Off Flavors

Accidents happen in beer-and it gets worse than a spilled mug. Slight, often unintentional changes in fermentation, equipment sterilization and ingredients can lend beer some undesirable tastes-referred to as "off flavors" in the brewing world. Below, we've highlighted common off flavors, but keep in mind that many taste nuances (even pleasant ones) can become off flavors when they inappropriately occur in beer styles they shouldn't.

3-methyl-2-butene-thiol Also known as Mercaptan. A compound causing a sulfuric, skunky aroma, typically due to a reaction between hops’ alpha acids and sulfur compounds in the presence of sunlight or electric light. Clear, green and blue bottles are most at risk for developing 3-methyl-2-butene-thiol.  
  
Acetaldehyde Rarely an intended component of a beer’s profile, acetaldehyde is an off flavor characterized by a green apple, pumpkin, paint or cut grass taste. Its most common cause is early removal of the beer from the yeast, before it has completed fermentation. Acetaldehyde is perhaps most famous as the component that causes hangover.
  
Acetic acid A bacterial byproduct, sour-tasting acetic acid is common in sour beer styles such as Flanders red ales, which are open-fermented to welcome such funk-inducing compounds. In most styles, though, acetic acid is an off flavor; it’s detectable by a green apple, vinegar or paint aroma and/or flavor. Its most common cause is a contamination in the brewing process or your pub’s tap system.
  
apple Tastes or scents or apples. Appropriate in ciders, but an off flavor in many other beer styles. See “Acetaldehyde” and “acetic acid.”
  
artificial Unnatural in flavor, smell or character, such as a simulated berry taste in fruit beers.
  
barnlike, barnyard Earthy, haylike, musty nuances; common in spontaneously fermented beers such as saisons and lambics. An off flavor in other styles. Also called farmlike and horselike.
  
buttery A buttery flavor often signifies diacetyl—an off flavor—but very low levels (akin to a chardonnay’s butteriness) is acceptable.
  
catty Musty, urinelike aroma; see “oxidized.”
  
cheesy A cheeselike off flavor caused by old hops; see “Trans-2-nonenal.”
  
Chlorophenolic   A plasticlike, chemical-smelling aroma.
  
corked Having the slight taste of cork; denotes a beer that has been aged improperly or was sealed with a faulty cork.
  
DiacetylUsually an off flavor, but a desired characteristic of some English ales. Identified by a butter or butterscotch flavor and a milkiness on the palate (sometimes, soapy or oily flavors are present, too). Caused by cold, interrupted or inhibited fermentation.
  
Dimethyl Sulfide (DMS)An off flavor. With a taste evocative of creamed corn, cabbage, tomatoes and other vegetables, DMS is derived from malt, can be detected in wort, and is reduced by a vigorous boil. Acceptable in moderation in some styles, such as Munich helles, American lagers and pilsners.
  
dirty Contains off flavors or undesirable traits.
  
farmlike Earthy, haylike, musty nuances; common in spontaneously fermented beers such as saisons and lambics. An off flavor in other styles. Also called "barnyard" or "barnlike."
  
Isovaleric acidAn off flavor introduced by old hops. The compound, which arises as hops age and lose their bitter alpha acids, produces a flavor typified by flavors of cheese, sweat or must.
  
lightstruck An off flavor characterized by a sulfuric, skunky aroma, typically due to a reaction between hops’ alpha acids and sulfur compounds in the presence of sunlight or electric light, indicating the presence of 3-methyl-2-butene-thiol. Clear, green and blue bottles are most at risk for lightstruck flavors. Also called "skunky."  
  
medicinal Akin to medicine or chemicals in aroma or taste.
  
MercaptanThe common name for 3-methyl-2-butene-thiol. A compound causing a sulfuric, putrid, skunky aroma, typically due to a reaction between hops’ alpha acids and sulfur compounds in the presence of sunlight or electric light. Clear, green and blue bottles are most at risk for developing Mercaptan.
  
metallicAromas or tastes similar to iron, rust, coins or blood. Its presence has a number of causes: rusty pipes or water, high malt roasting temperatures or staleness. Usually an off flavor.
  
milky Creamy and similar to milk in taste or texture; acceptable in some styles, but also an off-flavor. See “diacetyl.”
  
moldy Mildewed, damp or decaying aromas.
  
musty Mildewed, moldy or damp in aroma. An off-flavor.
  
oily Slick in mouthfeel; usually an off-flavor.
  
Ortho-Chlorophenol An off flavor and a phenolic compound. With a taste akin to Band-aids or bearing a medicinal quality, Ortho-Chlorophenol may appear from bacteria, but also from chlorinated water or sanitizers.
  
oxidationProlonged exposure to oxygen, which gives beer a cardboardlike flavor and/or aroma. Almost always an off flavor.
  
oxidized Synonymous with "stale." An off flavor that transpires when beer is exposed to oxygen or high temperatures, or is otherwise past its prime. Stale or oxidized flavors come in a variety of permutations—papery or cardboardlike, akin to rotten produce, diaperlike, leathery, sherrylike and bready are common forms.
  
papery See “stale” and “Trans-2-nonenal.”
  
phenolic Plastic or chemical in taste or scent; acceptable in low levels in some beer styles, but usually an off flavor.
  
rancid Offensive or rotten in odor or taste; an off flavor.
  
refined Made pure or clear by a filtering process; edited or polished in flavor; colloquially, sophisticated or superior.
  
sherrylike Akin to sherry in taste or smell; common in old ales, but a possible indicator of staleness in other styles.
  
skunky An off flavor characterized by a sulfuric, skunky aroma, typically due to a reaction between hops’ alpha acids and sulfur compounds in the presence of sunlight or electric light, indicating the presence of 3-methyl-2-butene-thiol. Clear, green and blue bottles are most at risk for lightstruck flavors. Also called "lightstruck."  
  
soapy Having a scent or taste by soap. An off flavor or defect.
  
solventlike An off flavor identified by a sharp, acrid scent and an unpleasant burning sensation in the swallow. Usually caused by high fermentation temperatures, and sometimes, wild yeast.
  
stale Synonymous with "oxidized." An off flavor that transpires when beer is exposed to oxygen or high temperatures, or is otherwise past its prime. Stale or oxidized flavors come in a variety of permutations—papery or cardboardlike, akin to rotten produce, diaperlike, leathery, sherrylike and bready are common forms.
  
sulfidic Bearing the putrid, rotten-egg aroma of sulfide. An off flavor.
  
sulfitic Having a burnt-rubber or -match aroma of sulfites, once used as preservatives in beer and still naturally occurring in wine. An off flavor.
  
sulfur An element when in its pure form has a matchlike aroma—an off flavor when too apparent, but desirable in some styles and common in beers from England’s Burton-on-Trent region.
  
Trans-2-nonenal An off flavor typified by tastes of paper or wet cardboard, usually detected in old or stale beer. Developed during aging, the compound can be thwarted by minimizing air in aging containers and bottles.
  



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