Sourdough ferments through wild yeast and bacteria living in your starter

Sourdough rises because of wild yeast and lactic acid bacteria that live in your starter—a mixture of flour and water you maintain over time. Unlike commercial yeast, which is a single strain added to dough, sourdough starter is a living ecosystem. The yeast ferments the dough and produces carbon dioxide gas, which creates the rise. The bacteria produce lactic acid, which gives sourdough its tangy flavor and changes how the gluten network behaves.

The starter itself is not magic or mysterious. It is flour, water, and time. When you mix flour and water together and leave it at room temperature, wild yeast and bacteria naturally present on the grain colonize the mixture. After a few days of feeding (adding fresh flour and water), the population of useful microbes grows strong enough to reliably ferment dough. That starter can then be used to leaven bread for years or decades.

The reason sourdough tastes and behaves differently from yeasted bread comes down to fermentation speed and acid production. Commercial yeast works fast and produces mainly carbon dioxide. Sourdough ferments more slowly and produces both gas and acid, which affects flavor, crust color, and how long the bread stays fresh.

Key Takeaways

  • Sourdough starter is a living culture of wild yeast and lactic acid bacteria that you feed regularly with flour and water.
  • The yeast produces carbon dioxide gas to make the dough rise, while bacteria produce lactic acid for flavor and dough conditioning.
  • Fermentation happens in two stages: bulk fermentation (dough rising in a bowl) and final proof (shaped dough rising before baking).
  • Temperature controls fermentation speed—warmer kitchens ferment faster, cooler ones slower, which is why recipes give time ranges rather than exact hours.
  • The long fermentation breaks down gluten and starches, making sourdough easier to digest and giving it a longer shelf life than yeasted bread.

How the starter creates rise in your dough

When you mix your sourdough starter into flour and water, the yeast cells in the starter begin eating the sugars in the flour. As they consume those sugars, they produce carbon dioxide gas as a waste product. That gas gets trapped in the gluten network of the dough, creating thousands of tiny bubbles. Over hours, more gas is produced, and the bubbles expand, causing the dough to rise.

The bacteria in the starter work alongside the yeast. They also ferment sugars, but they produce lactic acid instead of (or in addition to) gas. This acid serves two purposes: it flavors the bread, and it weakens the gluten slightly, making the dough easier to stretch and shape. The acid also slows down the yeast's activity, which is why sourdough ferments more slowly than commercial yeast dough—the acid creates an environment where the yeast works at a measured pace.

The strength of your starter determines how fast fermentation happens. A starter that has been fed recently and is at peak activity will ferment dough quickly. A starter that was fed hours ago and is past its peak will ferment more slowly. This is why bakers "feed" their starter a few hours before mixing dough—they want the yeast population to be active and ready to work.

Bulk fermentation: the first rise

After you mix your dough, it enters bulk fermentation, which typically lasts four to eight hours at room temperature (though this varies widely based on how warm your kitchen is). During this time, the yeast and bacteria are actively fermenting. The dough roughly doubles in size, though it does not need to double exactly—bakers watch for the dough to become puffy and show visible bubbles on the surface.

During bulk fermentation, the gluten network is also developing. Fermentation produces acids and enzymes that break down proteins and starches in the flour, which strengthens the gluten and makes it more extensible (able to stretch without tearing). This is why sourdough dough becomes smoother and more elastic over the course of bulk fermentation, even without vigorous kneading.

Many sourdough recipes call for "stretch and folds" during bulk fermentation—you gently pull the dough up from the sides and fold it over itself every 30 minutes for the first few hours. This is not kneading. It is a way to build strength in the dough without degassing it completely. The stretching also redistributes nutrients and yeast throughout the dough, keeping fermentation even.

Final proof: the second rise after shaping

Once bulk fermentation is complete, you shape the dough into a round or oval and place it in a banneton (a proofing basket) or on a floured cloth. The dough then enters final proof, which can happen at room temperature for two to four hours, or in the refrigerator overnight (or even for several days). During final proof, fermentation continues, but more slowly than during bulk fermentation.

Cold fermentation in the refrigerator is popular because it gives you flexibility—you can shape dough in the evening and bake it the next morning. The cold slows yeast activity dramatically, so the dough rises very slowly. This also allows more acid to develop, which deepens the flavor. Many bakers find that cold-fermented sourdough has a more complex, tangier taste than dough that proofed at room temperature.

You know the dough is ready to bake when it has risen noticeably and passes the "poke test": you gently poke the dough with a floured finger, and the indent springs back slowly but does not disappear completely. If it springs back when ready, the dough needs more time. If the indent does not spring back at all, the dough may be overproofed.

