Cream of Tartar

Recipes that call for whipping egg whites often add cream of tartar to the whites. What is it, and why is it added?

Cream of tartar, also known as potassium bitartrate, is a powdered byproduct of the winemaking process and, along with baking soda, is one of the two main ingredients in baking powder. Cream of tartar’s acidic nature lowers the pH of egg whites, which encourages the eggs’ proteins to unfold, thus creating more volume, greater stability, and a glossier appearance.

We compared whipped egg whites and cooked meringue (an egg white and sugar mixture) prepared with and without cream of tartar. For the whipped egg whites, the volume of the batch with cream of tartar was nearly double that of the batch without it. Both versions of the cooked meringue were similar right out of the oven, but after two days the meringue without cream of tartar had begun to separate and exude liquid (or “weep”), and the meringue made with cream of tartar was fully stable. Since meringues are often used to top pies and frost cakes, adding the cream of tartar is a wise step.