Leavening agents, cream of tartar

Cream of tartar is a fine white powder, the acidic byproduct of winemaking, used almost exclusively as a kitchen ingredient rather than eaten on its own. Its main jobs are stabilizing whipped egg whites so they hold stiff peaks for meringues and souffles, preventing sugar syrups from crystallizing when making candy or frosting, and acting as the acid component in homemade baking powder when mixed with baking soda. It also gets a pinch added to boiling water when cooking cauliflower or other pale vegetables to help them keep their white color.

Because it is a dry, stable powder, cream of tartar keeps almost indefinitely in a sealed container in the pantry, with no refrigeration needed. A single teaspoon is only 3 grams and about 8 calories, so it barely registers as a food source in normal use, but per 100 grams it is exceptionally concentrated in potassium, at 351% of the Daily Value, reflecting its origin as a purified salt rather than a whole food. It also provides 21% of the Daily Value for iron per 100 grams. In practice it is used in such small quantities per recipe, a teaspoon or less, that it functions as a technical baking and cooking aid rather than a meaningful source of any nutrient in the finished dish.