by Scott Bickham
This section is intended to give an overview of the more important flavors and flaws that may be encountered while judging. Some of these flavors may be desirable in some styles, but not in others, and the appropriateness will depend on the concentration. For this reason, not all of these characteristics are considered to be off-flavors. This one-page summary of common beer faults is another resource on the BJCP web site, but this section of the BJCP Study Guide has more complete descriptions that would be considered Master-level answers for the Beer Characteristic (T1) and Body/Mouthfeel (T3) questions on the BJCP Beer Written Proficiency Exam. These descriptions were updated and expanded in 2017, with substantial edits that reflect advancements in our understanding of the origin of beer flavor characteristics and how they are perceived. While some of the descriptions do mention the chemical compound(s) responsible for the flavor, the emphasis is not so much on the chemistry as on how they are perceived and the beer styles in which they are appropriate or inappropriate.
Acetaldehyde
This compound has the taste and aroma of fresh-cut green apples, and sometimes has notes of grass, green leaves or latex paint. It is normally reduced to ethanol by yeast during the secondary fermentation, but oxidation of the finished beer may reverse this process, converting ethanol to acetaldehyde. Elevated levels are generally present in green beer or if the beer is prematurely removed from the yeast. It can also be a product of bacterial spoilage by Zymomonas or Acetobacter. This flavor can also be the result of inadequate wort oxygenation, and while the resultant yeast by-products are normally metabolic intermediates, they can remain after fermentation in some cases. While acetaldehyde is normally considered to be a flaw, a low level is acceptable in the aroma and flavor of Kellerbiers, which typically have more yeast character than German beers that have been lagered.
Alcoholic
This flavor may be detected as a spicy, vinous character in the aroma and taste and is often accompanied by a warm or prickly mouthfeel. The simplest and most prevalent alcohol in beer is ethanol, which is produced by the fermentation of glucose and other simple sugars. Higher, or fusel, alcohols are usually present at sub-threshold concentrations, but elevated levels are associated with underpitching yeast, low levels of dissolved oxygen prior to pitching or low levels of free available nitrogen (FAN). These deficiencies force the yeast to metabolize fatty acids in the trub as a source of oxygen and carbon, producing a greater fraction of long chain alcohols. High gravity worts and high fermentation temperatures also tend to increase the concentration of these higher alcohols through increased yeast activity. Alcoholic characteristics are desired in strong ales and lagers such as Eisbock, Imperial Stout and English and American Barleywines, as long as they are not coupled with any appreciable solvent notes associated with elevated ester or fusel alcohol levels.
Astringency
This flavor is a mouth-puckering sensation that is comparable to chewing on grape skins or grape seeds, or by steeping black tea leaves for an extended period of time. It can be produced by the extraction of tannins from grain husks due to overcrushing, oversparging, or sparging with water having a pH over 6.0 and/or a temperature higher than 170 °F (77 °C). Water with high sulfate or magnesium content can enhance the level of astringency. Astringency may also be produced by polyphenols that result from spoilage by acetobacter or wild yeast. Another possible source is oxidation, in which case the responsible compounds are polyphenols and aldehydes. Finally, spices such as coriander, orange peel and cinnamon can also contribute astringent flavors, but these tend to mellow with age. Over-attenuation and low dextrin levels can increase the perception of astringency.
While astringency is undesirable in most beer styles, a low level is acceptable in some beer styles. Some heavily-hopped ales such as American IPA are allowed to have light astringency as long as the finish is not harsh. Low astringency is also permissible in some dark ales such as American Porter due to the use of roasted malts. Flanders Red Ale may have a low to medium astringency, which is one distinguishing characteristic from Oud Bruin, in which astringency is inappropriate.
Bitterness
The main sources of bitterness in beer are iso-alpha acids, which are produced when alpha acids in hops are isomerized during the boil. When properly executed, the bitterness in hoppy beers will linger into the aftertaste but should not be harsh. Excessive bitterness, on the other hand, is perceived as a harsh, dry taste, mostly on the back of the tongue. It can be due to over-hopping, especially when high alpha hops are used. It can also be a consequence of using aged hops, since the oxidation of beta acids produces unpleasantly bitter compounds that are water soluble and can end up at high levels in the finished beer. Bitter compounds may also be produced by oxidation or contamination by wild yeast, in which case there are usually other off-flavors such as astringency.
