Gas bubbles

Breads and cakes are aerated to the point that as much as 80% of their volume is empty space. Bakers use yeast and chemical leavenings to enhance the bubbles in their baked goods. It is important to note that leavenings do not create all of the gas present in the batter or dough. In fact, most of the bubbles form from air introduced when the baker kneads the dough, creams the butter and sugar, or whips the eggs. The carbon dioxide produced by leavenings is released into the water phase of the dough, diffuses to the bubbles already present, and merely enlarges them. Therefore, the initial aeration of doughs and batters strongly influence the final texture of the baked goods.

Starch

The word starch comes from a German root that means "to stiffen or make rigid." While starch has been used in paper making and textiles to stiffen, it has also served the same purpose bread making. Gluten proteins make only 10% of flour by weight, whereas starch makes up 70%. Starch granules absorb water, swell, and set to form the rigid bulk. They account for more than half the volume of dough. Though starch provides gives structure to bread, it also tenderizes it by penetrating and breaking up the gluten network. As carbon dioxide is produced, the rigidity of starch-formed walls stops the expansion of the bubbles. Pressure forces the water vapor inside to pop the bubbles and allow carbon dioxide to escape. As it does, a spongy network is left behind. This accounts for the crumbly texture of bread.

Starch is particularly important in batters where gluten proteins are too dispersed in water and sugar to contribute to the solidity of the cake.