Sourdough rises because wild yeast in the starter ferments sugars from flour and produces carbon dioxide gas. That gas gets trapped inside an elastic gluten network, inflating the dough like thousands of tiny balloons. But the rise isn’t just about yeast. It depends on a partnership between yeast and bacteria, the strength of the gluten structure, temperature, hydration, and timing.
Wild Yeast Produces the Gas
Commercial bread uses a single domesticated yeast strain from a packet. Sourdough relies on wild yeast species that live naturally in flour and in the environment around your kitchen. One of the most common is Kazachstania exigua, though dozens of species have been identified in starters worldwide. These yeasts break down carbohydrates in flour through a process called glycolysis, converting sugars into two byproducts: carbon dioxide and ethanol. The carbon dioxide is what physically pushes the dough upward. The ethanol mostly evaporates during baking.
Before yeast can do its work, the sugars need to be available. Flour is mostly starch, which is a long chain of sugar molecules locked together. Enzymes naturally present in flour, particularly amylase, snip those starch chains into simple sugars the yeast can actually consume. This is why freshly milled or high-quality flour often produces a more vigorous rise: it has more enzymatic activity and more accessible nutrients for the yeast to feed on.
Bacteria Shape the Environment
A sourdough starter isn’t just yeast. It’s a dense colony of lactic acid bacteria living alongside the yeast, typically outnumbering them by a ratio of 10:1 to 100:1. A study of 19 traditional Italian sourdoughs found that bacteria consistently reached concentrations above 100 million cells per gram, with yeast populations at least ten times lower. This bacterial dominance is normal and essential.
The bacteria produce lactic acid and acetic acid as they ferment, which is what gives sourdough its tangy flavor. More importantly for the rise, these organic acids drop the pH of a mature starter to somewhere between 3.8 and 4.3. That acidic environment does several things at once. It strengthens the gluten network, creating a more elastic dough that holds gas better. It suppresses harmful microbes that could compete with the yeast. And it influences flavor compounds that develop during the long fermentation.
The relationship is genuinely symbiotic. The bacteria create conditions the yeast thrives in, and the yeast produces carbon dioxide the bacteria can’t generate on their own. Neither one alone would make good sourdough bread.
Gluten Traps the Gas
Producing carbon dioxide is only half the equation. Without something to trap that gas, it would simply bubble out of the dough and escape. That’s where gluten comes in. When flour and water are mixed and kneaded (or folded, in the case of most sourdough recipes), proteins in the flour link together into a stretchy, elastic matrix. This gluten network acts as the structural backbone of the dough.
As yeast releases carbon dioxide into the watery phase of the dough, the gluten network stretches around each expanding gas pocket. The high viscosity of the hydrated gluten slows the diffusion of gas, preventing it from escaping. At the same time, the thin films of liquid between gas chambers maintain their integrity, keeping bubbles separate and intact. The result is a dough that gradually inflates with thousands of discrete air pockets. When the dough finally goes into the oven, heat sets the gluten structure permanently and the gas expands one last time, producing the final burst of rise known as oven spring.
Temperature Controls the Speed
Temperature is one of the biggest levers you have over how fast and how well your sourdough rises. Wild sourdough yeast is most active between 80 and 86°F (27 to 30°C). At these temperatures, fermentation is relatively fast, and the dough rises noticeably within a few hours.
The bacteria in the starter have a different preference. At cooler temperatures, between 60 and 72°F (15 to 22°C), they produce both lactic and acetic acid along with some carbon dioxide. At warmer temperatures, from 86 to 99°F (30 to 37°C), they shift to producing mainly lactic acid and ethanol. The catch is that most sourdough yeast species shut down in that warmer range. So if your kitchen is very warm, the bacteria can overwhelm the yeast, producing lots of acid but not enough gas to get a good rise.
This is why many bakers aim for a dough temperature around 78 to 82°F. It keeps both the yeast and the bacteria active without letting either one dominate. Cold retarding, where shaped dough goes into the refrigerator overnight, slows fermentation dramatically and gives the bacteria more time to develop complex flavors without overproofing the dough.
Hydration Affects Dough Structure
The ratio of water to flour in your dough, expressed as a hydration percentage, changes both the texture and the rise profile. Lower-hydration doughs (around 60 to 68% water relative to flour) tend to be stiffer and hold their shape more easily. They typically produce a taller rise with more pronounced oven spring, because the tighter gluten network holds gas in a more compact structure.
Higher-hydration doughs (75 to 85% or above) are looser and more extensible. They can produce a beautifully open crumb with large, irregular holes, but the loaf often spreads wider rather than rising tall. Too little water makes the dough tight and resistant, limiting how much the gluten can stretch. Too much turns it into a slack mass that can’t support its own weight. Most home sourdough recipes land somewhere in the 70 to 78% range as a sweet spot between structure and openness.
Two Stages of Rising
Sourdough goes through two distinct rising phases, each with a different purpose. The first is bulk fermentation, which happens after you mix the dough and before you shape it. During bulk fermentation, the yeast begins producing gas, the gluten network develops through folds or stretches, and the bacteria start generating acid. This stage typically lasts anywhere from 4 to 8 hours at room temperature, depending on how warm your kitchen is and how active your starter was. Most of the structural development happens here.
The second phase is the final proof, which happens after you’ve shaped the dough into its final form. This is when the shaped loaf fills out with gas and develops the tension on its surface that will translate into oven spring. Many bakers do this proof in the refrigerator overnight, which slows the rise and gives them a wider window to bake at the right moment. The dough continues to ferment in the cold, just much more slowly.
Why Sourdough Sometimes Doesn’t Rise
The most common reason sourdough fails to rise is an inactive or immature starter. If the yeast population hasn’t built up enough, there simply aren’t enough organisms producing gas. A healthy starter should roughly double in volume within 4 to 6 hours of feeding at room temperature. If yours isn’t doing that, it needs more time and consistent feedings before it’s ready to leaven bread.
Overproofing is the other major culprit. As fermentation continues, the yeast and bacteria consume the available nutrients in the flour. Past a certain tipping point, the gluten network begins to weaken and break down. The dough loses its ability to hold gas, and the structure collapses. An overproofed loaf looks flat, feels slack, and won’t spring back when you poke it. The fix is simply to shorten the fermentation time or reduce the dough temperature so you catch the dough before it passes its peak.
Other factors that limit rise include using low-protein flour (which produces a weaker gluten network), fermenting at temperatures above 86°F where yeast activity drops off, or using too much or too little starter relative to the flour in the recipe. Even salt matters: it tightens the gluten and moderates fermentation speed, so leaving it out or adding too much can throw off the balance.