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Un horno de microondas moderno (2016)
Dentro de un horno microondas usado: foto de 360 ​​°
( ver como un panorama interactivo de 360 ​​° )

Un horno de microondas (comúnmente conocido como microondas ) es un horno eléctrico que calienta y cocina alimentos exponiéndolos a radiación electromagnética en el rango de frecuencia de microondas . [1] Esto induce a las moléculas polares en los alimentos a rotar y producir energía térmica en un proceso conocido como calentamiento dieléctrico . Los hornos de microondas calientan los alimentos de forma rápida y eficaz porque la excitación es bastante uniforme en los 25-38 mm (1 a 1,5 pulgadas) exteriores de un alimento homogéneo con alto contenido de agua.

El desarrollo del magnetrón de cavidad en el Reino Unido hizo posible la producción de ondas electromagnéticas de una longitud de onda suficientemente pequeña ( microondas ). Al ingeniero estadounidense Percy Spencer generalmente se le atribuye la invención del horno microondas moderno después de la Segunda Guerra Mundial a partir de la tecnología de radar desarrollada durante la guerra. Llamado "Radarange", se vendió por primera vez en 1946.

Más tarde, Raytheon obtuvo la licencia de sus patentes para un horno microondas de uso doméstico que fue introducido por Tappan en 1955, pero aún era demasiado grande y costoso para el uso doméstico general. Sharp Corporation introdujo el primer horno de microondas con plato giratorio entre 1964 y 1966. El horno de microondas de mostrador fue introducido en 1967 por Amana Corporation . Después de que los hornos microondas se volvieran asequibles para uso residencial a fines de la década de 1970, su uso se extendió a las cocinas comerciales y residenciales de todo el mundo. Además de cocinar alimentos, los hornos microondas se utilizan para calentar en muchos procesos industriales.

Los hornos de microondas son un aparato de cocina común y son populares para recalentar alimentos previamente cocinados y cocinar una variedad de alimentos. Calientan rápidamente alimentos que pueden quemarse fácilmente o formar grumos si se cocinan en sartenes convencionales, como mantequilla caliente, grasas, chocolate o papilla . Los hornos de microondas generalmente no doran ni caramelizan directamente los alimentos, ya que rara vez alcanzan la temperatura necesaria para producir reacciones de Maillard . Se producen excepciones en los casos en que el horno se utiliza para calentar aceite para freír y otros elementos aceitosos (como el tocino), que alcanzan temperaturas mucho más altas que las del agua hirviendo. [ cita requerida ]

Los hornos de microondas tienen un papel limitado en la cocina profesional, [2] porque las temperaturas del rango de ebullición de un horno de microondas no producirán las sabrosas reacciones químicas que producirán al freír, dorar u hornear a una temperatura más alta. Sin embargo, estas fuentes de calor elevado se pueden añadir a los hornos microondas en forma de un horno microondas de convección. [3]

Historia [ editar ]

Desarrollos tempranos [ editar ]

Demostración de Westinghouse de cocinar sándwiches con un transmisor de radio de onda corta de 60 MHz en la Feria Mundial de Chicago de 1933

La explotación de ondas de radio de alta frecuencia para calentar sustancias fue posible gracias al desarrollo de los radiotransmisores de tubo de vacío alrededor de 1920. En 1930, la aplicación de ondas cortas para calentar el tejido humano se había convertido en la terapia médica de la diatermia . En la Feria Mundial de Chicago de 1933 , Westinghouse demostró la cocción de alimentos entre dos placas de metal conectadas a un transmisor de onda corta de 10 kW y 60 MHz . [4] El equipo de Westinghouse, dirigido por IF Mouromtseff, descubrió que los alimentos como filetes y papas se podían cocinar en minutos.

La solicitud de patente de los Estados Unidos de 1937 de Bell Laboratories establece: [5]

Esta invención se refiere a sistemas de calentamiento para materiales dieléctricos y el objeto de la invención es calentar dichos materiales de manera uniforme y sustancialmente simultánea en toda su masa. ... Por tanto, se ha propuesto calentar dichos materiales simultáneamente en toda su masa mediante la pérdida dieléctrica que se produce en ellos cuando se someten a un campo de alta tensión y alta frecuencia.

