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I'm reading Harold McGee's "On Food And Cooking", and in the "Microwave Cooking" section he writes:

Since the air in the oven is not heated, microwave ovens can’t brown meat surfaces unless they’re assisted by special packaging or a broiling element. (An exception to this rule is cured meats like bacon, which get so dry when cooked that they can brown.)

My question is: if the radio waves generated by a microwave are passing through air in order to reach the food that is cooked, and since air is made of molecules just like food is, why don't the radio waves affect the air molecules the way they do the food molecules?

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    Read up on dielectric heating. – Caleb Mar 31 at 18:33
  • molecules just like food - Well, not all molecules are created equal ;-) – TaW Apr 1 at 9:35
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    Same reason light goes through a window and not through a wall. Microwaves go through some things and are absorbed by others. – J... Apr 1 at 11:56
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    It might help to think of the air as being mostly transparent to the radio waves. Similar to the way light waves from the sun will heat your face more than it heats the air. If the air weren't mostly transparent, then the heat would never get to your face. – James Apr 1 at 12:11
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    Because microwaves heat up water molecules; not a huge constituent of air – Caius Jard Apr 2 at 19:40
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With all electromagnetic radiation, including visible light and microwaves, absorption depends on the molecules doing the absorption. Air is mainly oxygen and nitrogen, and these don't absorb very well at the 2.4GHz frequency used in microwaves, while foods do. A lot of this is down to the very efficient absorption by water, and almost all foodstuffs contain a fair amount of water.

Note that the air does warm up with heat from the food. For short cooking times this isn't a big effect, but for things that cook longer it's significant

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    The microwaves heat up the water in the air, heating the air a little bit, but it's not enough to have much of an effect on the food. – GdD Mar 31 at 16:51
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    @GdD, true, but how much, I'm not sure. The gas phase absorption spectrum of air is mostly in the mid/far infrared (mid-IR spectroscopy is my job and water value features are a pain). I couldn't (easily) find a GHz spectrum, but expect absorption to be very weak - weak enough to ignore given that only a few percent of air is water vapour . I did wonder about it as of course hot food generates steam, so more water vapour in the air. I suspect but can't prove that the biggest effect will be when this condenses on the walls, forming a liquid absorbing layer. – Chris H Apr 1 at 6:12
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    -1: This answer is misleading. The frequency is not critical -- it's that water (unlike oxygen, nitrogen, and most other molecules) is polar, i.e., has positive charge on one side and negative charge on the other, so the molecules are rapidly rotated by the oscillating electric field of the microwaves. See here: "It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. As noted microwave ovens can operate at many frequencies." – nanoman Apr 1 at 11:54
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    @nanoman you'll notice that I never said anything about resonance just the broad absorption. I specifically avoided going into the level of detail that would be required if I started discussing rotational spectroscopy, or any explanation of why the absorption differs between molecules, just that it does differ. Yes it's the polar nature of the molecules that's important, but that was a detail too much. – Chris H Apr 1 at 13:23
  • @Vino my Note that the air does warm up with heat from the food. covers transfer to and convection in the air. My comment to GdD addresses microwave absorption by water vapour – Chris H Apr 1 at 21:26
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Microwaves are actually lower energy photons than even visible light. So they don't cook because they carry high amounts of energy per photon, to impart to whatever they touch.

They cook for one reason only: they might be lower energy, but water and some other food molecules are electric dipoles (meaning their structure puts distinct positive and negative charges separated at different places in the molecule) and therefore they will rapidly rotate as they try to align themselves with electromagnetic waves across several ranges of microwave frequencies (even though these are not ionising radiations). Microwaves used in ovens typically have frequencies between 900 million and 2.5 billion Hz (cycles per second). That rotation in effect transfers energy to the molecule and its nearby molecules as heat. So a passing microwave of the right frequency can easily transfer its energy to a water molecule, and cause it to rotate quickly or jostle nearby molecules - which translates as being hotter. It can't easily transfer its energy to nitrogen or oxygen molecules in the air, most plastic/ceramic/glass used for food containers, and so on.

And that's what a microwave oven does. It sends a torrent of low energy photons into the cavity, they bounce around, and when they interact with molecules (and their energy is transferred), its very likely to be water, fat and some other molecules which mainly exist in the food. They don't get absorbed by the air, so the air itself isn't heated up. (A bit like how air isn't hot due to visible light travelling through it.)

Apart from perhaps some specialist materials engineered for the purpose, and some low-level absorption, any significant heating of containers, air, etc, only happen because of energy transferred to water in the food, which then warms the air and containers in turn., or evaporates as steam.

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    Quote nanoman, comment from other answer: "This answer is misleading. The frequency is not critical -- it's that water (unlike oxygen, nitrogen, and most other molecules) is polar, i.e., has positive charge on one side and negative charge on the other, so the molecules are rapidly rotated by the oscillating electric field of the microwaves. See here ( wikipedia.org/wiki/Microwave_oven#Principles ): "It is a common misconception that microwave ovens heat food by operating at a special resonance of water molecules in the food. As noted microwave ovens can operate at many frequencies." – mishan Apr 2 at 15:38
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    Thanks. Fixing. That's really worth catching. Sorry, ignorant me.+1 – Stilez Apr 2 at 16:50
  • ok now :) It also allows for a nice segway about why putting conductive stuff (metals) into microwave is and is NOT a bad idea: engineering.mit.edu/engage/ask-an-engineer/… – mishan Apr 2 at 21:55
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Every material (air, water, metal, plastic...) are more or less conductive and proportionally more or less absorbent of radiowave.

The more it absorbs the less material it needs to absorb a significant amount (let say 90%) of the wave energy.

For a given frequency and a given material you can calculate the amount of material you need to catch a given quantity of energy (heat).

This is a consequence of the skin effect.

For water the skin depth is about 4cm for microwaves and air is several meters, like km.

In other words, 90% of the microwave energy is absorbed by 4cm of anything with water in it, e.g., food.

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Buy an oven which combines a grill and microwave or even a full oven and microwave.

But remember that when you use the grill or oven settings you need containers that can take the heat while if you use the microwave settings you need containers that do not have metal.

So if you combine microwave and traditional over heat, you need a container that neither has metal nor will melt.

Or first use one setting, than move your meat to an other container and use the other setting.

In the microwave setting, the air will warm up a bit, as does the container, but not enough to brown the meat. That is how microwave ovens work, they heat the 'water' in the food, so will easily get it to boiling heat, but they do not heat the outside of the food.

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    This doesn't answer the question – GdD Mar 31 at 16:50
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    It did give an option to get what OP asked but now it also has an explanation to why it does not in a MW oven. – Willeke Mar 31 at 17:40
  • @GdD: To be fair to Willeke, this question belongs on the physics site. – James Apr 1 at 11:28
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    Which is why I pointed out that there are Microvvave ovens that can do the task, by having more options than mircowaves alone. It is the culinary help, not the scientific explanation, (which was already given in an other answer.) – Willeke Apr 1 at 15:19
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    @Flater: Well, the OP isn't asking what a microwave can do, they are asking "why". The question "Can a microwave brown meat?" is on topic. The question "Why don't the radio waves affect the air molecules?" is a physics question (in my opinion). I'm fine if the question is allowed on this forum, I just think the OP would get better answers on the physics site. – James Apr 1 at 15:23

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