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DEFECTS & DISEASES IN WINE

It is extremely important for wine consumers to be aware of and familiar with the most common diseases from which wine may suffer. The appearance of such diseases can, first of all, be caused by the low quality of the grapes themselves, but diseases can also appear during vinification or develop during storage and ageing. Wine very often contracts diseases due to a lack of proper care and attention. One should never forget that hygiene is the most important factor in wine-making.

Possible problems resulting from a lack of proper hygiene:

  • An earthy taste, usually accompanied by an smell of trees, humus, compost, or mushrooms, can develop in qvevri or in other unwashed containers, but is sometimes cased by the poor quality of the grapes themselves.
  • A mouldy smell reminiscent of damp cellars or stagnant water is mainly caused by poorly-maintained fermentation vessels, but can also be the result of a lack of hygienic measures during grape processing or of a damp and dirty cellar.
  • Wine becoming corked is a common problem. Usually the result of the use of low quality corks, this phenomenon can, however, also be caused by wine being left in mouldy wooden barrels.
  • A very unpleasant taste of plastic can be caused by wine becoming contaminated by the smell of plastic or other "chemical" vessels. This problem can be avoided by never storing wine in plastic containers.

Grape quality is also more or less directly linked to the problems listed above. The following are other flaws which can be caused by the use of unconditioned, damaged raw materials or by that of technology whose quality is inferior to that of the raw materials themselves:

  • The smell and flavour of vegetation, which can take the form of a smell of crushed green shoots or a taste of unripe grapes, can be caused by the presence of unripe grapes in the grape must, by that of solid matter during fermentation, or by leaving the grape must in contact with the chacha (the marc and grape skins) for too long.
  • Atypical ageing (ATA): this problem mostly concerns European-style white wines and sparkling wines and prevents aromas of fruit from revealing themselves. These overtones take the form of a short and often unpleasant flavour reminiscent of naphthalene, mastic, damp dirty clothes, wet dog fur or disinfectant, and are usually caused by the effects of drought, excessive or early harvesting, etc.
  • Wine can go sour if too much astringency is given off by the stalks or the pips as a result of faulty storage. Excessive sourness is also characteristic of new, unrefined wine.
  • A smell of sulphur: the various forms of sulphur dioxide – sulphur steaming in an empty container, liquid, powdered potassium dihydrogen phosphate (KDP) – are indispensible to the storage of wine, but too much sulphur can give the wine sulphurous overtones (the smell of matches when they are struck).
  • The presence of hydrogen sulphide and methanethiol in wine reveals itself as the smell of rotten eggs, overripe blackcurrants, mould, burnt rubber or rotten onions. This problem can be caused by the excessive sulphur content of the grape must or by a deficiency in yeast during fermentation, by the failure to transfer wine from one vessel into another, or as a result of the wine being kept in contact with the yeast sediment for too long (rotting yeast can give wine rotten overtones).
  • The smell of the oxidized flesh of cut apples or pears, or of sherry or dried fruit is characteristic of oxidation (acetic aldehyde). Oxidation may be caused by vessels not being filled to the brim or by an insufficient amount (or the complete absence) of sulphur in a wine, which darkens when it oxidizes. In some wines, however, the presence of acetic aldehyde is not considered to be a problem.

 Wine diseases or defects can also be caused by the activity of various micro-organisms:

  • Acetic acid can turn the wine to vinegar: this phenomenon is characterized by a prickling sensation in the nose or on the palate, and is easy to ascertain as the wine has a vinegary taste. Acetic acid is also often accompanied by the presence of ethyl acetate (see below). This phenomenon is the result of the effect of acetic acid bacteria and wild yeast on grapes which have been damaged (by hail or rot, for example) or contaminated (by worms, birds, bees, etc.). After fermentation, acetic acid bacteria which come into contact with air can turn into acetic aldehyde. In order to prevent wine from turning rotten, it is important to discard damages grapes during the harvest and to ensure that both the wine and the wine cellar (including all vessels, stainless steel tanks, barrels, etc.) are kept rigorously clean. Souring is often also caused by a flawed and excessively long process of fermentation.
  • The smell of diluents (ethyl acetate) such as glue or acetone or that of chacha frequently appear in tandem with the presence of acetic acid. This phenomenon usually develops during fermentation or as the result of the presence of air in vessels not having been properly filled, and can be avoided by picking healthy and damaged grapes separately and by following hygienic guidelines.
  • The presence of lactic acid (diacetyl) can give a wine a smell of rancid butter or sour cabbage (in conjunction with acetic acid). This phenomenon is the result of the activity of malolactic bacteria, and is often caused by the flawed fermentation of malic or lactic acid. Also, the renewed form of diacetyl – butanediol (overtones of butter or yoghurt) – has positive organoleptic properties.
  • The Brettanomyces genus of yeast can give a wine an "animal" smell reminiscent of horse sweat or leather. Brettanomyces is a fungal disease which can contaminate wine, but which has positive organoleptic properties in low concentrations.
  • Overtones reminiscent of mouse urine in wine: this flaw can develop in wines whose acidity is low, which contain iron, or in wines predisposed towards oxidation.

© Malkhaz Kharbedia/Georgian Wine Guide

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