Farewell San Francisco Sourdough?
by Roberto
Back in 1980, when I was a post‑doc in the San Francisco Bay Area, I began to develop a taste for what I considered a truly unique bread: San Francisco sourdough. There was a certain mystique to it, everyone familiar with it claimed there was nothing like it anywhere else. It was almost as if for sourdough bread there was something akin to the equally mystifying concept of terroir in wine. San Francisco sourdough was so special that when the dominant lactic acid bacterium present in its starters was identified, it was named Lactobacillus sanfranciscensis (recently re-named Fructilactobacillus sanfranciscensis). This particular species is widely used in the commercial preparation of sourdough starters
Figure 1. Overview of the process of serial transfer of a sourdough starter. Adapted from Source. Frontispiece: Sourdough bread (Photo credit Ryan Mueller). Source
However, the real charm of sourdough bread is not in obtaining a commercially available starter. It is in making and maintaining one's very own starter through serial passaging. The common lore is that by using the very simple mixture of flour and local water and whatever microbes are in the local environment (be it the air, the dust or even the hands of the home baker), each starter will acquire its very own unique local character. Thus, one would have expected strong biogeographic patterns in the microbial community compositions of sourdough starters. Alas, such presumptions were put to rest by a recent paper that describes the microbial communities of 500 sourdough starters. Somewhat surprisingly, the authors found very little biogeographic patterns in these starters. These are their ideas as to the possible reasons for this: "The limited role of geography in explaining sourdough diversity may be driven by the widespread movement of starters across large geographic distances through starter sharing or commercial distribution. Flour, a major potential source of microbes in de novo starters is also moved across large spatial scales. This geographic homogenization of starter and flour microbes likely swamps out any regional differences in potential yeasts or bacteria that can disperse into starters." There is a caveat to these results: the vast majority of the starters studied came from the United States. We'll have to wait and see if starters from regions remote from (and culturally isolated from) the United States show geography-determined patterns in their microbial composition. While home bakers round the world wait for such results it will be perfectly fine to still feel that there is something truly unique about their own sourdough starters.
