San Francisco sourdough

San Francisco sourdough is real twice: as a city’s long commercial habit of sour French bread, and as a 1971 USDA isolation of the microbes in that process. It is not real as a law of nature that only this fog can sour dough, or that a bacterium named for the city lives nowhere else.

What bakeries actually did #

French-style bakeries were part of Gold Rush San Francisco. Boudin’s own timeline dates the bakery to 1849, a move to Dupont (now Grant) in 1852, and an 1868 decision to keep leavening with mother dough after commercial cake yeast was widely available ¹. The company’s Gold Rush origin for that mother dough is presented as belief (“It’s believed that Isidore Boudin received the mother dough from one of these 49ers”) ¹. That is house history, not a lab notebook.

San Francisco’s Legacy Business staff report treats Boudin as founded in 1849 with a first storefront in 1852, and records that Louise Boudin saved the mother dough in the 1906 earthquake and fire ². Civic documentation of a business is not microbiology. It is evidence that a sour, naturally leavened loaf was a commercial product here for a long time — while much of American baking went over to manufactured yeast.

The popular loaf — large, blistered, very sour, often a round “French” bread — is a style shaped by bakeries, flour, and a market that wanted that tang. Tartine Bakery is a later artisan chapter of the same city, not a continuation of Wharf tourism bread. Both belong in the atlas; they are not the same object.

What the laboratory actually isolated #

In 1971, at USDA’s Western Regional Research Laboratory in Albany, California, Sugihara, Kline, and Miller sampled sour-dough French bread fermentations from five San Francisco bakeries. The leavening yeasts were not bakers’ yeast. Most isolates were maltose-negative, identified then as asporogenous Saccharomyces exiguus (Torulopsis holmii) ³. Names have moved since; the finding that stands is the ecology: a yeast that does not compete for maltose in an acidic dough.

Kline and Sugihara then isolated the souring bacteria: gram-positive rods that wanted maltose, produced lactic and acetic acids (acetic from 3 to 26% of total acid in their measurements) plus carbon dioxide, and were fussy about medium ⁴. They hesitated even to file them neatly among heterofermentative lactobacilli. Later taxonomy named the species Lactobacillus sanfrancisco / L. sanfranciscensis. In 2020 the genus split moved it to Fructilactobacillus sanfranciscensis ⁵. The city is in the epithet because of those isolates, not because the organism is a municipal employee.

Is it unique to San Francisco? #

No. The species is a sourdough specialist found in traditional Type I doughs far from California. Comasio, Verce, Van Kerrebroeck, and De Vuyst, writing about bakeries in more than one country, treat the San Francisco pairing of F. sanfranciscensis with Kazachstania humilis as a famous consortium — and as one of several maltose-sharing LAB–yeast pairings ⁶. In their own samples, F. sanfranciscensis was the prevalent LAB in sourdoughs from two of three bakeries in different countries ⁶. House microbiota, flour, and backslopping select the community. Fog is not a required reagent.

The tourist sentence “the bacteria only live here” collapses three true statements into a false one: (1) the species was described from here; (2) SF commercial doughs of 1971 had a characteristic yeast–LAB pairing; (3) that pairing is an ecological type, not a geographic fence.

What to do with this as a baker #

If you want sourness, manage Fermentation: temperature, hydration, flour, and time select lactic versus acetic notes. Calvel’s warning still applies on the Lactic acid bacteria page — city’s name is not an acid profile. Maintain a Sourdough starter. Visit Boudin Bakery as history you can still buy; visit other SF bakeries as other answers to the same craft. The general bread is Sourdough.