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.
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, Kloeckera apiculata, and Hanseniaspora uvarum. High levels of ethyl acetate are also produced by lactic acid bacteria and 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.
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 (H2S) is generally thought to be a metabolic by-product of yeast fermentation in nitrogen limited 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 racking the wine. Mercaptans have a very low sensory threshold, around 1.5 µg/L, 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 involved. 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 falsely identified as it.
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.
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:
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 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 terroir. 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.
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 anthocyanins and other phenols present within the wine to form the taint. As red wines contain high levels of anthocyanins they are generally more susceptible.
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.
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/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 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.
Mannitol is a polyol, and in wine it is produced by heterofermentative lactic 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.
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 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 neutral pH of saliva they can become very apparent on the palate, especially at the back of the mouth, as mouse cage or mouse urine.