Ask a Scientist: How does a beer get skunky?

Hops are added to beer for flavoring, and are sometimes called “beer bitters.”
Hops are added to beer for flavoring, and are sometimes called “beer bitters.” Getty Images

Malcolm Forbes is director of the Center for Photochemical Sciences at Bowling Green State University, in Ohio, a post he took after being on the faculty at UNC-Chapel Hill for 25 years. Here he explains what has gone wrong when your beer tastes like it has been spiked with skunk spray. Questions and answers have been edited.

Q: Is it true that letting cold beer warm to room temperature will cause it to skunk? If not, what is the real culprit?

A: That musky aroma has little to do with temperature and a lot to do with light. As long as you don’t go to extremes, heating and cooling shouldn’t change the chemical composition of your beer. However, if you let your favorite brew get too much sun, that can be a problem. Beers become skunky or “lightstruck” when ultraviolet light reacts with bitter hop compounds to create a chemical that mimics skunk spray.

Hops are added to beer for flavoring, and are sometimes called “beer bitters.” The molecules responsible for this bitterness are known as iso-alpha-acids. When hit by ultraviolet light, these iso-alpha acids fragment into what chemists call free radicals, molecules with unpaired electrons that are eager to pair up. These highly reactive molecules interact with sulfur atoms present in small amounts in beer, generating a compound called 3-methylbut-2-ene-1-thiol or “skunky thiol.” This molecule is similar in structure to the chemical that is released by skunks in response to threats from predators. In fact, it is the exact compound found in cat urine, which is why many people who own cats recognize the smell of skunked beer quickly.

Q: How quickly can a beer become lightstruck?

A: Skunky thiol is detectable to humans at very low concentrations. Therefore, it does not take much conversion of light energy to free radicals to produce what the industry refers to as “the malodorous beverage.” In a German beer garden in the summer, a 1- or 1.5-liter mug of beer will have a detectable “skunked” smell and flavor after only 10 or 15 minutes. Beer stored in clear glass is especially susceptible to this photochemistry, which is why you see beers such as Corona stored in boxes in the supermarket, and served with lime in bars and restaurants (the citric acid in the lime reacts with the thiol to neutralize the off-flavor, similar to how the pungent odor of amines from cooked fish is neutralized by adding lemon).

Q: What can be done to save beer from this terrible fate?

A: Beer sold in cans is an obvious solution, and barrier coatings technologies for can interiors are now available to ensure that beer does not suffer from contact with the metal surface. Craft beer in cans is an emerging industry, and consumers are gradually being won over, just as wine drinkers have gotten used to the screw cap.

Another method has been to chemically modify the hop compounds to remove the light sensitivity (these are called “advanced hop products” in the industry) and add them at different stages of the brewing process. Such products are used in beers such as Newcastle Brown Ale, which is sold in a clear bottle. While chemical modification of the hop compounds improves the lifetime of the beer during or after light exposure, this also alters the flavor of the beer, and not always for the better.