One Great Exception: San Francisco Sourdough

One notable exception to the starter microbiology I sketched out below is San Francisco sourdough. Set out a bowl of flour-water slurry there, and a different thing will likely happen. Namely, that your starter will very likely culture a very different and interesting yeast by the name of Candida humilis (formerly called Candida milleri, formerly called Saccharomyces exiguus) which is notable in that it doesn’t consume maltose, one of the primary products of enzymatic activity. And that leaves the door wide open to a unique bacterium that lives in the Bay Area and goes by the name of Lactobacillus sanfrancisco. L. sanfranciso, as you might expect, is a prodigious producer of acid (lactic acid as well as acetic acid, which is what gives vinegar its sour tang). L. sanfranciso also produces an antibiotic-type compound by the name of cycloheximide which not only suppresses competing bacteria, but also competing yeast (though obviously not the Candida humilis, which the L. sanfranciso further indulges by excreting a little glucose as it works).

Some makers of commercial sourdough starters claim that the cycloheximide produced by L. sanfranciso makes it impervious to invasion by microbes in climates outside of San Francisco. It’s the kind of balderdash you only get when someone is trying to sell you something. All kinds of microbes (including most other types of lactic acid bacteria) employ similar sorts of anti-competitive countermeasures. To imply that L. sanfranciso has the biggest gun in the microbial world regardless of environment is just plain silly.

God only (and I mean God only) knows what other kinds of yeast and bacteria (and yeast-bacteria relationships) are out there, and that might produce fabulous bread. We know so much about San Francisco sourdough because it’s been studied exhaustively by food scientists. Outside of that micro-climate, very little similar work has been done. So who knows what might be going on in the micro-climate of your own back yard? There may be something exceptional there just waiting for discovery.