A wine fault or defect is an unpleasant characteristic of a
wineoften resulting from poor winemakingpractices or storage conditions, and leading to wine spoilage. Many of the compounds that cause wine faults are already naturally present in wine but at insufficient concentrations to adversely affect it. In fact, depending on perception, these concentrations may impart positive characters to the wine. However when the concentration of these compounds greatly exceeds the sensory threshold, they replace or obscure the flavours and aromas that the wine should be expressing. Ultimately the quality of the wine is reduced, making it less appealing and sometimes undrinkable.
oxidationof wine is perhaps the most common of wine faults, as the presence of oxygenand a catalystare 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 other phenolspresent in wine are those most easily oxidised duToit, W.J. (2005). [http://www.wynboer.co.za/recentarticles/200508oxygen.php3 Oxygen in winemaking: Part I] . WineLand. URL accessed on 2 April 2006.] , which leads to a loss of colour, flavour and aroma - sometimes referred to as "flattening". In most cases compounds such as sulfur dioxideor erythorbic acidare added to wine by winemakers, which protect the wine from oxidation and also bind with some of the oxidation products to reduce their organoleptic effectGoode, Jamie (05/16/05). [http://www.wineint.com/storyprint.asp?sc=1810 Oxidation] . Wine International. URL accessed on 2 April 2006.] . Apart from phenolic oxidation, the ethanolpresent within wine can also be oxidised into other compounds responsible for flavour and aroma taints.
Acetaldehydeis an intermediate product of yeast fermentation; however, it is more commonly associated with ethanol oxidationcatalysed by the enzyme ethanol dehydrogenase. Acetaldehyde production is also associated with the presence of surface film forming yeasts and bacteria, such as acetic acid bacteria, which form the compound by the decarboxylationof pyruvate. The sensory thresholdfor acetaldehyde is 100-125 mg/L. Beyond this level it imparts a " sherry" type character to the wine which can also be described as "green apple", "sour" and "metallic". Acetaldehyde intoxicationis also implicated in hangovers.
Acetic acidin wine, often referred to as volatile acidity (VA) or "vinegar taint", can be contributed by many wine spoilage yeastsand bacteria. This can be from either a by-productof fermentation, or due to the spoilage of finished wine. Acetic acid bacteria, such as those from the genera " Acetobacter" and " Gluconobacter" produce high levels of acetic acid. The sensory thresholdfor acetic acid 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. [ [http://www.wineint.com/story.asp?storyCode=1908 Volatile Acidity] - great article from
Wine & Spiritmag ]
Ethyl acetateis formed in wine by the esterificationof 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 by wine spoilage yeasts, particularly " Pichia anomala", " Kloeckera apiculata", and " Hanseniaspora uvarum". High levels of ethyl acetate are also produced by lactic acid bacteriaand acetic acid bacteria. The sensory threshold for ethyl acetate is 150-200 mg/L. Levels below this can give an added richness and sweetness, whereas levels above impart "nail polish remover", " glue", or " varnish" type aromas.
Sulfuris used as an additivethroughout the winemaking process, primarily to stop oxidation as mentioned above but also as an antimicrobialagent. 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 dioxideis a common wine additive, used for its antioxidantand preservativeproperties. When its use is not managed well it can be overadded, with its perception in wine reminiscent of "matchsticks", "burnt rubber", or "mothballs". Wines such as these are often termed "sulfitic".
Hydrogen sulfide(H2S) is generally thought to be a metabolic by-product of yeast fermentation in nitrogenlimited environments. It is formed when yeast ferment via the sulfate reduction pathway. Fermenting wine is often supplemented with diammonium phosphate(DAP) as a nitrogen source to prevent formation. The sensory threshold for hydrogen sulfide is 40-50 μg/L, with levels above this imparting a distinct "rotten egg" aroma to the wine. Hydrogen sulfide can further react with wine compounds to form mercaptans and disulfides.
Mercaptans(thiols) are produced in wine by the reaction of hydrogen sulfide with other wine components such as ethanol or sulfur containing amino acids, such as methionine. They can be formed if finished wine is allowed prolonged contact with the lees. This can be prevented by rackingthe wine. Mercaptans have a very low sensory threshold, around 1.5 µg/L [http://www.etslabs.com/pagetemplate/blank.asp?pageid=350 Technical Bulletin - Sulfides in Wine] . ETS Laboratories. URL accessed on 12 March 2006.] , with levels above causing "onion", "rubber", and "skunk" type odours.
