Wine fault

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.

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.

Anthocyanins, catechins, epicatechins and other phenols present in wine are those most easily oxidised,[5] which leads to a loss of colour, flavour and aroma - sometimes referred to as flattening.

[6] Apart from phenolic oxidation, the ethanol present within wine can also be oxidised into other compounds responsible for flavour and aroma taints.

Beyond this level it imparts a sherry type character to the wine which can also be described as green apple, sour and metallic.

It is a common microbial fault produced by wine spoilage yeasts, particularly Pichia anomala or Kloeckera apiculata.

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.

Hydrogen sulfide (H2S) is generally thought to be a metabolic by-product of yeast fermentation in nitrogen limited environments.

The sensory threshold for hydrogen sulfide is 8-10 μg/L, with levels above this imparting a distinct rotten egg aroma to the wine.

Mercaptans have a very low sensory threshold, around 1.5 μg/L,[8] with levels above causing onion, rubber, and skunk type odours.

Like ethyl acetate, levels of DMS below the sensory threshold can have a positive effect on flavour, contributing to fruityness, fullness, and complexity.

Wines exposed to extreme temperatures will thermally expand, and may even push up between the cork and bottle and leak from the top.

Others, notably the Asian lady beetle, release unpleasant-smelling nitrogen heterocycles as a defensive mechanism when disturbed.

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.

[citation needed] The yeast Brettanomyces produces an array of metabolites when growing in wine, some of which are volatile phenolic compounds.

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.

[11] The geosmin fault occurs worldwide and has been found in recent vintages of red wines from Beaujolais, Bordeaux, Burgundy and the Loire in France.

The sensory threshold for the compound can vary depending on the levels of certain wine components, such as sulfur dioxide.

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.

Mannitol is a sugar alcohol, 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.

[12] Mannitol is usually produced in wines that undergo malolactic fermentation with a high level of residual sugars still present.

Expert winemakers oftentimes add small amounts of sulfur dioxide during the crushing step to reduce early bacterial growth.

The compounds responsible are lysine derivatives, mainly; The taints are not volatile at the pH of wine, and therefore not obvious as an aroma.

Refermentation, sometimes called secondary fermentation, is caused by yeasts refermenting the residual sugar present within bottled wine.

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.