If you’ve ever tried to drink a beer that’s been sitting on the shelf a little too long, you’ve no doubt experienced that “skunky” taste beer can have.

Malcolm Forbes, professor of chemistry, explains that the photodegradation of beer—the culprit behind the skunky taste—is similar to what happens to paint when it is exposed to light over long periods of time and begins to fade in color. Light causes the molecules in paint to break apart, and this creates free radicals—molecules with unpaired electrons—which are highly reactive, changing the composition of the paint and consequently its color.

Forbes first got the idea for studying the photochemistry of beer after giving a conference talk on a technique known as time-resolved electron paramagnetic resonance spectroscopy (TREPR) that his lab uses for investigating all sorts of thermal and photochemical reactions involving free radicals. For example, his lab is working on a project aimed at figuring out how free radicals move in soaps. By placing free radicals in an oil drop and then watching to see how they react, researchers can understand better, for instance, how free radicals do damage to cell walls.

After Forbes gave his talk, Denis De Keukeleire, a chemist from the University of Ghent in Belgium, went up to him and wanted to know if he was interested in using TREPR to study the photochemistry of beer. Of course he was interested. Being a beer connoisseur himself, Forbes could mix business with pleasure, so to speak. So the two, along with a couple doctoral students, set out to show for the first time real proof of the degradation of beer in sunlight.

Their first clue was the hops, as beer made without hops does not generate that skunky taste. With funding and hop compounds provided by brewing companies, the team used TREPR to figure out what happens chemically to hops when exposed to light.

As the light source for the TREPR experiments, the researchers use high-powered ultra-violet excimer lasers, which produce unfocused beams strong enough to give you a sunburn if you were to point them at your hand. To test the hop compounds, researchers aim a laser beam at the sample, and free radicals form. Then on a very short time scale—about 100 nanoseconds—the spectrometer captures the electron paramagnetic resonance (EPR) signal that provides information on both the structure and kinetics (how quickly the sample changes) of the hops sample.

“This is what you call fast high-resolution spectroscopy,” Forbes says, “Because you get a lot of structural information very quickly.”

From those experiments, the researchers were able to figure out exactly when free radicals form within the hop compounds, known as isohumulones. They also worked out the radicals’ chemical structures. “What we found is that the isohumulones are extremely light sensitive,” Forbes says, “and they break down very quickly into free radicals that are then trapped by sulfur sources, which come from proteins in the beer that turn the free radicals into a compound known as a thiol. And that thiol happens to be structurally related to the thiol found in the glands of a skunk. Hence, the skunky taste.”

What’s more is that the human nose and tongue are very sensitive to even low concentrations of the thiols created from the free radicals. “A few parts per trillion can make beer unpalatable,” Forbes says. “And a glass of beer exposed to light for even less than twenty minutes can turn skunky.”

So now that they’ve found the culprit, they just need to solve the other part of the mystery—how to keep those free radicals from forming so quickly. “Of course the best solution we offer is to drink your beer as fast as possible,” Forbes says with a grin.

While brewers typically bottle beer in brown bottles to prevent light from getting in, it would be cheaper and easier to recycle beer bottles if they were clear glass. And beer makers are working on that. Forbes explains that some producers such as Miller Genuine Draft and Newcastle Brown Ale use what’s called a modified hop product in which they extract certain compounds from the hops and chemically modify it so that it is no longer photochemically active. “But according to beer purists—such as a particular group of Germans—that product is no longer considered beer,” Forbes says. “In addition to giving beer its bitter flavor, it’s the hops that provides for the stability of the foam in the head and inhibits bacterial growth.”

There’s also Corona, which does not contain a modified hop product but, instead, is boxed to exclude light.

“What we really need to do,” Forbes says, “is to protect beer by developing a process that will alter and retard the photochemical process but retain the original flavor, and some of my Belgian collaborators are working on that right now.”

Even cheaper would be to manufacture beer in plastic bottles. To do that, though, researchers first need to figure out how to prevent organic material from the polymer (plastic) from leaching into the beer, which affects its flavor. “Once we solve that problem,” Forbes says, “I believe plastic bottles will be the next big thing for the beer industry.”