I went to an excellent seminar on wine faults at the Ecole du Vin de Bordeaux this morning, given by one of the fellow tutors who is also an oenologist. We went through the theory, then did the tasting. I always find these fault seminars (they are held once a year for all the tutors at the wine school) really fascinating and useful, so thought I would share it here.

Most of the major wine faults found in past decades have nearly all been eradicated - particularly the visual ones - but it is still important to be able to identify potential issues to know how to resolve them. The main fault categories are:
0. Faults from grape growing and harvesting
1. Visual and physio-chemical faults
2. Microbacterial faults
3. Oxidative and reductive issues
4. Ageing and barrel faults

Grapes and Harvesting
Vegetal Aromas
Causes: Usually this comes from havresting too early before the grapes are fully ripe.
How you can tell: Vegetal aromas, Green pepper, asparagus, herbaceous tastes. The threshold for most tasters to identify this is from 8-15 nanogram per litre (the chemical molecule responsible is 3-isobutyl 2 methoxpyrazine)

Moisi terreux
Causes: Presence of grey rot, also can come from dirty equipment used during vinification,
How you can tell: can get a mouldy, rotted aroma and smell, of wet earth. The threshold for this is 40-50 nanograms per litre, so again can discern it at low levels - although sometimes confused with brettonmyces.

Causes: The must has not been protected against oxygen, vinification has taken place without temperature control, and so there is an oxidation of the phenols in the wine - specifically, an enzyme called tyrosinase quinone combines w other phenols
How you can tell: The wine takes on brown hints (it is particularly obvious with a white wine) and loses its fruit aromas

Protein deposits
Causes: Lack of filtering and fining, especially common with white and rose wines, leading to the formation of insoluble complexes of tannin-protein or protein-protein. There have been lots of advancements made in this area and understanding of which graoe varieties are most at risk. Known as La Casse Proteique in French.
How you can tell: Cloudy, milky turbidity in the wine that affects above all its visual aspect

Iron or copper deposits
Causes: Use of copper in the winery (this was far more common in previous decades, now winery equipment is almost never made out of this substance) or iron rich soils excess of sulphite use or even from light. La Casse Metallique in French.
How you can tell: The copper combines with phosphorus acid in the wine, causing a white precipitation in white wines, or blue in red wines

Tartric precipitation
Causes: Not cold enough during ageing, lack of effective filtration. This can cause the precipitation of the tartaric acid as potassium bitartrates or calcium tartrates, which then form tartric cyrstals,
How you can tell: These crystals - that look like small lumps of sugar or salt - are visible in the bottles of neck, or the base. No quality problem for the wine, but consumers don't like it, and most producers know to cold stabilise to avoid it

An essential component in winemaking, but if too much used during bottling, or when wine has just been bottled, can get the bad eggs smell. Very rare, but unpleasant if you experience it. A wine without sulphur is very fragile, susceptible to oxygenation. Many oenologists have looked at what can replace it but its antioxidative, anti bacterial, antiseptic... It is highly useful!
NB: EU levels 160mg per litre for red, 260mg per litre for sweet 180 mg per litre for dry whites ( there might be slight difference depending on appellation, some up to 210mg per litre for dry white)

Microbacterial faults
Microbacterial faults we do still come in to contact with. They often can be subtle problems, but the taster just dont feel the wine is clean or pure.

Causes: Comes from unclean hygiene in winery, or insuffuciant sulphur, or temperature of the elevage (warm cellar conditions encourage it), detectable at 570 micrograms per litre. Brettanomyces is the transformation of phenolic acids to brettanomyces, found in the yeast.
How you can tell: This is the horsey, stables aroma more common in red wines, that some people identify as 'terroir'. Coinnoculation has grown as a result of fears of this, certainly a fault that people are very aware of now
This is a fairly controversial fault, and is well explained here: http://www.aromadictionary.com/articles/brettanomyces_article.html

Acetic spoilage
Causes: Lack of anti-oxygenation protection, lack of topping up carefully during ouillage, or stocking the wine at too high temperature. All of this case cause an oxidation of the ethanol (alcohol) into acetic acid, which then is turned (by a acetic bacteria) into ethyl acetate. Usually discernable at 150mg per litre for acetic ethyl
How can you tell: The wine will smell and/or taste of nail varnish remover, vinegar. It will also be enormously dry and bitter on the finish.

Lactic spoilage
Causes: Usually winemakers like lactic bacteria! But here it can stop the fermentation, and transorm the residual sugars in the grape must into to acetic acid and lactic acid.
How you can tell: A sour, sharp taste in the wine. More of a concern in hot years

La tourne
Causes: Usually from bad hygiene in the winery, not following vinification, leading to the spoilage of tartaric acid, so the wine loses too much acidity.
How you can tell: This again is rare nowadays, but completely unbalances a wine, making its PH levels too high and can be limp, flat wine, plus gives an unpleasant yoghurty aroma

Maladie de la graisse (roundness)
Transformation of polysaccharides by lactic acid to small amounts of aceitic acid, but more importantly loses the viscosity , so it is more of a visual fault, but can lead to limp, flat wine.

Maladie de l'amer (bitterness)
Causes: Transformstion of glycerol into acetic acid, comes from harvesting too early, but rare today
How can you tell: bitter finish to the wine, detectable at 10mg per litre

Le gout de souris (mousey taste)
This is more common with white wine, comes again from brettonomyces, and from lack of sulphites

Oxidation and reduction
Causes: Certain wines such as madeira or rancio this is looked for, otherwise it is a fault. Problems can come from badly managed pumping over, often found in sample bottles but any badly controlled bottling can be a problem.
How can you tell: Green apple, bitterness

Causes: From transformstion of yeasts intomethanethiol or ethanethiol
Not giving the wine enough potentisl for oxygen exchange, or fermenting the must in complete absense of oxygen w azote, using certain phytosanitaires products etc.
Slight amounts can be corrected by decanting etc, nut if it goes too far, the wine is spoilt
How can you tell: Onions, garlic, cauliflower, rotten eggs

Ageing faults
Corked wine

Causes: This is the TCA, can come from bad quality ccorks or from use of bad hygiene practises
2-5 nanograms per litre can begin to detect it, and sparkling wines even less. If a winery is infected, the only option is to replace everything w stainless steel
How can you tell: Musty smell, mouldy smells, generally dampening of the fruit.

Causes: Can arive from bad storage facilities, too much light or vibrations, 8 micrograms per litre can begin to detetct it.
How can you tell: Curry, nut smells, mainly in white wines

How to get rid of bad smells
Charcoal fining (must filter afterwards)
Using lees of yeasts to diminish reduction aromas
Oak chips are sometimes used to help mask minor odour issues without marking the flavour of the wine too much, can help mitigate, eg, some green aromas
But of course best way is to protect in advance - prevenir pas guerir!