Awine fault is a sensory-associated (organoleptic[1]) characteristic of a wine that is unpleasant, and may include elements of taste, smell, or appearance, elements that may arise from a "chemical or a microbial origin", where particular sensory experiences (e.g., an off-odor) might arise from more than one wine fault.[2] Wine faults may result from poorwinemaking practices orstorage conditions that lead towine spoilage.[citation needed]
In the case of a chemical origin, many compounds causing wine faults are already naturally present in wine, but at insufficient concentrations to be of issue, and in fact may impart positive characters to the wine; however, when the concentration of such compounds exceed asensory threshold, they replace or obscure desirableflavors andaromas that the winemaker wants the wine to express. The ultimate result is that the quality of the wine is reduced (less appealing, sometimes undrinkable), with consequent impact on its value.[3][verification needed]
There are many underlying causes of wine faults, including poorhygiene at the winery, excessive or insufficient exposure of the wine tooxygen, excessive or insufficient exposure of the wine tosulphur, overextendedmaceration of the wine either pre- or post-fermentation, faultyfining, filtering and stabilization of the wine, the use of dirtyoak barrels, over-extended barrel aging and the use of poor quality corks. Outside of the winery, other factors within the control of the retailer or end user of the wine can contribute to the perception of flaws in the wine. These include poorstorage of the wine that exposes it to excessive heat and temperature fluctuations as well as the use of dirtystemware duringwine tasting that can introduce materials or aromas to what was previously a clean and fault-free wine.[3][verification needed][4][verification needed]
In wine tasting, there is a big distinction made between what is considered aflaw and afault. Wine flaws are minor attributes that depart from what are perceived as normal wine characteristics. These include excessivesulfur dioxide,volatile acidity,Brettanomyces or "Brett aromas" anddiacetyl or buttery aromas. The amount to which these aromas or attributes become excessive is dependent on the particular tastes andrecognition threshold of the wine taster. Generally, a wine exhibiting these qualities is still considered drinkable by most people. However, some flaws such as volatile acidity andBrettanomyces can be considered a fault when they are in such an excess that they overwhelm other components of the wine. Wine faults are generally major attributes that make a wine undrinkable to most wine tasters. Examples of wine faults includeacetaldehyde (except when purposely induced in wines likeSherry andRancio),ethyl acetate andcork taint.[3]
The vast majority of wine faults are detected by the nose and the distinctive aromas that they give off. However, the presence of some wine faults can be detected by visual and taste perceptions. For example, premature oxidation can be noticed by the yellowing andbrowning of thewine's color. The sign of gas bubbles in wines that are not meant to besparkling can be a sign ofrefermentation ormalolactic fermentation happening in the bottle. Unusual breaks in the color of the wine could be a sign of excessivecopper,iron orproteins that were not removed during fining or filtering. A wine with an unusual color for its variety or wine region could be a sign of excessive or insufficient maceration as well as poor temperature controls during fermentation. Tactile clues of potential wine faults include the burning, acidic taste associated withvolatile acidity that can make a wine seem out of balance.[3][4]
| Wine fault | Characteristics |
| Acetaldehyde | Smell of roasted nuts or dried out straw. Often described as green apples and emulsion paint. Commonly associated with Sherries where these aromas are considered acceptable |
| Amyl-acetate | Smell of "fake" candy banana flavoring |
| Brettanomyces | Smell of barnyards, fecal and gamey horse aromas |
| Cork taint | Smell of a damp basement, wet cardboard or newspapers and mushrooms |
| Butyric acid | Smell of rancid butter |
| Ethyl acetate | Smell of vinegar, paint thinner and nail polish remover |
| Hydrogen sulfide | Smell of rotten eggs or garlic that has gone bad |
| Iodine | Smell of moldy grapes |
| Lactic acid bacteria | Smell of sauerkraut |
| Mercaptans | Smell of burnt garlic or onion |
| Oxidation | Smell of cooked fruit and walnuts. Also detectable visually by premature browning or yellowing of the wine |
| Sorbic acid pluslactic acid bacteria | Smell of crushedgeranium leaves |
| Sulfur dioxide | Smell of burnt matches. Can also come across as a pricking sensation in the nose. |
Theoxidation of wine is perhaps the most common of wine faults, as the presence ofoxygen and acatalyst are the only requirements for the process to occur. Oxidation can occur throughout the winemaking process, and even after the wine has been bottled.Anthocyanins,catechins, epicatechins and otherphenols present in wine are those most easily oxidised,[5] which leads to a loss of colour, flavour and aroma - sometimes referred to asflattening. In most cases compounds such assulfur dioxide orerythorbic acid are added to wine by winemakers, which protect the wine from oxidation and also bind with some of the oxidation products to reduce their organoleptic effect.[6] Apart from phenolic oxidation, theethanol present within wine can also be oxidised into other compounds responsible for flavour and aroma taints. Some wine styles can be oxidised intentionally, as in certainSherry wines andVin jaune from the Jura region of France.
