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This Dud's for You

Chemists tackle the science of bad beer, and find the bitter truth starts with the hops

FRIDAY, Nov. 9, 2001 (HealthDayNews) -- Beer lovers no longer need wonder about what turns a good brew bad.

Chemists at the University of North Carolina have deciphered the science of "skunky" beer, and it boils down to the formation of a compound that smells much like, well, skunk spray. Light is the culprit here, turning iso-a-acids, which give beer its naturally bitter taste, into nasty free radicals that resemble the chemical those striped members of the weasel family release when they are startled.

The researchers say the findings might be useful for brewers who are always looking to extend the shelf life of their beers, but who want to use less expensive clear glass bottles. Their work appears in the Nov. 5 issue of Chemistry - A European Journal.

"The fact that beer goes 'skunky' when you expose it to light has always been known, but knowing the exact molecular mechanism that makes it happen means maybe we can stop the chemistry," says lead author Malcolm D. Forbes. "A longer shelf life in a clear container; that's the goal."

Beer is made of barley, malt, hops and water. And the hops are where the problem begins, Forbes explains. As it turns out, hops help flavor beer, inhibit bacterial growth and contribute to the stability of the foam in the head. But they are also extremely sensitive to light. The final product of this photodegradation is a free radical the researchers call "skunky thiol," and it's an analog of skunk spray. Because humans are sensitive to this compound in very small doses -- a few parts per trillion -- it doesn't take much to ruin the brew.

In the UNC study, Forbes used a sophisticated imaging technique called time-resolved electron paramagnetic resonance (TREPR). Powdered hops were placed in a quartz container, which was then placed between two large magnets. Laser light was then aimed at the hops, and the scientists were able to watch how free radicals form in real time. They nailed down the exact molecular process by which free radicals become "skunky."

Knowing this now gives brewing chemists a road map of the process, so they can try to divert the disastrous end results, Forbes says. If the beer's photosensitivity could be reduced with small amounts of an additive, light would no longer be so daunting an enemy of a smooth swig, he explains. He adds that his collaborators in Belgium are already working on a patent that would use riboflavin to slow down the photodegradation process.

Another chemist who has studied the makeup of beer and its positive effects on heart disease says the findings answer an age-old question definitively.

"I think people already have known about the nature of this 'skunky' beer. What is important is they've nailed down how it happens. It tells people exactly how this happens, and they might be able to put additives in the brewing process to prevent it from happening. Maybe brewers can come up with some way to try and prevent this," says Joe Vinson, a chemistry professor at the University of Scranton in Pennsylvania.

But beer's problems don't end with iso-a-acids, Vinson notes: "The other thing is with beer there are some other off-flavors that are also a result of the brewing process."

There are some lipid-based free radicals that can make beer taste like cardboard, he explains. "Even if you keep the light out and do your things with free radicals, you still have problems at some point, because beer doesn't last very long in a bottle."


Because the crinkled, metal cap on a beer bottle isn't impermeable, so oxygen can get inside the bottle and create free radicals. That's why most brewers put vitamin C, an antioxidant, in the liner of the bottle cap, Vinson says. "It grabs the oxygen that tries to come in," he explains.

But no matter how hard brewers try to make beer last longer, that timeless pub beverage made from grains was never meant to last long, Vinson notes.

"Beer was made to be drunk as soon as it was brewed," he says. "If you want to store it, you're going to have problems eventually."

What To Do

The American Society of Brewing Chemists has the latest in beer news.

Check out RealBeer to dispel all the common myths about beer.

SOURCES: Interviews with Malcolm D. Forbes, chemistry professor, University of North Carolina, Chapel Hill, N.C.; Joe Vinson, chemistry professor, University of Scranton, Scranton, Pa.; Nov. 5, 2001, Chemistry - A European Journal
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