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Skunky beer culprit identified An analog of a compound found in skunk glands has been shown to be responsible for the offensive taste and skunky odor of beer that has been exposed to light. Chemistry professor Malcolm D. E. Forbes at the University of North Carolina, Chapel Hill; pharmaceutical sciences professor Denis De Keukeleire at the University of Ghent, in Belgium; and coworkers used laser flash photolysis and time-resolved electron paramagnetic resonance spectroscopy to investigate the photodegradation of a group of light-sensitive compounds known as isohumulones that are found in hops [Chem. Eur. J., 7, 4553 (2001)]. Hops help flavor beer, inhibit bacterial growth, and maintain the foam in the head of the beer. Forbes's team showed that isohumulones decompose in the presence of visible light to form radicals, which are trapped in beer by sulfur-containing species such as sulfur-rich proteins derived from barley malt. The final product of the process is the "skunky thiol"--3-methylbut-2-ene-1-thiol (shown). "This molecule has an extremely low taste and smell threshold in humans, just a few parts per trillion," Forbes says. "Historically, beer has been stored in brown or green bottles to protect hop-derived compounds from light. Understanding mechanisms behind changes in beer tastes is important because the world beer industry is hoping to save money by storing, shipping, and selling beer in less expensive clear glass." |
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