Subject: 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.
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