What primarily causes skunking in beer?

Skunking in beer is primarily caused by exposure to ultraviolet (UV) light, which is why fluorescent or direct sunlight is the correct answer. When beer, particularly those in clear or green bottles, is exposed to UV light, a chemical reaction occurs with the hops in the beer. This reaction leads to the formation of compounds known as thiols, which produce the distinct and unpleasant skunky aroma associated with spoiled beer.

While heat exposure and contact with oxygen can negatively affect beer quality in other ways—such as causing off-flavors or stale notes—they do not result in skunking specifically. Improper fermentation can lead to various issues like phenolic off-flavors or diacetyl but does not contribute to the skunky character that comes from light exposure. Understanding the role of light in causing skunking can help consumers make better choices about beer storage and enjoyment.