The bittering agent generated from hops while boiling beer wort is a compound called isohumulone. Ultraviolet light can degrade isohumulone all by itself. But it turns out that visible light can also induce isohumulone degradation -- it just needs a helper molecule, in the form of riboflavin. Once the proverbial ball starts rolling (or, in this case, the electrons start hopping), a series of reactions take place that eventually produce the compound 3-methylbut-2-ene-1-thiol. That mouthful, known colloquially as 3-MBT, is your skunk. In fact, 3-MBT is chemically very similar to one of the three main compounds found in a skunk's defense spray.