High levels of bitterness are appropriate in many well-hopped beers styles, including IPAs and American Barleywine. In some dark beer styles such as American Porter and Imperial Stout, the bitterness from hops can be accented by the roasted notes from the dark malts, but the finish should not be overly acrid, burnt or harsh. High sulfate water is sometimes used when brewing British Bitters, English IPA and German Pils, and in these styles, the mineral character can enhance the dryness of the finish and enhance the bitterness from the hops.
Body
The body of a beer is characterized as its fullness, viscosity, or thickness on the tongue and palate. Descriptors range from watery or characterless to satiating or thick. Body is a component of mouthfeel, which encompasses physical sensations such as astringency, alcoholic warmth and carbonation; the combination of all those components determines how the beer stimulates the palate. The body is primarily determined by the levels of dextrins and medium-length proteins. Lack of dextrins is caused by low saccharification temperatures, excessive use of completely fermentable adjuncts or by using highly attenuative yeast strains. A low protein level may be caused by excessively short or long protein rests, or by executing the step at temperatures that are too high or too low. A high alcohol content can also lighten the apparent body of the beer since the density of ethanol is less than that of water. Similarly, high carbonation levels will decrease the sensation of the fullness of the beer on the palate due to the effect of the dissolved gas on the density of the beer. Common remedies for light body include adding dextrin malt or unmalted flaked grains such as barley, oats or wheat to the grist. Light body is appropriate in American Light Lagers and Lambics, but not in strongly malt-accented styles such as Barleywines, Scotch Ales, and Doppelbocks.
Cheesy (Isovaleric acid)
Isovaleric acid is perceived primarily in the aroma with notes compared to aged hard cheese or described as goaty, sweaty or putrid. Individual sensory thresholds for this off-flavor vary by several orders of magnitude, so levels that some judges will regard as offensive may not be detected by others. Isovaleric acid usually originates from the use of old hops or hops that have been improperly dried or stored at high temperatures in the presence of oxygen. In this oxidative process, an isovaleryl group is removed from the humulene alpha acid and becomes flavor-active.
Isovaleric acid is also produced by Brettanomyces, which at low levels can contribute to the complexity of English Old Ales. Higher levels may be encountered in European Sour Ales (with the exception of Oud Bruin) and American Wild Ales. Otherwise, it is considered to be a flaw and should be addressed by using fresh hops that have been stored correctly – ideally in the freezer in an oxygen-free, vacuum-sealed container, or by proper sanitation if produced by Brettanomyces.
Diacetyl
This compound is responsible for an artificial butter, butterscotch or toffee-like aroma and taste. It may also produce a slickness on the palate. A significant number of tasters cannot perceive diacetyl at any concentration, so judges should be aware of their limitations. Low levels of caramel are sometimes mistaken for diacetyl, but off-flavor training should help judges recognize the difference. Diacetyl is a fermentation by-product which is normally absorbed by the yeast and reduced to more innocuous compounds called diols. High levels can result from prematurely separating the beer from the yeast or by exposing the wort to oxygen during the fermentation. Low FAN levels or mutation may also inhibit the ability of yeast to reduce diacetyl. Diacetyl is also produced during the initial stages of contamination by lactic acid bacteria, notably Pediococcus damnosus.
Note that high fermentation temperatures promote both the formation and the elimination of diacetyl, but the latter is more effective. For that reason, lager breweries often employ a diacetyl rest, which involves holding the beer in the 60-65 °F (15-18 °C) range for a few days after racking to the conditioning tank. Though rarely used by homebrewers, kräusening is a technique that can be used to eliminate diacetyl in beer. The technique introduces fresh yeast that is actively multiplying and is thus able to rapidly remove diacetyl. A low level of diacetyl is permissible in some styles, but it should never be obtrusive. These styles include Czech Lagers, English Bitters, Brown and Dark British Beers and Irish Red Ale. Diacetyl is not appropriate in German lagers (Kellerbier being the exception), Irish Stout, American ales or Belgian ales.
Dimethyl Sulfide (DMS) or Vegetal
DMS, or dimethyl-sulfide, is one member of a family of chemical compounds that produce the aroma and taste of cooked vegetables, notably corn, celery, cabbage, onion or parsnips. In extreme cases, it may even be reminiscent of shellfish or water in which shrimp has been boiled. A related compound is dimethyl-trisulfide (DMTS), which is perceived as having a flavor similar to cooked onions or onion soup. This flavor profile is different from the freshly-cut onion character of some members of the mercaptan family (see the Onion-like/Catty descriptor below).
