My evidence is entirely anectodal and non-scientific, but I've noticed food gets cold faster when it's been heated in a microwave instead of a stove. Is this true? And if it is, why does it happen?
[Physics] Why does microwaved food get cold faster
heatthermodynamics
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Good question. The rate of temperature increase scales as the power absorbed by the food divided by mass of the food. So to understand your question, you need to understand how power is absorbed.
There is a finite amount of power in the microwaves being produced. These microwaves bounce around in the metal cage where you put your food, until they come into contact with the food. (Well, some of them will get absorbed in the metal by imperfect reflection, but let's ignore that at first.) Once they get absorbed by the food, they turn into heat. Because they bounce around until they hit some food, the efficiency of a microwave is pretty high, in the sense that most of the power generated in the form of microwaves goes into heating the food, regardless of how much food you have.
So, at lowest-order, increasing the mass will increase the amount of water, but won't increase the amount of power being absorbed by the food. But now, that thing about absorption by the metal comes in. The power absorption will be slightly greater with a lot of food, since the food will be more likely to absorb the microwave before it gets absorbed by the metal. This is a lower-order effect, but it's there.
Of course, then the issue of skin depth comes in. Microwaves only penetrate a certain distance into the food. (Of order an inch, depending on the food.) So increasing the mass isn't really what you want; you want to increase the mass that's within the skin depth. For example, a wide dish of water that's one inch deep will absorb better than a jug of water with the same volume. This is why you want to split apart chicken breasts when defrosting them, for example.
To answer your question, then, the more food you put in, the more efficiently your food will capture the power being produced by the microwave oven. So you will capture the microwave's power better, but you will still heat slower because you have more mass. But this is not a dominant effect, and you might be better off redistributing your food to maximize the surface area.
Your friend is, very, very theoretically, right, but the risks on both theoretical grounds and also epidemiological grounds - i.e. microwave ovens have been used by many people for a long time without obvious illnesses showing themselves - are extremely small.
There are two ways wherein microwave cooking might "change the molecules": the first
They might break and reconfigure bonds within organic molecules. However, whilst this theoretically happens, it happens unbelievable seldom if practically at all. Bond energies and bond dissociation energies are of the order of electron volts or tens thereof. So they are a few or a few tens of optical photons' worth of energy: bond reconfiguration is thus driven by photons with frequency of the order 1000THz. Microwave oven photons, on the other hand, at 1 to 2 gigahertz, are six orders of magnitude less energetic. However, from quantum mechanics, there is a nonzero probability that bond breaking by microwave photons will happen, but it will be fantastically low. This is the idea of quantum tunnelling: if and event, through energy considerations, is forbidden classically, it still happens, albeit seldom. Cold hydrogen fusion happens, for example, when you pull sticky-tape off something, but the events are fantastically seldom.
Microwaves denature proteins through their pure heating effect, i.e. change their three dimensional shape without changing the chemical bonds within them. An analogy is supercoiling and curliness in a telephone receiver cable. The basic cable can stay intact, but different amounts of winding can get it "stuck" in configurations of different 3D shape (like the kind where it's supercoiled so much the knots wrap themselves around your hand when you're trying to talk on the telephone and your interlocutor, if unlucky, thinks they're getting sworn at). However, this denaturing is exactly the same effect as wrought by any other kind of heating. Protein denaturing is essentially the difference between cooked food and raw, whatever the heat source used for the cooking was.
So yes, the molecules do change, but in ways that are pretty much the same as changes wrought by any kind of heating, or even folding (as with an egg white - the whitening of whipped egg is owing to mehcanically wrought denaturing).
This article here is a more learned exposition on some of my ideas above.
Edit After Interesting Comment:
User Davidmh made the following comment on Volker's Answer:
Recipe: potatoes sliced in the microwave. Some of them, the ones in contact with the container can get very toasted, as if you grilled them.
This raises an interesting point. Although I believe the potato toasting is still a pure heating effect, there may indeed be an effect at work here that's peculiar to microwave cooking. The food in the microwave is interacting with the electomagnetic radiation, and so there must be a reaction - or scattered - electromagnetic field so that the food changes the field distribution within the resonant cavity. What you're seeing here is probably a combination of all four of the following:
- Microwaves are excluded more from deep within a big mass of food than they are from the edges: the changed field configuration means the edges get more heat. This is a bit like the electromagnetic skin effect;
- Near an edge, the food is less well "heat sink-ed". Heat can diffuse off in all directions away from a locally hot region within the body, so the heating tends to be made even within the body of a mass of food. At the edges, there are fewer diffusive paths for the heat to get away from locally hot regions;
- Water boils off in a microwave oven. It cannot get away from the inside of a mass of food, and so it tends to set up water-steam equilibriums inside the food body and thus tend to keep the temperature nearer to 100C. But water can boil off the surface of a food body. So the edges tend to dry out swiftly and, lacking the liquid water-steam equilibrium that tends to constrain the temperature, can rise to a much higher temperature.
- If the container is at all conductive, it will absorb the microwaves and become a local hot spot. I have found some earthenware dishes and pots do tend to get extremely hot on their own in a microwave oven. One of the factors in declaring something "microwave safe" is whether it absorbs in this way: not only does it heat the food unevenly if it does, it can destroy itself.
You could test how much 4. is a factor with a particular pot by putting it into the microwave with nothing in it and seeing whether it heats. BTW make sure you switch the microwave on for the test: I was trying to debug a test setup a few days ago and took two hours to twig that I hadn't switched the power on to a key piece of kit!
Best Answer
There are two physical processes that are important in a microwave oven. First is dielectric heating: basically, the oven sets up an electromagnetic wave inside itself, and certain molecules in the food absorb energy from the wave. Specifically, an absorber molecule needs a nonzero electric dipole moment, and it has to be free to rotate. Liquid water is a very efficient microwave absorber, fats and oils are somewhat less so, but a lot of molecules are not. So only some of the molecules in the food can actually absorb energy from the microwave oven.
The other important process is called thermalization. All that means is that the hotter molecules (which in this case happen to be the absorbers) will move around and bump into other molecules, and transfer some of their energy away. This is how heat spreads from the absorbers throughout the food.
Now, the thermalization process takes some time, typically more time than the food actually spends in the microwave oven. So when you take it out, it's unevenly heated: some of the molecules still have more energy than others. The temperature does even out after some time, but that means that the energy from the absorbers has to be shared out among all the molecules in the food (and its container), so they're not as hot as they were when they came out of the oven. This is the cooling process you've noticed.
In contrast, when you heat food on a stove or in a conventional oven, the only process involved is thermalization, and every molecule in the food (and pot) is equally capable of receiving energy by that process. So when you take your food off the stove, all the molecules have been completely heated up. The energy doesn't have to be "shared out" any more. For that reason, the food has more total thermal energy which keeps it hot longer.