Acetaldehyde is an intermediate product ofyeastfermentation; however, it is more commonly associated withethanoloxidation catalysed by the enzymeethanol dehydrogenase. Acetaldehyde production is also associated with the presence of surface film forming yeasts and bacteria, such asacetic acid bacteria, which form the compound by thedecarboxylation ofpyruvate. Thesensory threshold for acetaldehyde is 100–125mg/L. Beyond this level it imparts asherry type character to the wine which can also be described asgreen apple,sour andmetallic. Acetaldehydeintoxication is also implicated inhangovers.
Acetic acid in wine, often referred to as volatile acidity (VA) orvinegar taint, can be contributed by many wine spoilageyeasts andbacteria. This can be from either aby-product of fermentation, or due to the spoilage of finished wine. Acetic acid bacteria, such as those from the generaAcetobacter andGluconobacter produce high levels of acetic acid. Thesensory threshold for acetic acid in wine is >700 mg/L, with concentrations greater than 1.2-1.3 g/L becoming unpleasant.
There are different opinions as to what level of volatile acidity is appropriate for higher quality wine. Although too high a concentration is sure to leave an undesirable, 'vinegar' tasting wine, some wine's acetic acid levels are developed to create a more 'complex', desirable taste.[7] The renowned 1947Cheval Blanc is widely recognized to contain high levels of volatile acidity.
Ethyl acetate is formed in wine by theesterification of ethanol and acetic acid. Therefore, wines with high acetic acid levels are more likely to see ethyl acetate formation, but the compound does not contribute to the volatile acidity. It is a common microbial fault produced bywine spoilage yeasts, particularlyPichia anomala orKloeckera apiculata. High levels of ethyl acetate are also produced bylactic acid bacteria andacetic acid bacteria.
Sulfur is used as anadditive throughout the winemaking process, primarily to stop oxidation as mentioned above but also asantimicrobial agent. When managed properly in wine, its presence there is often undetected, however when used recklessly it can contribute to flavour and aroma taints which are very volatile and potent. Sulfur compounds typically have low sensory thresholds.
Sulfur dioxide is a common wine additive, used for itsantioxidant andpreservative properties. When its use is not managed well it can be overadded, with its perception in wine reminiscent ofmatchsticks,burnt rubber, ormothballs. Wines such as these are often termedsulfitic.
Hydrogen sulfide (H2S) is generally thought to be a metabolic by-product of yeast fermentation innitrogen limited environments. It is formed when yeast ferments via thesulfate reduction pathway. Fermenting wine is often supplemented withdiammonium phosphate (DAP) as a nitrogen source to prevent H2S formation. The sensory threshold for hydrogen sulfide is 8-10 μg/L, with levels above this imparting a distinctrotten egg aroma to the wine. Hydrogen sulfide can further react with wine compounds to form mercaptans anddisulfides.
Mercaptans (thiols) are produced in wine by the reaction of hydrogen sulfide with other wine components such as ethanol. They can be formed if finished wine is allowed prolonged contact with thelees. This can be prevented byracking the wine. Mercaptans have a very low sensory threshold, around 1.5μg/L,[8] with levels above causingonion,rubber, andskunk type odours. Note that dimethyl disulfide is formed from the oxidation of methyl mercaptan.
Dimethyl sulfide (DMS) is naturally present in most wines, probably from the breakdown of sulfur containing amino acids. Like ethyl acetate, levels of DMS below the sensory threshold can have a positive effect on flavour, contributing tofruityness,fullness, andcomplexity. Levels above the sensory threshold of >30 μg/L in white wines and >50 μg/L for red wines, give the wine characteristics ofcooked cabbage,canned corn,asparagus ortruffles.
Cork taint is a wine fault mostly attributed to the compound2,4,6-trichloroanisole (TCA), although other compounds such asguaiacol,geosmin,2-methylisoborneol,1-octen-3-ol,1-octen-3-one,2,3,4,6-tetrachloroanisole,pentachloroanisole, and2,4,6-tribromoanisole are also thought to be involved.[9] TCA most likely originates as ametabolite ofmould growth onchlorine-bleached wine corks and barrels. It causesearthy,mouldy, andmusty aromas in wine that easily mask the natural fruit aromas, making the wine very unappealing. Wines in this state are often described as"corked". As cork taint has gained a wide reputation as a wine fault, other faults are often mistakenly attributed to it.
