Photochemistry and Beer

The skunky odor occasionally experienced in beer is caused by an undesirable photochemical reaction. Hops, which are used to help preserve beer from oxidation and also to add bitterness to the flavor of beer, contain compounds known as humulones which are converted to iso-humulones during the brewing process.

It has been postulated that upon exposure to uv light from the sun, a photochemical reaction converts the iso-humulones to a thiol similar or identical in structure to the compounds responsible for the essence of skunk, primarily 3-methyl-1-butanethiol and 2-butene-1-thiol.

Colorless and green bottles afford little protection from uv radiation, so beers in these bottles are susceptible to what is known in the industry as light-struck beer. Some brands of beer found in colorless glass bottles prevent formation of skunky-smelling compounds by adding a chemical reagent which converts the iso-humulone to a photochemically stable compound.

When it comes to beer, storage in brown glass bottles is preferred. In fact undesirable photochemical reactions can degrade vitamins in foods as well, so protection of food from light is helpful in maintaining the food's nutrients.

Return to Photochemistry and the Electromagnetic Spectrum Menu