Sourdough starter

A sourdough starter is a deliberately-maintained community of wild yeasts and lactic acid bacteria in a flour-and-water medium, used to leaven bread without commercial baker's yeast. It is one of the oldest documented food preparations — bread leavened by wild fermentation predates pure-yeast leavening by thousands of years, with archaeological evidence in Egyptian baking dating to at least 1500 BCE. The modern sourdough starter is the continuous-tradition direct descendant of that practice.

The microbiology is mixed and stable. Wild yeasts (typically Candida milleri, Saccharomyces exiguus, sometimes S. cerevisiae) produce CO₂ for leavening; lactic acid bacteria (multiple Lactobacillus species, including L. sanfranciscensis, identified and named for the San Francisco sourdough tradition) produce lactic and acetic acids that give sourdough its characteristic tang. The yeast-bacteria ratio in a healthy mature starter is roughly 1:100 by cell count; the two communities are mutually dependent (the bacteria's acid environment is what selects for and protects the wild yeasts).

A new starter takes 5-10 days to establish from just flour and water — the first 2-3 days show enthusiastic activity from non-yeast bacteria (Leuconostoc, others) which dies off and gives way to the actual sourdough community in days 4-7. The visible result during this transition is a starter that 'fails' (becomes sluggish, smells off) around day 3-4, then recovers; many first-time sourdough makers discard their starter during the dip phase and never see it through to maturity.

A mature starter, fed daily and kept active, becomes a permanent kitchen organism. The San Francisco sourdough tradition includes starters with documented continuous lineages stretching back to the Gold Rush era (1850s) — kept alive by 170 years of daily feedings. Most fermenters' starters are younger, but the practice of feeding-as-permanence is real: a starter can persist indefinitely with care.