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Why don't we use infrared (IR) or even the far IR just to heat food in a microwave oven instead of, of course, the conventional 2.45 GHz microwaves? Don't people call IR heat waves?

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    $\begingroup$ The question should motivate a bit more why a microwave oven with infrared radiation instead of conventional microwaves would be a good idea actually. Possible advantages (cheaper, faster, ...) are not immediately clear to me. $\endgroup$ – Trilarion Sep 6 '18 at 19:54
  • $\begingroup$ This appears to be more of an engineering question than a physics question. $\endgroup$ – Kyle Kanos Sep 6 '18 at 21:05
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    $\begingroup$ @KyleKanos Eh, I don't think the question itself is complex enough to warrant an engineer's approach. $\endgroup$ – can-ned_food Sep 7 '18 at 4:11
  • $\begingroup$ I would be surprised if infrared isn't the most ancient way of cooking. If our ancestors hang meat next flame and it's effectively cooking with infrared, due to the lack of an enclosure that makes convection more effective. $\endgroup$ – user3528438 Sep 7 '18 at 21:05
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    $\begingroup$ One of the benefits of a microwave is that the oven itself (and air inside) doesn't heat, only the food your cooking does. $\endgroup$ – Andy Sep 8 '18 at 17:09
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We do use (near) infrared radiation to heat food – whenever we toast food or grill (UK)/broil (US) by beaming infrared downwards on to food! The point is that the infrared is strongly absorbed by the food we cook in this way, and doesn't penetrate significantly beyond about a millimetre. So the surface of the food is strongly heated – seared, toasted or scorched! What lies below the surface is cooked much more slowly, mainly by conduction of heat from the surface.

Microwaves are not as strongly absorbed and penetrate much further, so the food is 'cooked from the inside'. The microwaves are mainly absorbed by water molecules that are sent into a vibratory/rotatory motion by the electric field of the microwaves acting on the (polarised) molecules. These are forced oscillations, but not at resonance; the frequency of the microwaves (about 2.4 GHz) is not a natural frequency for the molecule. If it were, the microwaves would be absorbed by the surface layer, and we'd have another grill or toaster!

Edit (prompted by comment below). I don't mean to give the impression that water molecules are the only ones that absorb microwaves. Fats are also strong absorbers.

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    $\begingroup$ @Travis They make those and are used in places like major chain sub sandwich and coffee shops. Those "rapid cook ovens" combine microwave, convection, and radiant heating (IR) elements. $\endgroup$ – user71659 Sep 6 '18 at 16:46
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    $\begingroup$ Grilling in the US means putting meat or vegetables above an outdoor heat source such as a red hot bed of wood that has been burnt to coals, a red hot bed of charcoal, or, a red hot bed of volcanic rock in an outdoor gas grill. That these appears to be red to us means that the vast majority of the energy is in the near infrared. $\endgroup$ – David Hammen Sep 6 '18 at 17:29
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    $\begingroup$ @DavidHammen When grilling, cooking by convection (closing the lid) and conduction (grill marks) are desired. If you compare a broiler to a grill, the broiler has the heat source above the food, and has no door to encourage the hot air to leave. $\endgroup$ – user71659 Sep 6 '18 at 17:52
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    $\begingroup$ It doesn't actually cook from the inside out, @rus9384. That's supposed to be a reference to how they were marketed. It cooks from the outside in, but it penetrates more deeply inside as it's not absorbed as well. $\endgroup$ – William Grobman Sep 7 '18 at 4:19
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    $\begingroup$ @WilliamGrobman, I know it doesn't and I heard it is bad in defrost exactly because they were designed to affect water, not ice (which has crystal structure). $\endgroup$ – user168013 Sep 7 '18 at 5:58
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We do.

That iron on the top of the device is a simple ohmic resistor, which gets the 230V network current directly. There is no need for any converter or stabilization of the temperature or the electricity, because

  • increased resistivity of the iron due to the temperature
  • and the second law of the thermodynamics

safely keeps the iron to around 700K, while it is in power.

enter image description here enter image description here

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One reason is that although there are devices that heat/cook food primarily by IR, they only work well for specialized sorts of cooking. You'd probably get better answers, or at least ones that address quality results, by asking on the cooking site. However, I'll attempt a short answer.

