Getty ImagesBy Elizabeth Gunnison
Call me paranoid, but I have long harbored an absolute conviction that food heated up in the microwave cools faster than food heated on the stovetop or in the oven. I don't think I'm being delusional here. Whether I'm nuking a half-finished cup of coffee - which I do almost daily - or cooking a frozen pizza, or heating up last night's leftovers, it all comes out of the microwave sizzling hot and then seems to cool to room temperature before I can find a clean fork. Given that I am a person who once weasled my way out of a high-school physics class by inventing an independent study in poetry-writing, I have always simply chalked this observation up to karma: food heated hastily will cool hastily, just to spite your impatient ass. 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:
1. 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. So once my pizza leaves the microwave, it doesn't benefit from the same ongoing source of heat as one that was in the oven. To avoid this, transfer food from the microwave directly to a heated surface.
2. For thick foods like, say, leftover chili or take-out pad Thai, the issue here is uneven heating. Microwaves heat food unevenly due in part to differing amounts of energy in different parts of the appliance. This often leaves the center of the food cold while the edges are piping hot. So once my microwaved food has been left to sit for a few minutes, the heat from the edges will have migrated to warm up the center, for an overall cooler temperature. Avoid this issue by stirring your food midway through the heating process, or if it's something un-stirrable (like lasagna), move it to another part of the microwave partway through cooking.
3. When it comes to coffee and other liquids, there's enough fluid circulation that uneven heating doesn't occur. But my half-finished cup of coffee has a much smaller volume and only slightly smaller surface area than the full cup I started with. Even if it's heated to the same temperature as a full cup, the half-finished cup will always cool faster due to a proportionately larger surface area through which heat can escape. This is the case with any small piece of food (which is very often exactly what we're microwaving): smaller pieces of food cool faster than larger ones due to their higher surface area to volume ratio. Nothing to be done about this, besides drink your coffee faster and cut yourself a bigger piece of pie next time.
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