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.