Watch this page
Wine fault
A wine fault or defect is an unpleasant characteristic of awine often resulting from poorwinemaking practices 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 flavors andaromas that the wine should be expressing (or that the winemaker wants the wine to express). Ultimately the quality of the wine is reduced, making it less appealing and sometimes undrinkable.[1]
There are many causes for the perception in wine faults, including poorhygiene at the winery, excessive and/or insufficient exposure of the wine tooxygen, excessive or insufficient exposure of the wine to sulphur, overextended maceration of the wine either pre- or post-fermentation, faultyfining, filtering and stabilization of the wine, the use of dirty oak 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 during wine tasting that can introduce materials or aromas to what was previously a clean and fault-free wine.[1][2]
Differences between flaws and faults
In wine tasting, there is a distinction made between what is considered aflaw and a fault. Wine flaws are minor attributes that depart from what are perceived as normal wine characteristics. These include excessive sulfur 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 and recognition 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 and Brettanomyces 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 like Sherry andRancio), ethyl acetate and cork taint.[1]
Detecting faults in wine tasting
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 and browning of the wine's color. The sign of gas bubbles in wines that are not meant to be sparkling can be a sign of refermentation ormalolactic fermentation happening in the bottle. Unusual breaks in the color of the wine could be a sign of excessivecopper, iron or proteins 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 or 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.[1][2]
Wine faultCharacteristicsAcetaldehydeSmell of roasted nuts or dried out straw. Commonly associated with Sherries where these aromas are considered acceptableAmyl-acetateSmell of "fake" candy banana flavoringBrettanomycesSmell of barnyards, fecal and gamey horse aromasCork taintSmell of a damp basement, wet cardboard or newspapers and mushroomsButyric acidSmell of rancid butterEthyl acetateSmell of vinegar, paint thinner and nail polish removerHydrogen sulfideSmell of rotten eggs or garlic that has gone badIodineSmell of moldy grapesLactic acid bacteriaSmell of sauerkrautMercaptansSmell of burnt garlic or onionOxidationSmell of cooked fruit and walnuts. Also detectable visually by premature browning or yellowing of the wineSorbic acidplus lactic acid bacteriaSmell of crushedgeranium leavesSulfur dioxideSmell of burnt matches. Can also come across as a pricking sensation in the nose.
Oxidation
See also: Premature oxidation
The oxidation of wine is perhaps the most common of wine faults, as the presence of oxygen and a catalyst 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 other phenols present in wine are those most easily oxidised,[3] which leads to a loss of colour, flavour and aroma - sometimes referred to asflattening. In most cases compounds such as sulfur dioxide or 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 effect.[4] Apart from phenolic oxidation, the ethanol present within wine can also be oxidised into other compounds responsible for flavour and aroma taints.
The oxidation of ethanol
Acetaldehyde
Acetaldehyde is an intermediate product of yeast fermentation; however, it is more commonly associated withethanol oxidation catalysed 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 decarboxylation ofpyruvate. The sensory threshold for 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 andmetallic. Acetaldehyde intoxication is also implicated in hangovers.
Acetic acid
Acetic acid in wine, often referred to as volatile acidity (VA) or vinegar taint, can be contributed by many wine spoilageyeasts and bacteria. This can be from either a by-product of fermentation, or due to the spoilage of finished wine. Acetic acid bacteria, such as those from the genera Acetobacter andGluconobacter produce high levels of acetic acid. The sensory 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.[5] The renowned 1947 Cheval 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 by wine spoilage yeasts, particularly Pichia anomala or Kloeckera apiculata. High levels of ethyl acetate are also produced by lactic acid bacteria and acetic acid bacteria.
Sulfur compounds
Sulfur is used as an additive throughout the winemaking process, primarily to stop oxidation as mentioned above but also as antimicrobial 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
Sulfur dioxide is a common wine additive, used for its antioxidant and preservativeproperties. When its use is not managed well it can be overadded, with its perception in wine reminiscent ofmatchsticks, burnt rubber, or mothballs. Wines such as these are often termedsulfitic.
Hydrogen sulfide
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
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 the lees. This can be prevented byracking the wine. Mercaptans have a very low sensory threshold, around 1.5µg/L,[6] with levels above causing onion,rubber, and skunk type odours.
Dimethyl sulfide
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, 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 that dimethyl disulfide is formed from the oxidation of methyl mercaptan.
Environmental
Cork taint
2,4,6-trichloroanisole
Main article: Cork taint
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, 1-octen-3-ol, 1-octen-3-one, 2,3,4,6-tetrachloroanisole,pentachloroanisole, and 2,4,6-tribromoanisole are also thought to be involved.[7] TCA most likely originates as a metabolite of mould growth onchlorine-bleached wine corks and barrels. It causes earthy, 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 damage
Heat damaged wines are often casually referred to as cooked, which suggests how heat can affect a wine. They are also known as maderized 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 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 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.