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 to "fruityness", "fullness", and "complexity". Levels above the sensory threshold of >30 µg/L in white wines and >50 µg/L for red wines, give the wine characteristics of "cooked cabbage", "canned corn", "asparagus" or "truffles". Note: dimethyl sulfide is not formed from the oxidation of mercaptans (as had previously been reported here)...dimethyl disulfide is formed this way (from the oxidation of methyl mercaptan), but dimethyl sulfide is not.
Cork taint is a wine fault mostly attributed to the compound 2,4,6-trichloroanisole (TCA), although other compounds such as
guaiacol, geosmin, 2-methylisoborneol, octen-3-ol, octen-3-one, 2,3,4,6-tetrachloroanisole, pentachloroanisole, and 2,4,6-tribromoanisole are also thought to be involvedLaMar, Jim (09/25/02). [http://www.winepros.org/wine101/vincyc-tca.htm Cork Taint] . URL accessed on 12 March 2006.] . TCA most likely originates as a metaboliteof mouldgrowth on chlorinebleached wine corks and barrels. It causes "earthy", "mouldy", and "musty" 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 falsely identified as it.
Heat damaged wines are often casually referred to as "cooked", which suggests how heat can affect a wine. 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 will thermally expand, and may even push up between the cork and bottle and leak from the top. When opening a bottle of wine, if a track 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 don't read 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 to poor quality or other factors.
Lightstruck wines are those that have had excessive exposure to
ultravioletlight, particularly in the range 325 to 450 nmDrouhin, R.J. (01/23/98) [http://www.drouhin.com/wineviti/glasses.html Bottle Glass] . URL Accessed 3 April 2006] . Very delicate wines, such as Champagnes, are generally worst affected, with the fault causing a "wet cardboard" or "wet wool" type flavour and aroma. Red wines rarely becomes lightstruck because the phenolic compounds present within the wine protects it. Lightstrike is thought to be caused by sulfur compounds such as dimethyl sulfide. In France lightstrike is known as "goûts de lumière", which translates to "tastes 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 types of
ladybirds, release unpleasant volatile compounds as a defensive mechanism when disturbed. In sufficient quantities this can affect the bouquet and taste of wines. With an olfactory detection threshold of a few ppb, the principal active compounds are methoxypyrazines, or just pyrazines, that are perceived as rancid peanut butter, bitter herbaceous, green bell pepperor cat urine. This is also a naturally occurring compound in Sauvignon grapes and so ladybugs taint has been known to make Rieslings taste like Sauvignon Blanc.
The yeast "
Brettanomyces" produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds. Together these compounds are often referred to as "Brettanomyces character", or simply "Brett". The main constituents are listed below, with their sensory threshold and common sensory descriptors:
4-Ethylphenol(>140 µg/L): Band-aids, barnyard, horse stable, antiseptic
4-ethylguaiacol(>600 µg/L): Bacon, spice, cloves, smoky
*isovaleric acid: Sweaty saddle, cheese, rancidity
Geosminis a compound with a very distinct "earthy", "musty", "beetroot", even "turnip" 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 filamentous actinomycetes such as " Streptomyces", and moulds such as " Botrytis cinerea" and "Penicillium expansum", on grapes. Wines affected by but not attributed to geosmins are often thought to have earthy properties due to terroirKennel, Florence (14/12/05). [http://www.decanter.com/news/60674.html Bordeaux boffin solves geosmin conundrum] . Decanter.com. URL accessed on 2 April 2006.] . The geosmin fault occurs worldwide and has been found in recent vintages of red wines from Beaujolais, Bordeaux, Burgundyand the Loirein France. Geosmin is also thought to be a contributing factor in cork taint.
Lactic acid bacteria
Lactic acid bacteriahave a useful role in winemaking converting malic acid to lactic acid in malolactic 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 becoming turbid, "swampy", and slightly effervescent or "spritzy". 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 or "amertume" is rather uncommon and is produced by certain strains of bacteria from the genera "
Pediococcus", " Lactobacillus", and " Oenococcus". It begins by the degradation of glycerol, a compound naturally found in wine at levels of 5-8 g/L, via a dehydrataseenzyme to "3-hydroxypropionaldehyde". During ageing this is further dehydrated to acroleinwhich reacts with the anthocyanins and other phenols present within the wine to form the taintduToit, M., Pretorius, I.S. (2000). “Microbial spoilage and preservation of wine: Using weapons from nature's own arsenal - A review”. South African Journal of Enology and Viticulture 21: 74-96.] . As red wines contain high levels of anthocyanins they are generally more susceptible.
