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13 beer off-flavors to know and avoid

Things that make you go “ew.”
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beer off flavors

As beloved as beer may be, the beverage also has many enemies. Outside forces like oxygen, sunlight and time do their best to ruin beer, but even compounds found in and on malt, hops, water and yeast—the very ingredients that make beer delicious—can act as spoilers. With all the things that can go awry when making a beer, it’s a small miracle that most of them make it to the shelf free of flaws. Every now and then, however, you do run into a stinker, and it’s important to know exactly what caused the offending flavor. Here’s a baker’s dozen of those we encounter most often.

Acetaldehyde
Tastes like: green apples, fresh cut grass, cucumbers
Caused by: yeast. Acetaldehyde is naturally produced in the early stages of fermentation, but is usually converted into ethanol (AKA sweet, sweet booze) later on. Too much of the green apple flavor in a beer usually means the brewer used unhealthy or inactive yeast, fermented at too-low temperatures, or packaged the beer before the yeast was finished fermenting. (Fun fact: acetaldehyde is also one of the compounds produced by our bodies when we digest alcohol. Certain groups of people have a hard time breaking the compound down further, so it accumulates—this is why some people will become flushed in the face after drinking.)

Astringency
Tastes like: Not a taste so much as a sensation of dryness on the tongue
Caused by: polyphenols in malt, hops or spices. Most commonly it’s the result of poorly managed sparging—the brewing stage during which a brewer rinses malt with water to extract any residual sugar. Sparge too long or at too high a temperature and polyphenols from the grain husks will end up in the finished beer, making it astringent. An overzealous addition of spices—such as those commonly used in pumpkin ales and winter warmers—can also contribute some astringency.

Autolysed Yeast
Tastes like: meat, sulfur, vegemite, barbecue potato chips
Caused by: dead yeast. Yeast are hardy little critters, but they’re not immortal; they do eventually die, and when they do, they basically burst open (the word autolysis literally means “self-destruction”) and release their innards into the beer. This has a number of effects: It reduces the head on a beer, accelerates the creation of haze, and can even restart fermentation, resulting in overcarbonation. But the largest effect is in the flavor and aroma: The meaty bouquet of autolyzed yeast is so intense that it’s often used to add flavor to soups and barbecue potato chips. Yeast autolysis usually only occurs in very old bottles or cans, so make sure the beer you’re buying is fresh.

Bromophenol
Tastes like: ink, an old TV, an electric fire
Caused by: contamination of brewing ingredients via packaging materials. Malt or hops packaged in recycled paper or cardboard or inside material treated with fire retardant will sometimes impart this off-flavor to a finished beer.

Butyric Acid
Tastes like: parmesan cheese, rancid butter, vomit
Caused by: bacterial infection, usually by a bug called Clostridium. The offending microorganism is sometimes found in glucose and cane sugar syrups used in brewing, but can also contaminate a beer during the long, warm stand of a sour mash, which is why butyric acid is commonly encountered in poorly made Berliner weisses.

Chlorophenol
Tastes like: duct tape, antiseptic, Band Aids, plastic
Caused by: chlorinated water or chlorine-based santizers. Brewers and homebrewers who use untreated tap water commonly run into this off-flavor, which is formed through reactions between alcohol and chlorine.

Diacetyl
Tastes like: butter, butterscotch, buttermilk
Caused by: yeast. Dactyl is a natural byproduct of fermentation, usually created by yeast in the early stages but later reabsorbed. It can also be a sign of bacterial contamination in draft beer lines. The flavor of diacetyl is so buttery it’s also used to flavor popcorn, and though unpleasant in most beers, it is appropriate in some English-style ales.

Dimethyl Sulfide (DMS)
Tastes like: cooked corn, overcooked broccoli, dirty vegetable oil
Caused by: a compound in malted barley that’s transformed by heat. DMS usually develops during the boiling stage of the brewing process; it’s formed when temperatures reach 140 degrees but is driven off with a vigorous boil. This is part of why brewers strive to cool wort as quickly as possible after boiling: The longer the wort stays warm, the more chance there is for DMS to develop.

Indole
Tastes like: dirty sponges, halitosis, diapers
Caused by: coliform bacteria. These bugs, a family to which the dreaded E. coli belongs, are usually an indication of unsanitary food or water, but they can also thrive on improperly cleaned brewing surfaces and equipment. Brewers who make beer or store ingredients near farm pens or litter boxes have to be especially careful to avoid it.

Isovaleric Acid
Tastes like: American cheese, sweaty socks
Caused by: old or improperly stored hops. Isovaleric acid is a fatty acid found naturally in many plants, cheeses and, yes, foot sweat; it becomes a problem in hops that have been stored warm or for too long. Brettanomyces can also sometimes produce this compound.

Metallic
Tastes like: iron, blood, pennies, 9-volt batteries
Caused by: metal ions in brewing water. Municipal water left untreated by the brewer may contain some metallic elements, but non-passivated brewing and serving equipment such as kegs, keg couplers or draft faucets may also leach ions into the beer.

Papery/Oxidized
Tastes like: wet paper, cardboard
Caused by: Oxygen. Exposure of beer to air causes the creation of a compound called trans-2-nonenal, which has a distinct papery flavor and aroma. It often occurs over time in very old packaged beers, but can also be found in fresher beers aged warm or exposed to oxygen at some point during the brewing process.

Skunky/Lightstruck
Tastes like: skunk, really bad weed
Caused by: ultraviolet light. Hops, when exposed to sunlight or some fluorescent lighting, react with other elements in beer to form an incredibly pungent compound with the telltale aroma of skunk must. If you’ve ever tasted beer packaged in a clear or green bottle, you’ve probably encountered this off-flavor. Brown bottles offer decent protection from the ultraviolet light that gets the reaction started; cans are even better. But even beer poured into a glass from an un-skunked bottle, can or keg isn’t safe—a glass exposed to sunlight can skunk in as little as 10 seconds.

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