Why Your Sourdough Starter Tastes Like Where You Live

A jar of flour and water left on a counter doesn't sit inert. Within a few days it bubbles, sours, and starts to smell faintly like beer or yogurt. What's living in there is a stable partnership between two kinds of microbes: wild yeast for the rise, lactic acid bacteria for the tang. Bakers call it a starter. The species mix in any given starter depends, in part, on where the jar was sitting.

Microbiologists in the 1970s isolated the dominant bacterium in San Francisco sourdough and named it Lactobacillus sanfranciscensis. It thrives at the cool, slow fermentation temperatures common in Bay Area kitchens and produces the city's famously sharp tang. The dominant yeast paired with it, Kazachstania humilis, is acid-tolerant in ways most baker's yeast isn't.

You can carry a San Francisco starter to New York and feed it religiously, but the population inside the jar will drift. A 2021 study in eLife sampled starters from bakers around the world and found that flour, kitchen temperature, and feeding rhythm all shifted the microbial community within weeks. Geography contributes; technique probably contributes more.

The oldest evidence of this kind of fermentation is much older than commercial yeast. Archaeologists in Jordan have dated charred flatbread crumbs to about 14,000 years ago, predating agriculture itself. Until Louis Pasteur identified yeast in the 1850s, every loaf made for thousands of years was, technically, sourdough — leavened by whatever had drifted into the dough.

A jar on your counter is doing something humans have been doing, mostly without understanding it, since the last ice age.