A wine fault is a sensory-associated (
/ref>) 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.
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 a sensory threshold, they replace or obscure desirable flavors and aromas 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.[M. Baldy: "The University Wine Course", Third Edition, pp. 37-39, 69-80, 134-140. The Wine Appreciation Guild 2009 .]
There are many underlying causes of wine faults, including poor hygiene at the winery, excessive or insufficient exposure of the wine to oxygen, excessive or insufficient exposure of the wine to sulphur, overextended maceration of the wine either pre- or post-fermentation, faulty fining, 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 poor storage of the wine that exposes it to excessive heat and temperature fluctuations as well as the use of dirty stemware during wine tasting that can introduce materials or aromas to what was previously a clean and fault-free wine.[D. Bird: "Understanding Wine Technology", pp. 31-82, 155-184, 202-222. DBQA Publishing 2005 .]
Differences between flaws and faults
In wine tasting, there is a big distinction made between what is considered a
flaw 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" and
diacetyl 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 include
acetaldehyde (except when purposely induced in wines like
Sherry and
Rancio),
ethyl acetate and
cork taint.
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 or malolactic fermentation happening in the bottle. Unusual breaks in the color of the wine could be a sign of excessive
copper,
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 as well as poor temperature controls during fermentation. Tactile clues of potential wine faults include the burning, acidic taste associated with
volatile acidity that can make a wine seem out of balance.
|
| Wine fault | Characteristics |
| 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 |
| Smell of "fake" candy banana flavoring |
| Smell of barnyards, fecal and gamey horse aromas |
| Smell of a damp basement, wet cardboard or newspapers and mushrooms |
| Smell of rancid butter |
| Smell of vinegar, paint thinner and nail polish remover |
| Smell of rotten eggs or garlic that has gone bad |
| Smell of moldy grapes |
| Smell of sauerkraut |
| Smell of burnt garlic or onion |
| Smell of cooked fruit and walnuts. Also detectable visually by premature browning or yellowing of the wine |
| Smell of crushed geranium leaves |
| Smell of burnt matches. Can also come across as a pricking sensation in the nose. |
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.
,
, epicatechins and other phenols present in wine are those most easily oxidised,
[duToit, W.J. (2005). Oxygen in winemaking: Part I. WineLand, 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 dioxide or
erythorbic 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.
[Goode, Jamie (06/25/19). Oxidation in wine . internationalwinechallenge.com, accessed on 10 February 2021.] Apart from phenolic oxidation, the
ethanol 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 certain
Sherry wines and
Vin jaune from the Jura region of France.
Acetaldehyde
Acetaldehyde is an intermediate product of yeast fermentation; however, it is more commonly associated with
ethanol 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 of
pyruvate. The sensory threshold for acetaldehyde is 100–125
Micrograms/
litre. Beyond this level it imparts a
sherry type character to the wine which can also be described as
green apple,
sour and
metallic. Acetaldehyde intoxication is also implicated in
.
Acetic acid
Acetic acid in wine, often referred to as volatile acidity (VA) or
vinegar taint, can be contributed by many wine spoilage
yeasts 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 and
Gluconobacter 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.[ Volatile Acidity - article from Wine & Spirit magazine.] The renowned 1947 Cheval Blanc is widely recognized to contain high levels of volatile acidity.
Ethyl acetate is formed in wine by the esterification 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
food 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
preservative properties. 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
Hydrogen sulfide (H
2S) is generally thought to be a metabolic by-product of yeast fermentation in nitrogen limited environments. It is formed when yeast ferments via the sulfate reduction pathway. Fermenting wine is often supplemented with diammonium phosphate (DAP) as a nitrogen source to prevent H
2S formation. The sensory threshold for hydrogen sulfide is 8-10 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
.
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 by
racking the wine. Mercaptans have a very low sensory threshold, around 1.5
Micrograms/
litre,
[ Technical Bulletin - Sulfides in Wine . etslabs.com, accessed on 12 March 2006.] with levels above causing
onion,
rubber, and
skunk type odours. Note that dimethyl disulfide is formed from the oxidation of methyl mercaptan.
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 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.
Environmental
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.
