What is Skunked Beer?

Pepe Le Pew preferred wine. So what’s all this reference to “skunked” beer?

It’s actually pretty simple, and yes, a fairly accurate comparison. Skunked is a term we use when referring to beer that’s been compromised by exposure to UV rays. (The technical term is, appropriately, “lightstruck.”) When that bottle of beer is opened—and skunked beer only happens in bottles—there will be an unmistakable aroma of “skunk,” which may or may not influence your desire to drink or discard the beer.

The reason only bottled beer can be skunked is because UV rays can only reach the beer through glass bottles. Brown bottles do the best job at protecting the beer (about four times more protected), while green bottles are more susceptible, and clear glass bottles are (clearly) the most susceptible to skunking. While humans can slather on the SPF 50 and wear hats the size of small boats, we can’t quite do the same for our beer. Sure, we could put all of our beer in cans or brown bottles, but the industry is made up of hundreds of breweries, each with its own entrenched marketing and branding. Green glass and even clear glass bottles are not about to go away.

So what actually happens when a beer is skunked? The fairly complex chemical reaction involves blue spectrum and some ultraviolet light in sunshine (we trusted you, sunshine!) messing with certain compounds produced by hops called isohumulones, or iso-alpha acids. The whole reaction is slightly overwhelming (for us, because we’re chemistry dummies) but the rough sense we got is this: parts of sunlight “excite” parts of the isohumulone molecule to separate off and party with other compounds, which have separately been “excited” by sunlight to muck with the isohumulone, steal an electron (bad form, dude) and party with an amino acid to create a sulfhydrl radical (sulf=sulfur=skunk). Those guys party together and it gets nasty and no one cleans up before mom and dad get home. Or something like that. (We don’t know, we blacked out.) The most important thing is that the result of this sunlight-induced molecular recklessness is the creation of a compound called 3-MBT, which gives off that “je ne sais quoi” skunk flavor.

Among the misperceptions about skunked beer is that it takes a long time for a beer to get skunked, when in fact the reaction can happen fairly rapidly. So your best bet, especially if you’re drinking out of a green or clear glass bottle, is to keep your beer out of the sun.