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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.
Acid: A compound present in all grapes and an essential component of wine that preserves it, enlivens and shapes its flavors and helps prolong its aftertaste. There are four major kinds of acids--tartaric, malic, lactic and citric--found in wine. Acid is identifiable by the crisp, sharp character it imparts to a wine.
Malolactic Fermentation (ML): A bacterial fermentation occurring in most wines, this natural process converts sharper malic acid (found in green apples) into softer lactic acid (found in milk). Total acidity is reduced; the wines become softer, rounder and more complex. In addition, malolactic fermentation stabilizes wines by preventing an undesirable fermentation in the bottle. Often called the secondary fermentation. Frequently associated with big, rich, buttery Chardonnay, malolactic fermentation is prevented when fresher, crisper styles are desired.
Tartaric Acid: The principal acid in grapes and wine; contributes to taste and stabilizes color. Unlike malic acid, tartaric acid does not decline as grapes ripen. Tartaric acid can precipitate out of solution in bottled wine to form harmless tartrate crystals resembling shards of glass.
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