Tartaric is the acid occurring naturally in ripe grapes. It is the softest of all the principle acids, and is the preferred choice for most winemakers. It has three distinct advantages:
Tartaric acid can combine with the alcohol in wines and produce beneficial esters that improve the bouquet. It provides a higher level of protection against any contaminating organisms present than either malic or citric.
If present in excess it will tend to precipitate out as crystals of the bitartrate, especially if the wine is stored cool.