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BeerSci: Why You Should Never Drink Beer From A Clear Glass Bottle | Popular Science
Skunked beer smells just how it sounds: You crack open a bottle and an unmistakeably mustelid pong hits your nostrils. This can happen in any beer, but skunking seems most prominent in lagers due to their otherwise-mellow aroma profiles. Further, many of these self-same lagers (think Corona, Heineken, etc) are also shipped in clear or light green bottles.
The industry term for skunking is "lightstruck," and it's a beer fault that has been written about since about 1875. By the 1960s, scientists had narrowed the culprits down to a triple-threat of hops, a sulfur compound and a molecule known as a flavin. But despite more than a hundred years in the literature, the mechanism and chemical reactions that caused skunking were only elucidated in 2001 with the paper Mechanism for Formation of the Lightstruck Flavor in Beer Revealed by Time-Resolved Electron Paramagnetic Resonance by Burns et al. in Chemistry--A European Journal. In it, researchers used a special kind of spectroscopy, the aforementioned time-resolved electron paramagnetic resonance (TREPR), to look at how certain compounds in beer behaved as they were irradiated with light. TREPR is similar to what happens when you get an MRI, except that in MRI the technician is looking for the spins of atomic nuclei, while TREPR is looking for spins of unpaired electrons. This is important because most photochemical reactions, at some point in the pathway, create unpaired electrons. Following where those go, and to which molecules they are attached, is key to understanding the entire reaction mechanism.
What those researchers found is that there are two distinct pathways to getting skunky-smelling compounds in your beer. The two main actors is this tale of woe: hop alpha acids and light. Not heat. Not oxygen. Light.