Sugar

When you whip sugar into the foam, the whipping time increases before you attain the proper stiffness. This foam is smoother as well as more stable than a foam without sugar and doesn’t break down and become liquid if you delay a bit before using it.

The sugar unites with the egg whites, giving it binding power so the foam will not separate quickly. At the same time it slows coagulation so you whip longer to get to the same consistency. Adding some sugar when whipping an egg-white foam reduces the risk of over-whipping.