What causes beer to skunk when exposed to certain light?

Beer skunks primarily due to a reaction involving hop compounds, specifically isohumulones, when exposed to UV light. This process occurs predominantly in pale beers that utilize a significant amount of hopping. Isohumulones are bitter acids derived from hops that contribute to the flavor profile of beer. When these acids are subjected to light, particularly UV light, a reaction occurs that generates sulfur compounds, such as 3-methyl-2-butene-1-thiol, which have a strong, unpleasant odor reminiscent of skunk spray.

This phenomenon is particularly evident in beers packaged in clear or green bottles, as these provide less protection against light compared to brown glass bottles, which block more UV rays. Therefore, the correct answer reflects the key role of hop isohumulone in the light-induced skunking process, emphasizing how the chemistry of certain hop compounds interacts specifically with light to create undesirable flavors and aromas in beer.