What causes royal icing to be brittle?

Royal icing becomes brittle primarily due to the presence of egg whites, which contribute to its structure and texture. When egg whites are whipped, they create a stable foam that helps to give royal icing its characteristic hard finish once dried. However, the high sugar concentration in royal icing promotes quick drying, and as it dries, the moisture level decreases. This results in a hard and brittle consistency.

Egg whites also form proteins that can become rigid upon drying. The balance of sugar to egg whites is crucial; if there is too much sugar or insufficient moisture, the icing may dry out completely, leading to a brittleness that can make it fragile and prone to cracking. The other options, such as sugar concentration, butter content, and water content, play a role in the icing's overall texture but do not specifically contribute to its brittleness in the same significant way that egg whites do when the icing dries.