22. What is the Microbiology of San Francisco Sourdough?

Several studies have been conducted on the natural microbiological
flora of sourdoughs from around the world.  In terms of understanding
the basis of the symbiosis between yeast and lactobacilli the most
successful studies have been by Sugihara and colleagues.  Despite the
existence of several varieties of yeast and lactobacilli they showed
that the dominant yeast was a non spore forming variety of
Saccharomyces exigus called Torulopsis holmii and now reclassified as
Candida milleri sp. nov.  The dominant lactobacillus was a new
species christened Lactobacillus sanfrancisco sp. nov.

Yeast and bacteria occur in a ratio of 1:100.  The unique symbiosis
is explained thus: Though most strains of yeast can metabolise the
sugar maltose Candida milleri cannot.  Dough abounds in maltose which
is a released from "damaged starch" through the action of amylase
enzymes. Thus maltose is freely available to the lactobacilli which
have an absolute requirement for this sugar and they cannot utilise
other sugars present in dough.  The yeast can utilise all other
sugars present in dough thus the two critters do not compete for a
carbon source.  In addition, the lactobacilli have an enzyme maltose
phosphorylase which while assimilating maltose releases glucose into
the media to give the yeast a small boost.

The lactobacilli also secrete an antibiotic cycloheximide which
"sterilises" the dough since it kills many organisms but of course
Candida milleri is resistant to cycloheximide.

Lastly, Candida milleri is moderately tolerant to the acetic acid
which the lactobacilli produce. I should also note that the
nutritional requirements of the lactobacilli is complex - they
require a number of amino acids and fatty acids which may be derived
from dead yeast cells.

Spicher in Germany characterised German sour rye. He found the
dominant yeast species were Candida krusei, Saccharomyces cerevisiae,
Pichia saitoi and Candida milleri.  The Lactobacilli included L.
brevis, casei, fermenti, pastorianus, bucheneri, delbrueckii,
leichmannii, acidophilus, farciminis, alimentarius, brevis
var.lindneri, fermentum, fructivorans and Pediococcus acidilactici!
(This zoo of organisms present naturally in Rye flour is the reason
why it is so easy to start a good sourdough culture from rye for
example see "manuels starter" in the Laurel's Kitchen bread book.)

Pure culture studies showed that he could reconstitute a starter that
was close to the original with the yeast Candida krusei and
Lactobacillus brevis var. lindneri. The basis of the symbiosis is not
well understood to the best of my knowledge but is probably similar
in principle to the one described above for San Francisco sourdough.

On a final note, I should point out that pure cultures of
Lactobacillus sanfrancisco are grown on defined media, harvested and
freeze dried and supplied to bakeries around the world to make
instant sourdough!

Should there be sufficient interest in this sort of information, I
can post periodic updates on the scientific lore of breadmaking.

-Roland