Recipes that require stiffly beaten egg whites often call for potassium acid tartrate, better known as cream of tartar. Is this an ingredient you can skip, if you don’t happen to have it on hand?
To find out, we made two lemon meringue pies, one with the requisite 1/2 teaspoon cream of tartar for the four-egg-white meringue and one without.
After a stay in the refrigerator, the meringue without cream of tartar showed beads of moisture on its surface. The pie prepared with cream of tartar emerged in much better shape, with a dry, smooth, bead-free meringue.
The explanation? When egg whites are whipped, the protein strands within begin to unwind and form a network that holds water and air bubbles in place. But egg whites contain sulfur atoms, which form strong bonds that can over-strengthen this network. Acidic cream of tartar slows the formation of the sulfur bonds, preserving the stability of the network so that air and water stay put.
The bottom line:
For smooth, stiff beaten egg whites that keep their shape, don’t skip the cream of tartar. If you don’t have cream of tartar, substituting 1/4 teaspoon lemon juice or white vinegar per egg white works almost as well.