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Lasers Get to the Bottom of Bad Beer

Photonics Spectra
Apr 2002
Brent D. Johnson

Anyone who has tasted beer that has been exposed to too much sunlight will recall a uniquely unpleasant flavor, followed by shudders. The light transforms the beer, producing a sulfur compound that is not unlike the noxious chemical produced by skunks.

To help solve this problem, Malcolm Forbes, a chemistry professor at the University of North Carolina, and Denis De Deukeleire of Ghent University in Belgium exposed beer to laser light. They used a spectroscopy technique developed by Forbes called time-resolved electron paramagnetic resonance, because De Keukeleire believed that isohumulones -- hops' light-sensitive compounds -- break down to produce free radicals in beer.

Forbes explained that this technique requires an abundance of molecules with unpaired electrons and is typically performed in steady-state mode with a large lamp or with UV radiation. However, he and his colleagues Arne Heyerick and Colin Burns have pulsed the production of radicals at 25 ns with an excimer laser, resulting in a large, simultaneous yield of free radicals.

Forbes uses an LPX 100i excimer laser from Lambda Physik of Fort Lauderdale, Fla., that emits at 308 nm. It has a very short pulse width, which is significant. With a broad pulse width -- for example, 10 µs --many radicals would be present while the light was still being generated, which makes it difficult to discriminate among the various breakdown products.

The excimer produces a high-intensity pulse with very good repetition rates and good time and energy stability, he said. A two-minute scan can produce 6000 to 12,000 shots at a 10- to 200-Hz repetition rate. This is important for degradation chemistry because prolonged exposure can damage the sample.

The researchers identified how each isohumulone absorbs light at a five-carbon ring in its structure. The energy is transferred to a carbon-chain appendage that liberates a free radical, which bonds with sulfur to produce the offensive smell and taste.

Recently, Forbes and his team experimented with visible light at 532 nm to view the riboflavins left over in beer that are specific to hops. This is a more complicated process, but sensitization by riboflavins leads to free radicals comparable to the isohumulones. Forbes and De Keukeleire plan to publish their latest findings this summer.

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