiation Emissions from Microwave ovens: How safe are Microwave Ovens?|publisher=]|accessdate=5 March 2009}}</ref> According to the ]'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 {{nowrap|5 cm}} (2&nbsp;in) from the surface of the oven.<ref>{{cite web|title=Microwave Oven Radiation: Microwave Oven Safety Standard|url=http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/HomeBusinessandEntertainment/ucm142616.htm#4|publisher=U.S. ]|accessdate=16 February 2009|date=13 January 2010}}</ref> This is far below the exposure level currently considered to be harmful to human health.<ref>{{cite web|title=Advanced Measurements of Microwave Oven Leakage|url=http://www.arpansa.gov.au/pubs/emr/microwave.pdf|year=2004|publisher=]|accessdate=8 January 2011}}</ref> 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.<ref> Retrieved 12 Aug 2014.</ref> Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary.<ref>{{cite web|url=http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#6|title=Radiation Emissions from Microwave ovens: How safe are Microwave Ovens?|publisher=]|access-date=5 March 2009|archive-url=https://web.archive.org/web/20090306112844/http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#6|archive-date=6 March 2009|df=dmy-all}}</ref> According to the ]'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 {{nowrap|5 cm}} (2&nbsp;in) from the surface of the oven.<ref>{{cite web|title=Microwave Oven Radiation: Microwave Oven Safety Standard|url=https://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsandProcedures/HomeBusinessandEntertainment/ucm142616.htm#4|publisher=U.S. ]|access-date=16 February 2009|date=13 January 2010}}</ref> This is far below the exposure level currently considered to be harmful to human health.<ref>{{cite web|title=Advanced Measurements of Microwave Oven Leakage|url=http://www.arpansa.gov.au/pubs/emr/microwave.pdf|year=2004|publisher=]|access-date=8 January 2011|archive-url=https://web.archive.org/web/20110124131856/http://www.arpansa.gov.au/pubs/emr/microwave.pdf|archive-date=24 January 2011|df=dmy-all}}</ref>


The radiation produced by a microwave oven is non-ionizing. It therefore does not have the ] risks associated with ] such as ]s and ]. Long-term rodent studies to assess cancer risk have so far failed to identify any carcinogenicity from {{nowrap|2.45 GHz}} microwave radiation even with chronic exposure levels (i.e. large fraction of life span) far larger than humans are likely to encounter from any leaking ovens.<ref>{{cite journal | doi= 10.2307/3579874 | last1= Frei | first1= MR | last2= Jauchem | first2= JR | last3= Dusch | first3= SJ | last4= Merritt | first4= JH | last5= Berger | first5= RE | last6= Stedham | first6= MA | title= Chronic, low-level (1.0 W/kg) exposure of mice prone to mammary cancer to 2450 MHz microwaves | journal= Radiation research | volume= 150 | issue= 5 | pages= 568–76 | year= 1998 | pmid= 9806599 }}</ref><ref>{{cite journal | last1= Frei | first1= MR | last2= Berger | first2= RE | last3= Dusch | first3= SJ | last4= Guel | first4= V | last5= Jauchem | first5= JR | last6= Merritt | first6= JH | last7= Stedham | first7= MA | title= Chronic exposure of cancer-prone mice to low-level 2450 MHz radiofrequency radiation | journal= Bioelectromagnetics | volume= 19 | issue= 1 | pages= 20–31 | year= 1998 | pmid= 9453703 | doi= 10.1002/(SICI)1521-186X(1998)19:1<20::AID-BEM2>3.0.CO;2-6 }}</ref> However, with the oven door open, the radiation may cause damage by heating. Every microwave oven sold has a protective ] so that it cannot be run when the door is open or improperly latched. The radiation produced by a microwave oven is non-ionizing. It therefore does not have the cancer risks associated with ] such as ]s and ]. Long-term ] to assess cancer risk have so far failed to identify any carcinogenicity from {{nowrap|2.45 GHz}} microwave radiation even with chronic exposure levels (i.e. large fraction of life span) far larger than humans are likely to encounter from any leaking ovens.<ref>{{cite journal | doi= 10.2307/3579874 | last1= Frei | first1= MR | last2= Jauchem | first2= JR | last3= Dusch | first3= SJ | last4= Merritt | first4= JH | last5= Berger | first5= RE | last6= Stedham | first6= MA | title= Chronic, low-level (1.0 W/kg) exposure of mice prone to mammary cancer to 2450 MHz microwaves | journal= Radiation Research | volume= 150 | issue= 5 | pages= 568–76 | year= 1998 | pmid= 9806599 | jstor= 3579874 | bibcode= 1998RadR..150..568F }}</ref><ref>{{cite journal | last1= Frei | first1= MR | last2= Berger | first2= RE | last3= Dusch | first3= SJ | last4= Guel | first4= V | last5= Jauchem | first5= JR | last6= Merritt | first6= JH | last7= Stedham | first7= MA | title= Chronic exposure of cancer-prone mice to low-level 2450 MHz radiofrequency radiation | journal= Bioelectromagnetics | volume= 19 | issue= 1 | pages= 20–31 | year= 1998 | pmid= 9453703 | doi= 10.1002/(SICI)1521-186X(1998)19:1<20::AID-BEM2>3.0.CO;2-6 }}</ref> However, with the oven door open, the radiation may cause damage by heating. Microwave ovens are sold with a protective ] so that it cannot be run when the door is open or improperly latched.


Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off. They do not make the food or the oven radioactive. There is some evidence that nutritional content of some foods may be altered differently by cooking in a microwave oven, compared to conventional cooking, but there is no indication of detrimental health issues associated with microwaved food. <ref></ref> Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off, any more than light from an electric lamp remains in the walls and furnishings of a room when the lamp is turned off. They do not make the food or the oven radioactive. In contrast with conventional cooking, the nutritional content of some foods may be altered differently, but generally in a positive way by preserving more ]s ]. There is no indication of detrimental health issues associated with microwaved food.<ref>{{Cite web |url=http://www.arpansa.gov.au/radiationprotection/factsheets/is_microwave.cfm#safe |title=ARPANSA - Microwave Ovens and Health<!-- Bot generated title --> |access-date=26 March 2015 |archive-url=https://web.archive.org/web/20090306112844/http://www.arpansa.gov.au/RadiationProtection/Factsheets/is_Microwave.cfm#safe |archive-date=6 March 2009 |df=dmy-all }}</ref>


There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action.<ref name="childinjury">{{cite book|last=Frost|first=Joe L.|title=Children and Injuries|publisher=Lawyers & Judges Publishing|page=593|isbn=<!--0-913875-96-1, -->978-0-913875-96-4|url=http://books.google.com/?id=pHXdUbiRiA8C&pg=PA87&dq=microwave+injury&cd=2#v=onepage&q=microwave%20injury&f=false|accessdate=29 January 2011|date=30 September 2001}}</ref><ref name="handbelechaz">{{cite book |last1=Geddesm |first1=Leslie Alexander |last2=Roeder |first2=Rebecca A. |isbn=0-913875-44-9 |title=Handbook of electrical hazards and accidents |pages=369ff |url=http://books.google.com/?id=Pb4lUnSsMa0C&pg=PA370&dq=microwave+injury&cd=66#v=onepage&q=microwave%20injury&f=false |publisher=Lawyers & Judges Publishing |year=2006}}</ref> There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action.<ref name="childinjury">{{cite book|last=Frost|first=Joe L.|title=Children and Injuries|publisher=Lawyers & Judges Publishing|page=593|isbn= 978-0-913875-96-4|url=https://books.google.com/books?id=pHXdUbiRiA8C&q=microwave+injury&pg=PA87|access-date=29 January 2011|date=30 September 2001}}</ref><ref name="handbelechaz">{{cite book |last1=Geddesm |first1=Leslie Alexander |last2=Roeder |first2=Rebecca A. |isbn=978-0-913875-44-5 |title=Handbook of electrical hazards and accidents |pages=369ff |url=https://books.google.com/books?id=Pb4lUnSsMa0C&q=microwave+injury&pg=PA370 |publisher=Lawyers & Judges Publishing |year=2006}}</ref> 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 === === Chemical exposure ===
]
Some magnetrons have ceramic ] with ] (beryllia) added. The ] in such oxides is a serious chemical hazard if crushed and ingested (for example, by inhaling dust). In addition, beryllia is listed as a confirmed human carcinogen by the ]; therefore, broken ceramic insulators or magnetrons should not be handled. This is obviously a danger only if the microwave oven becomes physically damaged, such as if the insulator cracks, or when the magnetron is opened and handled directly, and as such should not be a concern during normal usage.
The use of unmarked plastics for microwave cooking raises the issue of ]s leaching into the food,<ref name="harvhealth">{{cite web|title=Microwaving food in plastic: Dangerous or not?|date=December 2019|url=https://www.health.harvard.edu/staying-healthy/microwaving-food-in-plastic-dangerous-or-not|publisher=Harvard Health Publishing (Harvard U.)}}</ref> or the plastics chemically reacting to microwave energy, with by-products leaching into the food,<ref name="microwavesafe">{{cite web|title=Microwave-Safe Plastics|url=http://www.skoozeme.com/techramblings/plastics.html|website=skoozeme.com}}{{unreliable source?|date=September 2022}}</ref> suggesting that even plastic containers marked "microwavable" may still leach plastic by-products into the food.{{citation needed|date=September 2022}}

