Water is seldom called for, but in small amounts it increases the volume and lightness of the foam. Because water thins the whites, however, it’s more likely that some liquid will drain from the foam. Albumen diluted by 40% or more of its volume in water cannot produce a stable foam.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.