How does a beer get skunky?

A skunked beer tastes like a skunk smells. It’s commonly thought that subjecting beer to variations in temperature will skunk it. However, skunkiness in beer is caused not by heat, but by light.

Isohumulones, the bitter compounds in hops used in beer, are very sensitive to natural light (artificial light affects them, too, but not nearly as fast). “If light reaches them, they break down very quickly and react with traces of sulfur compounds in the beer,” says Charles Bamforth, chair of the Food Science and Technology Department at the University of California–Davis and a top researcher in brewing science. This process produces MBT (3-methyl-2-butene-1-thiol), which not only smells like skunk, it’s also chemically very similar to the noxious compound in a skunk’s spray. And it’s potent. Some people can detect MBT at concentrations as low as one-billionth of a gram in a 12-ounce beer.

Cans offer the best protection against damaging light waves, and brown bottles rate a close second. “If you have really strong light for a very long time, then even in brown glass, the beer goes skunky,” says Bamforth. “But in a clear glass or a green glass, it’ll happen very, very quickly”—as in a matter of seconds, not hours. Pilsners, traditionally bottled in green glass, are very susceptible to skunking.

Allowing chilled beer to get warm and then trying to chill it again still isn’t a good idea. Heat speeds up oxidation, and the beer begins to taste like cardboard. To preserve the flavors in your beer, always store it in a cool, dark place, like, say, your refrigerator.