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Lactic Acid is one of six different acids found in wine and created by the winemaking process. A chemical compound usually found in dairy products, this mild acid is created when a wine undergoes Malolactic Fermentation (MLF) - the process that converts Malic Acid into Lactic Acid. Lactic Acid also appears naturally in grapes during the fermentation process when the yeast converts sugar to alcohol, though that is in minute, almost unnoticeable quantities; and it also appears when bacteria, called Lactic Acid Bacteria (LAB), converts sugar into Lactic Acid and Acetic Acids.
Softer on the palate than Malic Acid, Lactic Acid is only created in a wine if the winemaker feels that the wine is too harsh or sour. Two types of Lactic Acid are produced during the winemaking process: L-lactic Acid, produced through MLF and through regular fermentation; and D-lactic Acid; produced through regular fermentation. Both types of Lactic Acid can be produced by yeast or bacteria. Many winemakers will prevent the creation of Lactic Acid, for fear that "Lactic Souring" will occur. This is what happens when the bacteria introduced into the wine converts sugars into Lactic Acid and Acetic Acid (instead of converting Malic Acid into Lactic Acid). The end wine will have a sour milk smell that is entirely unpleasant. Winemakers can prevent this by racking wines quickly off their lees; shocking the wines with sulphur dioxide (which kills bacteria); and by monitoring used barrels and the wine itself for any sign of lactic souring.