Grape growing, winemaking and wine maturation

After the grapes are picked, they are transported to the winery in bins that hold anywhere from 15 pounds to one ton. In smaller containers, the grapes suffer less damage and express less juice. Any juice that leaks out of the grapes during transport is subject to oxidation, premature fermentation and spoilage. If harvest is done mechanically, sulfur dioxide, dry ice, or other types of refrigeration are sometimes used to slow down these reactions.

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White wines are pressed prior to fermentation. There are a variety of presses used today, but the most advanced technology is an enclosed bladder press in which the grapes and juice are flushed with nitrogen to prevent oxidation. The grapes are run through a crusher and destemmer and then loaded into the press. The bladder slowly inflates pressing the grapes against a grill to extract the juice.

At the opposite end of the technology spectrum is the vertical or basket press. This type of press has been used since medieval times. Whole clusters of grapes are loaded into the press and pressure is evenly applied to grapes. Although some basket presses are now enclosed, they were traditionally open vats, leaving the must open to oxidation and contamination.

Whether the design is bladder, membrane or vertical press, today's grape presses are highly sophisticated, with computerized programs that can be customized to vary time and pressure over the entire length of pressing. Remarkably, today's cutting edge presses can complete an extremely slow, gentle process of crush without breaking the skin of the pressed grapes.

Once the grapes are pressed, the unfermented juice (must) is allowed to settle in order to remove skins, stems, leaves and any other debris. Some winemakers use pectolytic enzymes to break down the pectin in the grape juice and speed up the settling. The must is also sometimes allowed to cold settle, filtered, fined or run through a centrifuge.

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Following settling, fermentation is begun, either by inoculating the must with cultured yeast or by allowing the naturally existing wild or ambient yeasts to start the fermentation. Some winemakers add food for the yeast in the form of diammonium phosphate (DAP) and other additives like the vitamin thiamin to help the fermentation proceed quickly and fully.


There are many other additives that can be used in winemaking. Some are regularly used in quality winemaking. In some regions, sugar is added to increase the alcohol level of the wine, a process called chaptalization. For example, other additives more widely used in mass-produced wines to accentuate acidity are the addition of tartaric acid; the addition of oak chips or tannin flavor to add flavor; or the addition of glycerol to add texture.

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Fermentation can be carried out in oak barrels, temperature-controlled stainless steel tanks or concrete vats. Each of these vessels has a particular advantage.

Oak barrels allow for more interplay between oxygen and the fermenting must, which can lead to more depth and complexity in the wine. Since each barrel contains a separate small batch of wine, the winemaker has more opportunity for blending to attain the desired wine.

Stainless steel tanks are the most convenient fermentation vessels because they make it easier to control the temperature and speed of the fermentation without negatively influencing the flavors in the wine. And, from a purely practical standpoint, they’re easier to clean than oak.

Concrete vats are prized for their ability to maintain a steady temperature; they are much more moderate than either wood or steel. Many winemakers feel that the many microscopic pits within the inside surface of concrete tanks allows for a softer process of micro oxygenation than wood vats.

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The optimal temperature for white wine fermentation is 18–20°C (64-68°F), but there is considerable variation from winery to winery depending on the desired resulting wine. Cooler fermentations preserve primary fruit aromas but they can also accentuate 'tutti-frutti' or tropical notes on the nose. Warmer temperatures can give more depth, but at the cost of losing some of the primary fruit character.

In non-jacketed tanks, submerging pipes containing coolant in the fermenting must can help in controlling the temperature of the fermentation. Oak barrels, however, don’t require such efforts because the wood is a self-regulating medium. The heat from the fermentation is released through the tiny pores in the wood. Fermentation can be stopped using many different methods including refrigeration, the addition of sulfur or by applying pressure.

After fermentation, the wine can either be aged on the lees, dead yeast cells suspended in the wine, or immediately racked off this deposit. Lees aging gives certain tangy complexity and creamy texture to the wine. Some winemakers choose to periodically stir the lees in a process called batonnage, while others will leave it still. Aging on the lees can be done in barrel or in tank and is increasingly practiced to make red wines as well as white.

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After this initial fermentation, malolactic fermentation takes place, nearly always in red wines and sometimes in whites. Malolactic fermentation (or MLF) is the conversion of the sharp malic acid, present in all wine grapes, into softer lactic acid. Malic acid is the crisp, tart type of acid found in granny smith apples, while lactic acid is the creamy acid found in milk. In malolactic fermentation, lactic acid bacteria consume the malic acid, producing lactic acid. MLF gives the wine a distinct buttery character that comes from a chemical known as diacetyl, produced during the fermentation process.

Not all white wine goes through MLF. Some winemakers prefer to retain the bright acidity in the wine, particularly in warm climates, while others simply don’t want the buttery character to be present. Adding sulfur or using sterile filtration can prevent MLF. When MLF is not performed some winemakers choose to chemically deacidify the wine using calcium carbonate.

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