Exposure of certain beers to light triggers a photochemical reaction that creates a strong skunky off-flavor commonly called lightstruck. The effect is rooted in beer chemistry and has been described in the brewing science literature as a predictable interaction among light, beer components, and packaging choices.
Chemical mechanism
The key players are riboflavin, a naturally occurring vitamin in malt, and iso-alpha-acids, the bittering compounds derived from hops. When beer is irradiated with ultraviolet or high-energy visible light, riboflavin acts as a photosensitizer, absorbing light and transferring energy that leads to cleavage of hop-derived iso-alpha-acids. One of the cleavage products is 3-methyl-2-butene-1-thiol, a sulfur-containing thiol with an extremely low sensory threshold that smells like a skunk. Charles Bamforth University of California, Davis has written extensively about photochemical pathways in beer and explains how riboflavin-initiated reactions produce these volatile thiols. Tom Shellhammer Oregon State University has noted the vulnerability of hop-derived compounds to oxidative and photochemical degradation, reinforcing the mechanistic picture.
Packaging and cultural choices
Packaging determines how much light reaches beer. Brown glass absorbs much of the damaging ultraviolet and short-wavelength visible light, while green and clear glass transmit more light and therefore increase the risk of lightstruck flavor. Historically, some European brewers favored green bottles for marketing and cultural reasons, a choice that made products more susceptible to light damage unless other protections were used. Modern breweries respond by using opaque cans, brown bottles, light-protective labels, or hop extracts formulated for greater light stability.
Economic and sensory consequences follow directly from the chemistry. Because humans detect 3-methyl-2-butene-1-thiol at very low concentrations, even brief exposure can render a beer unacceptable to consumers, shortening shelf life and increasing product returns. Exposure during retail display or outdoor events in sunny regions magnifies the problem, and smaller craft producers who prefer clear or decorative bottles must weigh aesthetics against quality risks.
Understanding the photochemical cause of skunky off-flavor has guided practical solutions in brewing practice and packaging design, and the consensus in brewing science supports choosing materials and storage conditions that minimize light exposure to preserve intended beer flavor.