What aroma is produced when beer is light struck by sunlight or indoor lighting?

The aroma produced when beer is light struck by sunlight or indoor lighting is typically associated with the formation of skunky odors, most notably from the compound 3-methyl-2-butene-1-thiol, which is a form of mercaptan. This happens when light interacts with certain compounds (like hops) in the beer, leading to the production of these unwanted sulfurous compounds that impart a strong, undesirable skunky aroma. This phenomenon is particularly common in beers that are packaged in clear or green glass bottles, which provide minimal protection against UV light.

Other options such as isovaleric acid, trans-2 nonenol, and diacetyl have distinct characteristics and origins. Isovaleric acid is typically associated with off-flavors related to yeast autolysis and can have a cheesy or rancid aroma. Trans-2 nonenol can contribute to fruity or floral notes but is not related to light exposure. Diacetyl is a byproduct of yeast activity and fermentation, resulting in a buttery flavor and aroma, rather than a skunky smell from light exposure. Therefore, mercaptan is the compound responsible for the notorious skunky aroma experienced in light-struck beers.