Cream of tartar is a white crystalline compound that exists in grapes, tamarinds, and other fruits. It is prepared from the sediments in wine barrels and is called argol. The argol is dissolved in hot water, and the colouring matter is removed by means of clay or egg albumin. After being purified by crystallization, it is ready for the market.
Cream of tartar has no leavening capacity by itself, but can be used in combination with sodium bicarbonate to provide the acid necessary for leavening. It was formerly used for making baking powder but that practice has been largely discontinued because of cost and its too quick reaction time. In some recipes, it is used alone. Cream of tartar is also an excellent stabilizer for egg whites.
Further refining of cream of tartar will isolate the principal active ingredient, tartaric acid, which is clear and odourless. It is soluble in water and has a rapid reaction in a batter when mixed with baking soda. It is also used to make soft cheeses such as mascarpone.