Sourdough, Fermentation and Rising — the Microbiology Behind Bread
Why the dough bubbles, and how you control the living cultures in the flour
When dough rises, it's because living microorganisms eat sugars in the flour and release gases. The gas is trapped in the gluten network that kneading has built, and the dough swells up. Baking good bread is largely about understanding and controlling this microbiological work—whether you're conscious of it or not.
§Yeast and sourdough — two ways to rise
Common baking yeast is a pure culture of the fungus Saccharomyces cerevisiae. It is fast, predictable, and produces neutral, mild bread. Sourdough, on the other hand, is a living ecosystem of wild yeast species and lactic acid bacteria that you keep alive by feeding it flour and water. The bacteria produce lactic acid and acetic acid, which give sourdough bread its characteristic sour taste, longer shelf life, and better structure.
- 01Baker's Yeast: Fast Rising (Typically 1–3 Hours), Mild Flavour, Consistent Results Every Time.
- 02Sourdough: slow rising (often 4–24 hours), sour taste, requires continuous feeding and care.
- 03Poolish and biga: pre-doughs where part of the flour ferments in advance to provide more flavor and strength.
§Temperature controls the pace
Microorganisms work faster when it is warm and slower when it is cold. Dough temperature is therefore the baker's most important control. A warm dough rises quickly but risks over-fermentation and becoming sour and slack. A cold dough rises slowly but develops more flavour. Many bakers aim for a finished dough temperature of 24-26 °C for wheat dough and use the water temperature to hit the target.
§What happens in the dough along the way?
| Phase. | What's happening | The Baker's Focus |
|---|---|---|
| Kneading | Gluten develops, air is incorporated | Correct consistency, not over-mixed |
| First lift | Yeast produces CO2, the dough doubles | Temperature and time, folding |
| Forming | Dough mixer | Gentle handling, preserve air |
| Over-proofing (quick) | Last run before oven | Finger test — the dough should spring back slowly |
§Fingertip sensation and finger test
An experienced baker feels in the dough when it is ready. The classic finger test is used for the final rise: press gently with a finger on the dough. If it springs back slowly and leaves a faint mark it is ready for the oven. If it springs right back it needs to rise more; if the hole remains it is over-proofed and has lost its lift.
§When fermentation goes wrong
Over-fermentation produces a flat, sour loaf with coarse crumb, because the gluten network has lost its ability to hold the gas. Under-fermentation produces a dense, compact loaf with small volume. Both are usually due to incorrect proportion between temperature and time. The solution is rarely more yeast — it is better control of the two.
“Time is the cheapest ingredient in the bakery. If you give the dough time, it gives you taste back.”
— Craft bakery luck