In the Can

It can now be revealed why bottled beer and beer from a tap tastes different from beer in a can.

Be forewarned: if you're a six-pack enthusiast, you're not going to like the explanation.

When you sip a can of your favorite brew, you are savoring not only fermented grain and hops but just a hint of the same preservative that kept the frog you dissected in 10th-grade biology class lily-pad fresh: formaldehyde.

What is formaldehyde doing in beer? The same thing it's doing in pop and other food and drink packaged in steel and aluminum cans: killing bacteria. But not the bacteria in the drink, the bacteria that attacks a lubricant used in the manufacture of the can.

Notre Dame's Steven R. Schmid, associate professor of aerospace and mechanical engineering, is an expert in tribology — the study of friction, wear and the lubrication — applied to manufacturing and machine design. The co-author of two textbooks, Fundamentals of Machine Elements and Manufacturing Engineering and Technology (considered the bible of manufacturing engineering), Schmid has conducted extensive research on the manufacturing processes used in the production of beverage and other kinds of cans.

Schmid explains that back in the 1940s, when brewers and other beverage makers began putting drinks in steel (and, later, aluminum) cans, the can makers added formaldehyde to a milk-like mixture of 95 percent water and 5 percent oil that's employed in the can manufacturing process. The mixture, called an emulsion, bathes the can material and the can-shaping tooling, cooling and lubricating both.

Additives in the oil part are certain bacteria's favorite food. But if the bacteria eat the emulsion, it won't work as a lubricant anymore. So can makers add a biocide to the emulsion to kill the bacteria.

Before a can is filled and the top attached, this emulsion is rinsed off, but a small residue of the oil-water mixture is inevitably left behind, including trace amounts of the biocide. The amounts remaining are not enough to be a health hazard, but they are enough to taste, and the first biocide used back in the 1940s was formaldehyde.

In the decades since, can makers have devised new formulas for emulsions, always with an eye toward making them more effective, more environmentally friendly and less costly. But because formaldehyde was in the original recipe, people got used to their canned Budweiser or whatever having a hint of the famous preservative's flavor. For this reason, Schmid says, every new emulsion formula since then has had to be made to taste like formaldehyde, "or else people aren't going to accept it." Extensive tests are run to make sure the lubricant and additives taste like formaldehyde.

"It's not that it tastes okay. It's just what people are used to tasting," he says. (Miller Genuine Draft and similar brews, Schmid says, use biocides that have no flavor.)

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, Schmid says, 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 can manufacturers are forever searching for ways to improve efficiency in their struggle to stay price competitive with plastic and glass bottles. 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.

The cost of a can accounts for only about 4 cents of the price of a canned beverage, Schmid says. About 10 cents goes for advertising. The 12 ounces of beverage in the container typically costs less than a penny to produce.

"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
Summer issue 2000