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Doing a bit of reading about the effect light can have during fermentation & came across the term 'skunking'.

What is skunking? What happens to the beer and how do you know if you've skunked a batch?

Other questions address how it occurs & how to prevent it, I'm just not sure what it actually is.

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As a side note, Google produced some interesting results. Particularly at Urban Dictionary (nsfw). –  C4H5As Mar 14 '11 at 12:46
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1 Answer

up vote 4 down vote accepted

http://beeradvocate.com/articles/527

Skunking is described in this article as UV radiation caused breakdown of "hop derived molecules, called isohumulones", which then bond with sulfur, giving you a skunk-like smell.

As other articles have said over and over, keep your beer cool and dark. You should have a safe, cool, dark storage place for your beer to ferment (if you're like me without temperature controlled fermentation), condition, and age if you so choose. If you fear several of your bottles have been exposed to too much light, you can then have a taste test with one from your "cellar" (laundry room closet for me).

The article also mentions the misinformed opinion that Heineken, and other German or dutch Pilsner style beers are "skunked", when in truth the flavors common to them are intended, having nothing to do with spoiling or light exposure.

EDIT:

Heineken Bottle vs Can

After being challenged in the comments to compare a bottle and can of Heineken, I have to concede, I and the article were wrong. I did purchase from a specialty beer and wine store, but their display cases weren't much different than a gas station. I poured both into a glass, which should have normalized the experience (refer to the article). I immediately noticed the smell difference, and took them to my wife, who was easily able to pick out the skunk smell without knowing anything about my experiment. The taste is remarkably similar, considering how much smell can affect taste. The smell is not a question, bottle skunk confirmed. I love my Grolsch bottles, but I'll be extra careful to keep them in the dark from now on.

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I think you have the misconception backwards. A lot of people assume that imported beer is supposed to have that characteristic twang, but it is actually skunking that has occurred due to using green or clean bottles and the long transit and poor handling coming across the Atlantic ocean. Taste a heinie fron a can vs. the bottle, there is a big difference. NO beer is intended to have skunk flavors. –  therulebookman Mar 14 '11 at 16:06
    
Agree with the above. –  Denny Conn Mar 14 '11 at 21:17
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Are you saying that it is impossible for green bottled beers from Europe to arrive in the US un-skunked, or that statistically most people complaining about skunking are drinking bottles that were poorly handled? For my part, I'll be picking up a bottle and a can on my way home for a taste test, you've got me curious, though I'm not excited about drinking Heineken. –  Mlusby Mar 14 '11 at 21:31
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I agree with all of the first comment, except for "NO beer is intended to have skunk flavors." I'm quite certain that the mild skunking of Sleemans (Canada) is deliberate. If I'm wrong about Sleemans, then there's my own beer which I sometimes skunk slightly on purpose. –  Jeff Roe Mar 15 '11 at 0:02
    
Wow! That's awesome. I've had imports that taste fine, Newcastle for example, so I assume that some Heinie's make it over un-skunked, but a lot of them are skunked. Huge props for taking the challenge, and taking pictures. Hopefully you had some homebrew around to chase the heineken with! –  therulebookman Mar 15 '11 at 20:17
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