" Acid aids in stabilizing whipped egg whites by encouraging more, finer air pockets, helping to increase the volume of the egg white foam. Though cream of tartar—a by-product of wine-making—is the form of acid traditionally added to egg whites as they’re whipped for meringues, cakes, and soufflés, a few drops of vinegar or lemon juice per egg white will yield a similar result. "

― Samin Nosrat , Salt, Fat, Acid, Heat: Mastering the Elements of Good Cooking