Sin embargo, el calentamiento dieléctrico de baja frecuencia , como se describe en la patente antes mencionada, es (como el calentamiento por inducción ) un efecto de calentamiento electromagnético , el resultado de los llamados efectos de campo cercano que existen en una cavidad electromagnética que es pequeña en comparación con la longitud de onda. del campo electromagnético. Esta patente proponía un calentamiento por radiofrecuencia, de 10 a 20 megahercios (longitud de onda de 30 a 15 metros, respectivamente). [6] El calentamiento de microondas que tienen una longitud de onda pequeña en relación con la cavidad (como en un horno de microondas moderno) se debe a efectos de "campo lejano" que se deben a la radiación electromagnética clásica.que describe la propagación libre de luz y microondas convenientemente lejos de su fuente. Sin embargo, el efecto de calentamiento primario de todos los tipos de campos electromagnéticos en las frecuencias de radio y microondas se produce a través del efecto de calentamiento dieléctrico, ya que las moléculas polarizadas se ven afectadas por un campo eléctrico que se alterna rápidamente.

Magnetrón de cavidad [ editar ]

El magnetrón de cavidad desarrollado por John Randall y Harry Boot en 1940 en la Universidad de Birmingham , Inglaterra.

La invención del magnetrón de cavidad hizo posible la producción de ondas electromagnéticas de una longitud de onda suficientemente pequeña ( microondas ). El magnetrón fue un componente crucial en el desarrollo del radar de longitud de onda corta durante la Segunda Guerra Mundial . [7] En 1937-1940, el físico británico Sir John Turton Randall, FRSE y compañeros de trabajo, construyó un magnetrón de múltiples cavidades para las instalaciones de radar militares británicas y estadounidenses en la Segunda Guerra Mundial. [8] Se necesitaba un generador de microondas de mayor potencia que funcionara en longitudes de onda más cortas , y en 1940, en la Universidad de Birminghamen Inglaterra, Randall y Harry Boot produjeron un prototipo funcional. [9] Inventaron una válvula que podía producir pulsos de energía de radio de microondas a una longitud de onda de 10 cm, un descubrimiento sin precedentes. [8]

Sir Henry Tizard viajó a los Estados Unidos a fines de septiembre de 1940 para ofrecer el magnetrón a cambio de su ayuda financiera e industrial (ver Misión Tizard ). [8] Una versión temprana de 6 kW, construida en Inglaterra por los Laboratorios de Investigación de la Compañía General Electric , Wembley , Londres, fue entregada al gobierno de los Estados Unidos en septiembre de 1940. El magnetrón fue descrito más tarde por el historiador estadounidense James Phinney Baxter III como "[t ] El cargamento más valioso jamás traído a nuestras costas ". [10] Se adjudicaron contratos a Raytheon y otras empresas para la producción en masa del magnetrón.

Descubrimiento [ editar ]

Hornos de microondas, varios de los años 80

En 1945, Percy Spencer , un ingeniero autodidacta estadounidense de Howland, Maine, descubrió accidentalmente el efecto de calentamiento de un haz de microondas de alta potencia . Empleado por Raytheon en ese momento, notó que las microondas de un conjunto de radar activo en el que estaba trabajando comenzaron a derretir una barra de chocolate que tenía en el bolsillo. El primer alimento cocinado deliberadamente con el horno de microondas de Spencer fueron palomitas de maíz, y el segundo fue un huevo, que explotó en la cara de uno de los experimentadores. [11] [12]

Para verificar su hallazgo, Spencer creó un campo electromagnético de alta densidad al alimentar energía de microondas de un magnetrón a una caja de metal de la que no tenía forma de escapar. Cuando se colocó comida en la caja con la energía de microondas, la temperatura de la comida se elevó rápidamente. El 8 de octubre de 1945, Raytheon presentó una solicitud de patente en los Estados Unidos para el proceso de cocción por microondas de Spencer, y pronto se colocó en un restaurante de Boston para su prueba un horno que calentaba alimentos utilizando energía de microondas de un magnetrón. [13]