The plasticizers which received the most attention are ] (BPA) and ],<ref name="harvhealth" /><ref>{{Cite journal|title=Phthalates and attributable mortality: A population-based longitudinal cohort study and cost analysis|first1=Leonardo|last1=Trasande|first2=Buyun|last2=Liu|first3=Wei|last3=Bao|date=1 January 2022|journal=Environmental Pollution|volume=292|issue=Pt A |pages=118021|doi=10.1016/j.envpol.2021.118021|pmid=34654571 |pmc=8616787 }}</ref> although it is unclear whether other plastic components present a toxicity risk. Other issues include melting and flammability. An alleged issue of release of ]s into food has been dismissed<ref name="harvhealth" /> as an intentional ] distraction from actual safety issues.

Some current plastic containers and food ] are specifically designed to resist radiation from microwaves. Products may use the term "microwave safe", may carry a microwave symbol (three lines of waves, one above the other) or simply provide instructions for proper microwave oven use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.<ref>{{cite web | url=http://www.plasticsinfo.org/s_plasticsinfo/sec_level2_faq.asp?CID=703&DID=2837 | title=FAQs: Using Plastics in the Microwave | publisher=American Chemistry Council | access-date=12 May 2010 | archive-url=https://web.archive.org/web/20100926171011/http://plasticsinfo.org/s_plasticsinfo/sec_level2_faq.asp?CID=703&DID=2837 | archive-date=26 September 2010 }}</ref>

Plastic containers can release ] into food when heated in microwave ovens.<ref>{{Cite web |date=2023-07-21 |title=Nebraska study finds billions of nanoplastics released when microwaving containers |url=https://news.unl.edu/newsrooms/today/article/nebraska-study-finds-billions-of-nanoplastics-released-when-microwaving/ |access-date=2023-09-04 |website=news.unl.edu |language=en}}</ref>

=== Uneven heating ===
Microwave ovens are frequently used for reheating ], and bacterial contamination may not be repressed if the microwave oven is used improperly. If ] is not reached, this can result in ], 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.<ref name="FSIS">{{cite web |title=Microwave Ovens and Food Safety |url=https://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/appliances-and-thermometers/microwave-ovens-and-food-safety/ct_index |website=Food Safety and Inspection Service |publisher=United States Department of Agriculture |access-date=1 June 2018 |date=8 August 2013}}</ref>

===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 ] and ] and other devices that communicate on the ], particularly at close range.<ref>
{{cite news
|first=Karl S. |last=Kruszelnicki
|date=25 September 2012
|title= WiFi frozen? Blame the microwave oven
|series=]
|publisher=]
|url=http://www.abc.net.au/science/articles/2012/09/25/3595484.htm
|access-date=19 January 2019
}}
</ref>
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.<ref>
{{cite web
|title=Part&nbsp;1
|department=Interference from non-WiFi sources
|series=WiSE Article
|website=cwnp.com
|url=https://www.cwnp.com/the-wise-article-series-interference-from-non-wifi-sources-part-1/
}}
</ref>


== See also == == See also ==
{{portal|Cooking}}
* ]
* ]
*]
*]
* ] * ]
* ] * ]
* ] * ]
* ]
* ] * ]
* ] * ]
*]

