When Good Beer Goes Bad
Chemists at the University of North Carolina at Chapel Hill decoded what goes wrong with beer to give it that offensive "light-struck" flavor (www.unc.edu
"Historically, beer has been stored in brown or green bottles to protect hop-derived compounds from light in a process we call photodegradation," said Dr. Malcolm D. Forbes, professor of chemistry.
"Hops help flavor beer, inhibit bacterial growth and are largely responsible for the stability of the foam in the head," Forbes said. "Hops, however, are light-sensitive, and the three main compounds in them identified as being light-sensitive are called isohumulones. When attacked by either visible or ultraviolet light, these break down to make reactive intermediates known as free radicals that lead to the offensive taste and skunky odor."
The end result of the light-on-hops chemical reaction is termed 'skunky thiols,' which are similar to compounds in skunk glands that produce a terrible taste and smell.
MGD and Corona
"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 said. "Producers of Miller Genuine Draft, for example, already do that by adding a chemically modified hop compound to the beer. We have found evidence that Miller beer is still photochemically active, but it doesn't make the same free radicals, and so those can't lead to bad-tasting skunky thiols as quickly."
Corona beer also is sold in clear bottles, but does not contain a modified hop product. Instead, they usually keep their bottles boxed to exclude light, and encourage drinkers to add a slice of lime to improve the odor - a great marketing solution.