Disponibilidad comercial [ editar ]

Raytheon RadaRange a bordo del buque de carga de propulsión nuclear NS Savannah , instalado alrededor de 1961

En 1947, Raytheon construyó el "Radarange", el primer horno microondas disponible comercialmente. [14] Medía casi 1,8 metros (5 pies 11 pulgadas) de alto, pesaba 340 kilogramos (750 libras) y costaba alrededor de US $ 5,000 ($ 57,000 en dólares de 2019) cada uno. Consumía 3 kilovatios, aproximadamente tres veces más que los hornos microondas actuales, y se enfría con agua. El nombre fue la obra ganadora en un concurso de empleados. [15] Se instaló (y permanece) uno de los primeros Radarange en la cocina del buque de carga y pasajeros de propulsión nuclear NS Savannah . Un modelo comercial temprano introducido en 1954 consumía 1,6 kilovatios y se vendía por US $ 2,000 a US $ 3,000 ($ 19,000 a $ 29,000 en dólares de 2019). Raytheon concedió la licencia de su tecnología a la empresa Tappan Stove deMansfield, Ohio en 1952. [16] Bajo contrato con Whirlpool, Westinghouse y otros importantes fabricantes de electrodomésticos que buscan agregar hornos microondas a juego a su línea de hornos convencionales, Tappan produjo varias variaciones de su modelo incorporado desde aproximadamente 1955 hasta 1960. Debido debido al mantenimiento (algunas unidades se enfriaron por agua), los requisitos incorporados y el costo (US $ 1,295 ($ 12,000 en dólares de 2019)), las ventas fueron limitadas.

Sharp Corporation de Japón comenzó a fabricar hornos microondas en 1961. Entre 1964 y 1966, Sharp introdujo el primer horno microondas con plato giratorio, un medio alternativo para promover un calentamiento más uniforme de los alimentos. [17] En 1965, Raytheon, buscando expandir su tecnología Radarange en el mercado doméstico, adquirió Amana para proporcionar más capacidad de fabricación. En 1967, introdujeron el primer modelo de hogar popular, la encimera Radarange, a un precio de 495 dólares (4.000 dólares en dólares de 2019). A diferencia de los modelos Sharp, un agitador de modo impulsado por motor en la parte superior de la cavidad del horno giraba permitiendo que los alimentos permanecieran estacionarios.

En la década de 1960, [ especificar ] Litton compró los activos de Franklin Manufacturing de Studebaker , que habían estado fabricando magnetrones y construyendo y vendiendo hornos microondas similares al Radarange. Litton desarrolló una nueva configuración del horno de microondas: la forma corta y ancha que ahora es común. La alimentación del magnetrón también fue única. Esto resultó en un horno que podría sobrevivir a una condición sin carga: un horno de microondas vacío donde no hay nada que absorba las microondas. El nuevo horno se mostró en una feria comercial en Chicago, [ cita requerida ]y ayudó a iniciar un rápido crecimiento del mercado de hornos microondas domésticos. El volumen de ventas de 40.000 unidades para la industria estadounidense en 1970 aumentó a un millón en 1975. La penetración del mercado fue aún más rápida en Japón, debido a un magnetrón rediseñado menos costoso. Varias otras empresas se unieron al mercado y, durante un tiempo, la mayoría de los sistemas fueron construidos por contratistas de defensa, quienes estaban más familiarizados con el magnetrón. Litton era particularmente conocido en el negocio de los restaurantes.

