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See previous story about cans and their manufacture

The formaldehyde flavor legacy is one little-known aspect of can-making. Another involves the smooth coating applied to the inside of cans. The rinse cycle that attempts to wash off the emulsion also aims to remove particulate metal debris that forms on the metal's surface during the bending and shaping of a can. Like the emulsion, some of the microscopic debris always remains after rinsing. Unlike the emulsion, it can be dangerous to swallow.

To keep powdered metal out of a can's contents, says Notre Dame's Steven R. Schmid, associate professor of aerospace and mechanical engineering, manufacturers spray-coat the inside with a polymer dissolved in a solvent. When the can is heated, the solvent boils away, leaving only the protective polymer coating.

The coating not only plasters any microscopic debris to the can wall and away from the food, it keeps the food from interacting with can material, an especially important consideration with steel cans.

"Say you've got tomato soup in this steel can. You don't want that acidic soup corroding your can. It would kill your can, and the can would adulterate your food," Schmid says. "It's also why you're advised that when you go camping and you have Spaghettios you don't cook them in the can, because the polymer will degrade and you're going to be eating polymer." (Industry sources tell Schmid that the typical consequences of such a culinary blunder are headaches and constipation.)

Schmid says a single can-tooling machine can form 400 cans a minute. In a typical process, all but the top is shaped during a single stroke through a disk of aluminum or steel. The top, seamed on after filling, is made of a more expensive aluminum alloy, rich in magnesium. The added ductile strength of the magnesium is necessary so another machine can mash down a pillar of the metal to form the rivet that attaches the pop top. Today's beverage cans are "necked" near the top for a reason. The narrower-diameter means less of the expensive lid alloy is needed. It saves a minuscule fraction of a cent per can, but it adds up, Schmid says.

"In this country alone we use about a can per person per day, so you have to make 250 million cans per day. It's an amazing thing to watch these machines kick out these cans."

— Ed Cohen

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