Acidification The addition of acid (usually tartaric) during fermentation, frequently necessary in hot climates where grapes tend to overripen and become deficient in acidity, thereby losing freshness.

Acidity The acids in a wine (principally tartaric, malic, citric and lactic) provide liveliness, longevity and balance: too much leaves a sour or sharp taste on the palate, while too little results in a flabby, shapeless wine. If tannin is the spine of a wine, then acidity is its nervous system.

Barrel or Cask Most of the world's greatest wines are at least partially aged in barrels, usually made from oak. A barrique is the standard Bordeaux barrel, holding 225 liters or the equivalent of about 300 bottles of wine. But casks may be as large as 100 hectoliters (i.e., 10,000 liters) or more.

Chaptalization The addition of sugar during fermentation to increase a wine's alcoholic strength.

Fermentation The conversion of grape juice into wine through the action of yeasts present in the juice, which turn sugar into alcohol. This alcoholic fermentation is also known as primary fermentation. (See Malolactic Fermentation.)

Filtration A method of clarifying and stabilizing wine to give it a pleasingly lucid color and to remove yeasts, bacteria or other solid matter that might otherwise spoil the wine after it has been bottled. Excessive filtration, like excessive fining, can strip a wine of aroma, body, texture and length.

Fining A method of clarifying wine by pouring a coagulant (such as egg whites) on top and letting it settle to the bottom. In general, a fining agent is allowed to fall through the wine, while in filtration, the wine is passed through a filter.

Lees Solid residue (mostly dead yeast cells and grape pulp, pips and skins) that remains in the cask after the wine has been drawn off. Many white wines and some reds are kept on their lees for a period of time to protect them from oxidation, enrich their textures and add complexity. Wines protected by lees contact can often be made with less sulfur addition, but careful technique is essential to ensure that off aromas don't develop.

Malolactic Fermentation A secondary fermentation in which the more tart malic acid is converted into softer lactic acid and carbon dioxide. Malolactic fermentation, which generally follows the alcoholic fermentation, is nearly always carried out in red wines. Some producers of white wines encourage malolactic fermentation, while others, especially those in hot regions that produce grapes with low levels of acidity, avoid it in order to retain the wine's freshness.

Must Grape juice not yet fermented or in the process of being fermented into wine.

Racking Transferring the wine from one cask to another to separate it from its lees.

Sediment Solid matter deposited in a bottle during the course of the maturation process. Sediment is generally a sign that the wine was not excessively filtered prior to bottling.

Sulfur The most common disinfectant for wine. Most winemakers feel that it is nearly impossible to produce stable wine without judicious use of sulfur products at one or more stages of vinification: just after the harvest to thwart fermentation by the wrong yeasts, in the cellar to prevent microbial spoilage and oxidation and at the time of bottling to protect the wine against exposure to air. But as a general rule, the amount of sulfur used in the production of fine wine has never been lower than it is today.

Tannin A bitter, mouth-drying substance found in the skins, stalks and pips of the grapes--as well as in wood barrels. Tannin acts as a preservative and is thus an important component if the wine is to be aged over a long period. Tannins are frequently harsh in a young wine, but gradually soften or dissipate as the wine ages in the bottle.

Yeast The various microorganisms that cause fermentation. Wild yeasts are naturally present on grape skins, but cultivated yeasts are generally used to control fermentation more carefully.