Editing Microwave oven (section)

== Heating characteristics ==
[[File:Microwave tunnel closeup.jpg|thumb|In addition to their use in heating food, microwave ovens are widely used for heating in industrial processes, such as this 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&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=June 1, 2018 |lang=en}}</ref><ref name="FDA">{{cite web  |title=Microwave oven radiation |date=December 12, 2017 |department=Radiation-emitting products / Resources for you |website=fda.gov |series=Center for Devices and Radiological Health |publisher=U.S. [[Food and Drug Administration]] |url=https://www.fda.gov/Radiation-EmittingProducts/ResourcesforYouRadiationEmittingProducts/ucm252762.htm |access-date=June 1, 2018 |lang=en }}</ref>

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 [[Reflection (physics)|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.<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 = July 20, 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 [[RF shielding|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.<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 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=[[University of Nebraska]] |place=Lincoln, NB |url=https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1038&context=foodscidiss |access-date=August 28, 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 [[Longitudinal-section mode|LSM mode]]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 [[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.<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 = September 23, 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 = November 4, 2011 | df = dmy-all }}</ref>

=== Effects on food and nutrients ===

Any form of cooking diminishes overall nutrient content in food, particularly [[water-soluble]] [[vitamin]]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=February 6, 2019 |series=Family Health Guide |publisher=[[Harvard Medical School]] |website=health.harvard.edu |url=http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml |access-date=April 13, 2021 |archive-url=https://web.archive.org/web/20110717050842/http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml |archive-date=July 17, 2011 }}</ref><ref name=NYTimes>{{cite news |first=Anahad |last=O'Connor |date=October 17, 2006 |newspaper=[[The New York Times]] |title=The claim: Microwave ovens kill nutrients in food |url=https://www.nytimes.com/2006/10/17/health/17real.html|access-date=April 13, 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=February 6, 2019|access-date=April 13, 2021|archive-url=https://web.archive.org/web/20110717050842/http://www.health.harvard.edu/fhg/updates/Microwave-cooking-and-nutrition.shtml|archive-date=July 17, 2011}}</ref> Like other heating methods, microwaving converts [[vitamin B12|vitamin B{{sub|12}}]] 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 |bibcode=1998JAFC...46..206W |s2cid=23096987 }}</ref>

Spinach retains nearly all its [[folate]] 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 [[nitrosamine]]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 [[blanching (cooking)|blanching]] is 3–4&nbsp;times more effective than boiled-water blanching for retaining of the water-soluble vitamins, folate, [[thiamin]] and [[riboflavin]], with the exception of {{nobr|[[vitamin C]],}}  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=[[University of Georgia]] |url=http://www.uga.edu/nchfp/papers/2003/03iftturnipgreensposter.html |date=July 15, 2003 |access-date=July 23, 2011 }}</ref>

=== 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 {{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 [[Char (chemistry)|char]], which are [[carcinogenic]].<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=December 30, 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 [[acrylamide]] in potatoes,<ref>{{cite web | title = Acrylamide: Information on diet, food storage, and food preparation | date = May 22, 2008 | publisher = U.S. [[Food and Drug Administration]] | 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., [[solanine]]) levels.<ref>{{cite report | first1 = Raymond | last1 = Tice | first2 = Brigette | last2 = Brevard | date = February 1999  | title = 3-Picoline [108-99-6]: 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 [[kitchen sponge|domestic sponges]] which have been thoroughly wetted. A 2006 study found that microwaving wet sponges for 2&nbsp;minutes (at 1000-watt power) removed 99% of [[coliform]]s, ''[[E. coli]]'', and [[MS2 phage]]s. ''[[Bacillus cereus]]'' 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 = July 19, 2017 | 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>