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.
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.