Bakers use the term “crumb” to describe the interior consistency of breads and cakes. When a baked good is said to have an open crumb, it means that it is riddled with large, irregular holes, integrating a great deal of air into the dough. Depending on what one wants to do with a finished baked good, a variety of crumbs could be considered desirable, from the rich closed crumb of a brioche to the classic chewy open crumb of ciabatta, a traditional Italian bread. In addition to assessing whether or not a crumb is open or closed, bakers may describe it as moist, crumbly, chewy, or sweet.
Several factors go into the creation of open crumb bread. Open crumb bread has a much higher moisture content than breads with a closed crumb, and it also tends to undergo long fermentation periods. Many breads made with wild yeasts or old dough starters have an open crumb, because they do best when allowed to ferment for an extended period of time. Finally, the dough for open crumb bread is also gently handled during the kneading and shaping process.
The holes in the crumb are caused by bubbles of carbon dioxide, a gas which is emitted by the yeasts in the bread as they digest the flour. As the bread bakes, the gas disperses, but the bubbles remain behind. Many open crumb breads have a very chewy, moist texture, and they tend to be packed with flavor as a result of the long, slow fermentation they were subjected to.
Some of the finest artisan breads in the world are open crumb breads, revealing a lacy pattern of irregularly sized holes when they are sliced open. There are all sorts of exciting things one can do with open crumb bread. For example, such bread lends itself beautifully well to sandwiches, because the bubbles in the bread will trap dressings and sauces, creating a burst of flavor in the mouth. It is also quite enjoyable when spread with butter, cheese, or a vegetable spread, leading many people to take it along on picnics.
Many sourdough breads have an open crumb, along with a dense, chewy crust. If you aren't sure what kind of crumb a loaf of bread has, ask the baker, and he or she should be able to provide you with information. In some bakeries, you may even be allowed to have a taste, so that you can judge the look, feel, texture, and taste of the crumb yourself. When baking at home, a recipe will usually indicate what sort of crumb you should end up with; as a general rule of thumb, commercial yeast and a short fermentation will yield a dense closed crumb bread, while long fermentations and old dough starters or wild yeast starters such as sourdough starter will yield an open crumb bread.