Uso residencial [ editar ]

Si bien es poco común hoy en día, los principales fabricantes de electrodomésticos ofrecieron cocinas combinadas de microondas durante gran parte de la década de 1970 como una progresión natural de la tecnología. Tanto Tappan como General Electric ofrecían unidades que parecían ser estufas / hornos convencionales, pero que incluían capacidad de microondas en la cavidad del horno convencional. Tales cocinas eran atractivas para los consumidores, ya que tanto la energía de microondas como los elementos calefactores convencionales se podían usar simultáneamente para acelerar la cocción y no había pérdida de espacio en la encimera. La propuesta también resultó atractiva para los fabricantes, ya que el costo de los componentes adicionales podría absorberse mejor en comparación con las unidades de mostrador donde los precios eran cada vez más sensibles al mercado.

En 1972, Litton (División Litton Atherton, Minneapolis) introdujo dos nuevos hornos microondas, con un precio de $ 349 y $ 399, para acceder al mercado estimado en $ 750 millones para 1976, según Robert I Bruder, presidente de la división. [18] Si bien los precios se mantuvieron altos, se siguieron agregando nuevas características a los modelos domésticos. Amana introdujo el descongelamiento automático en 1974 en su modelo RR-4D, y fue el primero en ofrecer un panel de control digital controlado por microprocesador en 1975 con su modelo RR-6.

1974 Radarange RR-4 . A fines de la década de 1970, los avances tecnológicos provocaron una rápida caída de los precios. A menudo llamados "hornos electrónicos" en la década de 1960, el nombre "horno de microondas" ganó popularidad más tarde, y ahora se les llama informalmente "microondas".

A fines de la década de 1970, se produjo una explosión de modelos de encimeras de bajo costo de muchos de los principales fabricantes.

Los hornos microondas, que antes solo se encontraban en grandes aplicaciones industriales, se convirtieron cada vez más en un accesorio estándar de las cocinas residenciales en los países desarrollados . En 1986, aproximadamente el 25% de los hogares en los EE. UU. Poseían un horno de microondas, frente a solo alrededor del 1% en 1971; [19] La Oficina de Estadísticas Laborales de EE. UU. Informó que más del 90% de los hogares estadounidenses poseían un horno microondas en 1997. [19] [20] En Australia, un estudio de investigación de mercado de 2008 encontró que el 95% de las cocinas contenían un horno microondas y que El 83% de ellos se utilizaron a diario. [21] En Canadá, menos del 5 por ciento de los hogares tenían un horno microondas en 1979, pero más del 88 por ciento de los hogares tenían uno en 1998. [22]En Francia, el 40 por ciento de los hogares poseía un horno microondas en 1994, pero ese número había aumentado al 65 por ciento en 2004. [23]

La adopción ha sido más lenta en los países menos desarrollados , ya que los hogares con ingresos disponibles se concentran en electrodomésticos más importantes como refrigeradores y hornos. En la India , por ejemplo, solo alrededor del 5% de los hogares poseían un horno microondas en 2013, muy por detrás de los refrigeradores con un 31% de propiedad. [24] Sin embargo, los hornos microondas están ganando popularidad. En Rusia, por ejemplo, el número de hogares con un horno microondas aumentó de casi el 24% en 2002 a casi el 40% en 2008. [25] Casi el doble de hogares en Sudáfrica poseían hornos microondas en 2008 (38,7%) que en 2002 (19,8%). [25]La propiedad de hornos microondas en Vietnam era del 16% de los hogares en 2008, frente al 30% de propiedad de refrigeradores; esta tasa aumentó significativamente desde el 6,7% de propiedad de hornos microondas en 2002, con un 14% de propiedad de refrigeradores ese año. [25]

Los hornos de microondas domésticos de consumo generalmente vienen con una potencia de cocción de 600 vatios y más (con 1000 o 1200 vatios en algunos modelos). El tamaño de los hornos microondas domésticos puede variar, pero por lo general tienen un volumen interno de alrededor de 20 litros (1,200 pulgadas cúbicas; 0,71 pies cúbicos) y dimensiones externas de aproximadamente 45 a 60 cm (1 pie 6 pulgadas a 2 pies 0 pulgadas) de ancho. , 35 a 40 cm (1 pie 2 pulg. 1 pie 4 pulg.) De profundidad y 25 a 35 cm (9,8 pulg. 1 pie 1,8 pulg.) De altura.

