Causes of Skunky Beer

Introduction

Sometimes after purchasing beer, you may open it to discover it doesn't smell quite right. In fact, the smell may be unpleasantly overwhelming. This is a phenomenon that has come to be known as "skunky beer." However, the name in the industry for this problem is "light struck" beer.

Isohumulones

Skunky beer occurs when the hops become exposed to any kind of light such as ultraviolet from the sun or even the fluorescent lights of a store display case. When light hits the beer, a chemical reaction breaks the isohumulones, compounds relating to beer bitterness, into free radicals.

Thiol

Beer contains naturally occurring sulfur proteins that react with the isohumulones as they break down. The reaction causes a substance called 3-methyl-2-butene-1-thiol. The most important part of this equation as it applies to skunky beer is the thiol. Thiol is the same chemical responsible for the defensive smell that skunks produce when agitated or defending themselves.

Prevention

The best way to prevent skunky beer from occurring is to limit its exposure to light. Buying beer that has darker bottles, comes in cans or is held in cardboard containers will help dramatically decrease your chances of buying a skunked beer. Beers that have higher contents of hops are more likely to become skunky as there are more isohumulones to be broken down by the light.

References

• Skunky Beer
• Washington Post: Light and Beer