Popular Science writer Martha Harbison explains why “skunking” in beer is very different from a lot of the other ways beer can go bad. Harbison claims beer skunking seems most prominent in lagers such as Corona and Heineken which are typically shipped in clear or light green bottles.
The mechanism and chemical reactions that caused skunking was clarified 12 years ago in a paper — Mechanism for Formation of the Lightstruck Flavor in Beer Revealed by Time-Resolved Electron Paramagnetic Resonance — that appeared in a European Journal.
Researchers used spectroscopy to see how certain compounds in beer behaved as they were irradiated with light, and found that there are two distinct pathways that allow skunky-smelling compounds in your beer: hop alpha acids and light.
Not heat. Not oxygen. LIGHT.
The bittering agent generated from hops while boiling beer wort is a compound called isohumulone. Both ultraviolet light and visible light can degrade isohumulone creating a series of reactions that eventually produce the compound 3-methylbut-2-ene-1-thiol.
“That mouthful, known colloquially as 3-MBT, is your skunk. In fact, 3-MBT is chemically very similar to one of the three main compounds found in a skunk’s defense spray,” says Harbison.
With clear, green or blue bottles, the glass doesn’t filter out the ultraviolet and blue wavelengths that start the skunking reaction, adds Harbison.
Brown bottles are much better at keeping those wavelengths out of your beer.
In recent years, “advanced hop products” such as chemically modified hop extracts have proven to be “light stable.”
Harbison claims not to know if Heineken or Corona use these in their beers.
“But recent work by scientists in Belgium indicates that even these ostensibly light-impervious hop extracts still generate off-flavors from exposure to light, including an “onion-like” compound 2-sulphanyl-3-methylbutanol — just not to the degree that raw hops do.”
If you want to avoid the skunk entirely, Harbison advises buying a beer that has been packaged in a keg, cask or can.
“Those beers can (and do) develop bad flavors, but you’ll never get one that has been skunked.”