When You're Hot, You're Hot -- Or Not

By Robert L. Wolke
Sunday, November 19, 2006

Of all the vital questions facing our nation in these post- election weeks, the most vexing may be, "How do I know when my turkey is done?"

This would seem to be a simple question to answer, because almost every food publication in the country is referring its readers this week to a chart showing that if your turkey weighs X pounds it should be done after Y hours in a Z-degree oven. (The Post sends people to the one at http://www.fsis.usda.gov/Fact_Sheets/Lets_Talk_Turkey/index.asp.)

The label on the turkey tells you the value of X, and your clock tells you the value of Y, but nobody ever questions the value of Z. We blithely set our ovens for a certain temperature and figure that's the temperature we will get. But shouldn't we be a bit more agnostic, as scientists are when taking measurements? When you set your oven for, say, 325 degrees, does it really heat up to 325 degrees?

I know you're already anxious about your turkey and don't need anything new to worry about. Well, I never worried about the Z factor either until a few weeks ago, when my wife was baking pumpkin pies. The pies just wouldn't cook in the expected amount of time; their crusts remained soft and the custard hadn't firmed up. Thinking there might be a glitch in the recipe, she made another batch, with the same result. That's when she called in the sage of all things chemical or physical, the solver of all problems (me).

"Let's check the oven temperature," I said.

I set the oven for several different temperatures, allowing time for it to reach each one, and measured them with an oven thermometer. (Those cheap little metallic ones that you hang on the oven rack are just fine, and you have plenty of time to buy one before the Big Day.) My actual oven temperatures averaged 22 degrees lower than their settings. I wondered why.

Summoning my finely honed powers of scientific observation, I noticed that there was a 14-by-16-inch slab of stone resting on the bottom shelf of the oven. It was our baking stone, also known as a pizza stone, which we leave in the oven after using it because it's just too darned heavy to be schlepped in and out.

"Aha!" I said ("eureka" having been vastly overused). "The stone is casting an infrared shadow."

"Huh?" my wife inquired.

"You see, electric elements heat the oven mainly by infrared radiation, which travels in straight lines. That slab of stone is shielding the middle of the oven from the heating element at the bottom and preventing it from reaching the set temperature."

"Yeah, right," she said. "Just fix it, okay?"

I removed the stone and tested the temperatures again, and all the Z's were right on the button.

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