Have you ever bought a bottle of wine described on the back label as having an "attractive clean bouquet evoking meadow flowers, honey and linden-tree in full bloom, which carries on to the crisp, refreshing palate, pleasing and well balanced, leading to a lingering long finish" and when you opened it you had the impression it smelt like the bottom of a bird's cage?

Much has already been written by experts in this country on technological problems which occur during the wine-making process and subsequent causes of a wine's instability.  Let us concentrate on those faults which are most common and that you encounter in everyday life as a consumer shopping for wine or as a customer in a restaurant.  In any event, if you come across a fault in the wine you ordered or purchased, you should send it back.

Corked wine, one of the most common faults, is not a description of a wine with bits of cork floating in it, but rather it indicates a presence of cork taint caused by a chemical compound called 2, 4, 6 trichloroanisole (TCA for short) which enters wine via a poor-quality cork.  TCA imparts distinct mouldy and musty smells often likened to old socks, damp cellars and even hamster cages in bad need of a clean!  Cork contamination can vary.  It may be very slight, barely recognisable and only detected by experienced tasters.  On the other hand, strongly affected wine is overwhelmed and any semblance of a varietal character is destroyed.  Although cork taint does not represent a health hazard, there is no remedy for corked wine other than to throw it away.  You can imagine the disappointment of a wine lover treasuring a unique bottle of a very special wine which is then opened on that special occasion...  and suddenly - horror!  I personally remember one such occurrence and you don't have to be a mathematical genius to work out the cost.  The culprit was none other than the famous wine from Pomerol, Bordeaux - Pétrus 1979!

In order to train yourself in cork-taint spotting, you can try the following test.  Once you have a seriously corked wine at your disposal, label five glasses from 1 to 5.  Sample number 1 will be 100% tainted wine, whilst number 5 will be a perfectly normal, unspoilt wine.  Numbers 2, 3 and 4 will have a certain measure of contaminated wine added to the normal wine, this being from 1 teaspoonful to 2 tablespoons.  Then you compare them all.

Maderised or oxidised wine (derived from the French word Madčre, meaning Madeira and thus madérisé, meaning oxidised, which is the process by which Madeira wine is deliberately made) is easily recognisable by a browning of the colour.  The colour of a young white wine will have an ochre to pinkish hue, whilst a red will be turning tawny.  Other characteristics are acetic smells and a dull, flabby palate, generally unpleasant.  Oxidation is caused by the reaction of oxygen with various wine components.  When oxidation occurs during the wine-making process, the winemaker is able to treat the condition by using sulphur dioxide or ascorbic acid.  If the wine has oxidised once in the bottle, due to contact with air through improperly fitted corks or leakages, you should return it.  However, there are many styles of wine that may have been deliberately oxidised, such as Madeira, Sherry, Vin Santo, Rancio, Vin Jaune and other styles vinified "sous voile" (literally under a veil, for example the thin film of yeast referred to as flor which forms on the surface of Fino Sherry) and they are greatly appreciated.

Volatile acidity (VA) or acetic spoilage can also be easily recognised due to its strong vinegary smells coming from an excessive amount of acetic acid or the related compound, ethyl acetate, which is reminiscent of nail polish (acetone).  Again, certain styles of wine, such as straw or passito wine (vin de paille) and even Sauternes, will have a higher content of volatile acidity present, which is desirable.  However, when contamination by acetobacter occurs, there is no cure other than turning it into vinegar.  It is pointless to add normal healthy wine to a volatile example, as this will simply serve to infect the healthy wine.

Over-sulphured wine will have been infused with an increased amount of sulphur dioxide.  This is characterised by a sharp pungent sensation in the back of the nose which can be likened to the smell of a freshly struck match.  Although sulphur dioxide is a necessary compound used in winemaking for its preservative properties, some people may be allergic to it.  Its use is strictly controlled and levels permitted are governed by law.  In the USA it goes to such extremes that wine producers and importers must put on the label a warning (apart, from the one by the Surgeon General that alcohol is dangerous to pregnant women etc.) that the product contains sulfites.

Hydrogen sulphide (H₂S) is another easily spotted wine fault characterised by the smell of bad eggs.  It often reacts with the wine's other components to create compounds called mercaptans which  have an unpleasant odour reminiscent of burnt rubber and rotting vegetables.

Tartrate crystals, on the other hand are harmless, tasteless, odourless and quite natural deposits of potassium or calcium tartrate that precipitate in colder temperature.  Often they are referred to as a sign of good winemaking, though they are undesirable in certain markets (USA, Japan) for aesthetic reasons.

Bubbles in an otherwise still wine do not usually indicate anything good, especially in a red wine where they would most certainly indicate some sort of instability.  However, before making a complaint, be sure this is not one of those types of wine in which this is a normal occurrence (Portuguese Vinho Verde, French Muscadet sur-lie) or that it is not down to modern winemaking methods where large doses of carbon dioxide are produced naturally during temperature-controlled fermentation in order to preserve the wine's youthful freshness.  Should you object to these bubbles, let the opened bottle stand for 15 minutes before serving during which time they should escape.

Mousiness is not usually detected on the nose but a few moments after the wine has either been swallowed or spat and this spreads from the back of the throat to the tip of the tongue.  The aftertaste is very unpleasant, reminiscent of mouse droppings or stale dog biscuits and is very difficult to rinse away.  It is still found quite commonly in bulk wine, and occasionally in bottle.  Grape varieties such as Irsay Oliver and Moravian Muscat are particularly prone to this.  Poor quality reds can also suffer from this affliction, which is caused by an infection of lactic acid bacteria due to poor hygiene.

Other microbial spoilage you may encounter may be bacterial spoilage by fermentation yeasts and proteins.  This occurs during bottling if the wine has not been filtered properly.  It may be in the form of a sediment or cloudiness and is referred to as casse.

Brettanomyces or Brett for short is one of the yeast strains found occassionally on grapes and in wines. They can be both anaerobic and aerobic.  It thrives in dirty cellars where the fruit fly (Drosophila melanogaster) can also carry it. Brettanomyces are difficult to manage.  Fortunately, it is sensitive to SO₂ and nowadays there are tools available to winemakers for monitoring Brett that can be very advanced.  There are four key by-products of Brett growth which can affect the aroma and flavour of wine:  esterases, volatile fatty acids, tetrahydropyridines and volatile phenols.  Two critical volatile phenol compounds have been isolated from Brett activity: 4-ethylphenol (4-EP) and 4-ethylguaiacol (4-EG).  EP gives animal, sweaty saddles flavour, whilst EG smoky, spicy, cloves-like flavours which is appealing. Brett in its perfect form is known as Dekkera, but Australian winemakers are vehemently against it. They are obsessed with cleanliness in their cellars and winemaking equipment, describing the off flavours of some (especially European) wines as "mousy".  Brettanomyces are usually considered a spoilage yeast, but low levels in red wine can improve complexity. They are also essential in the production of Belgian beers, notably in the spontaneously fermented Lambic and Gueuze.