Lightstrike
Lightstruck wines are those that have had excessive exposure to ultravioletlight, particularly in the range 325 to 450 nm.[8] Very delicate wines, such asChampagnes, are generally worst affected, with the fault causing a wet cardboard or wet 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 to a 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.
Ladybird taint
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, or the multicoloured Asian lady beetle, release unpleasant volatile compounds as a defensive mechanism when disturbed. In sufficient quantities these can affect the wine's bouquet and taste. With an olfactory detection threshold of a few ppb, the principal active compounds aremethoxypyrazines, or pyrazines, that are perceived as rancid peanut butter, bitter herb, green bell pepper, or cat urine. This is also a naturally occurring compound in Sauvignon grapes and so ladybird taint has been known to makeRieslings taste like Sauvignon blanc.
Microbiological
Brettanomyces(Dekkera)
Main article: Brettanomyces
The yeast Brettanomyces produces an array of metabolites 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:
4-ethylphenol (>140 µg/L): Band-aids, barnyard, horse stable, antiseptic4-ethylguaiacol (>600 µg/L): Bacon, spice, cloves, smokyisovaleric acid: Sweaty, cheese, rancidity
Geosmin
Geosmin is a compound with a very distinct earthy,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 filamentous actinomycetes such asStreptomyces, and moulds such asBotrytis cinerea and Penicillium expansum, on grapes. Wines affected by but not attributed to geosmins are often thought to have earthy properties due toterroir.[9] The geosmin fault occurs worldwide and has been found in recent vintages of red wines from Beaujolais,Bordeaux, Burgundy and the Loire in France. Geosmin is also thought to be a contributing factor in cork taint.
Lactic acid bacteria
Main article: Malolactic_fermentation § Wine_faults
Lactic acid bacteria have 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 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
Acrolein
Bitterness taint or amertume is rather uncommon and is produced by certain strains of bacteria from the generaPediococcus, Lactobacillus, andOenococcus. It begins by the degradation of glycerol, a compound naturally found in wine at levels of 5-8 g/L, via a dehydratase enzyme 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 taint.[10] As red wines contain high levels of anthocyanins they are generally more susceptible.
Diacetyl
Diacetyl
Diacetyl in wine is produced by lactic acid bacteria, mainly Oenococcus oeni. In low levels it can impart positive nuttyor caramel characters, however at levels above 5 mg/L it creates an intensebuttery 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 ofcitric acid when all of the malic acid has been consumed. Diacetyl rarely taints wine to levels where it becomes undrinkable.[11]
Geranium taint
2-ethoxy-3,5-hexadiene
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 1 ng/L.[12] In wine it is formed during the metabolismof potassium sorbate by lactic 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 of sorbic acid to the alcohol sorbinol. The alcohol is thenisomerised in the presence of acid to3,5-hexadiene-2-ol, which is then esterified with ethanol to form 2-ethoxy-3,5-hexadiene.[12] As ethanol is necessary for the conversion, the geranium taint is not usually found inmust.
Mannitol
Mannitol
Mannitol is a sugar alcohol, and in wine it is produced by heterofermentativelactic acid bacteria, such asLactobacillus brevis, by the reduction offructose. 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.[10] Mannitol is usually produced in wines that undergo malolactic fermentation with a high level ofresidual sugars still present.
Ropiness
Ropiness is manifested as an increase in viscosity and a slimey or fattymouthfeel of a wine. In France the fault is known as "graisse", which translates to fat. The problem stems from the production of dextrins andpolysaccharides by certain lactic acid bacteria, particularly of the generaLeuconostoc and Pediococcus.
Mousiness
2-acetyl-3,4,5,6-
tetrahydropyridine
Mousiness is a wine fault most often attributed to Brettanomyces but can also originate from the lactic acid bacteriaLactobacillus brevis, Lactobacillus fermentum, and Lactobacillus hilgardii,[10]and hence can occur in malolactic fermentation. The compounds responsible are lysine derivatives, mainly;
2-acetyl-3,4,5,6-tetrahydropyridine2-acetyl-1,4,5,6-tetrahydropyridine2-ethyltetrahydropyridine[13]2-acetyl-1-pyrrolene
The taints are not volatile at the pH 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,[14]especially at the back of the mouth, asmouse cage or mouse urine.
Refermentation
Refermentation, sometimes calledsecondary 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 andZygosaccharomyces bailii.[10] The main issues associated with the fault includeturbidity (from yeast biomassproduction), excess ethanol production (may violate labelling 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 as dimethyl dicarbonate. The Portuguese wine style known as "vinhos verdes" relies on this secondary fermentation in bottle to impart a slight spritziness to the wine.
Bunch rots
Organisms responsible for bunch rot of grape berries are filamentous fungi, the most common of these being Botrytis cinerea (gray mold) However, there are a range of other fungi responsible for the rotting of grapes such as Aspergillusspp., Penicillium spp., and fungi found in subtropical climates (e.g., Colletotrichumspp. (ripe rot) and Greeneria 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 and fenchone.[15]
See also
References
External links
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.