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| Grape growing, winemaking and wine maturation |
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| Making white wine |
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Once the grapes have been picked, they need to be transported to the winery. This can be done in bins that hold anywhere from about 15 pounds to one ton. The advantage of using a smaller container is that the grapes suffer less damage, expressing less juice. Juice that leaks out during transport is subject to oxidation, premature fermentation, spoilage from lactic and acetic acid bacteria and other dangers. These dangers can be reduced during mechanical harvesting by the use of SO2 and/or dry ice (or other types of refrigeration) to slow these reactions.
 Once the grapes are in the winery, the process of turning them in to wine begins. Let’s examine white wine first. White wines are pressed prior to fermentation. There are a variety of presses in use today. The state of the art technology is an enclosed bladder press, where the atmosphere can be flushed with nitrogen to prevent oxidation. The grapes run through a crusher / destemmer and then loaded into the press, and a bladder slowly inflates, pressing them against a grill.
At the opposite end of the spectrum is the vertical or basket press. This is essentially the same type of press that has been in use since medieval times. Here the whole clusters of grapes are loaded into the press, which then slowly presses them. Although some are now enclosed, they traditionally were open, and thus exposed to oxidation and contamination.
Once the grapes have been pressed, the unfermented juice or must is settled to remove foreign matter (material other than grape or MOG, as well as bits of skin, stem, leaves and other debris. This process may be expedited by the use of pectolytic enzymes that break down the pectins in the grape juice. It may also be filtered, fined or run through a centrifuge.
 After settling, the fermentation is begun, either by inoculating the must with cultured yeast or by allowing the natural wild or ambient yeasts to begin to ferment. Some winemakers will add food for the yeast in the form of diammonium phosphate (DAP) and other items, such as the vitamin thiamin that help the fermentation to proceed quickly and to continue to term.
In addition to these additives, other things are occasionally added as well. In some regions, sugar is added to increase the alcoholic strength of the resultant wine, a process known as chaptalization. Growers in other regions might find it necessary to add acidity in the form of tartaric acid prior to fermentation. Still other winemakers might add oak chips, oak essence, or tannin powder prior to fermentation. Some winemakers may also add artificial flavoring to the wine, and others have been known to add glycerol.
 Fermentation can be carried out either in oak barrels, temperature controlled stainless steel vats and concrete vats. Each of these vessels has their own particular advantages. Oak barrels give the wine greater complexity, in part because of the interplay between oxygen and the fermenting must. In addition, each barrel is a separate batch, and blending many batches together tends to increase complexity. Stainless steel provides a very convenient vessel, because it is easier to control the temperature and easy to clean. Concrete vats are prized because they hold their temperature well (thermal inertia).
The optimal temperature for white wine fermentation is 18–20° C, although there is considerable variation in this temperature. Cooler fermentations help preserve primary fruit aroma, but they can also accentuate the tutti-frutti, tropical character of the nose. Warmer temperatures can give more depth, but some primary fruit character is inevitably lost.
 Temperature control is accomplished in non-jacketed tanks by submerging pipes containing coolant in the fermenting must. Previous generations added ice to the fermenting tanks or sprayed the outside with cold water. Oak barrels are a self-regulating medium – the surface to air ratio is small enough that the heat of the fermentation dissipates in the air. Fermentation can be stopped by refrigeration, the addition of sulfur, pressure, sterile filtration, pasteurization or fortification.
After fermentation, the resultant wine may be aged on the lees, or dead yeast cells, or it may be racked off this deposit. If aging proceeds on the lees, they may be stirred or not stirred. Lees aging provides a certain tangy complexity and creamy texture to the wine. Aging on the lees can be done either in barrel or in tank, and is increasingly practiced on red wines as well as white.
It is also after this initial fermentation that malolactic fermentation takes place, both in red wines (nearly always) and white (sometimes - see below). Malolactic fermentation (or MLF) is a transformation of the harsh malic acid prevalent in wine grapes into softer lactic acid. Not a fermentation in the same sense as the alcoholic fermentation caused by yeast, this is done by lactic acid bacteria that consumes the malic acid and produces lactic acid. Malic acid is the type found in granny smith apples, while lactic acid is the acid found in milk. MLF gives the wine a distinct buttery character that comes from a chemical known as diacetyl that is produced during the process.
Not all white wine goes through MLF, for two reasons – either it is desirable to the keep the acidity for the wine (particularly in warm climates), or because the buttery character is not wanted in the wine (particularly with aromatic varieties such as Riesling). MLF can be prevented with sulfur or sterile filtration. When MLF is not performed, the wine is sometimes chemically deacidified. Calcium carbonate or its derivatives are used for this process.
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Wine Regions of the World
