If Chicago has deep dish pizza and Boston has cream pie, San Francisco has sourdough bread. And just like the pizza and pie, San Francisco sourdough just isn’t the same outside its hometown.
But that’s because only San Francisco is home to a certain bacterium that bears its name– Lactobacillus sanfranciscensis.
Of course bread uses another microbe– the yeast that turns sugar into the air bubbles that lighten the loaf. For sourdough, though, local bacteria then add their secret ingredient. They eat up the yeast’s waste and turn it into acid, making the bread San Francisco sour.
The bacteria also make the dough inhospitable for other microbes, keeping all that doughy goodness for the yeast and itself. The yeast and bacteria make such great partners because the yeast can’t eat the sugar maltose, which the bacteria absolutely need.
San Franciscans have been noshing on this local concoction since at least the Gold Rush. Boudin Bakery first baked buns in 1849. Some bakeries even claim to have a “starter”– the bit of dough that contains yeast and bacteria– that’s over a century old. They pinch off a piece of starter for every new loaf, and care for the dough with regular feedings of flour and water.
If you’ve got a favorite brand, chances are it’s because of the unique mix of yeast and bacteria from that bakery. Other towns’ sourdough will taste a little bit different because their bacteria aren’t the San Francisco kind.
Want to whip up a loaf unique to your backyard? The Exploratorium has a recipe to make your own starter that will pick up local yeast and bacteria. Or if you prefer that authentic San Francisco flavor, buy the original.
For more on Lactobacillus sanfranciscensis, check out the Berkeley Science Review.
Amber Dance is the Quest Intern and a science communication student at UC Santa Cruz.