Los microondas pueden ser de mesa giratoria o de cama plana. Los hornos giratorios incluyen una placa o bandeja de vidrio. Los de cama plana no incluyen plato, por lo que tienen una cavidad plana y más ancha. [26] [27] [28]

Por posición y tipo, US DOE los clasifica en (1) encimera o (2) sobre la estufa y empotrados (horno de pared para un modelo de mueble o cajón ). [26]

Los microondas tradicionales dependen de la energía de una bobina magnética, pero muchos modelos más nuevos funcionan con un inversor. Los microondas inverter pueden ser útiles para lograr resultados de cocción más uniformes, ya que ofrecen un flujo continuo de potencia de cocción. Un microondas tradicional solo tiene dos configuraciones de calor, ENCENDIDO y APAGADO, pero un modelo de inversor puede soportar temperaturas más bajas durante un período prolongado sin tener que apagarse y encenderse nuevamente. Además de ofrecer una capacidad de cocción superior, estas microondas generalmente son más eficientes en términos de energía. [29] [28] [30]

A partir de 2020 , la mayoría de los hornos microondas de encimera (independientemente de la marca) vendidos en los Estados Unidos fueron fabricados por Midea Group . [31]

Consumo de energía [ editar ]

A partir de 2006, un horno microondas típico consumía más energía en el uso de energía de reserva durante todo el día que en el calentamiento de alimentos. [32]

Principios [ editar ]

Más información: calentamiento dieléctrico
Un horno de microondas, c. 2005
Reproducir medios
Simulación del campo eléctrico dentro de un horno microondas durante los primeros 8 ns de funcionamiento

Un horno de microondas calienta los alimentos pasando radiación de microondas a través de ellos. Las microondas son una forma de radiación electromagnética no ionizante con una frecuencia en la denominada región de microondas (300 MHz a 300 GHz). Los hornos microondas utilizan frecuencias en una de las bandas ISM (industrial, científica, médica) , que de otro modo se utilizan para la comunicación entre dispositivos que no necesitan una licencia para funcionar, por lo que no interfieren con otros servicios de radio vitales.  

Los hornos de consumo funcionan alrededor de 2,45 gigahercios (GHz) nominales, una longitud de onda de 12,2 centímetros (4,80 pulgadas) en la banda ISM de 2,4 GHz a 2,5 GHz, mientras que los grandes hornos industriales / comerciales suelen utilizar 915 megahercios (MHz), 32,8 centímetros (12,9 pulgadas). ). [33] El agua , la grasa y otras sustancias de los alimentos absorben energía de las microondas en un proceso llamado calentamiento dieléctrico.. Muchas moléculas (como las del agua) son dipolos eléctricos, lo que significa que tienen una carga positiva parcial en un extremo y una carga negativa parcial en el otro, y por lo tanto giran mientras intentan alinearse con el campo eléctrico alterno de las microondas. . Las moléculas en rotación golpean otras moléculas y las ponen en movimiento, dispersando así la energía.

Esta energía, dispersada como rotaciones moleculares, vibraciones y / o traslaciones en sólidos y líquidos, eleva la temperatura del alimento, en un proceso similar a la transferencia de calor por contacto con un cuerpo más caliente. [34] Es un error común pensar que los hornos microondas calientan los alimentos al operar con una resonancia especial de moléculas de agua en los alimentos. Como se señaló, los hornos microondas pueden funcionar en muchas frecuencias. [35] [36]

Descongelar [ editar ]

El calentamiento por microondas es más eficiente en agua líquida que en agua congelada, donde el movimiento de moléculas es más restringido. La descongelación se realiza a una potencia baja, lo que permite que la conducción lleve el calor a las partes aún congeladas de los alimentos. El calentamiento dieléctrico del agua líquida también depende de la temperatura: a 0 ° C, la pérdida dieléctrica es mayor a una frecuencia de campo de aproximadamente 10 GHz, y para temperaturas del agua más altas a frecuencias de campo más altas. [37] Una potencia de vataje más alta del horno de microondas resultará en tiempos de cocción más rápidos.

