Fermentation Mechanics and Timing
What fermentation produces
As I mentioned, a sourdough starter is made up of a stable community of lactic acid bacteria and wild yeasts. The lactic acid bacteria and yeasts live in balance within a dough system, working in harmony and keeping other microorganisms out. The acid produced by the bacteria and the ethanol (alcohol) produced by the yeasts make the dough environment inhospitable to other microbes (including unwanted pathogens). Remarkably, suitable bacteria can tolerate ethanol, and suitable
The acid produced by the bacteria and the ethanol produced by the yeasts make the dough environment inhospitable to other microbes. This is why sourdough bread keeps so much longer than commercial yeast bread.
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The yeast and bacteria are waking up. The dough is dense and flat. This is normal — give it time.
The two fermentation phases
Bulk fermentation: the main phase, after mixing, before shaping. The dough ferments as one mass. This is where most of the rise and flavor development happens.
Final proof: after shaping, in the banneton. Usually done cold (refrigerator) for 8-16 hours. Slower, more controlled, develops flavor without overproofing. The cold also makes the dough easier to score and load into the oven.
Pronounced tang, vinegary notes
What drives fermentation speed
Temperature is the primary driver. A few degrees makes hours of difference. Warmer dough = shorter bulk fermentation. Cooler dough = longer bulk fermentation.
Starter activity matters too — a sluggish starter means slow fermentation. Hydration affects it: wetter doughs ferment faster. Flour type matters: whole grain flours ferment faster than white because they contain more enzymes and nutrients for the microbes.
pH and flavor
Why Are pH Levels So Important in Sourdough Baking?
Longer, cooler fermentation tends to produce more acetic acid (sharp, vinegary tang). Shorter, warmer fermentation produces more lactic acid (mild, yogurt-like sourness). Most home bakers find the cold overnight proof hits the right balance.