One of the most important and underutilized items in the home baker’s arsenal is steam. When bread goes into a hot oven, it gets a shell of dried dough almost instantly, which makes it so that any further expansion of the gas bubbles creates a compact crust that lacks a crispy finish. The introduction of steam ensures the crust is kept hydrated for longer, so it is able to relax and allow the gases to fully develop in the first 10 to 15 minutes of baking, a period known as “oven spring” which gives bread most of its height and openness of crumb as well as its dramatic blooming.
The science behind steam is elegant and exact. Water vapor rising from the oven condenses on the bread, retarding the formation of crust while helping to enhance the Maillard reaction — the sugar-protein reaction responsible for the browning, flavor, and smell of the bread — later in the baking process. As the steam vaporizes (around the first one-fourth of the way through the bake), the crust hardens and holds in the water within the bread, forming a crunchy, crackly crust that shatters under the right amount of pressure. A great crumb contrasted with a brittle crust is the hallmark of an excellent bread.
If you don’t have a pro-style steam-injection oven, don’t worry! Most home bread recipes involve one of two common practices: either you shape your dough and then place it inside a very hot Dutch oven (preheating the pot in the oven is crucial) and cover it with a lid for the first part of the bake. The steam from your own dough will provide all the moisture you need. Or, you preheat a stone or baking steel in the oven, add your dough, and then pour about a cup of hot water into a hot skillet or sheet pan at the bottom of the oven. The water will flash into steam and help the bread rise. Some recipes suggest spraying the oven with water from a spray bottle every few minutes during the initial rise. All of these tricks keep the dough moist as it rises.
Using steam requires timing and observation too: too much too long and the crust will be pale and soggy; too little and the loaf will crack open too wide or not at all. Generally, it is most valuable in the first 12-20 minutes, depending on the moisture content of the dough and the personality of the oven. Experienced bakers learn to pay attention to the sheen on the surface, the sound of the steam and the rate of spring. These signs become automatic after a while and what may at first seem like a shot in the dark is actually a skill learned by experience.
Finally, there’s the steam itself, which is the unsung hero of bread baking. It requires almost nothing from us—just water, some heat, and a modicum of thought—but it gives back so much: a blooming oven spring, an attractive bloom to the crust, and a rich, dark color that crackles and shatters when touched, providing what might be the most magical sound in all of bread making—the crack of a freshly baked loaf.