==Notes==
{{notelist}}


== References == == References ==
{{reflist|30em}} {{reflist}}


== External links == == External links ==
{{Commons category|Microwave ovens}} {{Commons category|Microwave ovens}}
* {{US patent|2495429}}: Percy Spencer's original patent
*
* {{Webarchive|url=https://web.archive.org/web/20150226223720/http://newton.dep.anl.gov/askasci/chem00/chem00636.htm |date=26 February 2015 }}, Argonne National Laboratory
* Description with circuit diagrams
* ] ] suitable for young people
* *
* from ] magazine * from '']'' magazine
* {{dead link|date=March 2015}}
*
* *
* : Short explanation of microwave oven in terms of ] and ], intended for use in a class in ]
* {{dead link|date=March 2015}} Videos of the inside of the microwave oven compartment
* , David Ruzic, University of Illinois
* {{dead link|date=March 2015}} Short explanation of microwave oven in terms of ] and ], intended for use in a class in ]
{{Home appliances}}
* {{US patent|2147689}} - ''Method and apparatus for heating dielectric materials''
{{Authority control}}


] ]
] ]
] ]
] ]
] ]
]
]

Latest revision as of 17:36, 20 December 2024

Kitchen cooking appliance

Microwave oven
A modern microwave oven (2022)
TypeAppliance
Inception1947; 77 years ago (1947)
ManufacturerVarious
AvailableGlobally

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 vibrate and produce thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogeneous, high-water-content food item.

The development of the 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 post World War II from British radar technology developed before and during the war. Named the "RadaRange", it was first sold in 1947.

Raytheon later licensed its patents for a home-use microwave oven that was introduced by Tappan in 1955, 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. The countertop microwave oven was introduced in 1967 by the Amana Corporation. After microwave ovens became affordable for residential use in the late 1970s, their use spread into commercial and 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 and are popular for reheating previously cooked foods and cooking a variety of foods. They rapidly heat foods which can easily burn or turn lumpy if cooked in conventional pans, such as hot 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, such high-heat sources can be added to microwave ovens in the form of a convection microwave oven.

History

Early developments

Demonstration by Westinghouse of cooking sandwiches with a 60 MHz shortwave radio transmitter at the 1933 Chicago World's Fair

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:

This invention relates to heating systems for dielectric materials and the object of the invention is to heat such materials uniformly and substantially simultaneously throughout their mass. ... It has been proposed therefore to heat such materials simultaneously throughout their mass by means of the dielectric loss produced in them when they are subjected to a high voltage, high frequency field.

However, lower-frequency 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

Main article: Cavity magnetron
The 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. 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 "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

Microwave ovens, several from the 1980s

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. Employed by Raytheon at the time, he noticed that microwaves from an active radar set he was working on started to melt a Mr. Goodbar candy bar he had in his pocket. The first food deliberately cooked 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 8 October 1945, Raytheon filed a United States patent application for Spencer's microwave cooking process, and an oven that heated food using microwave energy from a magnetron was soon placed in a Boston restaurant for testing.

Another independent discovery of microwave oven technology was by British scientists, including James Lovelock, who in the 1950s used it to reanimate cryogenically frozen hamsters.

Commercial availability

Raytheon RadaRange aboard the NS Savannah nuclear-powered cargo ship, installed circa 1961

In 1947, Raytheon built the "Radarange", the first commercially available microwave oven. It was almost 1.8 metres (5 ft 11 in) tall, weighed 340 kilograms (750 lb) and cost about US$5,000 ($68,000 in 2023 dollars) each. It consumed 3 kilowatts, about three times as much as today's microwave ovens, and was water-cooled. The name was the winning entry in an employee contest. An early Radarange was installed (and remains) in the galley of the nuclear-powered passenger/cargo ship NS Savannah. An early commercial model introduced in 1954 consumed 1.6 kilowatts and sold for US$2,000 to US$3,000 ($23,000 to $34,000 in 2023 dollars). Raytheon licensed its technology to the Tappan Stove company of Mansfield, Ohio in 1952. Under contract to Whirlpool, Westinghouse, and other major appliance manufacturers looking to add matching microwave ovens to their conventional oven line, Tappan produced several variations of their built-in model from roughly 1955 to 1960. Due to maintenance (some units were water-cooled), in-built requirement, and cost—US$1,295 ($15,000 in 2023 dollars)—sales were limited.

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 ($5,000 in 2023 dollars). Unlike the Sharp models, a motor driven mode stirrer in the top of the oven cavity rotated allowing the food to remain stationary.

