Laut Wörterbuch: "Cream of tartar": gereinigter Weinstein, doppelweinsteinsaures Kali.

Hierzu einige Auszüge aus amerikanischen Echos:

Baking powder vs. Baking soda [...] Baking powders, by contrast, also contain a dry acid, usually one of a variety of acidic calcium phosphates, or else potassium hydrogen tartarate, commonly known as cream of tartar, and some other stuff to keep it from [...] The acid phosphates and potassium hydrogen tartarate are used because they are among only a few convenient solid acids which are acceptable in baked goods. There are alternative acids available, but these are at least common. [...]

Tartar ? [...] Tartaric acid is a by-product of the fermentation process. Tartaric acid is known as a "diacid" (pronounced dye-acid) since it has two acid groups. Cream of tartar is made when one of these acids has been neutralized and substituted for by potassium. Cream of tartar is therefore the potassium salt of tartaric acid and it is weakly acidic. Cream of tartar is also found in most baking powders along with baking soda and sodium aluminum sulfate. Cream of tartar is commonly used in the beating of eggs. It's action is to change the pH of the albumin slightly and to stabilize the foam. To understand how stabilization occurs you only need to know that albumin is made of proteins and proteins are made from acids (amino acids). Whipping eggs breaks these proteins apart and they don't like this very much and try to get back together. Cream of tartar keeps this from happening by providing a suitable acid medium to keep the proteins from coagulating (reforming their bonds). Thus eggs beaten with tartaric acid keep their loft. More is not better, only 1/16th of teaspoon is recommended per egg white. [...]