DMS is normally produced by the heat-induced conversion of S-methyl-methionine (SMM), but most of it evaporates during a long, open, rolling boil. A short, weak or closed boil, or slow cooling of the wort may lead to an abnormally high level of DMS in the finished beer. Some DMS is also scrubbed out during a vigorous fermentation, which is why lagers and cold-conditioned ales may have a slightly higher level than warm-fermented ales. Wild yeast or Zymomonas bacteria may produce a high enough level of DMS to make the beer undrinkable.
Pilsner malt contains as much as eight times the SMM of pale malt or other malts kilned at moderate to high temperatures, so beers based primarily on 2-row or 6-row Pils malt sometimes have a light DMS character. This is a much more common cause of cooked-corn character in beer than a covered boil. A low level of DMS is acceptable in many pale lagers, including American Lager, Cream Ale, International Lager, Munich Helles and German Pils. DMS is not appropriate in ales, Czech lagers, amber lagers or dark lagers.
Estery or Fruity
This is an aroma and taste that recalls bananas, strawberries, pears, apples, plums, papaya and/or other fruits. The responsible compounds are esters, which are formed from the combination of an alcohol and an organic acid. High ester levels are a product of the yeast strain, high fermentation temperature, high gravity worts, the metabolism of fatty acids in the trub, low yeast pitching rate, and/or low wort aeration. These flavors are desirable in most ales, particularly Belgian and British styles. The most prevalent ester in beer is ethyl acetate; however it has a high odor threshold and is usually only detected as a nail-polish remover (acetone) aroma at high concentrations. At low levels, this ester has an aroma reminiscent of pears. The three most commonly perceived esters in beer are: isoamyl-acetate, which provides the banana notes in German Weissbier; ethyl butyrate, which has notes of tropical fruits and bubble gum and is part of the flavor profile of many Belgian ales; and ethyl hexanoate, which has the aroma of red apples.
Fruity character can also be derived from hops, which contain hydrocarbons that are responsible for the woody, herbal, piney and resiny character of fresh hops. While these compounds are found at very low levels in kettle-hopped beers, they are unstable when heated and form aromatic compounds such as geraniol and linalool, which provide floral notes, and limonene, which has citrus and lemongrass notes. While these compounds are technically alcohols, the aromatics they contribute to hoppy beers are located in the same section of the Beer Flavor Wheel as esters and are often grouped with fruity esters when judging.
Grassy
This is the flavor and aroma of freshly cut grass or green leaves. Responsible compounds include hexanal and heptanal, which are aldehydes produced by the oxidation of alcohols in the finished beer or from the deterioration of improperly stored malt or hops. This is particularly true when these ingredients are stored in a moist environment, which is why “wet-hopped” beers made with fresh hops that have not been fully dried can have grassy notes.
Some English and American hop varieties such as Fuggles and Mosaic can produce grassy notes, especially when used as late-kettle or dry hops. Most hop varieties will lend a grassy flavor if they are left in contact with beer for too long, so limiting dry-hopping to ten days or less and using fresh hops will minimize the risk of developing this characteristic. Low levels of grassy flavors and aromas are acceptable in dry-hopped examples of English IPA, American Pale Ale and IPAs, but excessive amounts are considered to be a flaw.
Head Retention
Good head retention is measured visually in terms of the time required for the head to collapse to half of its initial height. This should be at least one minute in well-brewed and conditioned beers. The beading of the foam should also be uniform and tight, leaving lace on the glass as the beer is consumed. Good head retention is promoted by several factors, including isohumulones, high original gravity, moderate alcohol content, dextrins and the levels of high- and medium- molecular weight proteins. Adequate carbonation is also essential. Most of these variables are style-dependent, but in general, the brewer can increase the protein content by adjusting the length and temperature of the protein rest and using adjuncts such as flaked wheat and barley. Fatty acids carried over from the trub and unclean glassware are both detrimental to head stability, since they decrease the surface tension of the foam – causing the bubbles to collapse prematurely. Beer styles that should have good head retention include German Pils, Witbier, and Belgian Blond Ale. Styles which may have low or impaired head retention include Eisbock, Imperial Stout and English and American Barleywines.