Grasas y azúcares [ editar ]

El calentamiento por microondas es menos eficiente en grasas y azúcares que en agua porque tienen un momento dipolar molecular más pequeño . [38] Los azúcares y triglicéridos (grasas y aceites) absorben las microondas debido a los momentos dipolares de sus grupos hidroxilo o éster . Sin embargo, debido a la menor capacidad calorífica específica de las grasas y aceites y su mayor temperatura de vaporización, a menudo alcanzan temperaturas mucho más altas dentro de los hornos microondas. [37] Esto puede inducir temperaturas en el aceite o los alimentos grasos como el tocino muy por encima del punto de ebullición del agua, y lo suficientemente alto como para inducir algunas reacciones de pardeamiento, muy parecido al asado convencional (Reino Unido: asado a la parrilla), estofado o freír.

Calentar en el microondas alimentos con alto contenido de azúcar, almidón y grasa puede dañar algunos recipientes de plástico. Las frutas como los tomates tienen un alto contenido de azúcar. [ cita requerida ] Los alimentos con alto contenido de agua y poco aceite rara vez superan la temperatura de ebullición del agua.

Fuga térmica [ editar ]

El calentamiento por microondas puede causar fugas térmicas localizadas en algunos materiales con baja conductividad térmica que también tienen constantes dieléctricas que aumentan con la temperatura. Un ejemplo es el vidrio, que puede exhibir una fuga térmica en un horno de microondas hasta el punto de derretirse si se precalienta. Además, las microondas pueden derretir ciertos tipos de rocas, produciendo pequeñas cantidades de roca fundida. Algunas cerámicas también se pueden fundir e incluso pueden volverse transparentes al enfriarse. La fuga térmica es más típica de líquidos conductores de electricidad como el agua salada. [39]

Penetración [ editar ]

Otro concepto erróneo es que los hornos microondas cocinan los alimentos "de adentro hacia afuera", es decir, desde el centro de toda la masa de alimentos hacia afuera. Esta idea surge del comportamiento de calentamiento visto si una capa absorbente de agua se encuentra debajo de una capa secante menos absorbente en la superficie de un alimento; en este caso, la deposición de energía térmica dentro de un alimento puede exceder la de su superficie. Esto también puede ocurrir si la capa interna tiene una capacidad calorífica menor que la capa externa, lo que hace que alcance una temperatura más alta, o incluso si la capa interna es más conductora térmica que la capa externa, lo que la hace sentir más caliente a pesar de tener una temperatura más baja. En la mayoría de los casos, sin embargo, con alimentos de estructura uniforme o razonablemente homogéneos, las microondas se absorben en las capas externas del artículo a un nivel similar al de las capas internas.

Dependiendo del contenido de agua, la profundidad de la deposición de calor inicial puede ser de varios centímetros o más con los hornos de microondas, en contraste con el asado / asado a la parrilla (infrarrojos) o el calentamiento por convección, métodos que depositan calor en una fina capa sobre la superficie de los alimentos. La profundidad de penetración de las microondas depende de la composición de los alimentos y la frecuencia, y las frecuencias de microondas más bajas (longitudes de onda más largas) penetran más. [ cita requerida ]

Componentes [ editar ]

Un magnetrón sin sección (no se muestra el imán)
Espacio interior de un horno microondas y paneles de control.

Un horno microondas consta de:

  • a high-voltage power source, commonly a simple transformer or an electronic power converter, which passes energy to the magnetron
  • a high-voltage capacitor connected to the magnetron, transformer and via a diode to the chassis
  • a cavity magnetron, which converts high-voltage electric energy to microwave radiation
  • a magnetron control circuit (usually with a microcontroller).
  • a short waveguide (to couple microwave power from the magnetron into the cooking chamber)
  • a turntable and/or metal wave guide stirring fan.
  • a control panel.

