Grape growing, winemaking and wine maturation

Racking is the process of removing clear wine from the barrels or tanks by draining it carefully off the sediment into another container. The first racking is done to remove the wine from the lees after fermentation. As the wine continues to settle, it can be racked as often as needed to attain natural clarity. This state is called “falling bright” and is usually sufficient to ensure stability in a wine.

Along with clarifying the wine, racking also exposes it to oxygen, an essential component of wine maturation. Without the presence of oxygen, the sulfur present in wine can lose electrons in a process known as reduction which causes the development of hydrogen sulfide, which gives wine a "rotten egg" smell. This can easily be avoided by racking. If the hydrogen sulfide is not removed, it can lead to the development of more intractable mercaptans that smell even worse. An excess of oxidation is not desirable either because it leads to a loss of fruit and degradation of the wine, necessitating additional sulfur treatments.

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Fining is a technique of clarification in which a substance consisting of particles with a particular electric charge are added to the wine. These particles bind with particles of an opposite charge causing them to precipitate and fall out. The wine is then filtered to remove the precipitate. Fining 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, a traditional method used to fine red wine; isinglass, a fish by-product; milk, or 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 like using a centrifuge and pasteurization can be used to improve a wine’s stability but these methods are not used in quality winemaking because they are incredibly harsh on the wine.

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When tartaric acid and potassium in wine form potassium bitartrate, which precipitates easily, and calcium tartrate, which is difficult to precipitate, these compounds crystallize and form deposits in the wine. Most wine throws tartrate crystals over time. These crystals are harmless but can lead to consumer complaints. For this reason, they're often removed using several very debatable methods.

In cold stabilization, wine is chilled to -4ºC for table wines and -8ºC for sweet or fortified wines, and stored for eight days. This causes the tartrates to precipitate around minute crystals present in wine. The process is fairly inefficient because the crystals form at the bottom of the tank, so it is necessary to stir the wine to have it all come in contact with the crystals. The equipment for cold stabilization is expensive and the results are unpredictable.

In the contact process, the wine is chilled to 0ºC and crystals of potassium bitartrate 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.

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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 categorized according to how much they remove from the wine. In depth filtration, the wine is filtered through layers such as cellulose pads (plate and frame filtration) or diatomaceous earth (kieselguhr filtration). This can be a fairly light filtration depending on the density of the filtering material because only the largest particles are removed and little damage is usually done to the wine.

Another type of filtration is called absolute filtration. 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 performed and 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.

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