Why doesn't a microwave heat the air around food, in addition to the food itself?
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?
Best Answer
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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Quick Answer about "Why doesn't a microwave heat the air around food, in addition to the food itself?"
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.Does a microwave heat the air?
Microwave radiation causes water molecules go nuts, but the air itself isn't directly heated because its molecules aren't as polar as the molecules that make up water.Why does microwave heat the bowl and not the food?
What is this? With food safe glazes, no dangerous chemicals should leach into your food. That doesn't mean your dish should be used in the microwave. If the bowl gets hot, before the food, the microwaves are exciting molecules in the glaze.Why the microwave oven used for cooking heats up food but not the ambient air?
They are absorbed by these liquids and raise their temperature. In the gas phase, the absorption bands start at much higher frequencies, so the standard microwave frequencies of around 2.45 GHz can not heat air or water vapor.Does a microwave oven heat food from the outside in or from the inside out?
Microwave ovens do not \u201ccook from the inside out\u201d, as many people say. Microwaves actually heat from the outside in, very similar to other heating methods. Microwaves are actually pretty simple devices. The core components needed for a microwave are simply a magnetron and a high voltage source.More answers regarding why doesn't a microwave heat the air around food, in addition to the food itself?
Answer 2
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.
Answer 3
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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