Husky or Grainy
This is perceived in both the aroma and the flavor, with notes of raw or germinating grain and is often accompanied by a dry astringency. The responsible compounds are aldehydes in the malt, and elevated husky/grainy notes can result in beer if the malt is too finely crushed, the grain husks are shredded, the mashing time is excessive, the temperature or pH of the sparge water is too high, or the grains are oversparged. Pilsner malt has a high level of these aldehydes, and this can result in a light grainy character that is acceptable in most pale lagers, including International Pale Lager, German Pils and German Helles Exportbier. Roasted malts can impart a grainy character that is acceptable in some dark beer styles such as Dark Mild, Scottish Ales, Oatmeal Stout and American Porter as long as it is not harsh. Altbier and Irish Red Ales can also exhibit some graininess in the flavor and aroma.
Lightstruck
This off-flavor smells like a skunk or freshly-roasted coffee beans. The compound responsible for the lightstruck character is one of the mercaptans found in the scent glands of skunks. These compounds are formed in beer when ultraviolet light cleaves an isohumulone molecule, and the resulting radical combines with a sulfur compound. Beer stored in clear or green glass bottles is more susceptible to this reaction. Lightstruck flavor is not desirable in any style but is fortunately rare in bottle-conditioned homebrew because the yeast helps inhibit the reaction and because of the predominant use of brown bottles. However many filtered European imports possess this quality due to poor handling or storage conditions.
Metallic
This character is perceived in the aroma and flavor is usually caused when the metals such as iron, copper and nickel are leached into the brewing water, the mash or the wort. They can sometimes arise from oxidation of lipids in improperly stored malt or during beer ageing. Very small amounts of these minerals are essential for yeast health, but too much produces flavors described as blood-like, inky or like coins. Brewers who obtain their water from a well should ensure that the water is treated to remove any dissolved metals, and porcelain-enameled steel brewing kettles should be discarded if the porcelain becomes chipped or cracked. Stainless steel brewing equipment generally does not contribute any metallic character as long as the surfaces that come into contact with the wort are allowed to passivate (oxidize) prior to use.
Musty
Musty aromas and flavors are also described as being earthy, moldy or like damp cellars, old books or decomposing wood. This off-flavor is generally a result of the growth of mold or mildew on malts, malt extracts or brewing equipment and is not appropriate in any style. The remedies include using fresh ingredients, storing open bags of malt in a cool, dry environment and sanitizing brewing equipment prior to use. Mildew can also grow on the interior of refrigerators used for fermenting lagers, and this character can leech into the beer and produce musty flavors. This situation can be prevented by periodically cleaning the interior surfaces of the refrigerator with a sanitizer solution. Musty flavors can also result from oxidation, in which case the beer may have corked or “cellared” notes. These were once regarded as acceptable and even desirable in the Bière de Garde style, but the current consensus is that this characteristic is more of a feature of mishandled commercial exports rather than how a fresh, authentic example should taste.
Onion-like or Catty
These flavors and aromas are produced by mercaptans, which are sulfidic compounds that can have a wide range of characteristics, including grapefruit, floral, tropical fruit, fresh-cut onions or garlic, catty, burnt rubber or raw sewage. They are usually formed by yeast through the metabolism of sulfur-containing amino acids, but they are also found in hop oils – particularly in some of the New World hop varieties. The mercaptans derived from hops have very low perception thresholds and can significantly influence the aroma of beer, either positively or negatively. At low concentrations, some of the mercaptans found in Citra and Mosaic hops have pleasant aromas similar to passion fruit, grapefruit, tropical fruit and sauvignon blanc grapes. At higher concentrations, they develop aromas that some tasters negatively compare to blackcurrent leaves, cat urine or underarm sweat, but others positively label as being “dank.” The oils of Summit and Simcoe hops both contain a mercaptan which has an aroma like freshly-cut onions, and some dry-hopped examples of American IPAs and Double IPAs exhibit this characteristic. Higher mercaptans with aromas similar to raw sewage or burnt rubber are produced by yeast autolysis or bacterial contamination and are not appropriate in any style.