The most common is probably the ordinary toaster, where you insert say a couple of slices of bread (or toaster pastries &c) into slots, push a lever, and heating elements toast the outside surface until "done", when the toaster pops the bread up and shuts off the heat.

If you've ever examined the result on anything but thinly-sliced bread - say a bagel - you probably noticed that while the surface is nicely toasted, the inside might be barely warm. That same principle applies to more general-purpose devices like the toaster oven. The IR heats the surface: the inside must be heated by conduction from it. This is generally ok for things like sandwiches, but generally doesn't do a good job for baking, or roasting things for a long time. You too easily wind up with a burnt crust surrounding an uncooked center.

You even see this with conventional electric ovens, which use a heating element on the bottom of the oven that is heated to a red heat, thus giving off lots of IR. Place your cookie sheet or pan of brownies too close to this, and you get burned bottoms and underdone tops. (I use a baking stone myself, which shields the pans from the direct IR given off by the heating element, and gives much better results.)

OTOH, as others have explained, if you simply want to heat food quickly, a microwave is much more efficient, and quicker. A greater fraction of the energy is transferred to the food, and throughout the volume rather than just at the surface.

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  • $\begingroup$ Another food that is good to cook with IR is pizza: a lot of commercial pizza ovens are essentially an IR source mounted over a conveyor belt, so you can put uncooked pizzas in one side and take them out cooked on the other... $\endgroup$ – Jules Sep 8 '18 at 7:58
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Microwaves are absorbed by water, which works for the oven because most food has high water content. (If you had a completely dry food, it would not microwave well.) They also penetrate the food a bit, so you get heating throughout, which makes it fast.

Infrared ovens do exist, so that's also viable. They tend to cook on the outside more like a conventional oven. The ones that I personally know for homes are a variant of toaster oven in size and form factor. I don't know that there's any reason that it has to be that way. Search online. You'll easily find these.

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    $\begingroup$ "The ones that I personally know for homes are a variant of toaster oven in size and form factor" - well, the one in my kitchen (and there's nothing special about it) has a useable cooking volume of about 70 liters. That's a bit big for making toast! $\endgroup$ – alephzero Sep 5 '18 at 18:41
  • $\begingroup$ Yeah, well there you go. That's why I qualified it with "personally know." I do think there might be some subtly about the difference between a generic electric oven and an "infrared" oven in the degree to which convection plays a part, but I'm not sure that adds anything to the answer of the question that was actually asked. @alephzero $\endgroup$ – Brick Sep 5 '18 at 18:43
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    $\begingroup$ @JMac I originally thought the same, but then I tried to hack my regular, convective toaster oven into a reflow solder oven. There's less precision of control in the temperature. The IR ovens are on and then they are off. The convection ovens are take a while to heat up and then also take a while to cool. Not sure it matters for food, but for the reflow process the one is precise enough to control the process and the other is not. Obviously it depends on which product you find, but I think there's a much bigger convective element in the old ovens that people are saying heat primarily by IR. $\endgroup$ – Brick Sep 5 '18 at 19:16
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    $\begingroup$ @JMac The distinction is temperature of the element, thanks to the Stefan-Boltzmann law. One such device is the "infrared broiler". Heat is transferred from an electric element or flame to a high temperature ceramic grid, and is placed on top of the food, avoiding convection heating. Another example, which I'll have to mention by name, is the Panasonic FlashXpress toaster oven. It uses a conventional nichrome heating element, but it also has much higher temperature quartz elements. As mentioned, the idea for both is to induce browning at the surface of the food without overcooking the inside. $\endgroup$ – user71659 Sep 6 '18 at 0:35
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    $\begingroup$ The vast majority of the heat in a non-convection oven comes from infrared radiation from the hot oven walls, plus some from conduction from the oven rack, plus a bit more from the hot air. $\endgroup$ – David Hammen Sep 6 '18 at 17:57

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