Why temperature matters so much

Fermentation speed is almost entirely controlled by temperature. Yeast and bacteria work faster in warmth and slower in cold. A sourdough dough in a 75°F kitchen will ferment noticeably faster than the same dough in a 65°F kitchen. This is why sourdough recipes rarely give exact times—they give ranges (like "4 to 8 hours for bulk fermentation") and tell you to watch the dough instead of the clock.

If your kitchen is very warm (above 80°F), fermentation can happen so fast that the dough overproofs before you are ready to bake. If your kitchen is cool (below 65°F), fermentation will be slow, and you may need to leave dough to rise overnight or use the refrigerator strategically. Some bakers use a proofing box or place their dough in an oven with the light on to create a warm, stable environment.

Seasonal changes affect fermentation too. Summer doughs ferment faster than winter doughs, even in the same kitchen. Experienced sourdough bakers adjust their feeding schedule and fermentation times with the seasons, using warmer water in winter and cooler water in summer to keep fermentation on a predictable schedule.

How fermentation changes the bread's flavor and texture

The long, slow fermentation of sourdough produces flavors that quick yeasted bread cannot match. The bacteria in the starter produce lactic acid (mild, yogurt-like) and acetic acid (sharper, vinegary). The longer the fermentation, the more acid develops, and the tangier the bread tastes. Cold fermentation especially encourages acid production, which is why overnight-proofed sourdough often tastes more sour than dough that proofed at room temperature for the same total time.

Fermentation also breaks down starches and proteins in the flour through enzymatic action. This makes the bread easier to digest and may make it easier for some people to tolerate, though sourdough is not gluten-free. The breakdown of starches also affects how quickly the bread goes stale—sourdough stays soft and fresh longer than yeasted bread because the starches have been partially broken down and do not firm up as quickly.

The crust color and texture also depend on fermentation. The acids produced during fermentation lower the pH of the dough, which affects how the crust browns in the oven. Sourdough typically develops a darker, thicker crust than yeasted bread. The long fermentation also allows more flavor compounds to develop, which contribute to the complex taste you notice when you bite into the bread.

What happens if fermentation goes wrong

If your dough does not rise at all, your starter may not be active enough. A starter that has not been fed in days or weeks may have a weak yeast population. The fix is to feed your starter regularly (usually once or twice a day) until it reliably doubles in size within four to eight hours of feeding. Once it is active, use it to make dough.

If your dough rises too fast and overproofs, the gluten network becomes so stretched that it cannot hold gas anymore. The dough collapses, and the bread bakes flat and dense. This usually happens in very warm kitchens or when dough is left to ferment too long. The solution is to watch the dough carefully and bake it as soon as it passes the poke test, or to use the refrigerator to slow fermentation.

If your dough does not rise enough during final proof, it may not have enough gas to oven spring (the rapid rise that happens in the first few minutes of baking). The bread will be denser than you want. This usually means the dough needed more time to ferment, or your starter was not active enough. Next time, extend the fermentation or use a more active starter.

Frequently Asked Questions

Why does my sourdough starter smell like nail polish remover?

That smell is acetone, produced by yeast as a byproduct of fermentation. It is normal and harmless. It usually means your starter is hungry and needs to be fed. After you feed it, the smell typically fades within a few hours. If the smell persists for days after feeding, your starter may need more frequent feedings or warmer conditions to keep the yeast population strong.

Can I use my sourdough starter straight from the refrigerator?

You can, but the starter will ferment more slowly because it is cold. Most bakers feed their starter and let it sit at room temperature for a few hours until it is bubbly and active, then use it to make dough. This ensures the yeast population is awake and ready to work. If you use cold starter directly, your dough will ferment very slowly, which can work but requires patience.

How do I know if my dough is overproofed?

Overproofed dough fails the poke test—when you gently poke it with a floured finger, the indent does not spring back. The dough may also smell very sour and look slightly deflated or jiggly. If you catch it early, you can still bake it, but the bread will be denser and flatter than ideal. To prevent overproofing, use the refrigerator to slow fermentation or bake as soon as the dough passes the poke test.

Does sourdough fermentation kill gluten?

No. Fermentation breaks down some of the gluten proteins and starches, but it does not eliminate gluten. Sourdough contains gluten and is not safe for people with celiac disease or gluten sensitivity. The breakdown that does happen may make sourdough easier to digest for some people, but this varies individually.

Why do I need to feed my starter if I am not baking?

Your starter is a living culture of yeast and bacteria. They need food (flour) and water to survive. If you do not feed your starter, the yeast and bacteria will eventually die or go dormant, and your starter will become weak or inactive. Regular feeding keeps the population healthy and ready to ferment dough whenever you want to bake.