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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?

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My guess would be that, in the cases where you've noticed this effect, the microwave is heating unevenly. Some parts get very hot, which you notice when you touch the food, but since other parts are much cooler, there's a lot less thermal energy, and as the system starts to equilibrate, the hot parts cool quickly. – Ted Bunn Apr 8 '11 at 19:14
Belongs on Cooking? – Sklivvz Apr 8 '11 at 20:44
@Ted, In case the food is in big chunk, this effect is there. Inside of big meat lumps or dumplings are shielded somewhat from the MW. – Georg Apr 8 '11 at 22:39
@Sklivvz: this is about thermodynamics, it seems fine here. – David Z Apr 9 '11 at 0:16
up vote 10 down vote accepted

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.

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this : ""and it has to be free to rotate."" is a contradiction in itself. There is no free rotation of molecules in liquids. (aside from a limited number of molecules like Camphor or Adamantan, these rotate even in crystal lattice) MW heating of water is the relaxation of orientational polarisation. The resonances of water molecules rotation in gas phase is at other frequncies. – Georg Apr 9 '11 at 11:06

There are differences in the amount of moisture which affects the cooling time as well but I think that most of the commonly "observed" effect is simply due to the fact that the regular oven heats up the container - the dishes - while the microwave oven ideally heats up the content only.

Because the dishes are hot in the classical oven, their heat is keeping the food warm for a longer time than the relatively cooler dishes taken from a microwave.

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Oh, that's a better answer than the one in my comment above. I bet that the cool dish is the main effect. – Ted Bunn Apr 8 '11 at 19:14

Another effect is that instructions for heating in a microwave are of the order of minutes. Mostly the water molecules get heated to 100C during that time.

When heating in the oven one may set it 180C which is normal cooking temperatures, and one leaves it a longer time; of course then there is more heat content and it takes longer to cool.

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Not mostly, only the water is heated in a MWO! But not the water molecules, only bulk water is heated by microwaves. Because water is about 70 or more % of food, the MW heat food, not the dishes: that is why dishes stay cold as Lubos states in his answer. – Georg Apr 8 '11 at 22:13
Well, Georg, that is what I thought at first, that only water is heated, but I looked up the article in Wiki, and fat molecules also respond to the radiation. It seems fires can start in a microwave after all the water has gone. It is the few minutes that make the difference, keep the dishes cooler and the food at 100C. – anna v Apr 9 '11 at 6:39
I assume another item of Wikis limits. A lot of people (including scientists) do not know about the effect used in MW heating. Or is the article You read about "wave" heating in general? RE fat: try some oil in a glas dish in Your MW oven. – Georg Apr 9 '11 at 11:02
@Georg so this answer is wrong? Do you have an answer? – cambraca Apr 11 '11 at 3:02
@cambraca, Yes this is wrong. The right answer is Lubos and for big chumps, Davids answer. – Georg Apr 11 '11 at 9:21

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