Saturday, November 01, 2008

Wine Faults

During wine evaluation by judges, they are not only looking for the good things about wine but also for technical faults. These are many and varied and mainly come from poor wine making techniques.
Below are a few of the more common faults and how they express themselves.
Oxidation: The most common wine fault. It is caused when oxygen comes in contact with the wine during vinification or is a result of poor sulphur addition management. Phenolics in the wine are oxidised causing a loss of colour, flavour and aroma which produces a so called "flat" wine.
Acetyldehyde: This is caused mainly by the oxidation of alcohol and imparts a sherry like flavour to the wine.

Volatile Acidity (Acetic Acid): This is vinegar taint and is caused by spoilage yeasts and bacteria eg. acetobacter
Ethyl Acetate: Smells like nail polish remover and is caused by the esterification of alcohol and acetic acid. It is a common microbial fault produced by wine spoilage yeasts. High levels of ethyl acetate are also produced by lactic acid bacteria and acetic acid bacteria.
Sulphur Dioxide: Sulphur is used as an additive during the winemaking process, mainly as an antioxidant but also as an antimicrobial agent. At normal levels (up to 200ppm) its presence is undetected. However overuse can contribute to flavour and aroma taints which are very volatile and potent. Over sulphured wine will be reminiscent of burnt matches, burnt rubber, or mothballs.
Hydrogen Sulphide: A rotten egg gas smell and is caused by the fermentation of wine where the grape juice has a low nitrogen content. This is overcome by adding additional nitrogen sources eg. DAP
Mercaptans: An onion and rubber smell caused by the reaction of wine components with hydrogen sulphide.
Dimethyl Sulphide: Imparts a cooked cabbage, canned corn, asparagus character to wine.

Cork Taint: Cork taint is a wine fault mostly attributed to the compound 2,4,6-trichloroanisole or more commonly, TCA.
TCA most likely is the result 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". Cork taint is said to affect 5% of the world's wine. It has lead to the development of a whole range of new bottle closures including synthetic cork and screw caps.
Generally when judges come across a corked wine they will ask for another pour from another bottle as it is not considered technically a one hundred percent wine making fault.
Brettanomyces ("Brett"): This yeast produces a whole number of off flavours and aromas in wine eg. band-aids, bacon, sweaty saddle and rancidity.
Lactic Acid Bacteria (LAB): While LAB is useful in coverting the harsher malic acid in wine to lactic acid during malolactic fermentation, it can also produce wine faults. Wines that have not undergone malolactic fermentation may be contaminated with LAB which can result in refermentation of the wine. Wines then become turbid, swampy and slightly effervescent.

Diacetyl: This is produced by lactic acid bacteria. At low levels it can impart positive nutty or caramel characters which with some wines eg. Chardonnay, can be considered desirable. However at high levels it creates an intense buttery or butterscotch flavour and then it is perceived as a fault.
Geranium Taint: This is a flavour and aroma taint reminiscent of geranium leaves. It is formed during the metabolism of potassium sorbate by LAB. Potassium sorbate is sometimes added to wine as a preservative against yeasts.
Mousiness: A microbial derived fault which can become very apparent on the palate, especially at the back of the mouth, as mouse cage or mouse urine.
Refermentation: This is caused by yeasts refermenting the residual sugar present within bottled wine. The most common culprit is the standard wine fermentation yeast, Saccharomyces cerevisiae. The main problems associated with the fault include turbidity, a slight carbonation and some coarse odours.
Hazes: Of course there are a whole lot of protein and metal hazes as well as deposits (tartrates) that cause turbidity and affect the visual quality of wine (but not always taste) and are regarded as faults.

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