The scent of 3-methyl-2-butene-1-thiol can be most disturbing. Unless you’re a skunk. In that case it’s a welcome commodity because it effectively wards off predators. But you certainly don’t want it in your beer. How does it get in there? Skunks aren’t to blame, light is! It may be hard to believe that light can alter the flavor of beer, but it most assuredly can. Hops, or more specifically the blossoms of the female hop plant, are the secret to the taste of beer. They were originally added to compensate for the sweet taste of malt but turned out to have value in the control of undesirable microbes. There are many compounds present in hops, including some which have estrogen-like effects and have been blamed for breast growth in excessive drinkers. But it is one specific compound, iso-humulone, which seems of importance in the problem of "light struck flavor." Light cleaves the molecule to produce an active fragment that then reacts with some sulfur compounds found in beer to form the offensive 3-methyl-2-buten-1-thiol.
The fact that molecules can be affected by light should come as no surprise. After all, we know all about the damage that sunlight can do to molecules in our skin. It can do the same to iso-humulone. Is there a solution to this problem? Sure there is. The same way that we can protect our skin with sunscreens that filter out the most damaging wavelenghts of sunlight, we can also filter out the rays that damage beer. Of course applying sunscreen to beer bottles is not an option. But building protection directly into the bottles is. And that is why beer is sold in brown bottles! The wavelengths of light that cause the skunky smell are filtered out by the brown pigment. How can we then have beer in clear bottles? Chemical ingenuity has made this nuance possible. Through the process of hydrogenation, much as for margarines, the molecular structure of iso-humulone can be slightly altered to make it stable to light. The golden color of beer can now be enjoyed without having to open the bottle.