Mid-Baking: From Foam to Sponge

Oven spring stops when the crust becomes firm and stiff enough to resist it, and when the interior of the loaf reaches 155–180°F/68–80°C, the temperature range in which the gluten proteins form strong cross-links with each other and the starch granules absorb water, swell, gelate, and amylose molecules leak out of the granules. Now the walls of the gas cells can no longer stretch to accommodate the rising pressure inside, so the pressure builds and eventually ruptures the walls, turning the structure of the loaf from a closed network of separate gas cells into an open network of communicating pores: from an aggregation of little balloons into a sponge through which gases can easily pass. (If the dough were not transformed into a sponge, then cooling would cause each isolated gas cell to shrink, and the loaf would collapse.)