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Chemists Determine Cause of "Skunky" Beer

beer
Image: Courtesy MALCOLM FORBES

These days, beer is the beverage of choice at chemist Malcolm Forbes¿s laboratory meetings at the University of North Carolina. That¿s because their latest project aims to determine why beer exposed to light tastes bad. The results of their research, an explanation for this so-called light-struck flavor, will appear in the November 5th issue of Chemistry¿A European Journal.

The phenomenon of beer turning skunky after exposure to light has been reported in the literature for more than 100 years, Forbes notes, but only now have scientists pinpointed the underlying mechanism. Using a type of spectroscopy that exploits electron spin, the researchers compiled a computer simulation of the reaction by which light-sensitive molecules in hops degrade into unpleasant-smelling products. The mechanism is intriguing, Forbes says, because one part of the molecule absorbs light energy that then migrates through the molecule and causes a free radical to form at a different location. "The final product of the reaction turns out to be what we call 'skunky thiol,' an analogue of a compound found in skunk glands that produces a very bad taste and smell," Forbes says. The flavor threshold of the thiol is so low, the authors write, that concentrations of a few parts-per-trillion can make beer unpalatable.

Breweries typically avoid the degradation of their product by packaging it in brown or green bottles to protect it 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," Forbes explains. Currently beer manufacturers that package their product in clear bottles can use modified hops that produce different free radicals and result in less of the foul chemicals. But one company minimizes the impact of the skunky thiols through more ingenious means. "Corona is marketed extremely cleverly," Forbes says of that beer, which uses regular hops but has been sold in clear bottles all along. The company suggests that drinkers add a slice of lime to enhance the taste of the beer, not to mention its odor.

"The Mystery of Lambic Beer" by Jacques De Keersmaecker (Scientific American, August 1996) is available for purchase at the Scientific American Archive.
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