Humans share a lot of similar enemies with beer time (aging), heat, and dirty glassware (only half joking!). But there is one thing humans tend to love, that really messes up beer: sunshine. Lightwaves react with some ingredients in beer to create a sulfur compound that makes your beverage smell almost exactly like skunk spray. And there is no tomato juice cure for this skunking! Here’s what you need to know about lightstruck (commonly called “skunked” beer).
How Does Beer Get Skunky?
When beer is exposed to light a reaction occurs (known as photooxidation) with bitter hop compounds called iso-alpha acids which results in the intensely flavor-active compound 3-methyl-2-butene-1-thiol (3MBT or MBT). Sometimes MBT is called simply mercaptan, which is the name for the larger family of sulfur compounds it belongs to.
This reaction can happen very quickly especially in strong sunlight (ex. the parking lot during a tailgate!) according to The Practical Brewer, a handbook from the Master Brewers’ Association of the Americas:
“Lightstruck – While this defect is well known in both odor, taste and origin, it is sometimes not realized how little exposure can produce a noticeable lightstruck character. The wavelength of light causing this photochemical spoilage is 550 nm and below. Bottled beer can become lightstruck in less than one minute in bright sun, after a few hours in diffuse daylight and in a few days under normal fluorescent light.”
Brewers protect beer from light by using aluminum or dark glass packaging. Clear glass bottles and green tinted glass don’t offer enough protection from lightwaves to prevent the lightstruck reaction. Consumers can protect their beverage from the dreaded skunk aroma by using opaque glassware in direct sunlight and storing beers in a dark place.
But I Never Noticed This Flavor in My Favorite Beer That Comes in Green Glass Bottles?
Some brewers use treated hop extracts like Tetra Hop Extracts which have the specific hop compounds that react with light removed. These hop products allow beer to be exposed to any kind of light without developing MBT.
However, even though these beers won’t specifically smell like skunk spray, they will develop other flavors from age and light exposure. One study of so-called light stable beers shows exposure to sunlight develops an onion-like flavor in these beers.
The other answer to this question is that the beers you like in green and clear bottles (think heineken and Corona Light) do smell a little skunky, but you’re used to that as part of their flavor profile. Speaking of flavor profile….
What Does Skunky Beer Taste Like?
Lightstruck beer that has undergone the reaction to create MBT smells like skunk spray. People also describe the flavor and aroma as musky, foxy (who is smelling foxes?!), dirty, dank, and marijuana-like. The last descriptor makes sense as the “T” of MBT stands for “thiol” and just like hops marijuana gets much of its flavor from thiols. Some thiols smell like tropical fruits and fresh herbs, it just happens that this specific thiol isn’t so pleasant.
Beyond being pretty stinky, MBT is extremely flavor active. This means our human noses can sense it at extremely low levels. According to the American Chemical Society if you added one eyedropper’s worth of pure MBT to an Olympic sized swimming pool of beer, it would noticeably change the taste! Potent stuff!
I Thought it Was Warm Beer that Gets Skunked?
Yes and no. For a long time a favorite “myth” of beer writers was that was kegs couldn’t get skunked because they were protected from light. The assumption was that any off tastes in a keg came from other oxidation reactions or that the poured beer was skunked after it was poured. Afterall, a sunny tailgate is the perfect place for this reaction to happen. However, newer research found two yet-to-be identified compounds that develop in beer smell almost exactly like MBT happen when beer is stored warm, even without light.
So that myth bust, is now busted itself!
Try it Yourself
Want to taste a skunky beer? That’s easy! Put a clear glass of beer in direct sunshine for about 5 minutes. If there wasn’t shade in your area you’ll be able to taste that musty, skunky flavor. This experiment works best with low flavor intensity beer styles like pilsners or blond ales. It’s also best to stick to beers made by small craft breweries as the larger industrial breweries try to protect their beers from destabilizing in sunlight.
- A well cited heavy on the technicalities look: Beer Lightstuck Flavor: The Full Story
- A nice explanation of the actual chemical reaction from PopSci
- The Role of Hops in Brewing touches on lightstruck reactions
- The book where I learned the most about lightstruck beer The Practical Handbook for the Specialty Brewer (cited above!)