Heat damaged wines are often casually referred to ascooked, which suggests how heat can affect a wine. They are also known asmaderized wine, fromMadeira wine, which is intentionally exposed to heat. The ideal storage temperature for wine is generally accepted to be 13 °C (55 °F). Wines that are stored at temperatures greatly higher than this will experience an increased aging rate. Wines exposed to extreme temperatures willthermally expand, and may even push up between the cork and bottle and leak from the top. When opening a bottle of wine, if a trace of wine is visible along the length of the cork, the cork is partially pushed out of the bottle, or wine is visible on the top of the cork while it is still in the bottle, it has most likely been heat damaged. Heat damaged wines often become oxidized, and red wines may take on a brick color.
Even if the temperatures do not reach extremes, temperature variation alone can also damage bottled wine through oxidation. All corks allow some leakage of air (hence old wines become increasingly oxidized), and temperature fluctuations will vary the pressure differential between the inside and outside of the bottle and will act to "pump" air into the bottle at a faster rate than will occur at any temperature strictly maintained.
Reputedly, heat damage is the most widespread and common problem found in wines. It often goes unnoticed because of the prevalence of the problem, consumers don't know it's possible, and most often would just chalk the problem up to poor quality, or other factors.
Lightstruck wines are those that have had excessive exposure toultraviolet light, particularly in the range 325 to 450 nm.[10] Very delicate wines, such asChampagnes, are generally worst affected, with the fault causing awet cardboard orwet wool type flavour and aroma. Red wines rarely become lightstruck because of the phenolic compounds present within the wine that protect it. Lightstrike is thought to be caused by sulfur compounds such asdimethyl sulfide. In France lightstrike is known as "goût de lumière", which translates toa taste of light. The fault explains why wines are generally bottled in coloured glass, which blocks the ultraviolet light, and why wine should be stored in dark environments.
Some insects present in the grapes at harvest inevitably end up in the press and for the most part are inoffensive. Others, notably theAsian lady beetle, release unpleasant-smelling nitrogen heterocycles as a defensive mechanism when disturbed. In sufficient quantities, these can affect the wine's odor and taste. With an olfactory detection threshold of a few ppb, the principal active compound isisopropyl methoxypyrazine; this molecule is perceived as rancidpeanut butter, greenbell pepper, urine, or simply bitter. This is also a naturally occurring compound in Sauvignon grapes, and sopyrazine taint has been known to makeRieslings taste likeSauvignon blanc.[citation needed]
The yeastBrettanomyces produces an array ofmetabolites when growing in wine, some of which are volatilephenolic compounds. Together these compounds are often referred to asphenolic taint,"Brettanomyces character", or simply "Brett". The main constituents are listed below, with their sensory threshold and common sensory descriptors:
Geosmin is a compound with a very distinctearthy,musty,beetroot, eventurnip flavour and aroma and has an extremely low sensory threshold of down to 10 parts per trillion. Its presence in wine is usually derived as metabolite from the growth of filamentousactinomycetes such asStreptomyces, andmoulds such asBotrytis cinerea andPenicillium expansum, on grapes. Wines affected by but not attributed to geosmins are often thought to have earthy properties due toterroir.[11] The geosmin fault occurs worldwide and has been found in recent vintages of red wines fromBeaujolais,Bordeaux,Burgundy and theLoire in France. Geosmin is also thought to be a contributing factor incork taint.
Lactic acid bacteria have a useful role in winemaking convertingmalic acid to lactic acid inmalolactic fermentation. However, after this function has completed, the bacteria may still be present within the wine, where they can metabolise other compounds and produce wine faults. Wines that have not undergone malolactic fermentation may be contaminated with lactic acid bacteria, leading to refermentation of the wine with it becomingturbid,swampy, and slightlyeffervescent orspritzy. This can be avoided by sterile filtering wine directly before bottling. Lactic acid bacteria can also be responsible for other wine faults such as those below.
Bitterness taint oramertume is rather uncommon and is produced by certain strains of bacteria from the generaPediococcus,Lactobacillus, andOenococcus. It begins by the degradation ofglycerol, a compound naturally found in wine at levels of 5-8 g/L, via adehydratase enzyme to3-hydroxypropionaldehyde. During ageing this is further dehydrated toacrolein which reacts with theanthocyanins and other phenols present within the wine to form the taint.[12] As red wines contain high levels of anthocyanins they are generally more susceptible.
