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Are those draft lines clean? [Photograph: Beer tap on Shutterstock]

We've now explored the sources of a pretty good range of the many delicious flavors that can be found in beer. But what about the not-so-delicious ones?

Many supporters of the craft beer industry see their world through rose-colored glasses, but the fact of the matter is that the big boys over at Bud/Miller/Coors have one undeniable advantage over us craft folk: consistency. Craft beer, by nature of its experimental tendencies, doesn't always taste like it was intended to. Risky (but potentially innovative!) procedures can sometimes result in a flop of a beer, either due to recipe flaws or the production of off-flavors. And as if that weren't enough, the way retailers handle the beer can negatively impact flavor as well. There are a few things to look for if you want to figure out what went wrong with that funky , off-tasting beer in your glass.

Skunking

Skunky aromas in beer seem to be simultaneously among the most discussed and least understood by the beer drinking masses. Many folks seem to believe that these aromas, which unsurprisingly smell quite a bit like the business end of a skunk, stem from warm-cold cycling of your beer. These are the guys that refuse to buy beer out of the refrigerator at the grocery store because it will warm up on the car ride home. While this shift from cold to warm to cold again isn't exactly the best thing for beer, it isn't really that terrible for it either. The benefits associated with cold stored beer (the stuff from the refrigerator) over warm stored beer (the stuff from the back) almost certainly outweigh the detriment caused by warm-cold cycling. More to the point, though: warm-cold cycling has nothing to do with skunky flavors in beer.

Another name for skunked beer is "lightstruck." Skunky beer is caused by a reaction that occurs when the alpha acids from hops in beer are exposed to ultraviolet light from, say, the fluorescent light in your grocery store's fridge. Or, you know, the sun.

Thankfully, most craft beer you'll come across these days is housed within the protection of UV-filtering brown glass or UV-eliminating aluminum. The stuff you've got to be worried about comes in clear or green glass—this type of packaging offers little to no protection from harmful UV rays, and will allow for skunking to occur within just minutes of sun or fluorescent light exposure.

Want to taste a skunked beer? Pay attention to your beverage the next time you're at an outdoor beer garden on a sunny day. Grab a hoppy beer. The first sip in the bar tastes great, but after a few minutes under the sun's rays, you'll start to smell skunk.

Diacetyl

I remember the first time I learned about diacetyl (pronounced die-uh-SEE-tull by most American chemists, but die-ASS-it-ull by most American beer folk). It wasn't as a beer geek, but rather as a citizen concerned for the welfare of our nation's popcorn workers. Popcorn lung is actually a serious problem. Diacetyl, the buttery-tasting compound that flavors most microwave popcorn, can totally mess up your lungs if you inhale a lot of it.

If that doesn't sound like something you'd want in your beer, well, too bad. Diacetyl is naturally produced in almost every beer fermentation. Thankfully, if fermentation is done properly, yeast will clean up most or all of this flavor. Still, a subtle diacetyl presence is considered acceptable or even desirable in some beer styles.

But if there's too much, things taste gross. While there won't be enough diacetyl to hurt your lungs, an aggressive artificial-butter or butterscotch flavor paired with a slick, round mouthfeel are telltale signs of a beer gone bad.

Where it gets confusing is when you try to find the source of the problem. Yes, an incomplete or poorly-executed fermentation can cause diacetyl problems, but don't grab your pitchforks and torches and head toward your local brewer's house quite yet—diacetyl can also be an indicator of a bacterial infection in your beer. Infections occur not just from poor sanitization practices on the part of the brewer, but also from poor draft maintenance at your local pub. If you get a funny tasting pint at a bar, ask how often they clean their draft lines. If they look like you're speaking in a foreign language, head for the door and don't look back. If they clean their lines every two weeks or so, as is recommended by the Brewer's Association, it's time to write an email to the brewer—and the bar's manager.

DMS

Dimethyl Sulfide, or DMS, tastes a bit like cooked corn or cabbage. Until Dogfish Head inevitably decides to use cabbage in one of their beers, it's probably not what the brewer was going for. This stuff crops up as a result of heating lightly-kilned malt (as you do in the mashing process), and typically burns off in the boiling process. Some people are more sensitive than others to this flavor, but it's usually most noticeable in pale lagers, which makes sense—these beers are often made with exclusively lightly-kilned malt, and without big hop or yeast flavors getting in the way, it is easier to detect that cabbagey note.

Acetaldehyde

Commonly found in beer that has been served before it is ready, this is the flavor most closely tied to what brewers call "green beer," or beer that is too young. This stuff, like diacetyl, is produced naturally in all beer fermentations, but is eventually cleaned up before completion of the ferment. Serve that beer before it's ready or remove the yeast while the beer is still fermenting, and you'll likely have a mouthful of acetaldehyde, which tastes quite a lot like green apples.

Confusingly, acetaldehyde can also be produced as a byproduct of oxidation, so its possible to have it in a beer that is both too young and too old. It's a good thing this is probably the least offensive off-flavor in beer.

Oxidation

If you've ever described a beer as "stale," you are likely referring to one that has been oxidized. This can happen as a result of poor beer handling after fermentation or from contact with the oxygen held within a bottled beer's headspace between the beer and the cap. Over time, most beers will oxidize to some degree, so oxidation is especially prevalent in aged beer. However it occurs, the chemical reactions that occur in beer as it is exposed to oxygen can produce a whole array of flavors.

Least appealingly, these flavors can be similar to paper or cardboard. It is even possible, though less common, that diacetyl (and as previously mentioned, acetaldehyde) can be produced through oxidation as well.

But there is definitely some good that can come from oxidation in beer and some brewers are embracing it. Oxidation can bring on a honey, toffee, or sherry-like flavor to your beer, and thus can provide a pleasing complexity in aged beer. Italy's Birra Baladin has taken this idea about as far as it can go with their Xyauyù line, which is a still barleywine that the brewers intentionally oxidize.

Infection

One more term you may encounter if you're downwind of a beer geek's ramblings is infection.This is a beer that has unintentionally been partially fermented by wild yeast or bacteria. An infected beer could display a wide range of characteristics.

The byproducts of these accidental fermentations almost universally taste terrible. A good first indication of an infected bottle of beer is overcarbonation—wild yeast and bacteria can usually eat a range of sugars that normal brewer's yeast cannot, and the continued fermentation in the bottle will trap that excess CO2 in the form of bubbles. In dramatic cases, this can result in a "gusher." Your kitchen counter will be absolutely covered in signs of infection if that's the case.

You'll be able to taste other indications of infection. Look for an unexpected sourness or the aforementioned diacetyl and acetaldehyde—both could indicate an infection.

Again, let the pitchfork stay in its holster until you've sourced the cause of the problem. Poor sanitation is always to blame when it comes to infections in beer, but improperly maintained kegs and draft lines are just as likely to be the cause as the brewers themselves.

About the author: Mike Reis is a Certified Cicerone and Co-Director of Beer at the Monk's Kettle and Abbot's Cellar restaurants in San Francisco. Follow him on Twitter @beerspeaks or find him behind a pint near you.

More from Mike Reis

Have Beer Weeks Grown Too Big?
How to Identify Yeast Flavors in Beer: Esters, Phenols, and Alcohols
How to Identify Oats, Rye, Wheat, Corn, and Rice in Your Beer
How to Identify Hops in Your Beer: The Three C's
8 Tips for Hosting a Beer Dinner at Home
The Best Places to Drink Beer Outside in San Francisco and the East Bay
Aging Beer: 6 Tips to Get You Started
Hops From a Land Down Under

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