Vol. 28: Burnt Cookie Deflector
To avoid burnt cookie bottoms, make this infrared radiation deflector.
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+ More about the science behind infrared radiation, and other ideas to minimize burnt-bottomed baked goods.By Thomas R. Fox
Probably the hardest part of baking great tasting cookies is avoiding a hard, black bottom! Every December, my family makes an enormous amount of Christmas cookies, so we know one or two things about cookie disasters. We've tried all sorts of methods to alleviate this 'dark' aspect of our chocolate chip and sugar cookies, but not one has proven itself worthy of repeating – until now.
The best way to solve a problem is to find out what causes it. The problem is simple: discovering why the bottoms of cookies get burnt while the rest of the cookie is done just right! While it is possible that the problem is that the oven’s thermostat is not accurate, more often the problem is radiation striking the bottom of the cookie sheet – infrared radiation, to be more precise. All matter above absolute zero radiates electromagnetic waves that have a slightly longer wavelength than red visible light. This particular radiation is called infrared. The warmer the object, the more energy (shorter wavelength) the infrared radiation. If you get something hot enough, like a steel bolt, it will glow a dull red. What you don’t see is the huge amount of infrared energy being radiated by that hot bolt. If you could see infrared light, the bolt would be so bright it would blind you!
So, what does all of this have to do with burnt cookie bottoms? Well, when you bake cookies, you put the cookie dough on a cookie sheet and put them in a preheated oven, typically with the thermostat set at 350o F. In an electric oven, beneath the cookie sheet is the oven element, which gets close to 1500o F when it is on. This 1500o F element then radiates intense infrared to the cookie sheet, heating it several hundred degrees hotter than the oven’s air temperature, which is controlled by the thermostat. This really hot cookie sheet is what burns the bottoms of the cookies. In a gas oven, the gas burner is beneath the bottom of the oven, which – though gets quite hot – does not get quite as hot as an electric oven element. This means the cookie sheet in a gas oven won’t usually get quite as hot as it would if it was in an electric oven. This is why there are probably more complaints about an electric oven burning cookie bottoms than a gas oven burning them. However, even with a gas oven, it isn’t uncommon to get burnt cookie bottoms!
So now that we know the problem, how do we solve it? We solve it by keeping the cookie sheet as close to the oven’s air temperature as possible – which is the same thing as saying, keep it as cool as practically possible. There are several ways of attempting to do this – some are quite costly, some are a bit awkward to use, and some are both. The method I use, and describe here, is inexpensive and yet very effective.
One of the partial solutions to burnt cookie bottoms is the use of a convection oven. The reason a convection oven helps here is that the air movement tries to keep the cookie sheet close to the oven’s air temperature. While it never quite makes it, it does try. Of course, a quality convection oven isn’t cheap, although it does have other advantages than reducing burnt cookie bottoms. Another help is using heavyweight cookie sheets. The science principle behind why a heavy cookie sheets helps with the problem is its higher heat capacity. Heat capacity simply means the amount of heat energy that is needed to change a body's temperature by a certain amount. The heavier the cookie sheet, the greater the heat capacity. In more common terms, heavier cookie sheets will warm up slower than light ones. This means that in the oven, a heavier cookie sheet will be cooler than a light cookie sheet during the oven burner’s on-time, and so it won’t burn the bottom of the cookies as much. The disadvantages of a really heavy cookie sheet are obvious – expense and awkwardness.
A really simple way of helping keep the cookie sheet from getting much hotter than the oven’s air temperature is to simply cover the bottom of the cookie sheet with shiny aluminum foil. The shiny surface of the foil reflects many of the infrared rays that come from the oven’s burner. While this super-simple method helps, I discovered an even more effective way, and it is nearly as simple. It's a radiation shield, made by covering a cookie sheet with shiny aluminum foil and placing it on a rack below the cookie sheet with the cookies on it. This radiation shield protects the cookie sheet containing cookies from almost all the intense infrared radiation from the burner. In fact, the cookie sheet almost remains at the same temperature as the oven itself! I found that this is a simple and inexpensive way of not only stamping out burnt cookie bottoms, but burnt biscuits, brownies, cinnamon rolls, and more!
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