Malic acid is a naturally occurring acid found in grapes. It is also added to wine to adjust the acidity. Malic acid can help to balance the sweetness of the wine and make it more refreshing. It can also help to improve the clarity and color of the wine.
 
Malic acid can be added to wine at any stage of the winemaking process. However, it is most commonly added during fermentation. To add malic acid to wine, simply dissolve the desired amount in a small amount of water and then add it to the wine. The amount of malic acid that you need to add to your wine will depend on the desired acidity level. A good rule of thumb is to start with 1 tsp of malic acid per gallon of wine and adjust as needed, 1 tsp of malic acid will increase the acidity of wine by about 0.1%.