cream of tartar

a white, crystalline, acidic compound obtained as a byproduct of wine fermentation and used chiefly in baking powder. • Alternative name: potassium hydrogen tartrate; chem. formula: HOOC(CHOH) 2 COOK.

Potassium bitartrate, also known as potassium hydrogen tartrate, has formula KC4H5O6. It is a byproduct of winemaking. In cooking it is known as cream of tartar. It is the potassium acid salt of tartaric acid, a carboxylic acid.

Occurrence: Potassium bitartrate crystallises in wine casks during the fermentation of grape juice, and can precipitate out of wine in bottle. This crude form (known as beeswing) is collected and purified to produce the white, odorless, acidic powder used for many culinary and other household purposes.

In food, potassium bitartrate is used for:
Stabilizing egg whites, increasing their heat tolerance and volume;
Preventing sugar syrups from crystallising;
Reducing discolouration of boiled vegetables;
Frequent combination with baking soda (which needs an acid ingredient to activate it) in formulations of baking powder.
Commonly used in combination with potassium chloride in sodium-free salt substitutes
A similar acid salt, sodium acid pyrophosphate, can be confused with cream of tartar because of their common function as a baking powder.

Household use: Potassium bitartrate can be used with white vinegar to make a paste-like cleaning agent. It is a vital ingredient in Play-Doh and gingerbread house icing. This mixture is sometimes mistakenly made with vinegar and sodium bicarbonate (baking soda), which actually react to neutralise each other, creating carbon dioxide and a sodium acetate solution.