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 developed a new configuration of the microwave oven: the short, wide shape that is now common. The magnetron feed was also unique. This resulted in an oven that could survive a no-load condition: an empty microwave oven where there is nothing to absorb the microwaves. The new oven was shown at a trade show in Chicago, and helped begin a rapid growth of the market for home microwave ovens. Sales volume of 40,000 units for the U.S. industry in 1970 grew to one million by 1975. Market penetration was even faster in Japan, due to a less expensive re-engineered magnetron. Several other companies joined in the market, and for a time most systems were built by defence contractors, who were most familiar with the magnetron. Litton was particularly well known in the restaurant business.

Residential use

Finnish Fiskars 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.

1974 Radarange RR-4. 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.

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. Almost twice as many households in South Africa owned microwave ovens in 2008 (38.7%) as in 2002 (19.8%). Microwave oven ownership in Vietnam in 2008 was at 16% of households, versus 30% ownership of refrigerators; this rate was up significantly from 6.7% microwave oven ownership in 2002, with 14% ownership for refrigerators that year.

Consumer household microwave ovens usually come with a cooking power of between 600 and 1200 watts. Microwave cooking power, also referred to as output wattage, is lower than its input wattage, which is the manufacturer's listed power rating.

The size of household microwave ovens can vary, but usually have an internal volume of around 20 liters (1,200 cu in; 0.71 cu ft), and external dimensions of approximately 45–60 cm (1 ft 6 in – 2 ft 0 in) wide, 35–40 cm (1 ft 2 in – 1 ft 4 in) deep and 25–35 cm (9.8 in – 1 ft 1.8 in) tall. Countertop microwaves vary in weight 23 – 45 lbs.

Microwaves can be turntable or flatbed. Turntable ovens include a glass plate or tray. Flatbed ones do not include a plate, so they have a flat and wider cavity.

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).

A traditional microwave only has two power output levels, fully on and fully off. Intermediate heat settings are achieved using duty-cycle modulation and switch between full power and off every few seconds, with more time on for higher settings.

An inverter type, however, can sustain lower temperatures for a lengthy duration without having to switch itself off and on repeatedly. Apart from offering superior cooking ability, these microwaves are generally more energy-efficient.

As of 2020, the majority of countertop microwave ovens (regardless of brand) sold in the United States were manufactured by the Midea Group.

Categories

Microwave-safe symbol

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

Further information: Dielectric heating
A microwave oven, c. 2005
Simulation of the electric field inside a microwave oven for the first 8 ns of operation

A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency in the so-called microwave region (300 MHz to 300 GHz). Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are otherwise used for communication amongst devices that do not need a license to operate, so they do not interfere with other vital radio services.

It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. Instead, microwave ovens heat by causing molecules to spin under the influence of a constantly changing electric field, usually in the microwave frequencies range, and a higher wattage power of the microwave oven results in faster cooking times. Typically, consumer ovens work around a nominal 2.45 gigahertz (GHz) – a wavelength of 12.2 centimetres (4.80 in) in the 2.4 GHz to 2.5 GHz ISM band – while large industrial / commercial ovens often use 915 megahertz (MHz) – 32.8 centimetres (12.9 in). 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.

A microwave oven takes advantage of the electric dipole structure of water molecules, fats, and many other substances in the food, using a process known as dielectric heating. 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 constantly spin around as they continually try to align themselves with the electric field. This can happen over a wide range of frequencies. The electric field's energy is absorbed by the dipole molecules as rotational energy. Then they hit non-dipole molecules, making them move faster as well. This energy is shared deeper into the substance as molecular rotations, vibrations or other movement signifying an increase in the temperature of the food. Once the electrical field's energy is initially absorbed, heat will gradually spread through the object similarly to any other heat transfer by contact with a hotter body.

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. Microwave heating is less efficient on fats and sugars than on water because they have a smaller molecular dipole moment.

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. This can induce temperatures in oil or fatty foods like bacon far above the boiling point of water, and high enough to induce some browning reactions, much in the manner of conventional broiling (UK: grilling), braising, or deep fat frying.

The effect is most often noticed by consumers from unexpected damage to plastic containers when microwaving foods high in sugar, starch, or fat generates higher temperatures. Foods high in water content and with little oil rarely exceed the boiling temperature of water and do not damage plastic.

