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

It is used in cooking as an acidic temper for baking soda (leavening agent) and also to stabilize egg whites.You can use equal amounts of vineger or lemon juice s a substitute.

It has virtually no nutritional value.