That “skunky” aroma is actually the result of a beer getting “lightstruck.” Specifically, it’s the result of light reacting with certain hop compounds. This reaction creates a chemical compound that smells like a skunk’s spray. The reaction is known as photo-oxidation.
The “obnoxious odor” was first documented by German Chemist Dr. Carl Lintner in 1875. This seems pretty late in the history of beer, but my educated guess is that it coincides with 3 things: the increased use of clear glass in drinking vessels in 19th century, the industrialization of glass bottles in the mid to late 19th century, and the increased science dedicated to beer. Previous to commonly available glass products, beer was served from wooden barrels into some sort of opaque drinking vessel. This allowed for very little light to actually get to the beer. With the advent of glass drinking vessels and bottles, light now had more of an opportunity to mess with beer.
Specifically, it’s blue light in the visible spectrum, 350-500 nm, that causes the reaction in isohumulones. Isohumulones are created when brewers boil the hops to isomerize the alpha acids to create bitterness. In fact, IBU (International Bitterness Units) is a measurement of isohumulone in beer, 1 part isohumulone per million equals 1 IBU. When blue light strikes the isohumulones, it causes photodecomposition in conjunction with a photosensitizer; in this case, riboflavin.
3-methyl-2-butene-1-thiol – The Lightstruck Chemical: MBT
If you want to get really geeky, the chemical compound formed is called 3methyl-2-butene-1-thiol and is known as MBT for short. MBT is formed when light cleaves the isopentenyl-side chain from the alpha acids. This “photocleavage,” as it it’s known, creates dimethyl allyl radicals which then react with sulfur containing compounds, thiols, to create MBT. Yoshiro Kuroiwa first suggested it was MBT in 1960.
This process can occur virtually instantaneously in strong light. Tasting panels have been able to detect MBT after only 10 seconds of exposure to full sunlight. Artificial lighting also contributes. If you wish to slow or prevent this from happening to beer, use brown bottles or cans for packaging. Brown bottles are great at slowing down this reaction, but if given enough time and exposure, they too will succumb to the light. There are sleeves that retailers can buy to put around their beer cooler lights that will block the offending wavelengths and help protect their beer.
Green and clear bottles provide virtually no protection. This has led to “skunky” being a common adjective applied to many European beers who view the clear or green bottle as part of their marketing image. In reaction to this, Pilsner Urquell completely wraps their 6-packs in cardboard and will be converting to brown bottles when they run out of their stock of green bottles.
Light Transmission of various color bottles: We can see that the brown bottle transmits the least light in the dangerous spectrum.
Another method that some large Macro brewers use is a specially formulated hop liquid that modifies the side chain bonds to prevent this photodegradation. This special hop liquid is used most prominently by Miller Brewing. Their beers will not skunk in light, although with enough light exposure other problems will crop up.
Cans are a great way to block light from beer. They are impenetrable to light and thus you won’t get lightstruck. Although recent tests around Thermal Aging (subjecting the beer to excessive heat to test shelf life/stability) have shown development of similar compounds in canned beer that would qualify in the skunk aroma realm. Scientists are still working on why and how this happens.
And, Yes. MBT is very similar to many of the compounds that skunks produce for their spray. This link shows the various compounds that various skunks make.
And, No. There is no way to let the MBT “age” out of your beer. It is a permanent chemical change.
Homework: You can run you’re own experiment at home to learn exactly what real skunking tastes and smells like. It’s easiest to do with clear or green bottles, but you won’t really know if the beer is skunked when you bought it. If you’re willing to take the time, buy a hoppy beer in a brown bottle like a Sierra Nevada Pale Ale and place in a spot that gets direct sun for long periods of time. You’ll need to leave one bottle in sunlight for around 72 hours of sunlight, so this will take around a week! But if you’re in it for the science, it should be a fun way to play with beer. Keep the other bottle in a dark place where it won’t be exposed to light. Then when you’re ready, chill them down. I’d open the dark-stored bottle first and leave the other one closed. You don’t want to have the aroma waft out and taint your notes on the good bottle. Taste it. Then open the lightstruck bottle and taste it. There you go, beer science!
Sources:
The Oxford Companion to Beer, Edited by Garrett Oliver (Oxford University Press, 2012)
Tasting Beer, by Randy Mosher. (Storey Publishing, 2009)
The Chemistry of Beer Aging, by Bart Vanderhaegen, Hedwig Neven, Hubert Verachtert, & Guy Derdelinckx (Centre for Malting and Brewing Science, Katholieke Universiteit Leuven, 2005)