UNC Chemists Figure Out What Causes "Skunky Beer"

"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."

Using isohumulones supplied by brewing companies and a sophisticated technique called time-resolved electron paramagnetic resonance spectroscopy, the UNC scientists and colleagues determined what happens chemically during photo degradation. Lasers served as the light source for producing the chemical reactions they studied.

"This light problem is a phenomenon that was reported in the literature as early as 1875, but until now the detailed mechanism has not been unraveled," Forbes said. "The final product of the reaction turns out to be what we call "skunky thiol," an analog of a compound found in skunk glands that produces a very bad taste and smell. This molecule has an extremely low taste and smell threshold in humans, just a few parts per trillion."

A report on the findings will appear in the Nov. 5 issue of a publication called Chemistry - A European Journal and appeared online this weeek. Besides Forbes, authors are UNC chemistry doctoral student Colin S. Burns and Dr. Denis De Keukeleire and his doctoral student Arne Heyerick of the University of Gent in Belgium.

The new paper describes how the team succeeded in creating the free radicals, working out their structure, explaining reactions that made them and learning precisely where in the isohumulone compounds photo breakdown takes place.

"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 manufacturers do not used a modified hop product, he said. Instead, they usually keep their bottles boxed to exclude light and encourage drinkers to add a slice of lime to improve the odor.

"Corona is marketed extremely cleverly," the chemist said.

Forbes’ laboratory is one of the most sophisticated in the world for studying super-fast radical reactions that take place in billionths of seconds. His Belgian collaborators have applied for a patent on a process that will alter and retard the photochemistry of beer to make the beverage easier to store and give it a longer shelf life.

The National Science Foundation, the Interbrew-Baillet Latour Foundation of Leuven, Belgium, and the UNC chemistry department supported the research.