A skunked beer tastes like a skunk smells (not good). But what makes beer skunky? 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.

In fact, skunky beer is often called lightstruck beer, which sounds way cooler than it actually is.

So how does light make beer skunky?

Isohumulones (also known as alpha acids), the bitter compounds in hops used in beer, are very sensitive to natural light (artificial light affects them, too, but not nearly as fast), which triggers a chemical reaction.

“If light reaches them, they break down very quickly and react with traces of sulfur compounds in the beer,” says Charles Bamforth, former 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.

How to prevent skunked beer

Canned beer offers 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. You can even tip the balance by simply walking outside into sunlight with your beer, especially if it’s been poured into a glass.

According to Beeriety, though, some beer that comes in green glass bottles will never get that skunk smell because rather than actual hops, “they use a hop substitute known as tetra-hop, which thanks to the miracle of modern science avoids smelling like skunks when it’s exposed to sunlight.”

And then there are those super-hoppy, intentionally funky dank IPAs and barnyard sours, which might hit you with a wallop of skunky smell (or otherwise strike you as unpleasant), but aren’t actually bad. Serious Eats has advice on how to tell skunky from funky.

How to prevent other off flavors in beer

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, especially beer bottles, always store in a cool, dark place, like, say, your refrigerator. Or a properly packed cooler.

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That won’t always do the trick, as there are other ways for beers to be adversely affected—like bacteria, as in the case of diacetyl, which occurs naturally during fermentation due to the yeast, but can go out of control in a beer and cause overwhelming buttery flavors. Other compounds that can negatively impact beer smell and taste are acetaldehyde (aromas of grass or green apple) and dimethyl sulfide (sometimes likened to creamed corn).

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