What You’ll Learn
- What fermentation means in sourdough.
- Where the microorganisms get their food.
- How gas and acids change the dough.
- Why time and temperature alter fermentation speed.
- How to translate the science into better baking decisions.
Quick Answer
During sourdough fermentation, yeasts and lactic acid bacteria use sugars made available from the flour. Yeasts contribute carbon dioxide that inflates the dough, while bacteria produce organic acids and other flavour compounds. Temperature, starter quantity, flour and time change how quickly these reactions progress.
What is fermentation?
Fermentation is a collection of microbial and enzymatic processes rather than one single reaction. Flour contains starch, proteins, minerals and enzymes. When flour is hydrated, enzymes begin breaking some starch into simpler sugars that microorganisms in the starter can use.
The starter contains communities of wild yeasts and lactic acid bacteria. Their activity is what distinguishes sourdough from dough raised only with commercial baker’s yeast.
Where do the sugars come from?
Flour is rich in starch, but microorganisms cannot use all starch directly. Enzymes such as amylases break starch molecules into smaller sugars. Those sugars become part of the food supply for yeasts and bacteria.
The exact balance changes with flour type, temperature, fermentation time and the microorganisms in the culture.
Gas production
Yeasts metabolise available sugars and produce carbon dioxide. Gluten and the surrounding dough structure trap much of that gas, so the dough expands.
Gas production alone is not enough. A dough can ferment vigorously but still spread if its gluten network is weak. That is why fermentation and dough strength have to develop together.
Acid production
Lactic acid bacteria produce organic acids, including lactic and acetic acids. These contribute to sourdough flavour and lower the pH of the dough.
Acidity also affects enzymes and dough properties. If you want the chemistry in more detail, continue to pH in sourdough.
Time changes the dough
As fermentation continues, the dough accumulates gas and acids, aromas develop and the structure changes. More time is not automatically better. Underfermented dough has not developed enough gas or maturity; excessively fermented dough can weaken and lose its ability to hold shape.
Temperature changes the speed
Warmer dough generally ferments faster because microbial and enzymatic activity increases across a useful baking range. Cooler dough slows the same processes.
This is why a recipe’s five-hour bulk fermentation cannot be copied blindly into every kitchen. Learn to combine the clock with dough temperature and fermentation.
Flour and hydration matter too
Wholegrain and rye flours often produce different fermentation behaviour because they contain more of the grain and bring different enzyme, mineral and nutrient profiles. Hydration affects mobility inside the dough and changes handling as well.
For the practical side of water percentage, read sourdough hydration explained.
What fermentation changes that you can see
- The dough increases in volume.
- Bubbles become visible at the sides and surface.
- The dough feels lighter and more aerated.
- Aroma becomes more fermented and complex.
- The dough’s strength and extensibility change as fermentation progresses.
Why this matters in the kitchen
The useful lesson from the science is not to memorise microorganism names. It is to understand that sourdough is responsive. If the dough is colder, fermentation will take longer. If the starter is weak, gas production will be slower. If the dough has gone too far, more time will not repair the structure.
Bake Slower Tip
Treat time as a measurement, not a command. Record how long fermentation took, but decide when to move on by combining time with temperature, rise, bubbles and dough strength.
Common Mistake
Assuming that more fermentation always means more flavour and therefore better bread. Fermentation has an optimum window; beyond it, dough strength and baking performance can deteriorate.
From science to practice
The next practical skill is judging bulk fermentation. That page turns these reactions into observable cues you can use while the dough is actually in front of you.
Today You Learned
- Enzymes make flour sugars available to sourdough microorganisms.
- Yeasts produce carbon dioxide that helps the dough rise.
- Lactic acid bacteria produce acids and flavour compounds.
- Time and temperature strongly influence fermentation speed.
- Good baking comes from matching scientific understanding with visible dough cues.
Frequently Asked Questions
Is fermentation the same as proofing?
Proofing is a stage of fermentation. Fermentation begins when the starter is incorporated into the dough and continues through bulk and final proof.
Why does cold dough ferment slowly?
Lower temperature reduces the rate of microbial and enzymatic activity, so the same dough may need considerably more time.
Does sourdough fermentation stop in the fridge?
It slows greatly but does not instantly stop. The dough continues changing during cold proofing.
Why can overfermented dough become weak?
Acidity, enzymatic activity and prolonged fermentation can alter the dough structure until it no longer holds gas and shape as effectively.
Continue Learning
Next: pH in Sourdough
See how acidity develops during fermentation and why pH affects flavour and dough behaviour.
Written by Jim, founder of Sourdough by the Sea. Practical sourdough guidance for home bakers, based on clear process, careful observation and better bread.
What this means at the bench
The useful question is not only what is happening scientifically, but what you can observe. Connect the science to changes in rise, aroma, acidity, dough strength, extensibility and fermentation speed in your own kitchen.