Skunk Explained! Why Beer Gets That Inevitable Off-Flavor Under the Sun

Summer is here, and it’s time to enjoy a cold beer outdoors at breweries with comfortable patios like the ones found at TAPS, Bootlegger’s, or Tustin Brewing Co.

While doing so, you may notice a familiar flavor and aroma as you approach the end of your pint. Don’t worry, the brewers didn’t do it!

Lightstruck, commonly described as “skunked,” is one of the most detectible and easily identified compounds, for both the novice and expert taster.

So, what causes this? The short answer: Light can affect the hop acids in the beer. The acids are broken down, rearranged, and transform into a compound very similar to skunk spray. (Yuck!) When exposed to light, the process begins nearly instantaneously. While drinking a beer outside on a brewery patio, it may not be perceived until the end of your pint, if at all, but the process will happen.

Warning, chemistry ahead!

The detailed explanation of this process involves three key factors. They are: (1) isomerized humulone alpha acids, (2) riboflavin, and (3) light, particularly visible blue and ultraviolet.

1) Hops contain alpha acids known as humulones that provide the necessary bittering properties to beer. This is accomplished by boiling the hops during the brewing process.

2) Riboflavin is a B-vitamin naturally produced by most brewer’s yeast strains during fermentation. The higher the malt content, the higher the riboflavin in beer.

3) Visible light and UV with wavelengths from 350 to 500 nanometers.

Now that we have the suspects in custody, let’s recreate the crime.

When visible light strikes beer, the riboflavin is agitated, changing the chemical structure of the molecule. A brief series of chemical reactions then leads to production of a compound called 3-MBT, which has a structure eerily similar to skunk spray. The amount needed for human detection of one of the most recognizable off-flavors in beer is surprisingly low.

Prevention is possible by using certain hop-extract products in the brewhouse. However, these are not common and can be expensive for independent brewers. Clear or green glass bottles or growlers allow for 3-MBT development, but brown bottles or growlers do provide protection. Kegs and cans or crowlers block 100 percent of light, fully preventing this issue.

Sitting under the warm sun with your beer in a clear glass is inevitable. All you can do is understand this is a natural process and is not intended by our talented brewers in Orange County.

Bottom line: Know to expect some off-flavors if you have a beer outdoors in the sunlight. The odds are you will have a skunked beer at some point while sipping a cold one at RIIP, Bottle Logic, or Pizza Port. Yet, the hop content may be low enough to notice any lightstruck development, or you’ll finish your beer before it becomes perceptible. So, do not fear the summer sun and enjoy your locally produced beers.

Editor’s note: Charlie Perez is an Advanced Cicerone® who covers the Orange County beer scene for the Booze Blog.

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