Diacetyl in wine is produced bylactic acid bacteria, mainlyOenococcus oeni. In low levels it can impart positivenutty orcaramel characters, however at levels above 5 mg/L it creates an intensebuttery orbutterscotch flavour, where it is perceived as a flaw. The sensory threshold for the compound can vary depending on the levels of certain wine components, such as sulfur dioxide. It can be produced as a metabolite ofcitric acid when all of themalic acid has been consumed. Diacetyl rarely taints wine to levels where it becomes undrinkable.[13]
Geranium taint, as the name suggests, is a flavour and aroma taint in wine reminiscent ofgeranium leaves. The compound responsible is2-ethoxyhexa-3,5-diene, which has a low sensory threshold concentration of 1 ng/L.[14] In wine it is formed during themetabolism ofpotassium sorbate bylactic acid bacteria. Potassium sorbate is sometimes added to wine as apreservative against yeast, however its use is generally kept to a minimum due to the possibility of the taint developing. The production of the taint begins with the conversion ofsorbic acid to the alcoholsorbinol. The alcohol is thenisomerised in the presence of acid to3,5-hexadiene-2-ol, which is then esterified with ethanol to form2-ethoxy-3,5-hexadiene. As ethanol is necessary for the conversion, the geranium taint is not usually found inmust.
Mannitol is asugar alcohol, and in wine it is produced byheterofermentative lactic acid bacteria, such asLactobacillus brevis, by thereduction offructose. Its perception is often complicated as it generally exists in wine alongside other faults, but it is usually described asviscous,ester-like combined with a sweet and irritating finish.[12] Mannitol is usually produced in wines that undergo malolactic fermentation with a high level ofresidual sugars still present. Expert winemakers oftentimes add small amounts of sulfur dioxide during the crushing step to reduce early bacterial growth.
Ropiness is manifested as an increase inviscosity and aslimey orfatty mouthfeel of a wine. In France the fault is known as "graisse", which translates tofat. The problem stems from the production ofdextrins andpolysaccharides by certain lactic acid bacteria, particularly of the generaLeuconostoc andPediococcus.
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Mousiness is a wine fault most often attributed toBrettanomyces but can also originate from thelactic acid bacteriaLactobacillus brevis,Lactobacillus fermentum, andLactobacillus hilgardii,[12] and hence can occur inmalolactic fermentation. The compounds responsible arelysine derivatives, mainly;
The taints are not volatile at thepH of wine, and therefore not obvious as an aroma. However, when mixed with the slightly basic pH of saliva they can become very apparent on the palate,[16] especially at the back of the mouth, asmouse cage ormouse urine.
Refermentation, sometimes calledsecondary fermentation, is caused by yeasts refermenting theresidual sugar present within bottled wine. It occurs whensweet wines are bottled in non-sterile conditions, allowing the presence of microorganisms. The most common yeast to referment wine is the standard wine fermentation yeastSaccharomyces cerevisiae, but has also been attributed toSchizosaccharomyces pombe andZygosaccharomyces bailii.[12] The main issues associated with the fault includeturbidity (from yeastbiomass production), excess ethanol production (may violatelabelling laws), slightcarbonation, and some coarse odours. Refermentation can be prevented by bottling wines dry (with residual sugar levels <1.0g/L), sterile filtering wine prior to bottling, or adding preservative chemicals such asdimethyl dicarbonate. ThePortuguese wine style known as "vinhos verdes" used to rely on this secondary fermentation in bottle to impart a slightspritziness to the wine, but now usually uses artificial carbonation.
Organisms responsible for bunch rot of grape berries are filamentous fungi, the most common of these beingBotrytis cinerea (gray mold) However, there are a range of other fungi responsible for the rotting of grapes such asAspergillus spp.,Penicillium spp., and fungi found in subtropical climates (e.g.,Colletotrichum spp. (ripe rot) andGreeneria uvicola (bitter rot)). A further group more commonly associated with diseases of the vegetative tissues of the vine can also infect grape berries (e.g.,Botryosphaeriaceae,Phomopsis viticola). Compounds found in bunch rot affected grapes and wine are typically described as having mushroom, earthy odors and include geosmin,2-methylisoborneol,1-octen-3-ol,2-octen-1-ol,fenchol andfenchone.[17]
This summary document lists the common wine faults including the name of the fault, the type of the fault, the odor characteristics, and the chemical responsible. A wine fault is an unpleasant organoleptic characteristic including look, smell, or taste. Wine faults can come from a chemical or a microbial origin and some off-odors can be the result of multiple faults.
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