The Beer Off-Flavor Series: Skunk

By Drew Beechum

This is part 5 in The Beer Off-Flavor Series by Drew Beechum. Catch Drew at Homebrew Con 2019 where he’ll be presenting “Brewing Simple: Great Beer with Less Effort and More Fun.”

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Ah, “skunked” beer. The legendary off-flavor with oft-lored causes.

Also known as “lightstruck,” this sulfurous compound (3-methyl-2-butene-1-thiol) is generally avoided by all but the biggest professional brewers (more on that later) and a select few farmhouse brewers in pursuit of that subtle but “classic” touch of skunk.

Luckily for us, lightstruck beer has a simple cause that is easy to prevent.

Key Indicators

lightsturck molecule

3-methyl-2-butene-1-thiol (C2H6S)

Tasters commonly report these characteristics when detecting a “skunked” beer:

  • Sulfur
  • Skunk spray aroma
  • Biting sulfur sensation on the tongue

Humans are extraordinarily sensitive to sulfur compounds, with the average threshold of perception measured in parts per trillion (ppt). For comparison, most other off-flavor compounds are measured in parts per billion (ppb) or even parts per million (ppm).

If you want to experience a classic example of lightstruck beer, just head down to your local beer store and look for beer packaged in green or clear bottles. Beers such as Corona and Heineken are almost inevitably skunky from a combination of poor storage and marketing-driven packaging choices. (Marketers love green and clear bottles because they show off the beer, and many consumers still associate such glass with “premium imports.”)

Cause & Prevention

Ultraviolet (UV) Light

Remember those times your college buddy said, “Hey! Don’t let the cold beer get warm and chill it again. It’ll skunk!” Well, that’s not how it works.

Skunking is the result of UV and blue light interacting with isomerized alpha acids (compounds extracted from boiled hops). This is why you’ll hear people use the term “lightstruck” in place of “skunked.”

I won’t terrify you with the chemistry, but your primary sources to worry about are sunl</subight, intense, direct UV light, and “bright white” LEDs.

Solution: Your beer is a vampire: keep it out of the light! It only takes a few minutes of exposure to cause damage. Seriously! I’ve had pints of IPA sitting outside that have changed over the course of drinking them. Stupid sun!

The same precautions apply to fermentation as well, when vitamin B compounds critical to the skunking reaction begin to appear. Keep your fermenters in a dark spot if possible. If you don’t have such spooky luxury, covering fermenters—especially clear ones—with blankets, towels, or your favorite Dracula cape can help keep out the light.

When you package your beer in bottles, go for brown glass for a little extra protection from the light. And then break out the ultimate protection by putting those bottles in a box and closing it up. Light can’t cause harm if it can’t reach the beer, and light’s not going through cardboard!

There are UV-stabilized hop extracts that prevent this problem, but we homebrewers can’t seem to get our mitts on them yet.

homebrew fermenter

Even during fermentation, lightstruck is a risk if your wort is exposed to UV light.

Closely Related: Mercaptan

While not the same as the lightstruck compound (3-methyl-2-butene-1-thiol), mercaptan (CH3SH) can sometimes be confused for lightstruck beer. Mercaptan resembles rotten eggs more than it does skunk spray. Think of natural gas—mercaptan is added to this naturally odorless gas to help us detect leaks. Mercaptan usually shows up in beers as an infection; it can also appear in wine due to yeast health issues.

Solutions: As with every batch of beer you pursue, always practice thorough cleaning and sanitizing measures and ensure a healthy pitch of yeast. See my contamination and yeast health tips from the first part of the Beer Off-Flavor series.

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Drew Beechum lives in Los Angeles, Calif. When not busy playing with computers for pay, he plays with beer. Drew is a member of the Maltose Falcons, co-host of Experimental Brewing, and coauthor of Simple Brewing: Great Beer. Less Work. More Fun. 

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