UNC Study Explains Beer `Skunkiness'

Malcolm Forbes, a UNC chemistry professor and one of the experiment's conductors, said the study revealed that the biggest factor contributing to some foul-smelling beers is sunlight.

Forbes said sunlight provides energy for a chemical reaction to occur within the beer. "We've figured out what's going on and what causes skunky beer," Forbes said.

Forbes said sunlight causes a free radical reaction in the beer, which involves breaking hop compounds in the beer. The broken bonds each contain electrons that create the free radicals.

Sulfur then traps the free radicals and creates the main culprit in creating rotten beer -- thiol. Thiol produces a scent that most human beings identify as being "skunky," Forbes said, and most can't tolerate this odor even in small amounts.

Forbes said most beer is produced in brown or green bottles because they help shield sunlight. He said dark bottles prevent skunkiness more effectively.

"Brown and green glass prevent it to a large degree," said Forbes. "(But) I think down the road you'll see beer in clear bottles."

Forbes predicted that there could be a movement toward clear and plastic bottles in the beer industry because of the study's findings.

He said modifying a beer's ingredients beforehand could help solve the problem, something already being done by manufacturers of Miller Genuine Draft.

"In Miller Genuine Draft, they chemically modify hop compounds," Forbes said. "They pre-extract flavor compounds and react them with hydrogen."

Forbes said beer manufacturers can then use clear and plastic bottles, which are better because they are cheaper to make and are easier to recycle.

Producers of Corona do not chemically modify any of the hops used in their beer, Forbes said. He said Corona's basic strategy is to keep the beer out of the sunlight for as long as possible.

Forbes also said Corona specifically marketed the use of limes to hide any unpleasant odors. He said another strategy Corona uses to conceal the smell is by using longer beer necks. "It's a cleverly marketed product," Forbes said.

Forbes said representatives from Labatt Brewing Company have contacted him about the study and have requested that similar experiments be performed on their beer. But he said this might not be possible because hop compound concentrations are relatively low in beer.

Jon Connolly, the brewmaster at the Carolina Brewery, said the results of the experiment are good news to brewers around the world.

"We make every effort to make sure every part of the brewing process is done correctly," Connolly said. "We're excited about anyone who can get skunkiness out of beer."

The University Editor can be reached at udesk@unc.edu.