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Cream of tartar is a dry, powder-like, acidic byproduct of winemaking called for in a myriad of different baking recipes (from cookies to cakes to frosting). Grapes are a natural source of cream of tartar’s main ingredient: tartaric acid (hence the “tartar” in its name). And when tartaric acid is half neutralized with potassium hydroxide, cream of tartar is the result, crystalizing into a hard crust on the inside of wine barrels.