BI Answers: Why Does Beer Get Skunked?

It's fall. Time for football tailgates, baseball playoffs, and Oktoberfest — which technically began in September, but no one is going to stop you from celebrating. So go ahead and pop open a beer.

But sometimes an awful thing happens when you lift that cool glistening bottle to your lips. Something tastes off. The beer is bad.

It's skunked.

Skunked is really bad. If you've experienced it, you know, and you don't want it to happen again.

Can't go wrong with a can of Heady Topper! walknboston/flickr

Some people blame bad refrigeration practices — letting the beer go from cool to warm and back again — but even though that can make beer stale by increasing the rate of oxidation, it's not the culprit for that skunky taste.

Skunked beer is caused by a specific chemical reaction triggered by exposure to light, as explained in the latest Reactions video by the American Chemical Society.

Brewers know this — there's a reason why craft beer comes in brown bottles or cans, as opposed to green or — shudder — clear glass.

What's more, the name skunk is particularly relevant.

Beer gets its bitterness and a lot of its flavor from hops, one of the main ingredients needed to make the delicious beverage.

They're added to the wort, or not-yet-beer, during the brewing process. When boiled, hops release iso-alpha acids into the liquid.

So far so good.

But if beer is exposed to sunlight, the sun's power breaks down those iso-alpha acids. The resulting compounds bind with proteins that contain sulfur.

This creates a new chemical — one that's almost exactly identical to the one released by skunks.

It's incredibly potent too.

People can taste this chemical in concentrations of one part per billion. As the video explains, "if you filled an Olympic-sized swimming pool with beer, one eyedropper of this stuff would change the way it tasted." Which would ruin an otherwise delightful Olympic-sized swimming pool full of beer.

So treat your beer with respect and keep it out of the sun, especially if the beer is in a clear glass. Supposedly, enough light can get through a green bottle to skunk it, if given enough time.

Watch the full video for more.

This post is part of a continuing series that answers all of your "why" questions related to science. Have your own question? Email science@businessinsider.com with the subject line "Q&A"; tweet your question to @BI_Science; or post to our Facebook page.