Cream of tartar is potassium hydrogen tartrate, which is an acidic salt. Grapes are the most common natural source of tartaric acid. During the wine production process, a sediment forms as a result of combining potassium hydroxide with the tartaric acid, leaving a white sediment: cream of tartar.
A combination of cream of tartar and baking soda was the original baking powder. Baking soda reacts almost instantly when exposed to wet/acidic ingredients, but the addition of cream of tartar tempers the effect of the baking soda and delays the rising reaction – which will produce a higher rise in the oven. Unlike baking powder, however, cream of tartar will not lose it potency over time, so you can buy a jar and keep it in your kitchen cupboard forever.
While not generally used in dependently, it serves a variety of purposes in the kitchen. The most common purpose is in helping to beat egg whites. Egg whites (and baking soda) are two of the most common non-acidic foods that are found in the kitchen and adding cream of tartar when beating them will help them to stabilize, resulting in an increase in volume in the final beaten egg whites. A pinch of salt or a small amount of vinegar can also help, but there is no exact substitute for cream of tartar.
Cream of tartar can also be added to sugary confections to produce a creamier texture, because the acid helps counteract crystallization as the sugar cooks in frostings and candies. For more recipes that use cream of tartar, take a look at these: