Tartaric acid (E472d), not to be confused with tartrazine, is a tart tasting white crystal that is found in grapes, bananas, and tarmarinds, and that accumulates on the sides of the barrel during the aging of wine. It's occasionally added to grape and berry juices to add additional tartness or to mask the aftertaste of artificial sweeteners.
Unlike many other additives to fruit and vegetable juices, there's nothing especially harmful about tartaric acid. It may even make fruit and vegetable juices and other beverages safer by lowering the pH of the liquid below the point at which bacteria can multiply. A grape beverage that (1) is not made with sugar but (2) has to have added tartaric acid added for tartness usually is especially high in vitamin C, since grapes use vitamin C to make tartaric acid to protect themselves from disease.