What substance in flour, milk, and eggs adds structure to baked cakes?

The correct answer is proteins. In baking, proteins play a crucial role in creating structure within cakes and other baked goods. When you combine flour, milk, and eggs, the proteins, particularly glutenin and gliadin from the flour and the proteins found in eggs, form a network that gives the cake its shape and stability as it bakes.

As the mixture is mixed and heated, the proteins coagulate, setting the cake's structure. This is why recipes often emphasize proper mixing; insufficient mixing can lead to a less cohesive structure, while overmixing can lead to a tough texture.

While fats do contribute to the texture and moisture of cakes by providing tenderness, and carbohydrates, including sugars and starches, add sweetness and can assist in browning, it is the proteins that are primarily responsible for providing the essential structure that holds the cake together. Starches also contribute to the overall texture but are secondary to the role of proteins in imparting strength.