In short, if a beer is not protected by the right kind of glass, it quickly becomes “light-struck”, or in the beer biz what we call “skunked”. Beer, as it turns out, is very sensitive to light. Or, more accurately, the hops in beer are sensitive to light.
Here is what happens. Hops contain something called alpha acid, the substance that gives beer its bitterness. When dissolved in beer it isomerizes and turns into something called iso-humulone. Beer also naturally possesses undetectable trace amounts of sulphur.
When iso-humulone comes into contact with light, it triggers a photo-chemical reaction with the sulphur to create 3-methyl-2-butene-1-thiol – not a very pleasant compound. In fact, it is the same substance skunks spray to ward off attackers. Yummy!
Yep, you read that right. Skunky beer is called “skunky” for a reason. It literally contains the same chemical compound as skunk spray.
Sherbrooke Liquor Blog » Blog Archive » Beer 101: Lesson #7 – Beer Gone Bad: Light-struck beer.