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Give It a Rest

By Robert L. Wolke
Wednesday, February 2, 2005; Page F01

I have a question regarding the temperature of a roast. Emeril Lagasse, the famous Food Network chef, says to remove the meat from the heat source before it gets to the desired temperature. He claims that as it "rests," its temperature will continue to rise and it will keep on cooking. My physics knowledge, poor as it is, tells me that with no heat source, the meat must cool down, not get hotter. Is Emeril correct?

Of course Emeril and other chefs are correct. Would I argue with Emeril Lagasse? He's bigger than I am.

It does seem counterintuitive that a piece of meat just resting on the counter could grow hotter. That would violate the most fundamental law of physics: You can't get energy from nowhere. But while the meat rests, its internal temperature indeed rises from 5 to 15 degrees, depending on its size and shape. This effect is called "carry-over cooking."

The source of the heat is the outer portions of the roast, which had been exposed most directly to the oven's hot air and are therefore hotter than the interior. The resting time allows some of the extra heat at the surface to work its way into the interior, thereby raising its temperature and, as a result, cooking it further.

A roast should always be taken out of the oven before its interior reaches the desired final temperature. That's not only to avoid overcooking, but also to allow the juices to redistribute themselves throughout the roast.

In baking, however, a cakemaker usually tests for the batter to be fully set before removing the cake from the oven. Shouldn't this result in overbaked cakes? After all, you can't turn off conduction. Or is there some chemistry going on that I, as a mere physicist, am unaware of? Finally, why can't a thermometer be used to determine when it is done?

A toothpick test -- whereby a toothpick inserted into a cake's center comes out dry -- is the ultimate test of doneness, signaling that all the wet batter has turned into cake.

But unlike the deepest parts of a rib roast or leg of lamb, all parts of the batter in a cake pan are relatively close to the surface, so there won't be a big temperature difference between the surface and the interior. Any such differences will even out in baking, because wet batter is a good conductor of heat -- much better than meat. Because the surface isn't much hotter than the interior, little if any carry-over cooking will take place after the cake is out of the oven.

A thermometer is not of much use, because all cakes and pans are different. We can't specify a desired "finish" temperature for cake as we can with beef, for example: 120 to 130 degrees for rare, 140 to 150 degrees for medium and 170 to 185 degrees for ruined. These are the final temperatures after resting.

In The Post's Jan. 12 Food section, the article on reheating takeout foods said, "Some foods take better to the moist heat of the microwave; others require the dry atmosphere of the oven." But microwaves are not moist.

This reader is right to be suspicious. Microwaves certainly aren't wet, are they?

The problem is in the expression "moist heat." We all know what it means: heating food in the presence of moisture, such as in braising, as opposed to roasting. But applying the adjective "moist" or "dry" to the noun "heat" is just a shortcut. Heat itself cannot be moist, dry, green or yellow. It's pure energy.

That said, under most circumstances the atmosphere in a microwave oven really is more moist, or more humid, than in a traditional oven. For one thing, home microwave ovens have an interior volume of less than one cubic foot, while traditional home ovens are typically more than three times as big. So any water vapor given off by the food while it's cooking will be more concentrated -- more water per cubic foot -- in the microwave oven's atmosphere.

More important, microwaves eject more water vapor from the food than regular ovens. A regular oven heats all kinds of food molecules indiscriminately, but a microwave's energy is absorbed primarily by the food's water molecules. It's as if the microwave is targeting water molecules, making them hot before anything else.

Many of the hot-water molecules escape as vapor or steam before they can share their heat with the other molecules in the food. The atmosphere in the microwave oven therefore becomes quite humid when compared with a conventional oven heating the same food. You can call it "moist heat" if you wish. Often there is so much water vapor in the air that it condenses on the walls of the microwave oven as actual "wetness." You don't see that happening in a regular oven.

My own "smart" microwave oven has an automatic reheating feature that takes advantage of these phenomena. With it, I don't have to guess how long and at what setting to reheat my food without cooking it or drying it out excessively. I just put the food in the oven and press the "Sensor Reheat" button. The oven heats the food until its "sensor" detects steam. It then continues heating the food for a couple of minutes to give the hot-water molecules time to distribute their heat throughout the food.

This feature is convenient, but I really hate appliances that are smarter than I am.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh. He can be reached at wolke@pitt.edu.

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