Makes sense! Today, in a burst of science-y curiosity, I decided it was time to seek a more logical explanation for the phenomenon. I called up Harold McGee , a man who has made a career of applying chemistry, physics, and biology to cooking, and attracted considerable awe from legions of science-phobic food journalists like me in the process.
McGee explained that my observations were "kind of right and kind of not"; the bottom line is that heat is heat, and whether produced by a gas flame or an electroamagnetic wave, it behaves in the same way.
However, my nuked food could really be cooling faster for three reasons:. In the case of something like a frozen pizza, the culprit is the container. Heated in an oven, a metal baking sheet or pizza pan will absorb heat from the oven and become much hotter than the pizza sitting on it, helping to cook the pizza while in the oven and continuing to keep it warm once it's sitting out on the counter. In a microwave, the containers we use are not heated directly by the microwaves.
The microwaves heat the food, which can in turn heat the container it sits in — but never make it hotter than the food itself. If you put it in the microwave, some parts can reach boiling point while other parts are colder. The whole thing might only have an average temperature of 50C rather than C. Microwaves ovens are very good at heating food unevenly. Where in one spot you might see gravy boiling madly, mere inches away the bulk of the food is still frozen.
When you pull a microwaved item from the oven, these temperature differences begin to even out. The heat travels by conduction from the hot bits to the cooler portions of the food. That cooling effect usually happens pretty fast by comparison to the rate at which a food loses heat to the air. Since heat leaves via the surface, you expect the cup to cool more quickly. There shouldn't be any difference in the 'quality of heat' of the coffee from different sources.
There can be special cases of materials which have hidden 'degrees of freedom' internal chemical properties, structures of little crystals. Then as those materials equilibrate, energy is traded with those internal modes, so the initial state of those modes affects how quickly the material cools. However, there isn't anything like that in a cup of coffee. Mike W. You should try a carefully controlled experiment, using a thermometer.
It's important to use the same kinds of cups at the same temperature. 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. 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.
0コメント