Why Does That Bottle Smell Wrong? Faults With Identifiable Causes

A fault is a compound present at a level that most tasters recognise as spoiling the wine, which makes the category partly objective and partly a matter of threshold and expectation. The same compound at a lower concentration may add complexity, and several compounds regarded as faults in one style are defining characteristics of another, which is why the honest definition involves the level and the context rather than the presence of anything. What distinguishes a fault from a preference is that the compounds involved have identifiable chemical sources, are measurable, and are recognised across trained tasters at concentrations above the relevant threshold, which puts them on firmer ground than most judgements about taste.

A handful account for nearly everything encountered:

• •Cork taint, giving damp cardboard and musty aromas and stripping fruit, caused by a compound formed by moulds acting on chlorinated material
• •Oxidation, giving bruised apple, sherry and flat colours, from too much air contact
• •Volatile acidity, giving vinegar and nail varnish, from acetic acid bacteria
• •Reduction, giving struck match, rubber and drains, from sulphur compounds forming in the absence of oxygen
• •Brettanomyces, a yeast giving farmyard, plaster and medicinal notes, which some styles tolerate deliberately
• •Heat damage, giving stewed flavours, from storage or transport at high temperature

Cork taint was serious enough to alter how wine is packaged worldwide. The responsible compound is detectable by many people at concentrations of a few parts per trillion, which is among the lowest thresholds for any aroma compound, and it arises when moulds present in cork act on chlorinated compounds, historically introduced by bleaching. Estimates of affected bottles ran from a few per cent to considerably more at its worst, which represents an enormous quantity of wine and an enormous commercial problem. The response was a shift to screw caps and synthetic closures for a large share of production, particularly in countries with no cork industry, alongside substantial work by the cork industry on sourcing and processing that has reduced the incidence markedly.

Several faults arise after bottling and are entirely the responsibility of whoever kept the wine. Heat is the worst, since warmth accelerates every chemical reaction in the bottle and drives expansion that can push the cork and admit air, which is why wine stored above a radiator or left in a car deteriorates within days. Temperature swings are worse than a steady warm temperature. Light causes reactions producing sulphur compounds in bottles that are not dark glass, which is why the fault has a name referring to light. A dried cork shrinks and lets air in, which is why bottles with natural cork are stored on their side and why screw-capped bottles need not be. Vibration is frequently blamed and the evidence for it is weak.

Several things commonly reported as faults are not, and distinguishing them saves argument. Crystals in the bottom of a bottle or on a cork are tartrate deposits, which are harmless, tasteless and indicate that the wine was not filtered to remove them. Sediment in older red wine is normal and is why such bottles are decanted. Sulphite additions are made to nearly all wine and cause the headaches attributed to them far less often than is believed, since the quantities are lower than in many dried foods. A closed or muted wine may open with air rather than being faulty. And a style somebody dislikes is not a fault, which is the distinction that most needs stating.

A fault is a compound above the level most tasters find spoiling, and the same compound lower down may add complexity, so the definition involves concentration and context. Cork taint, oxidation, volatile acidity, reduction and a particular yeast account for nearly everything. Cork taint is detectable at parts per trillion and drove the worldwide shift to screw caps.