Scientists have (relatively) recently discovered the compounds that give “skunked” beer that all-too-familiar aroma.
Isohumulones, one of the compounds that contributes bitterness to beer, decomposes when struck by ultraviolet light and creates 3-methylbut-2-ene-1-thiol (MBT) which is detectable with only a few parts per billion.
Over at khymos.org, a site dedicated to “molecular gastronomy” and the science of cooking, has put together a very nice explanation of the process, including diagrams for those of us who have already been drin…I mean, those of us without a chemistry background.
It’s also apparently not a problem restricted to beer, as olive oil, butter, and milk can suffer the same fate if left in sunlight.
The moral of the story: Buy beer in brown bottles!
Update: Here’s another excellent explanation of the phenomenon by George de Piro, the Brewmaster at C.H. Evans Brewing Company. While it lacks the really cool chemical reaction diagrams, it does explain why Miller beers, long packaged in clear bottles, don’t “skunk”.