Which ingredient in royal icing primarily contributes to its brittleness?

The brittleness of royal icing primarily arises from the use of egg whites as an ingredient. Egg whites contain proteins, particularly albumin, which, when whipped, create a stable foam. As the icing dries, these proteins coagulate and form a rigid structure. This transformation is what gives royal icing its characteristic hard and brittle texture once set.

While powdered sugar contributes to the overall sweetness and texture of royal icing, it does not impart the structural integrity that egg whites do. Water is essential for mixing and achieving the right consistency, but it evaporates as the icing dries, contributing to the firmness rather than brittleness. Egg yolks are not typically included in royal icing formulations; instead, they are known for enriching textures and flavors in other types of frostings or icing. Therefore, the combination of egg whites and the drying process is what primarily defines the brittleness of royal icing.