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 doesn't 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.[40][29][28][30]

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.[citation needed] 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.[citation needed] 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.[citation needed]

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.[41]

Control panel[edit]

Modern microwave ovens use either an analog dial-type timer or a digital control panel for operation. Control panels feature an LED, 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, frozen vegetables, frozen dinners, and popcorn. In the 90s 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 effect, but by repeatedly turning the power off and on. 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.[42]

Variants and accessories[edit]

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,[43] 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.[citation needed] 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[edit]

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).[44][45]

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 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.[46]

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.[47]

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 turn the power off and on repeatedly - 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-center on the turntable tray instead of exactly in the center, as this will result in more even heating of the food throughout.[48]

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.[49]

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. 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. The technical term for such a microwave-absorbing patch is a susceptor.[50]

Effects on food and nutrients[edit]

Any form of cooking will diminish 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.[51][52] 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.[51] 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.[citation needed] The higher rate of loss is partially offset by the shorter cooking times required.[53]

Spinach retains nearly all its folate when cooked in a microwave oven; when boiled, it loses about 77%, leaching nutrients into the cooking water.[51] Bacon cooked by microwave oven has significantly lower levels of nitrosamines than conventionally cooked bacon.[52] Steamed vegetables tend to maintain more nutrients when microwaved than when cooked on a stovetop.[52] 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).[54]

Safety benefits and features[edit]

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 will stay 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.[55] 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,[56] however unlike deep-frying, it is of only limited effectiveness in reducing glycoalkaloid (i.e., solanine) levels.[57] Acrylamide has been found in other microwaved products like popcorn.

Use in cleaning kitchen sponges[edit]

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 two minutes (at 1000 watt power) removed 99% of coliforms, E. coli and MS2 phages. Bacillus cereus spores were killed at four minutes of microwaving.[58]

A 2017 study was less affirmative: about 60% of the germs were killed but the remaining ones quickly re-colonized the sponge.[59]

Hazards[edit]

High temperatures[edit]

Superheating[edit]

Charred popcorn burnt by leaving the microwave oven on too long

Water and other homogeneous liquids can superheat[60][61] 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.[62]

Closed containers[edit]

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 will 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[edit]

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.

Metal objects[edit]

Any metal or conductive object placed into the microwave oven will act 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.[63] 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 (3×106 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.[64]

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 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. High reflected power may also cause magnetron arcing, possibly resulting in primary power fuse failure, though such a causal relationship isn't 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.[65] 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 Starbucks 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]

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[edit]

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.[66] Tests have shown confinement of the microwaves in commercially available ovens to be so nearly universal as to make routine testing unnecessary.[67] 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.[68] This is far below the exposure level currently considered to be harmful to human health.[69]

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.[70][71] 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 micronutrients - see above. There is no indication of detrimental health issues associated with microwaved food.[72]

There are, however, a few cases where people have been exposed to direct microwave radiation, either from appliance malfunction or deliberate action.[73][74] The general effect of this exposure will be 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[edit]

Microwave-safe symbol

The use of unmarked plastics for microwave cooking raises the issue of plasticizers leaching into the food,[75] or the plastics chemically reacting to microwave energy, with by-products leaching into the food,[76] 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,[75] 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[75] 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.[77]

Uneven heating[edit]

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 that parts of the food will not 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. [78]

Interference[edit]

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.[79]

See also[edit]

  • Countertop
  • Induction cooker
  • List of cooking appliances
  • List of home appliances
  • Microwave chemistry
  • Peryton (astronomy)
  • Robert V. Decareau
  • Thelma Pressman
  • Wall oven

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External links[edit]

  • U.S. Patent 2,495,429: Percy Spencer's original patent
  • Ask a Scientist Chemistry Archives, Argonne National Laboratory
  • Further Reading On The History Of Microwaves and Microwave Ovens
  • Microwave oven history from American Heritage magazine
  • Superheating and Microwave Ovens, University of New South Wales (includes video)
  • "The Microwave Oven": Short explanation of microwave oven in terms of microwave cavities and waveguides, intended for use in a class in electrical engineering
  • How Things Work: Microwave Ovens, David Ruzic, University of Illinois