White wines become darker with brown tints. Red wines become brown and the pigments may precipitate out of the wine. The aromas become stale, flat and cardboard-like, with characters of green apples or sherry-like, aldehydic aromas. In extreme cases, the acetobacter and film yeasts will grow on the surface, raising volatility to unacceptable levels, as characterised by a strong vinegar aroma.
a. Exposure of wine to air before bottling
During winemaking, the wine is constantly exposed to air during racking, fining, filtering, storage in tank and bottling. If levels of oxygen are too high, and the wine is not protected through the use of inert gases such as carbon dioxide (through the addition of dry ice which blankets the surface of the wine), then the wine will oxidise.
b. Wines bottled under cork closures
Wines bottled under cork have a greater risk of oxidation than wines bottled under screw cap, especially if the cork is of poor quality. If the wine is exposed to rapid fluctuations in temperature during transport or storage, the cork will expand and contract, drawing air in and out of the bottle, exposing the wine to air. Also if the wine is not stored laying down then the cork can dry out, allowing more air to penetrate through the natural holes in the cork causing oxidation.
2. Cork Taint (TCA)
At low levels, it dulls the wine’s aroma. At higher levels, it gives the wine a musty, wet newspaper, dank cellar aroma and reduces the fruit, oak and other desirable characters. It is estimated that between 2 and 6 per cent of all wines bottled under cork are affected by faulty corks. People vary in their ability to detect TCA.
If the cork has been bleached and sterilized with chlorine during manufacture, the chlorine can be converted to chloranisole by microorganisms, causing the chemical compound known as 2,4,6 – triachloroanisole, or cork taint.
3. Hydrogen Sulfide
A rotten egg smell.
Nitrogen deficiency in the grape. Yeasts require sulfur containing amino acids and nitrogen to convert the sugars from the grapes into alcohol. If the levels of nitrogen are too low, then enzymes in the grape will generate excess sulfur, creating a rotten egg smell.
4. Ethyl Mercaptan
An onion or rubber aroma.
Reaction of ethanol with hydrogen sulfide.
5. Dimethyl Sulfide
At low levels, it can contribute fruity, fullness and complexity. At high levels, there is an unpleasant cooked cabbage water, canned corn, asparagus or truffle character
Occurs during bottle ageing probably due to the breakdown of sulfur containing amino acids.
6. Excess Sulfur Dioxide
Subdues the wine aroma, and strips the wine of its character. At higher levels, it has a sharp, irritating smell.
Too much free (or unbound) sulfur dioxide due winemaker error.
7. Volatile Acidity / Volatility
1. Acetic Acid
At low levels, it can lift the nose and add flavour and complexity to the wine to help to balance the residual sugars in a dessert wine. At higher levels, there is a vinegar smell or sharpness on the nose and a ‘pricked’ sensation with a hard, sour finish, especially at the back of the roof of the mouth.
2. Ethyl Acetate
At low levels, it can impart a fruity character to the wine and add complexity. At higher levels, it creates a solvent-like aroma similar to nail polish remover.
a. Wild yeasts present on the berries produce acetic acid and ethyl acetate during fermentation.
b. Acetic acid bacteria present on the grape oxidise ethanol to acetic acid.
A mouse cage or mousy aroma that leaves a hard, bitter taste in the mouth.
Mousiness is produced by both lactic acid bacteria and species of the yeast, Brettanomyces. Red wines that are made using low levels of sulfur dioxide are most prone to mousiness as sulfur dioxide is normally used to destroy unwanted microorganisms.
9. Brettanomyces (or ‘Brett’)
Depending on the type, Brett can add to the wine’s complexity through smoky, bacon, spice and clove aromas. Other less pleasant Brett characters include sweatiness, cheese, rancidity, band aid, medicinal, antiseptic and barnyard characters. People vary in their ability to detect Brett and therefore in their tolerance to it.
Brettanomyces yeast exists on the surface of the grape skins and also in the winery. It is more evident in red wines that have low levels of sulfur dioxide.