Yeast

Many breads are leavened by yeast. The yeast used for leavening bread is Saccharomyces cerevisiae, the same species used for brewing alcoholic beverages. This yeast ferments carbohydrates in the flour, including any sugar, producing carbon dioxide. Most bakers in the U.S. leaven their dough with commercially produced baker's yeast. Baker's yeast has the advantage of producing uniform, quick, and reliable results, because it is obtained from a pure culture. Many artisan bakers produce their own yeast by preparing a growth culture that they then use in the making of bread. When this culture is kept in the right conditions, it will continue to grow and provide leavening for many years.
Both the baker's yeast and the sourdough methods of baking bread follow the same pattern. Water is mixed with flour, salt and the leavening agent (baker's yeast or sourdough starter). Other additions (spices, herbs, fats, seeds, fruit, etc.) are not needed to bake bread, but are often used. The mixed dough is then allowed to rise one or more times (a longer rising time results in more flavor, so bakers often "punch down" the dough and let it rise again), then loaves are formed, and (after an optional final rising time) the bread is baked in an oven.
Many breads are made from a "straight dough", which means that all of the ingredients are combined in one step, and the dough is baked after the rising time. Also, dough can be made using a "pre-ferment", when the leavening is combined with some of the flour and water a day or so ahead of baking and allowed to ferment overnight. On the day of the baking, the rest of the ingredients are added, and the rest of the process is the same as that for straight dough. This produces a more flavorful bread with better texture. Many bakers see the starter method as a compromise between the highly reliable results of baker's yeast and the flavor and complexity of a longer fermentation. It also allows the baker to use only a minimal amount of baker's yeast, which was scarce and expensive when it first became available. Most yeasted pre-ferments fall into one of three categories: "poolish" or "pouliche", a loose-textured mixture composed of roughly equal amounts of flour and water (by weight); "biga", a stiff mixture with a higher proportion of flour; and "pâte fermentée", which is simply a portion of dough reserved from a previous batch. Sourdough (also known as "levain" or "natural leaven") takes it a step further, creating a pre-ferment with flour and water that propagates naturally occurring yeast and bacteria (usually Saccharomyces exiguus, which is more acid-tolerant than S. cerevisiae, and various species of Lactobacillus).