Alcoholic Fermentation: Also called primary fermentation, this is the process in which yeasts metabolize grape sugars and produce alcohol, carbon dioxide and heat. The final product is wine.
Fermentation: The process by which yeast converts sugar into alcohol and carbon dioxide; turns grape juice into wine.
Malolactic Fermentation (ML): More accurately referred to as "malolactic conversion." A bacterial conversion occurring in most wines, this natural process converts sharper malic acid (the same acid found in green apples) into softer lactic acid (the same acid found in milk). Total acidity is reduced; the wines become softer, rounder and more complex. In addition, malolactic conversion stabilizes wines by preventing an undesirable fermentation in the bottle. Most red wines undergo malolactic conversion, but the practice is most frequently discussed in association with Chardonnay: When employed, ML results in rich, buttery whites; it's prevented when fresher, crisper styles are desired.
Secondary Fermentation: The process that creates the bubbles in sparkling wine. As the wine is bottled, a small amount of yeast and sugar is added before the bottle is sealed with a sturdy crown cap. The yeasts quickly start fermenting the sugars, producing alcohol and carbon dioxide. Since the gas cannot escape, it dissolves into the wine.
Temperature of Fermentation: As yeasts convert grape sugars into alcohol, they also produce heat. Excessively high temperatures can kill the yeasts and make the wine’s fruit flavors seem stewed or dull, whereas cooler temperatures maintain the freshness of the fruit. Just the right amount of warmth can contribute a richer, rounder mouthfeel.
Malic Acid: A sharp, tart acid found in grapes as well as in green apples. Less-ripe grapes or grapes grown in cooler climates can contain high levels of malic acid; the resulting wines often contain aromas and flavors reminiscent of green apples. It is converted to smoother lactic acid during malolactic fermentation.
Lactic Acid: A smooth (not sharp) acid created during malolactic fermentation. This acid is also found in milk.