|
| Grape growing, winemaking and wine maturation |
|
| Stabilization |
Take the Quiz |
| |
|
|
Once the fermentation process is complete, wines undergo different techniques of stabilization, finishing and maturation. Some of these techniques are intended to add things to the wine, others to remove things, and yet again others are meant to change the wine without adding or subtracting anything.
Freshly fermented wine can contain a number of impurities. These can include lees or active wine yeast cells or other types of yeast that can cause off flavors such as brettanomyces, bacteria (lactic or acetic acid bacteria), proteins, and excess iron or copper.
 Racking is the process of removing clear wine from sediment that has settled out by draining it carefully off the sediment into another container. The first racking is the removal of the wine from the lees of the fermentation. As the wine continues to settle, it can be racked as often as possible to ensure natural clarity. This is called “falling bright”, and is usually sufficient to ensure stability in a wine.
Racking also exposes wine to oxygen. This is an essential component of wine maturation that must be carried out. The reason for this is that sulfur present normally in wine combines with oxygen. If none at all is present, the sulfur can lose electrons (a process known as reduction), which causes the development of hydrogen sulfide. This gives a “rotten egg” smell to the wine. It can easily be avoided by racking, however, if it is not removed, it can lead to the development of more intractable mercaptans that stink even worse. An excess of oxidation, however, is not desirable either, because it leads to a loss of fruit and degradation of the wine, necessitating additional sulfur treatments.
 Another common treatment for wines after fermentation is filtration. This also removes sediment and impurities from the wine, and does so more quickly and thoroughly than simple racking. There are several types of filtration. They are usually categorized according to how much material they remove from the wine. One type is depth filtration, where wine is filtered through layers of a substance such as cellulose pads (plate and frame filtration) or diatomaceous earth (kieselguhr filtration). This is (or can be) a fairly light filtration – the density of the filtering material is variable). Only the largest particles are removed, and little damage is normally done to the wine.
Another type of filtration is called absolute filtration, and here the wine is filtered through a membrane with holes of a given size. This type of filtration can be used to remove much smaller particles, and is often accused of removing character from the wine. Examples of absolute filtration include membrane filters and cross-flow filters. These types of filters are often used as polishing filters, after depth filtration has been used. They can be used to sterile filter the wine, removing nearly all possibility of refermentation or spoilage from bacteria unless the wine is re-exposed to the hazard. “Ultra-filtration” uses membranes that remove the smallest possible particles from wine.
Fining is another technique of clarification that consists of adding a substance to wine that consists of particles with a particular electrical charge. These particles will bind with particles with an opposite charge, causing them to precipitate out. The wine is then filtered to remove the residue. It is useful for removing materials such as proteins that can cause a haze later in the wine. It also removes some particles of tannin and color from a wine.
Organic compounds used in fining include egg whites (red wine) isinglass (a fish by-product), milk (casein) and gelatin. Mineral elements such as bentonite and kaolin clay can also be used, as can synthetic chemicals such as PVPP or polyvinylpolypyrrolidone. Other techniques that are used to improve a wines stability include the use of a centrifuge and pasteurization.
David Bird is of the opinion in his book Understanding Wine Technology that tartrate stabilization is one of the most significant problems facing a modern bottling plant. It is essentially another form of clarification, since most wine throws a deposit of tartrate crystals eventually. These are the result of tartaric acid and potassium in the wine that form potassium bitartrate (which precipitates easily), and calcium tartrate (which is difficult to precipitate). These crystals are harmless, but can lead to consumer complaints. For this reason, they are often removed using several different methods.
In cold stabilization, the wine chilled to -4 ºC (for table wines and -8 ºC for sweet or fortified wines), and stored for eight days. This causes the tartrates to precipitate around minute crystals present in wine. The process, however, is fairly inefficient because the crystals form on bottom of tank, and it is necessary to stir the wine to have it all come in contact with the crystals. The equipment for cold stabilization is expensive to install and to run, and the results are unpredictable.
In the contact process, the wine is chilled to 0∞ C and crystals of potassium bi-tartrate are added. The wine is stirred, and crystals form. These are removed by filtration, ground down and reused. A continuous version of this system is now being used where cooled wine is pumped through a bed of crystals into a tank and siphoned off at the top of the tank
Metatartaric acid is an additive that prevents formation of tartrates for 6 - 18 months. It eventually breaks down to tartaric acid, encouraging more crystal formation. For this reason, it is used for short shelf-life wines.
|
|
|
|
Wine Regions of the World
