Sourdough

How wild yeast works

A sourdough starter is a mixed-microbe culture — wild yeasts (especially Saccharomyces exiguus) plus various species of lactic acid bacteria (Lactobacillus, Pediococcus). These microbes occur naturally on flour, on grains, in the air, and on the baker’s hands.

When flour and water are mixed and left at room temperature, the microbes feed on the carbohydrates, multiplying. Over 5–10 days of feeding (adding fresh flour and water daily), the culture stabilizes into a viable starter. This stabilization process is one of the slow pleasures of sourdough baking.

Slow rise, complex flavor

Compared to commercial yeast bread, sourdough:

• Rises more slowly — typically 8–24 hours per rise instead of 1–2.
• Develops complex flavor — lactic and acetic acids from the bacteria create the characteristic tang.
• Produces an open crumb — large irregular air pockets if handled correctly.
• Has lower glycemic index — fermentation breaks down some starches.
• Reduces phytic acid — a long fermentation makes minerals more bioavailable.

The flavor depth comes from hours to days of microbial fermentation developing dozens of distinct compounds — far more than commercial yeast, which mostly just produces CO₂.

A pandemic-era revival

Sourdough baking experienced a global revival during the COVID-19 pandemic in 2020. Stuck at home with extra time, millions of people started sourdough starters, often naming them (“Bread Pitt,” “Lord of the Rye”). Flour shortages followed; yeast became scarce in supermarkets but didn’t matter for sourdough bakers.

The trend established a new generation of home sourdough bakers, with Instagram and YouTube accelerating technique-sharing.

San Francisco sourdough

The San Francisco sourdough is a distinct local cultivar — characterized by Lactobacillus sanfranciscensis, a lactic bacterium specific to that region’s bakeries. The bacterium produces extra-sour flavor; commercial San Francisco bread (Boudin Bakery, since 1849) features it prominently.

The bacterium is not unique to San Francisco — similar strains exist elsewhere — but the regional flavor profile has become trademark-iconic.

Long-lived starters

Some sourdough starters have multi-generational continuity — passed down for decades or centuries. Boudin Bakery’s mother starter has been continuously fed since 1849. A famous Belgian baker’s starter has been documented continuously for over 100 years. These cultures are essentially living heirlooms; bakers pass small jars of the starter to friends, family, and successors.