"This beer tastes like a skunk!!!"
Do I have your attention? It should because this is possibly the only recognizable off-flavor and aroma that's easily identified by the novice beer drinker and expert alike.
I’m sure a lot of us don’t really care if a beer is in a clear growler or a green bottle. That’s fine. But, in case you want to know why you get the whiff of an overly in-love cartoon character, then read on for some clarification.
How does a beer become skunked, anyways? Well, I’ll explain this in two ways: the simple way and the scientific way (between the sets of ** below).
The short answer is as follows: Your beer will be Light-Struck, or “skunked,” when visible blue or UV light comes in contact with the hop acids in the beer. The acids are broken down, rearranged, and transform into something called 3-MBT. In other words, the hop acids become skunk spray! Well, something very close to it.
When exposed to light, this process begins to take place instantaneously. If you’re drinking your beer outside, it may not be perceived until the end of your pint, if at all, but it WILL happen.
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Warning: Chemistry explanation begins here. Scroll down to the past the ** below to skip.
Ok, here we go. I hope you’re sitting down for this one.
First, let’s start by identifying what is needed to skunk a beer:
1. Isomerized Humulone Alpha Acids,
2. Riboflavin.
3. Light, including the Visible Spectrum and Ultraviolet.
1. Hops contain Alpha Acids (known as Humulone(s) communally) that need to be Isomerized (transformation of a molecule into another while keeping the same number of atoms) in order to provide the necessary bittering properties to beer. This is accomplished when hops are boiled in the brewing process. These Isomerized Alpha Acids become Isohumulones. On a side note, Isohumulones add antibacterial properties to beer.
2. Riboflavin is a B-Vitamin naturally produced by the vast majority of Saccharomyces Cerevisiae (Ale Brewing Yeast) strains during fermentation. The higher the malt content, the higher the Riboflavin present in beer.
3. Visible light and UV with wave lengths between about 350 and 500nm is of concern here.
Now that we have the suspects in custody, let’s recreate the crime.
When visible light strikes beer, the Riboflavin is agitated and takes an electron from the Isohumulone, changing the chemical structure of the molecule. The Riboflavin then reacts with an amino acid containing sulfur to create a sulfhydryl radical. This radical then combines back to the already modified Isohumulone molecule to create “3-methylbut-2-ene-1-thiol,” or 3-MBT. This is what gives the “skunky” flavor to the beer. Its structure is basically the same to that of skunk spay. Yum!
The threshold of human detection for 3-MBT is surprisingly low (easily perceived at about 4PPT in beer) and is one of the most recognizable off-flavors in beer. We've all had beers out of clear or green bottles at some point, right?
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I’m not taking sides or trying to prove anyone right or wrong, I’m simply stating the facts. You have a clear growler of fresh beer? Fine, just keep it away from light as much as you can. We can’t help the marketing decisions some establishments make. All we can do, as craft beer enthusiasts, is do OUR best to enjoy the beer as the brewer intended.
Obviously, clear or green bottles/growlers allow for easy formation of 3-MBT while brown bottles provide just about enough protections. Kegs and cans (yes, cans!) are the way to go since they block 100% of light, preventing this issue all together... Until you pop it open and pour it out under the sun. You’re on your own, there.
Bottom line, know what to expect and the real possibilities of some off flavors if you have a beer in a clear glass or in sunlight. The odds are you will have a skunked beer at some point. However, the hop content may be low enough that you might not even notice or you’ll finish your beer before it becomes really noticeable. Take the proper, simple precautions and you’ll be fine.
In the end, it's all about enjoying your beer. So do just that, however that may be.
Cheers!!
-Gilbert "Charlie" Perez, Certified Cicerone®
Source/More Info: http://beersensoryscience.wordpress.com/2011/03/17/lightstruck/