The young woman is enjoying a sun-drenched, late summer day in Duesseldorf, Germany. The waiter has just delivered a wonderful beer: Uerige Alt. She looks like she can't wait to taste it, and she shouldn't. Every deceptively beautiful photon of light that hits the beer is causing a chemical reaction that renders it undrinkable.

Read on to see how...

The sun is setting over a vast expanse of suburban lawn. Dave has got the charcoal lit and the fumes from his lawn mower are starting to dissipate; time for a beer. Earlier in the day he moved a six-pack from its spot in his well-lit garage into the fridge. It should be perfectly chilled by now. In his alacrity he almost falls up the porch steps; he can’t get to the beer fast enough!

After fumbling around the kitchen utensil drawer for a few minutes he comes up with the bottle opener. A pffft and a pour later, he lifts the glass to his mouth. Hey! What’s that smell? Is there a skunk in the yard?

A quick look out the window reveals a yard devoid of musteline animal life, and also offers a reprieve from the offending aroma. Dave returns to his beer only to be assaulted again by the skunky stench. He holds the beer away from his nose; the smell grows weaker. He brings the beer back…that’s it! The beer smells like skunk! What happened?

Skunking is a very common beer flaw. So common, in fact, that some Americans think that European beers are supposed to exhibit skunk aroma. This misconception is reinforced by the European beer marketers who believe American drinkers expect that “special” aroma, and force their company’s brewers to produce skunk-prone product.

The saddest thing about skunking is that it is easily prevented. Once you understand the cause of this flaw, you’ll see how:

Almost all of the world’s commercial beers contain hops. In fact, American and German beers are required by law to contain at least a small amount of hops. Hops give the beer bitterness, and can also be used to impart a spicy, fruity sort of aroma best described as “hoppy.” (Obviously, I’ll never be writing a dictionary.)

The hop compounds that are responsible for making beer bitter are called isomerized alpha-acids. These chemicals, along with sulfur compounds found in beer, are also culpable in beer skunking. When light hits beer, it provides the energy necessary to drive a reaction that transforms the iso-alpha-acids into 3-methyl-2-butene-1-thiol. The “thiol” part of that somewhat cumbersome name indicates that there is sulfur present. Sulfur compounds often have strong, offensive aromas. Some musteline animals, like skunks, have evolved the ability to produce this chemical, and use it for self-defense.

In a sense, the aroma of light-struck beer doesn’t just resemble skunk spray, it is skunk spray! It’s the same stuff!

This photochemical reaction is the only cause of skunked beer. Warm storage, while damaging to the flavor of beer, does not skunk it. Cycling the temperature of beer from warm to cold and back again is also not implicated. Storing beer in the dark is the simple way to prevent skunking.

Blue light, and to a lesser extent green and a bit of near ultraviolet are the most damaging to beer. Most wavelenghts of ultraviolet light are not a concern because glass blocks them quite effectively (that’s why you don’t get sunburned in your car). The color of glass is the color of the light that it transmits, so green bottles allow the green light though. Similarly, blue light passes unhindered through pretty, cobalt-blue bottles. Clear bottles transmit all of the visible light. That is the reason beer in green, blue, and clear bottles is almost always skunked. Yes, even some very expensive imports.

The photochemical reaction that skunks beer occurs very quickly; a well-hopped beer in clear glass can become noticeably offensive with just 30 seconds of exposure to sunshine. Brown glass transmits less visible light than the previously mentioned colors, and therefore offers some protection from skunking. It does allow some light through, so beer in brown bottles will skunk after a few hours of light exposure.

Since light is an essential ingredient in the skunking process, beers packaged in kegs, cans, and opaque bottles cannot be skunked. Beers with very little hops, like all of the insipid mass-market American lagers, cannot become as skunky as overtly hoppy styles like pale ales, simply because there is less iso-alpha-acid available.

There is a way to skunk-proof beer, and it is used by the Miller Brewing Company. Instead of using hop flowers to bitter their beers, they use a special hop extract called tetra-hop. The isomerized alpha-acids in this product have a slightly different molecular structure than naturally isomerized alpha-acids, and are immune to the effects of light. This extract has the added benefit of greatly increasing the foam retention of a beer.

Tetra-hop extract is only used for bittering; it has no aroma. If one brews with any hop constituent other than tetra-hop, the beer will no longer be light-proof, so any beer with hop aroma is subject to skunking. Hop aroma is not desired by Miller’s marketing department, but clear glass bottles are, so tetra-hop works nicely for them.

The marketers that sell Corona figured out a different way to skunk-proof their product: they advise shoving a lime wedge or three into the bottle, and consuming the beverage directly from the package. The strong aroma of the citrus fruit serves to mask the inevitable skunkiness, while the narrow mouth of the bottle prevents the drinker from actually smelling the liquid.

It is easy to skunk a beer for educational purposes, but almost impossible to find beer packaged in green, blue or clear glass that is not already skunked. Homebrewers can simply put some of their beer into clear bottles, expose one to sunlight light for one-half hour while keeping the other in the dark, and then comparing their aromas.

Non-brewers can try this: get two clear drinking glasses. Pour some hoppy, fresh, beer with no skunk aroma into each glass, leaving adequate room to get your nose into the glass later. Cover each glass with clear plastic wrap, to preserve the volatile aromatics. Put one on a sunny windowsill and keep the other in a dark location of similar temperature. Sniff the two beers every ten minutes until you are tired of the torture.

It is quite unfortunate that many beers worth drinking, including many German imports, are packaged in green and clear glass at the whim of marketers. Some of these beers are packaged in brown glass in their homelands, because the marketers believe their domestic customers are erudite enough to reject skunked beer. Hopefully, they will soon learn that many American beer drinkers also recognize skunk aroma as a flaw.