Or, more succinctly and specifically, what is the Microbiology of San Francisco Sourdough?
Sourdough is a combination of natural microbiological flora and fauna that occur in and/or on grains. The wild yeasts live in a symbiotic relationship with the Lactobacillus. There are many different kinds of yeasts and bacteria. Of those, some are able to live in a close and mutually beneficial relationship. Despite there being such a wide variety of possible partners, the dominant yeast in San Francisco sourdough is the non spore forming variety of Saccharomyces exigus called Torulopsis holmii, recently reclassified as Candida milleri. The dominant lactobacillus is a species christened Lactobacillus sanfrancisco. The ratio of yeast to bacteria is roughly 1:100.
While most strains of yeast easily metabolize the naturally occurring sugar, maltose, "Candida milleri" cannot. Dough is rich in maltose which is a released from "damaged starch" in the underlying flour through the action of amylase enzymes. Thus maltose is freely available to the lactobacilli which require this sugar as they cannot utilize any of the other sugars also present in dough. Since the yeast can utilize all of the sugars present in dough, the two classes of life forms do not compete for a carbon source (carbon, as a part of the ample carbohydrate structure of the flour). Additionally, lactobacilli possess the enzyme 'maltose phosphorylase'. By assimilating the maltose, the LB's release glucose into the matrix (dough) and give the yeast a diet of sugar to fuel their growth.
In addition to the maltose, the lactobacilli also secrete antibiotic cycloheximides which "sterilizes" the dough since it kills many organisms. Natural selection has made Candida milleri resistant to cycloheximides. In addition, Candida milleri is moderately tolerant of the acetic acid produced by Lactobacillus sanfrancisco. The nutritional requirements of Lactobacillus sanfrancisco is complex. In addition to assimilating maltose, they also require some amino acids and fatty acids which are derived from expired yeast cells.
The time-line of bread from sourdough to modern.
Starting a sourdough starter.
Repairing a "broken" or poorly performing sourdough starter.
Tips about the sourdough process.
In search of the elusive wild sourdough culture.