Tartaric Acid exists naturally in Grapes (and Bananas) and is one of the main Acids in Wine. Addition of Tartaric Acid will lower the PH of the Must to a level where undesirable Spoilage Bacteria can't exist, thus acting as a Preservative after Fermentation. Presence of Tartaric Acid in wine can cause "Wine Diamonds" which appear like broken glass on the cork surface but are harmless.
