Sourdough bread is bread that is leavened using a sourdough culture containing wild yeasts and bacteria. The yeast that can be bought in small packages and jars at the store is commercial yeast . It is one strain of yeast, raised because it is really great for making bread quickly consistently. A sourdough culture will probably contain many different strains of yeast, and several types of bacteria. While an envelope of commercial yeast is innumerable copies of one organism, a sourdough culture is a diverse ecosystem. Because of this, sourdough breads have a more complex flavor and texture than breads made with commercially available yeast.
So where does the "sour" part of sourdough come from? While yeast produce carbon dioxide and alcohol, the bacteria in a sourdough culture produce lactic acid. This give some sourdough breads their distinctive tangy taste. The increased acidity and other compounds produced by the bacteria make sourdough breads more resistant to mold and staling than other artisan breads.
Although these bacteria can make the bread tangy, not all sourdough is noticeably tangy. It depends on what bacteria are in the culture being used. However, there are methods that can create the sour flavor that some people enjoy in their bread.
Making true sourdough bread is more difficult, and more time consuming than using instant yeast. That having been said; sourdough creates loaves of bread that have a flavor that can't be matched by breads made using commercial yeast.
It is a common misconception that sourdough bread can be produced using a commercial yeast that is allowed to develop in a batter overnight. This is called a preferment, poolish , or biga . This technique can make really great breads, but it isn't true sourdough and tastes very different. Here I present sourdough bread and describe the techniques used to create it.