[LaMar, Jim (09/25/02). Cork Taint, accessed on 12 March 2006.] TCA most likely originates as a
metabolite of mould growth on
chlorine-bleached 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 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, from
Madeira 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
ultraviolet light, particularly in the range 325 to 450 nm.
[Drouhin, R.J. (01/23/98) Bottle Glass , 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 become lightstruck because of the phenolic compounds present within the wine that protect it. Lightstrike is thought to be caused by sulfur compounds such as
dimethyl 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 (pyrazine) taint
Some insects present in the grapes at harvest inevitably end up in the press and for the most part are inoffensive. Others, notably the Asian 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 is isopropyl methoxypyrazine; this molecule is perceived as rancid
peanut butter, green
bell pepper, urine, or simply bitter. This is also a naturally occurring compound in Sauvignon grapes, and so
pyrazine taint has been known to make
taste like
Sauvignon blanc.
Microbiological
Brettanomyces (Dekkera)
The yeast
Brettanomyces produces an array of
when growing in wine, some of which are volatile phenolic compounds. Together these compounds are often referred to as
phenolic 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, antiseptic
-
4-ethylguaiacol (>600 μg/L): Bacon, spice, cloves, smoky
-
isovaleric acid: Sweaty, cheese, rancidity
Geosmin
Geosmin is 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
Actinomycetota 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
terroir.
[Kennel, Florence (14/12/05). Bordeaux boffin solves geosmin conundrum. Decanter.com, accessed on 2 April 2006.] The geosmin fault occurs worldwide and has been found in recent vintages of red wines from
Beaujolais,
Bordeaux,
Burgundy wine and the
Loire in France. Geosmin is also thought to be a contributing factor in
cork taint.
Lactic acid bacteria
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
turbidity,
swampy, and slightly
effervescence 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
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
dehydratase enzyme to
3-hydroxypropionaldehyde. During ageing this is further dehydrated to
acrolein which reacts with the
and other phenols present within the wine to form the taint.
[duToit, 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.
Diacetyl
Diacetyl in 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 acid when all of the
malic acid has been consumed. Diacetyl rarely taints wine to levels where it becomes undrinkable.
[Gibson, George; Farkas, Mike Flaws and Faults in Wine, accessed on 12 March 2006.]
Geranium taint
Geranium taint, as the name suggests, is a flavour and aroma taint in wine reminiscent of
pelargonium leaves. The compound responsible is
2-ethoxyhexa-3,5-diene, which has a low sensory threshold concentration of 1 ng/L.
In wine it is formed during the
metabolism of potassium sorbate by lactic acid bacteria. Potassium sorbate is sometimes added to wine as a
preservative 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 then
isomerisation 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.
Mannitol
Mannitol is a
sugar alcohol, and in wine it is produced by heterofermentative lactic acid bacteria, such as
Lactobacillus brevis, by the
redox 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 still present. Expert winemakers oftentimes add small amounts of sulfur dioxide during the crushing step to reduce early bacterial growth.
]
Ropiness
Ropiness is manifested as an increase in viscosity and 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 and by certain lactic acid bacteria, particularly of the genera Leuconostoc and Pediococcus.
Mousiness
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,[ and hence can occur in malolactic fermentation. The compounds responsible are lysine derivatives, mainly;
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,][Gawell, Richard Somellier, A Mouse Must Have Wee'd in My Wine!. aromadictionary.com, accessed on 12 March 2006.] especially at the back of the mouth, as mouse cage or mouse urine.
Refermentation
Refermentation, sometimes called secondary fermentation, is caused by yeasts refermenting the residual sugar present within bottled wine. It occurs when sweet wines are bottled in non-Asepsis 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 biomass production), excess ethanol production (may violate wine label 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 wine style known as "vinho verde" used to rely on this secondary fermentation in bottle to impart a slight spritziness to the wine, but now usually uses artificial carbonation.
]
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 Aspergillus spp., Penicillium spp., and fungi found in subtropical climates (e.g., Colletotrichum spp. (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.[Grapevine bunch rots: impacts on wine composition, quality, and potential procedures for the removal of wine faults. Steel CC, Blackman JW and Schmidtke LM, J Agric Food Chem., 5 June 2013, volume 61, issue 22, pp. 5189-5206, .]
See also
-
Oenology
-
Acids in wine
-
Browning in red wine
-
Storage of wine
External links
