Also known as a sourdough culture, a sourdough starter is a simple mixture of flour and water which acts as a natural leavening agent.
There are many species and strains of yeast and lactic acid bacteria found in sourdoughs. These set up the
environment inside a starter which selects for the particular microbial profile (out of many possible combinations) most suited to those conditions.
In traditional sourdoughs (those refreshed frequently to maintain leavening) Lactobacillus sanfranciscensis is perhaps the most common—not just in starters of the San Francisco sourdough bread process where it was first discovered, but in starters around the world.
Flour contains a variety of sugars that microorganisms can feed on. Most sourdough lactic acid bacteria will start fermenting glucose first, but Lb. sanfranciscensis is a notable exception in that it prefers maltose, the disaccharide of glucose. It uses half of the molecule and excretes the other, a glucose, back into its surroundings.
Maltose is the most abundant sugar in wheat doughs, but these yeasts lack the ability to break
it down for themselves. Other yeasts and bacteria that possess the ability to ferment maltose
generally prefer glucose, and will leave the maltose alone for as long as there is enough glucose
available. And so in providing glucose to other organisms, Lb. sanfranciscensis actually helps to
conserve maltose for itself—just one of the ways in which it gets along well with many sourdough
microorganisms, and perhaps one of the reasons it is found so often.
Developing and Perpetuating a Sourdough Culture
There are two distinct phases involved in sourdough: The first is developing the culture, a process
that generally takes 6 to 10 days; the second is perpetuating the culture, so that it can be successfully used in bread production for years. Developing a sourdough culture simply requires attracting yeasts and bacteria that will coexist, the wild yeasts providing leavening and the various strains of Lactobacillus providing flavor.
Wild yeasts live in abundance on seeds, grains, fruits, and vegetables. The skin of grapes and other
fruits contain wild yeast, as does the powdery film at the base of the outer leaves of cabbage. Flour also is
a favorable environment for wild yeast; in fact, there are tens of thousands of yeast cells in a single gram of
flour (a gram of commercial yeast, on the other hand, contains several billion yeast cells).
Once water is added to the flour, the life cycle of the incipient culture is begun. After 24 hours in
a warm room, the flour-water paste will show signs of having risen.