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.
"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."
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.
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