Describing your favorite beer isn't much different than reviewing your favorite Captain Beefheart record. Mostly, you detail how much and to what degree it reminds you of things that it isn't, and it's these lattices of difference that make microbrews (and Trout Mask Replica) so damn great. Unfortunately, they're both susceptible to unwelcome and corrupting influences.
For beers, off-flavors can originate during the brewing process or develop through subsequent mishaps in the bottle or on the draft line. After a nasty run-in with a foam-spewing, musty-dirty-laundry-water-swill disaster of a six pack earlier this month, I was motivated to describe the most commonly occurring brew nuisances, ranked from least to most heinous, with recommended precautions or cures.
Acetaldehyde is the least offensive of the lot (though it's the main culprit in causing hangovers), lending beer the flavor of Granny Smith apples. It often indicates that a beer is too green itself and needs some time to sit around and let the yeast gobble up the excess acetaldehyde, turning it into a more-desirable form of ethanol.
Diacetyl is a chemical compound that betrays itself by buttering (or butter-scotching) up your beer's flavor and mouth-feel. Like acetaldehyde, every beer has at least a little bit present during brewing and it's usually eliminated by the yeast. It's an acceptable feature of some styles (it always brings Stella Artois to mind for me), but if your hefeweizen or porter tastes like it was aged in an AMC popcorn bucket, you've got a problem. The problem may lie either in a brewing snafu or, if you're drinking at the bar, insufficient sanitation of the draft line. If you're in control of such things, the Brewer's Association recommends giving draft lines a solid cleanse at least every two weeks.
Skunked beer, as the term implies, tastes like the business end of Pepe Le Pew. Turns out that light hates your hops, breaking down alpha acids and setting off the chemical reaction that will make you reach for a tomato juice chaser. This is only a problem for beers that are a) bottled in green or clear glass or b) left in direct sunlight for a lengthy stretch, so be mindful of keeping your IPAs in the shade.
Oxidation occurs when your beer encounters too much oxygen either while brewing or during the aging process. This means your beer will assume the subtlety of wet cardboard or soggy paper, though in the right hands it can also introduce sherry impressions to the flavor profile. There isn't much to remedy oxidation once it has occurred, so let the imbiber beware.
For further analysis of where your beer could go wrong, consider checking out the Beer Judge Certification Program's handy flashcards or one of Beersmarts' off-flavor classes. Either way, you'll be better prepared to guard against a drain pour.
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