Paper or Cardboard
These characteristics are perceived in both the aroma and flavor and are primarily due to oxidation, which produces the aldehyde (E)-2-nonenal (formerly known as trans-2-nonenal). Depending on the beer style and extent of the oxidation, the flavors can be perceived as being like cut cucumbers, recycled paper, lipstick or wet cardboard. Pale-colored and low-gravity beers are more susceptible to developing these characteristics when oxidized, while darker beers with more malt complexity tend to develop the sherry-like flavors described below. (E)-2-nonenal has an extremely low flavor threshold of about 0.1 ppb and is inappropriate in any style. The threat of oxidation occurring in packaged homebrew can be minimized by avoiding splashing the beer while racking and bottling, ensuring that an adequate fill level with less than 2” (5 cm) of headspace, using oxygen scavenging bottle caps, and storing the beer at cool temperatures.
Phenolic (Clove-like or Spicy)
These flavors and aromas are due to phenols, notably 4-vinyl guaiacol, which produces the signature clove-like character in German Weissbier and some Belgian ale styles. These phenols are produced by special strains of S. cerevisiae, from the amino acid precursor, ferulic acid, and the level may be enhanced by incorporating a ferulic acid rest at around 111 °F, 44 °C (17). Some Belgian ale styles such as Saison sometimes exhibit peppery flavors and aromas which are also due to yeast-derived phenols. The levels of these phenols in the beer also depend on the fermentation temperature, with high temperatures favoring higher levels.
Phenolic (Medicinal or Wild Yeast)
As noted in Section IV, some wild yeast produce unpleasant phenols flavors and aromas described as medicinal, plastic-like, smoky or reminiscent of adhesive strips or electrical tape. Hence their control is usually a matter of proper sanitation. Chlorophenols are particularly offensive members of this family with additional bleach-like flavors that are derived from chlorinated water or sanitizer residue. Phenols may also be extracted from grain husks by overcrushing, oversparging or sparging with hot or alkaline water, and these compounds typically also produce an astringent mouthfeel. The leathery, horse blanket and barnyard characteristics in Brettanomyces-driven styles such as Lambic and American Wild Ale are also due to phenols, but these flavors are indicative of wild yeast contamination when they inadvertently appear in other styles.
Sherry-like
This is the aroma and taste of dry sherry and is often accompanied by hazelnut, almond or dried fruit notes. The flavor is produced by the oxidation of melanoidins, which are compounds that provide a rich malt character in fresh samples of some beer styles but transform into sweet sherry-like flavors when oxidized. One of the most common products of this oxidation is benzaldehyde, which has the flavor and aroma of bitter almonds, marzipan, dried cherries and cherry stones. A low to moderate sherry-like character is acceptable and adds complexity to Old Ales and English Barleywines. Well-aged examples of Weizenbock and Oud Bruin may also low sherry-like notes. High levels of these oxidative compounds give winey and slightly sour notes reminiscent of prune juice and indicate that the beer is past its prime. Sherry-like character is not appropriate at any level in lighter, low-gravity beer styles.
Solvent-like
This describes a pungent aroma and flavor similar to acetone, paint thinner or turpentine and is often accompanied by a hot, burning sensation in the mouthfeel. It is due to high concentrations of ethyl acetate and other esters, and possible fusel alcohols such as propanol and butanol. Possible sources include underpitching yeast, insufficient aeration of the chilled wort prior to fermentation, and fermenting on the trub – especially at elevated temperatures. Some yeast strains are more prone to producing these flavors than others, but this can usually be addressed through temperature control. Very low levels of solvent-like notes can add complexity and a pleasant “boozy” character to strong ale styles such as Old Ale, Barleywines and American Strong Ale but should never be excessively hot or unpleasant. Otherwise they are inappropriate and should be controlled by better management of the aeration and fermentation.
Sour or Acidic
This is one of the basic tastes, and the two most common acids responsible for this flavor in beer are lactic and acetic acid, which both have related esters that may be perceived in the aroma. Lactic acid is produced by Gram-positive bacteria such as Lactobacillus and Pediococcus, which are present in dust and saliva. It has the same sour character found in yogurt and sourdough bread. Acetic acid can be produced by several contaminants, including Acetobacter, Zymomonas, and yeast in the Kloeckera and Brettanomyces families. It’s the sour component of vinegar, which must contain at least 4% acetic acid, and should be a familiar flavor to judges due to its use in many marinades, condiments and pickled food products.
High levels of sour and acidic flavors generally indicate a sanitation problem, but they are an important part of the profile of European Sour Ales (Berliner Weiss, Flanders Red Ale, Oud Bruin and Lambic) and Gose. Light (lactic) sourness is acceptable, but not required, in Belgian Witbier and Saison. Note that some examples of Weissbier have light citrus notes derived from the wheat malt which can be perceived as being slightly acidic, but any appreciable sourness in this style is inappropriate. Some dark beers such as Imperial Stout and Baltic Porter can have a slightly acrid (pungent or sharp) character from roasted malts, but this is different from the sour flavors due to lactic or acetic acid, which are undesirable in these styles.
