Starch

The elastic gluten proteins are essential to the making of raised breads. But proteins account for only about 10% of flour weight, while about 70% is starch. Starch granules serve several functions in doughs and batters. Together with the water they hold on their surfaces, they make up more than half the volume of the dough, interpenetrate the gluten network and break it up, and so tenderize it. In the case of cakes, starch is the major structural material, the gluten being too dispersed in the large amount of water and sugar to contribute solidity. During the baking of bread and cakes, the starch granules absorb water, swell, and set to form the rigid bulk of the walls that surround the bubbles of carbon dioxide. At the same time their swollen rigidity stops the expansion of the bubbles and so forces the water vapor inside to pop the bubbles and escape, turning the foam of separate bubbles into a continuous spongy network of connected holes. If this didn’t happen, then at the end of baking the cooling water vapor would contract and cause the bread or cake to collapse.