A wine fault is a sensory-associated (organolepticMerriam-Webster. (n.d.). Organoleptic. In Merriam-Webster.com dictionary. Retrieved June 26, 2023, from https://www.merriam-webster.com/dictionary/organoleptic.) 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. Wine faults may result from poor winemaking practices or storage conditions that lead to wine spoilage.
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 exceeds 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 faultCharacteristics AcetaldehydeSmell 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-acetateSmell of "fake" candy banana flavoring BrettanomycesSmell of barnyards, fecal and gamey horse aromas Cork taintSmell of a damp basement, wet cardboard or newspapers and mushrooms Butyric acidSmell of rancid butter Ethyl acetateSmell of vinegar, paint thinner and nail polish remover Hydrogen sulfideSmell of rotten eggs or garlic that has gone bad IodineSmell of moldy grapes Lactic acid bacteriaSmell of sauerkraut MercaptansSmell of burnt garlic or onion OxidationSmell of cooked fruit and walnuts. Also detectable visually by premature browning or yellowing of the wine Sorbic acid plus lactic acid bacteriaSmell of crushed geranium leaves Sulfur dioxideSmell 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. Anthocyanins, catechins, 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 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 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 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 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 (H2S) 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 H2S 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 disulfides.
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 μg/L,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