That's the title of an in-depth article at BBC News. Here's an excerpt.
When James W Marshall discovered gold in the hills north of San Francisco in 1848, it sparked a migration of epic proportions. Within a year, tens of thousands of immigrants from both across the country and around the globe had relocated to the Californian city and its surroundings, carrying with them basic necessities like clothing; carpentry tools; and coffee, sugar and flour. However, there was one staple item that would become a part of the city’s history forever: starter for bread.
In a place where nourishment was scarce, bread starter (a dough that has fermented using naturally occurring bacteria and yeast) was a prized possession during the California Gold Rush, allowing miners to turn drab flour into loaves that were both nutritious and delicious. Somehow, the bread tasted tangier and more flavourful than it did elsewhere, and thus San Francisco sourdough was born.
More than 170 years later, San Francisco is synonymous with sourdough bread.
. . .
Rumours often swirl about the city’s relentless fog playing a role in the taste of its sourdough, cultivating a type of wild bacteria that only exists in San Francisco. There’s also the fact that it was California miners making their way to north Alaska and Canada for the Klondike Gold Rush, and bringing their sourdough starters with them, who earned the nickname “sourdoughs”, because they would actually cuddle with their starters on cold nights to keep the yeast active. These men became known for their fresh bread and assured their own nourishment in the process.
Then there’s Boudin Bakery, considered San Francisco’s oldest continuously operating business, which has been churning out loaves of sourdough bread since first opening its doors in 1849 – just one year after Marshall discovered gold at Sutter’s Mill. Founder Isidore Boudin, a French immigrant and bakers’ son, obtained the bakery’s original wild yeast starter from a gold miner, and they’ve been using the same mother dough (another name for sourdough starter) for more than 170 years. An interesting tidbit: it was actually Louise Boudin, wife of the then-late Isidore, who saved Boudin’s mother dough from destruction during San Francisco’s legendary 1906 earthquake.
. . .
But despite all the connections and stories, sourdough isn’t endemic to San Francisco. Actually, it’s one of the oldest bread types – a fermented and leavened dough that dates back at least 4,500 years to ancient Egypt.
There's much more at the link, including a recipe.
Miss D. and I are very fond of sourdough bread, particularly a blend that our local bakery calls "Bay City sourdough", allegedly similar to what you can buy in San Francisco. I don't know about that, but it's certainly got a flavor we enjoy.
If you enjoy cooking, particularly baking, the article has a lot of information about the history and culture of bread-making in San Francisco. It's interesting (and mouth-watering) reading.