Sourdough Fermentation: How Yeast and Bacteria Make Bread Rise

Two organisms, one dough

A sourdough starter is a stable culture of wild yeast (mostly Saccharomyces species, sometimes Kazachstania) and lactic-acid bacteria (mostly Lactobacillus and related genera), living together on the starches and sugars of flour. The bacteria acidify the environment; the yeast, tolerant of that acidity, ferments the sugars anaerobically. It's a mutualism: bacterial acid suppresses competing spoilage organisms, and yeast byproducts feed some of the bacterial metabolism.

The reaction that makes bread rise is simple glycolytic fermentation of glucose (from starch broken down by flour's own amylase enzymes):

C₆H₁₂O₆  →  2 C₂H₅OH  +  2 CO₂
glucose         ethanol       carbon dioxide

The ethanol mostly evaporates during baking; the CO₂ is what does the mechanical work. It doesn't escape the dough — gluten, the elastic protein network formed when flour's glutenin and gliadin proteins hydrate and are worked, traps it as discrete bubbles rather than letting it bubble away like in a glass of soda.

Bubble nucleation and growth

CO₂ doesn't appear from nowhere as new bubbles; it mostly diffuses into existing microscopic air pockets that were incorporated when the dough was mixed and kneaded. Each pocket acts as a nucleation site. As fermentation proceeds, dissolved CO₂ supersaturates the surrounding dough and diffuses down its concentration gradient into the nearest bubble, which grows. Growth rate depends on the local CO₂ production rate, the diffusion distance to the nearest bubble, and how much resistance the surrounding gluten network offers to expansion — a well-developed gluten sheet stretches elastically around a growing bubble instead of tearing, which is why kneading (or the stretch-and-folds of a no-knead method) matters as much as the microbiology.

What temperature, hydration and flour actually change

Temperature sets fermentation kinetics: yeast and lactic bacteria activity roughly follows an Arrhenius-type response to temperature within their viable range, with both groups accelerating from refrigerator temperatures up toward the high-20s to low-30s °C before activity falls off again near 40 °C and above as the organisms are stressed. But the two microbial populations don't respond identically — bacteria tend to favour slightly warmer conditions for lactic acid production while a cooler, longer proof shifts the balance toward more acetic acid and a sharper sourness, which is the whole reason bakers retard dough in the fridge overnight.

Hydration (water as a percentage of flour weight) changes both the rheology and the microbial environment: wetter doughs let gas cells expand more freely and give a more open, irregular crumb, but they also weaken the gluten network's ability to hold structure without support (hence the popularity of high-hydration loaves baked in a Dutch oven). Flour type matters because whole-grain and rye flours carry far more of the wild microbial population and more free sugars and minerals to feed it — a whole-wheat starter typically ferments faster than an all-white one — while also containing more bran, which physically cuts gluten strands and limits how much the dough can expand before bubbles coalesce and collapse.

Reading the rise

A dough's volume over time during bulk fermentation is not perfectly exponential: it starts slow (lag phase, as the culture reactivates and enzymes solubilise starch into fermentable sugars), accelerates through a roughly exponential middle phase as yeast populations and CO₂ output climb, and then plateaus or even collapses if fermentation runs long enough that the gluten network over-extends, bubbles coalesce into larger ones that escape more easily, and the structure can no longer hold the gas. That collapse point — dough that has "overproofed" — is exactly what the bubble-growth model predicts: past some critical bubble size and film thinness, coalescence and rupture beat further nucleation.

Frequently asked questions

Try it live

Everything above runs in your browser — open Sourdough Fermentation and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.