Sourdough starter (mixed culture)

Sourdough starter is the original leavening culture — the technique that produced every leavened bread in human history before commercial baker's yeast became available in the late 19th century. The culture is a mixed community of wild lactic acid bacteria (LAB) and wild yeasts established on a flour-water medium and maintained by regular feedings. The community composition varies meaningfully by region, baker, and substrate:

- *Lactic acid bacteria* (typically 100:1 cell ratio over yeasts): - Lactobacillus sanfranciscensis (the famously San Francisco-named species — actually a heterofermenter formally now classified as Fructilactobacillus sanfranciscensis) - L. plantarum, L. brevis, L. fermentum, L. casei, L. delbrueckii - Specific composition depends on region — French levain communities differ from German Sauerteig, Italian lievito madre, Russian zakvaska, etc.

- *Yeasts*: - Saccharomyces exiguus (now Kazachstania exigua) — historically the canonical sourdough yeast - Candida humilis — common in many starters - S. cerevisiae — present in some starters; typically arrives through environmental contamination - Various Kazachstania, Pichia, Hanseniaspora species

The relationship between LAB and yeasts in a sourdough starter is symbiotic rather than competitive. The LAB acidify the dough (lowering pH to ~3.5-4.5), which selects for acid-tolerant yeasts (S. exiguus, C. humilis) over the acid-sensitive S. cerevisiae that dominates in commercial baker's yeast environments. The yeasts produce CO₂ for leavening and small amounts of ethanol; the LAB produce lactic acid (homofermenters) plus acetic acid and CO₂ (heterofermenters). The combined effect produces the characteristic sourdough flavor: tangy from acids, complex from yeast-produced esters, and structurally leavened from yeast CO₂.

The famously regional Lactobacillus sanfranciscensis deserves note. The species was first isolated and named in 1971 from San Francisco bakery starters; it is unusually adapted to the cold, foggy San Francisco climate and produces a slightly different acid profile than LAB in warmer regions. Modern molecular analysis has detected L. sanfranciscensis in sourdough starters from many other regions worldwide, suggesting the species is more cosmopolitan than originally believed; the unique 'San Francisco sourdough' character may have more to do with bakery technique and microclimate than strain uniqueness. The Boudin Bakery in San Francisco has maintained the same mother starter since 1849 — 175+ years of continuous propagation.

Maintaining a sourdough starter requires regular feedings: equal parts (by weight) of flour and water added to existing starter, typically daily at room temperature or weekly when refrigerated. A starter that has missed feedings for 1-2 weeks becomes weak but is usually recoverable with 2-3 days of consistent feeding. A starter abandoned for months can sometimes be recovered but often must be discarded and restarted. Frozen or dehydrated starters can be archived long-term.

The starter community is also active in several non-bread ferments. *Bread kvass uses sourdough rye bread as the substrate and water as the medium for a separate fermentation — the same sourdough community contributes to the finished kvass alongside additional environmental microbes. Injera (Ethiopian teff-based) uses ersho, a starter culture that overlaps significantly with European sourdough cultures in composition. Pumpernickel* depends on long-aged rye sourdough for the deep acidic profile that defines the bread.