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.