Diacetylin wine is produced by lactic acid bacteria, mainly " Oenococcus oeni". In low levels it can impart positive "nutty" or "caramel" characters, however at levels above 5 mg/L it creates an intense "buttery" or "butterscotch" 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 of citric acidwhen all of the malic acidhas been consumed. Diacetyl rarely taints wine to levels where it becomes undrinkableGibson, George; Farkas, Mike [http://www.bcawa.ca/winemaking/flaws.htm Flaws and Faults in Wine] . URL accessed on 12 March 2006.] .
Geranium taint, as the name suggests, is a flavour and aroma taint in wine reminiscent of geranium leaves. The compound responsible is "2-ethoxyhexa-3,5-diene", which has a low sensory threshold concentration of 0.1 mg/LHühn, T; Sponholz, W.R. and Pulver, D. (1999). [http://www.beverages.ch/service/Microorganisms.PDF The influence of microorganisms in winemaking] . (PDF) URL accessed on 2 April 2006.] . In wine it is formed during the
metabolismof potassium sorbateby lactic acid bacteria. Potassium sorbate is sometimes added to wine as a preservativeagainst 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 of sorbic acidto the alcohol "sorbinol". The alcohol is then isomerised in the presence of acid to "3,5-hexadiene-2-ol", which is then esterified with ethanol to form "2-ethoxy-3,5-hexadiene". As ethanol is necessary for the conversion, the geranium taint is not usually found in must.
Mannitolis a polyol, and in wine it is produced by heterofermentativelactic acid bacteria, such as " Lactobacillus brevis", by the reduction of fructose. Its perception is often complicated as it generally exists in wine alongside other faults, but it is usually described as viscous, ester-like combined with a sweet and irritating finish. Mannitol is usually produced in wines that undergo malolactic fermentation with a high level of residual sugars still present.
Ropiness is manifested as an increase in
viscosityand a "slimey" or "fatty" mouthfeel of a wine. In France the fault is known as "graisse", which translates to "fat". The problem stems from the production of dextrins and polysaccharides by certain lactic acid bacteria, particularly of the genera " Leuconostoc" and " Pediococcus".
Mousiness is a wine fault most often attributed to "Brettanomyces" but can also originate from the
lactic acid bacteria" Lactobacillus brevis", " Lactobacillus fermentum", and " Lactobacillus hilgardii". The compounds responsible are lysinederivatives, mainly;
2-ethyltetrahydropyridineMarais, Johann [http://www.wynboer.co.za/recentarticles/0299nitrogen.php3 Flavourful nitrogen containing wine constituents] . Wynboer. URL accessed on 12 March 2006.]
2-acetyl-1-pyrroleneThe taints are not volatile at the pHof wine, and therefore not obvious as an aroma. However, when mixed with the neutral pH of saliva they can become very apparent on the palateGawell, Richard [http://www.aromadictionary.com/articles/mousey_article.html Somellier, A Mouse Must Have Wee'd in My Wine!] . aromadictionary.com. URL accessed on 12 March 2006.] , especially at the back of the mouth, as "mouse cage" or "mouse urine".
Refermentation, sometimes called
secondary fermentation, is caused by yeasts refermenting the residual sugarpresent within bottled wine. It occurs when sweet wines are bottled in non-sterile conditions, allowing the presence of microorganisms. The most common yeast to referment wine is the standard wine fermentation yeast " Saccharomyces cerevisiae", but has also been attributed to " Schizosaccharomyces pombe" and " Zygosaccharomyces bailii". The main issues associated with the fault include turbidity(from yeast biomassproduction), excess ethanol production (may violate labelling laws), slight carbonation, 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 as dimethyl dicarbonate. The Portuguese winestyle known as "vinhos verdes" relies on this secondary fermentation in bottle to impart a slight spritziness to the wine.
Acids in wine
* [http://people.ok.ubc.ca/neggers/Chem422A/Organoleptic%20Defects%20in%20Wine.pdf Organoleptic defects in wine] (PDF document)
* [http://www.beverages.ch/service/Microorganisms.PDF The influence of microorganisms in winemaking] (PDF document)
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