Not all "bad" wine is faulty
Wine that tastes too tannic, too acidic, or too oaky might not be to your taste, but it is not faulty. A wine fault is a specific, identifiable flaw caused by a problem in the winery or during transport or storage. Here are the most important ones to recognise.
TCA: cork taint (the "corked" bottle)
What it smells like: Damp cardboard, wet dog, mouldy basement. The wine smells musty and suppressed.
What causes it: TCA (2,4,6-trichloroanisole) is a compound that forms when natural cork reacts with certain chlorine-based compounds. At high concentrations it makes wine smell revolting; at low concentrations it simply strips the wine of its fruit and makes it seem flat and dull.
What to do: Return it. A corked bottle should always be replaced by any decent retailer or restaurant. About 2-5% of cork-sealed bottles have some degree of TCA, a significant problem the industry has worked hard to reduce.
Oxidation
What it smells like: Bruised apple, cider vinegar, sherry-like nuttiness in a wine that should not have it.
What causes it: Too much oxygen exposure during winemaking or after opening. A red wine left open for three days will taste oxidised.
What to do: Not a fault in all contexts, Sherry and Madeira are deliberately oxidised. But in a fresh table wine it is a defect. At a restaurant, oxidation usually means the bottle was opened too long ago.
Reduction
What it smells like: Struck match, rubber, cooked egg, or garlic.
What causes it: The opposite of oxidation, too little oxygen contact during winemaking creates sulfide compounds. Common in wines made without oxygen exposure (protective winemaking).
What to do: Decant vigorously or leave in the glass for 20 minutes. Reduction often blows off with air exposure. Not always a fault, some natural wines are deliberately reductive.
Refermentation
What it smells like: Unexpected fizz in a still wine, sometimes combined with yeasty notes.
What causes it: Residual sugar refermented in the bottle. Common in inexpensive wines, rare in quality wines.
Brett (Brettanomyces)
What it smells like: Barnyard, leather, band-aid, horse sweat.
What causes it: A wild yeast (Brettanomyces) contaminating the wine. At low levels many find it adds complexity (common in some Burgundy and Rhône). At high levels it overwhelms everything.
