Tartaric acid is an organic acid. It occurs naturally in many plants (ex: grapes, bananas). It is one of the main acids found in wine. It is added to other foods to give it a sour taste, and it's used as an antioxidant. Salts of tartaric acid are known as tartrates. You may even have a bottle of this in your kitchen that you use to cook with!
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Be careful not to confuse "Cream of tartar" with "Tartar sauce"..."Cream of Tartar" is the potassium salt of tartaric acid.