Tartaric acid is a natural occurring acid found in grapes. It is used in winemaking to adjust the acidity of the wine. It can also be used to prevent the wine from becoming too acidic during fermentation.
 
To use tartaric acid in winemaking, dissolve the desired amount in a small amount of water and add it to the wine. The amount of tartaric acid you need to add will depend on the acidity of the wine. A good rule of thumb is to add 1 teaspoon of tartaric acid per gallon of wine to increase the acidity by 0.1%.