Posted on June 29th, 2009

Regardless of what your favorite type of beer is, the last thing any of us want to happen is to see a sixpack of our ale or lager of choice to go bad and get spoiled or “skunked.”

Although the term is frequently used to describe beer that’s gone bad for any variety of reasons, to be precise “skunked beer” refers to beer that’s been over-exposed to sunlight, or “light-struck.” What exactly does that mean, and how can you avoid this happening to your beer? Read on to find out.

Although there are plenty of ways to ruin a beer, overexposure to light is the only way to skunk it. Storing beer at room temperature won’t do it; re-chilling cold beer that’s warmed up won’t do it either. These are common misconceptions, but the fact remains the only way to skunk a beer is to overexpose it to light.

The reasons why light is so damaging to your beer gets technical fast, but basically, the light causes alpha-acids (the key component of hops) to break down and combine with other chemicals in beer to create 3-methyl-2-butene-1-thiol, a sulfur-containing substance which produces the strong sulfur smell which is extremely similar to isopentyl mercaptan, or skunk spray. It’s easy to see why light-struck beer got its skunky nickname; it’s almost the exact same smell.

This is why most beer is sold in brown bottles or cans; the dark glass and opaque aluminum protect beer from most of the harmful UV rays that damage it, a good thing because beer without any protection can become skunked after just a few hours of exposure to direct sunlight. You shouldn’t worry about a glass of your favorite beer going skunky the next time you enjoy it on your patio, but give it an afternoon undisturbed and it might.

At this point you might be wondering about Newcastle or Miller High Life or another beer that comes in a clear bottle, then of course there are also some European beers like Beck’s that come in green bottles. How come every single one of those beers doesn’t get skunked? Because those beers don’t actually use 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. The downside of tetra-hops is it doesn’t smell like hops much at all either; it has almost no scent at all.

For the curious it’s easy to create skunky beer at home, just put a glass of your favorite beer on the windowsill for an afternoon and see how the smell compares before and after. You can watch the guys over at Basic Brewing try this experiment themselves if you’re curious but still don’t want to waste a perfectly good beer in the name of science.