Cookware

Cookware must be transparent to microwaves. Conductive cookware, such as metal pots, reflects microwaves, and prevents the microwaves from reaching the food. Cookware made of materials with high electrical permittivity will absorb microwaves, resulting in the cookware heating rather than the food. Cookware made of melamine resin is a common type of cookware that will heat in a microwave oven, reducing the effectiveness of the microwave oven and creating a hazard from burns or shattered cookware.

Thermal runaway

Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave oven to the point of melting if preheated. Additionally, microwaves can melt certain types of rocks, producing small quantities of molten rock. Some ceramics can also be melted, and may even become clear upon cooling. Thermal runaway is more typical of electrically conductive liquids such as salty water.

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.

Energy consumption

In use, microwave ovens can be as low as 50% efficient at converting electricity into microwaves, 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 magnetron with section removed (magnet is not shown)
Inner space of a microwave oven and its control panel

A microwave oven generally consists of:

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.

The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints. The first is that they should be in one of the industrial, scientific, and medical (ISM) frequency bands set aside for unlicensed purposes. For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in some countries (ITU Region 2) while 2.45 GHz is available worldwide. 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

In addition to their use in heating food, microwave ovens are widely used for heating in industrial processes. A microwave tunnel oven for softening plastic rods prior to extrusion.

Microwave ovens produce heat directly within the food, but despite the common misconception that microwaved food cooks from the inside out, 2.45 GHz microwaves can only penetrate approximately 1 centimeter (0.39 in) into most foods. The inside portions of thicker foods are mainly heated by heat conducted from the outer 1 centimeter (0.39 in).

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 B12 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 100 °C (212 °F) (the boiling point of water), whereas microwaved food can get internally hotter than this, leading to faster breakdown of vitamin B12. 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. Bacon cooked by microwave oven has significantly lower levels of nitrosamines than conventionally cooked bacon. Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop. Microwave blanching is 3–4 times more effective than boiled-water blanching for retaining of the water-soluble vitamins, folate, thiamin and riboflavin, with the exception of vitamin C, of which 29% is lost (compared with a 16% loss with boiled-water blanching).

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 100 °C (212 °F). Cookware used in a microwave oven is often much cooler than the food because the cookware is transparent to microwaves; the microwaves heat the food directly and the cookware is indirectly heated by the food. Food and cookware from a conventional oven, on the other hand, are the same temperature as the rest of the oven; a typical cooking temperature is 180 °C (356 °F). That means that conventional stoves and ovens can cause more serious burns.

The lower temperature of cooking (the boiling point of water) is a significant safety benefit compared with baking in the oven or frying, because it eliminates the formation of tars and 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

Charred popcorn burnt by leaving the microwave oven on too long

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 (3×10 V/m). 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.

A microwave oven with a metal shelf

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.

A microwaved DVD-R 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

Further information: Microwave burn and Microwave § Effects on health

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 5 cm (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 2.45 GHz microwave radiation even with chronic exposure levels (i.e. large fraction of life span) far larger than humans are likely to encounter from any leaking ovens. However, with the oven door open, the radiation may cause damage by heating. Microwave ovens are sold with a protective interlock so that it cannot be run when the door is open or improperly latched.

Microwaves generated in microwave ovens cease to exist once the electrical power is turned off. They do not remain in the food when the power is turned off, any more than light from an electric lamp remains in the walls and furnishings of a room when the lamp is turned off. They do not make the food or the oven radioactive. In contrast with conventional cooking, the nutritional content of some foods may be altered differently, but generally in a positive way by preserving more micronutrientssee 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

Microwave-safe symbol

The use of unmarked plastics for microwave cooking raises the issue of plasticizers leaching into the food, or the plastics chemically reacting to microwave energy, with by-products leaching into the food, suggesting that even plastic containers marked "microwavable" may still leach plastic by-products 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 as an intentional red herring distraction from actual safety issues.

Some current plastic containers and food wraps are specifically designed to resist radiation from microwaves. Products may use the term "microwave safe", may carry a microwave symbol (three lines of waves, one above the other) or simply provide instructions for proper microwave oven use. Any of these is an indication that a product is suitable for microwaving when used in accordance with the directions provided.

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

Notes

  1. Here "efficient" means that more energy is deposited and temperature rises faster, not necessarily that the temperature rises to a higher maximum. The maximum temperature is also a function of the material's specific heat capacity, which for most substances is lower than water. For a practical example, milk heats slightly faster than water in a microwave oven, but only because milk solids have less heat capacity than the water they replace.

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