Tartaric acid is the molecule that makes unripe grapes taste sour. It is a principal flavor element in wine.
Tartaric acid is used as a flavoring agent in foods to make them taste sour.
The potassium salt of tartaric acid (potassium bitartrate or potassium hydrogen tartrate) is weakly acidic, and is known as "cream of tartar".
Since it is a dry acid, cream of tartar is used in baking powders (along with sodium bicarbonate) to produce carbon dioxide gas when water is added. Other acids used in baking powder are fumaric acid and phosphoric acid
phosphoric acid: InChI=1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)/f/h1-3H