Sulfury
The most common sulfury characteristic noted in beer is due to sulfur-dioxide, which is natural by-product of fermentation that is usually scrubbed out by a vigorous fermentation or dissipated during lagering. At high levels, it recalls the aroma of a struck match, but at lower levels provides a fleeting sulfury background note that is appropriate in German light lagers and Kölsch. This sulfury character is also appropriate and can be more persistent in Kellerbier and Australian Sparkling Ale. Some British styles (including Bitters and English IPA) sometimes have a mineral or sulfury character, but in this case, is derived from sulfates in the water rather than yeast. Sulfury flavors are more common in wines and ciders due to the common use of potassium or sodium meta-bisulfite as a preservative and antioxidant; however they are rare in US commercial beers because sulfite additives are legally restricted (<10 ppm without labeling).
Another sulfury compound sometimes encountered in beer is hydrogen-sulfide, which at low to moderate levels, has the aroma of rotten eggs. It is produced early in fermentation – particularly by lager yeast strains – but as with sulfur-dioxide, is generally scrubbed out by carbon-dioxide. At very low levels, it gives a desirable “fresh” character to many German lagers. At higher levels, the rotten egg character is more noticeable and unpleasant and should be addressed by changing yeast strains, ensuring that there is an adequate supply of FAN during primary fermentation and/or lagering for a longer time period. Bacterial contamination by Zymomonas can produce very high levels of hydrogen-sulfide which recall wastewater and raw sewage (this compound is the main component of sewer gas).
Sweet
Sweet flavors are due to the presence of unfermented residual sugars in the beer. High levels of residual sugars can result from flocculent or low-attenuating yeast strains (as in Scottish Ales). Abnormally high levels can be a consequence of poor yeast health linked to low FAN levels or low levels of dissolved oxygen prior to pitching. Both of these factors can lead to a “stuck” fermentation, which leaves the beer with a high final gravity and sweet worty flavor. High gravity worts, high dextrin content and the addition of lactose (Sweet Stouts) also play a role in determining the sweetness of the finished beer. Other components of the beer that can increase the perceived sweetness include ethanol (up to around 5% ABV) and the level of chloride in the water.
The level of sweetness can be controlled by adjusting the original gravity, the mash temperature, the amounts of caramel malts, the amounts of unfermentable or completely fermentable adjuncts, the length of the boil and the attenuation of the yeast. The appropriate level of residual sweetness will be style-specific and depends on the malt-hop balance and desired level of dryness in the finish. A high residual sweetness is desirable in strong beers such as Wee Heavy, English Barleywine and Eisbock, but inappropriate in dry styles such as American Lager, Berliner Weiss, Gueuze and Saison.
Yeasty
Yeasty character is not the same as fermentation character, which includes the ester, phenol and alcohol flavors described elsewhere in this section. Rather, yeasty character describes either the bread dough notes of fresh yeast or the meaty/brothy notes of autolyzed yeast. In the former case, there may be enough yeast in suspension to produce a light sheen or haze in the appearance, and high levels of suspended yeast can also contribute to bitterness (yeast “bite”) and mouthfeel (a powdery sensation). The fresh yeast character is appropriate in unfiltered German lagers, Australian Sparkling Ale, Gose and some Belgian ales. Yeasty character is inappropriate in most other beer styles, particularly in lagers (unless they are designated as unfiltered) and British Bitters, in which finings are used to clarify cask-conditioned versions. Yeasty character can be controlled by using more flocculent yeast strains and by cold-conditioning the beer to facilitate sedimentation of the yeast after fermentation is complete.
Autolysis is the process whereby yeast cells self-destruct in the absence of other nutrients. It occurs in beers which were left on the yeast for too long after the fermentation has finished, or sometimes in bottle-conditioned beers that have been aged in a warm environment. Autolysis results in aromas and flavors of Vegemite, beef broth, or in extreme cases, like burnt rubber (mercaptan). These flavors are generally not desirable in beer, although low levels are often considered positive in vintage Champagnes, where their toasty notes are a key component of the “sur lie” (on sediment) character.