Steam

Hard-crusted breads are baked with steam injected into the ovens during the first part of the baking period. Rye breads also benefit from baking with steam for the first 10 minutes.

The steam helps keep the crust soft during the first part of baking so the bread can expand rapidly and evenly without cracking or breaking. If steam were not used, the crust would begin forming earlier and thus would become thick and heavy. Also, crust that forms too early is likely to split as the interior continues to expand. The steam also helps distribute the heat in the oven, further aiding oven spring. When the moisture of the steam reacts with the starches on the surface, some of the starches form dextrins. Then, when the steam is withdrawn, these dextrins, along with sugars in the dough, caramelize and turn brown. The result is a thin, crisp, glazed crust.