Lightstruck. Beer with off–aromas is often colloquially said to be “skunked,” but those who have encountered a skunk-like flavor in their beer rarely know how close they are to the truth. Certain compounds in hops are light sensitive and when exposed to strong light a photo-oxidation reaction takes place, creating the intensely flavor-active compound 3-methyl-2-butene-1-thiol (MBT). MBT is one of the most powerful flavor substances known to man. Commonly referred to as “skunky,” the pungent odor compound resembles that of the famously malodorous defense spray deployed by skunks.

In 1875 the German chemist Dr Carl Lintner first reported on the formation of an offending taste and obnoxious odor in beer exposed to light. In the 1960s, Yoshiro Kuroiwa suggested that the main constituent of the off-flavor was MBT derived from the photodecomposition of isohumulones, the beer bittering principles, in the presence of a photosensitizer, namely riboflavin (vitamin B₂). Further, the Kuroiwa group established that the blue part of the visible spectrum (350–500 nm) in particular is most efficient in generating lightstruck flavor. In strong sunlight, the reaction can be almost instantaneous, with tasting panels able to detect the aromatic effects of an exposure to less than 10 seconds of full sunshine. Under less deleterious conditions, for example a display cabinet with fluorescent lighting, these reactions still occur, though they may take a number of days or weeks to become noticeable.

This now well-known reaction involves the cleavage in the isopentenyl-side chain of the iso- alpha acids in hops. See iso-alpha acids. This photocleavage is light-catalyzed and results in the formation of a dimethyl allyl radical. Reaction of the radical with sulfur-containing compounds (thiols) forms MBT. Green or clear glass offers little to no protection against this reaction, but brown glass is highly effective, at least against short-term or low-intensity exposure. Aluminum cans or beer stored in kegs offers the best protection against exposure to light. Despite the lack of protection offered by clear and green glass bottles, some breweries persist in using them, the bottle color having become an important part of their branding strategy.

When the bottle offers no protection, modern hop chemistry has brought alternatives for preventing MBT formation from hops. “Tetra” and “hexa” hops are modified by reductions in side-chain double bonds that prevent the photodegradation reaction. These advanced hop products are based on liquid or supercritical carbon dioxide extracts of hops. See humulone. The resins in these are in turn isomerized in alkaline solutions into isohumulones which can be further reduced to produce bittering compounds that do not degrade into MBT. These advanced forms of bittering hops are known as “light stable” products. Their use in brewing yields a beer that is less vulnerable to the deleterious effects of sunlight, although it must be remembered that light can induce other flavor changes in beer that are not related to hops.

Research has recently revealed two unidentified compounds with aromas indistinguishable from the “skunky” aroma used to describe MBT. It is thought that these two new, yet to be elucidated compounds contribute to the overall lightstruck flavor in beer. Perhaps more interesting is the fact that MBT and one of the newly discovered compounds also formed during thermal aging in canned beer in the absence of light.