You crack that frosty brew. It's a hot day. You're salivating. You take a deep swig and... what the hell is that? Pepé Le Pew? A flatulent egg farmer? No. It's your precious beer, and it's gone bad—skunked beyond recognition. How could this happen?
More importantly, how could you have prevented it?
It's Friday afternoon, you've made it through the long week, and it's time for Happy Hour, Gizmodo's weekly booze column. A cocktail shaker full of innovation, science, and alcohol. If it stinks, don't drink.
The Science of Skunk
You may hear friends use the word skunky to describe several nuances that can ail a lager. Wrong. Skunky is specific. It does not mean flat. It does not mean it tastes stale, sour, bitter, or metallic. Skunky means skunky, as in it literally smells and tastes like a skunk. (Don't pretend you've never tasted skunk, you dirty liar.)
That is caused by a very specific chemical compound: 3-methyl-2-butene-1-thiol, or what Dr. Malcolm D. Forbes, professor of chemistry at the University of North Carolina calls "skunky thiol." Why? Because it's almost identical to the compound found in a skunk's notorious anal glands. These chemicals are called mercaptans and they contain a high amount of sulfur. We humans are extremely sensitive to mercaptans, and we're able to smell and taste them down to just a few parts per trillion.
Okay, so that's disgusting, but how did these chemical compounds get into your normally delicious beer?
It Isn't Heat
One of the most pervasive beer myths is that taking a cold beer out of the fridge, letting it warm up, then putting it back in the fridge causes it to get skunked. This is 100 percent false. A change in temperature—especially such a small change—cannot account for that chemical reaction. Beer being warm over a period of time can and will affect the flavor (more on that in the minute), but remember that skunkiness is the flavor of a specific chemical compound, and heat fluctuation does not create it. So what does?
Skunking is caused by exposure to light. End of story. Well, not really. How it happens is simple: Beer is flavored with hops. It's a bittering agent that helps protect the brew from bacteria, and it's been used for centuries. Reports of light degrading beer date back to 1875, but it wasn't until 2001 when Dr. Forbes at UNC figured out how and why, exactly.
Hops, are light-sensitive, and the three main compounds in them identified as being light-sensitive are called isohumulones. When attacked by either visible or ultraviolet light, these break down to make reactive intermediates, known as free radicals. These lead to the offensive taste and skunky odor.
This is why lighter, less-hoppy beers are generally less susceptible to skunking—there's less in them that can skunk. That's little solace to those of us who enjoy a smack in the mouth from a double-IPA. But fear not. Hop-lovers can take preventative action.
How To Avoid the Stinker
- 1. Storage. If you aren't keeping beer in your refrigerator (which is where it definitely should be), store it in a cool, dark place. Obviously, sunlight contains a ton of UV rays, so avoid that. But florescent lights pump out UV rays, too. It's a very small amount compared to sunlight, but if those lights are often shining on beer stored outside a fridge, it may be enough to produce the undesirable effect.
- 2. Packaging. Clear glass is the worst at filtering UV light. Green glass is a little bit better. Brown and amber glass, however, do a very good job of filtering UV, though you still want to minimize exposure to sunlight. Cans are the best—virtually no light can get through them at all. A lot of great beers come in cans now, and no, they don't taste metallic. They taste as they would coming out of a keg (which is also skunk-proof). That said, if you buy a 12-pack and it's all closed up in cardboard, it's almost certainly okay, even if it's in clear or green glass bottles.
- 3. Sourcing. You've probably got a few grocery stores in your neighborhood where you usually buy beer. How do they store it? Are the refrigerated displays right by the windows, or up against a lighted glass door? That may not the best place to buy. Chances are, if the store sells varieties more exotic than Heineken, then the store's distributor takes the beer and its storage seriously. Visit a craft beer bar—or, if you're feeling adventurous, head over to your favorite local brewery during tasting hours—to ask a pro where to get the best beer around. As long as you can trace the darkest, coldest path the beer can take, you'll enjoy the freshest beer you can find.
Even if heat and age don't cause skunkiness (in the absence of light), those factors can absolutely affect the flavor of your beer. When beer goes stale, it doesn't curdle or sour like a spoiled dairy product. This leads a lot of people to think that beer doesn't go bad. And it kind of doesn't. But age does take it further and further away from the flavor profile that the brewmaster intended, primarily due to oxidation.
This is where refrigeration is important. Beer starts getting stale (as in the flavors start changes) surprisingly fast. Like with eggs, meat, or dairy, keeping it cold will keep it fresher, longer. You may start to taste the flavor changes after as few as five days! That means if you're buying imported European beer—which generally comes via sea, unrefrigerated— in most cases it's stale before it gets to you. (Like you needed another reason to jump onto the American craft-beer bandwagon.)
So, despite what you learned freshman year of college, it has nothing to do with taking it out of the fridge and then re-cooling it. It's just time spent unrefrigerated, period. And, of course, even refrigerated beer will go stale. It'll just take longer. The responsible thing to do is drink it.
Take care of your beer. Pamper it a little. Respect its delicate chemistry, and make sure it's been treated tenderly. Raise a glass to brewmaster George di Piro, aka Professor Beer, who writes eloquently about the effects of oxidation and aging in beer. Then, next time you're at a summer party, and a friend putting beer on ice starts pontificating about skunking, go ahead and punch him in the brain with your newly acquired knowledge.