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The Straight Dope 

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When wrapping your baking spud in aluminum foil, should you wrap shiny side in or shiny side out? I could swear I was taught in school that shiny side in will help it retain heat better, making it cook faster. However, I know a guy who says that you should always keep the shiny side away from your food, the reason being that the shiny side is shiny because it's treated with a dangerous chemical, and you don't want the chemical getting into your edibles. When I asked him why on earth they'd cover one side of the foil with a dangerous substance, he replied that shiny foil sells better than dull foil, and the only reason they don't coat both sides is that they want one side to be safe enough to put next to food. Personally I think the guy's nuts, but then again he's got a PhD in engineering while I am just a humble product of the Louisiana public school system. Still. . . dangerous shiny chemicals? Come on. --Ray Shea, via the Internet

Makes perfect sense to me, Ray. Big corporation markets product used mainly in food preparation. Coats one side with poison to boost sales. Leaves other side uncoated so product can be used safely but--here's the best part--doesn't tell anybody. Half of uninformed consuming public wraps spuds wrong side in, eats toxic result, dies horribly! Big corporation conceals shocking death toll for 50 years! And I'm queen of Romania!

The truth is that the shiny side is not treated with a dangerous chemical. Mineral oil is used as a lubricant during the rolling process, some trace of which may remain on the finished foil--but it's not dangerous. The shiny side is shiny because of the way foil is made. During the last pass through the rolling mill, a double thickness of foil is run between the rollers. The side of each sheet that comes in contact with the polished steel rollers comes out shiny. The other side has a matte finish.

Having dispensed with the paranoid rumors, let's get down to the guts of the matter. Are you supposed to wrap stuff shiny side in or shiny side out? The official word from the Reynolds aluminum people is as follows: "It makes little difference which side of the Reynolds Wrap aluminum foil you use--both sides do the same fine job of cooking, freezing, and storing food. There is a slight difference in the reflectivity of the two sides, but it is so slight that laboratory instruments are required to measure it."

However, when I first called up Reynolds I got Terry, who was filling in for Joe, the regular foil guru. Terry had the idea you were supposed to wrap food shiny side in. The quote above came later from Mary, by fax. Cecil loves PR people and believes everything they tell him, but when they contradict each other he's suspicious. I decided to conduct an independent test. I briefly considered calling my brother-in-law the physicist at the Oak Ridge National Laboratory, figuring maybe they could fire up the cyclotron. But that wasn't in keeping with the do-more-with-less tradition of Straight Dope kitchen science. I went out and got two baking potatoes, which I determined were the same size by digital measurement (felt 'em). I also bought two identical meat thermometers and a roll of Reynolds Wrap aluminum foil and wrapped the aforementioned vegetables therein, one shiny side in, the other shiny side out. Finally I stuck a meat thermometer an equal distance into each spud, placed them in the Straight Dope Oven of Science, and cranked her up to 450 degrees Fahrenheit (232 Celsius). Result: the shiny-side-in potato heated up more slowly than the shiny-side-out one and after 40 minutes was less thoroughly cooked. Having been criticized in the past for generalizing from inadequate data, I repeated the experiment with another pair of potatoes. This time the shiny-side-out potato heated up more slowly. The Straight Dope Science Advisory Board, which is never satisfied, has sternly informed me that I should test another 15,000 spuds before I draw any firm conclusions. Given grossly inadequate federal funding for basic research, I wouldn't hold my breath. For now, however, my opinion is as follows: Shiny shminy. Put the foil on any way you want.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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