Sourdough Fermentation Simply Explained: What Happens in the Dough?

For the first several months that I baked sourdough, I had absolutely no idea what was actually happening inside my dough during fermentation. I knew I was supposed to mix flour and water and starter, then wait a long time, and eventually bread would appear. The waiting part felt like magic, or maybe a very slow and nerve-wracking form of magic where I just stared at a bowl of dough and wondered if it was doing anything. Spoiler: it was doing a lot. I just did not understand any of it.

Understanding fermentation changed my baking completely. Not because I suddenly needed to think about microbiology while making breakfast for my kids, but because once I understood what was actually happening in the dough, I could start making informed decisions instead of blindly following recipe timelines. Why does my dough need more time when the kitchen is cold? Why does it taste more sour some weeks? Why did my loaf turn out great on Tuesday and terrible on Thursday with the exact same recipe? Fermentation science answers all of these questions, and I promise I can explain it without a single equation.

The Cast of Characters

Your sourdough starter is a living ecosystem with two main types of microorganisms: wild yeast and lactic acid bacteria (LAB). They live together in a symbiotic relationship, meaning they help each other out. The yeast and bacteria in a sourdough starter are different from commercial baker’s yeast (Saccharomyces cerevisiae). The dominant yeast in most sourdough starters is Kazachstania humilis (formerly called Candida humilis), although there can be dozens of yeast species present depending on your particular starter. The bacteria are predominantly Lactobacillus species, particularly Lactobacillus sanfranciscensis (recently renamed Fructilactobacillus sanfranciscensis, because scientists love making names longer).

What the Yeast Does

The wild yeast is responsible for the rise. When yeast cells consume sugars (primarily glucose and fructose from the enzymatic breakdown of flour starch), they produce two things: carbon dioxide gas and a small amount of ethanol. The carbon dioxide is what inflates your dough. Those bubbles you see forming during bulk fermentation are carbon dioxide being produced by millions of yeast cells eating sugar and releasing gas. The ethanol mostly evaporates during baking, although it contributes a small amount to aroma.

Yeast activity is heavily influenced by temperature. At room temperature (around 72-78°F), yeast cells are active and multiplying. As temperature increases toward 85°F, activity speeds up. Above 95°F, yeast starts to struggle, and above 140°F it dies. Below 50°F, yeast activity slows dramatically but does not stop entirely, which is why cold retarding in the fridge still allows some slow fermentation to continue overnight. This temperature sensitivity is why a dough that takes four hours to rise in a warm kitchen might take eight hours in a cold one. The yeast is doing the same work, just at different speeds.

What the Bacteria Do

The lactic acid bacteria are responsible for the flavor. While the yeast is producing gas, the bacteria are producing acids, primarily lactic acid and acetic acid. These acids are what give sourdough its characteristic tangy flavor, and the balance between them determines the quality and character of that tanginess.

Lactic acid is mild, creamy, and smooth. Think yogurt. It provides a gentle tanginess that enhances the wheaty flavor of the bread without overwhelming it. Acetic acid is sharp, vinegary, and assertive. Think vinegar (because that is literally what acetic acid is). In small amounts, it adds complexity. In large amounts, it makes your bread taste aggressively sour and can produce an unpleasant aftertaste.

The Fermentation Timeline

Let me walk you through what is happening inside your dough at each stage of the process, from mixing to baking.

Stage 1: Mixing and Autolyse (0-1 hours)

When you first combine flour and water (with or without the starter, depending on whether you autolyse separately), enzymes in the flour become active. The two most important are amylase and protease. Amylase breaks down starch into simple sugars, creating the food supply that the yeast and bacteria will consume during fermentation. Protease breaks down some of the gluten proteins, which actually helps with extensibility (the ability of the dough to stretch without tearing).

During autolyse, the flour is also fully hydrating. Dry flour particles absorb water, gluten proteins begin to bond and form a network, and the dough transforms from a shaggy mess into a cohesive mass. If you have ever wondered why your dough looks so different after a 30-minute autolyse even though you did nothing to it, this is why. The enzymes and hydration are doing significant work while you scroll through your phone.

Stage 2: Early Bulk Fermentation (1-3 hours)

Once the starter is incorporated and you begin bulk fermentation, the yeast and bacteria get to work immediately. In the early hours, activity is relatively slow as the organisms adjust to their new environment and begin multiplying. You are building the microbial population during this phase. Your stretch-and-folds during this period serve two purposes: they develop gluten structure, and they redistribute the yeast and bacteria throughout the dough so fermentation happens evenly. Our bulk fermentation guide covers the mechanics of this phase in detail.

During these early hours, the dough temperature is critically important. If your dough is too cold (below 72°F), microbial activity will be sluggish and you will need to extend your timeline significantly. If it is too warm (above 85°F), the bacteria will outpace the yeast, producing more acid than gas, which can result in a dense but very sour loaf. The sweet spot for most bakers is 75-80°F, which keeps both yeast and bacteria active in a balanced way.

Stage 3: Active Fermentation (3-5 hours)

This is where things really start moving. The yeast population has grown substantially and gas production accelerates. You will notice the dough volume increasing visibly, bubbles appearing on the surface and along the sides of your container, and the texture becoming noticeably lighter and more airy. If you lift the edge of the dough, it should feel puffy and show a honeycomb-like structure of bubbles underneath.

The bacteria are also ramping up acid production during this phase. The pH of the dough, which started around 5.5-6.0 after mixing, drops steadily as acids accumulate. By the end of bulk fermentation, the pH will be around 4.0-4.5. This increasing acidity actually affects the gluten network. Moderate acidity strengthens gluten and improves gas retention. But too much acidity begins to break down the gluten structure, which is why over-fermented dough becomes slack and loses its ability to hold shape.

The volume increase during bulk fermentation is the best indicator of progress. Most recipes aim for a 50-75% increase in volume by the end of bulk. Some bakers push to a full double, but for most flour types and hydration levels, 50-75% is the sweet spot. Going beyond that risks over-fermentation, where the yeast has consumed most of the available sugars and the gluten is starting to degrade from acid exposure.

Stage 4: Shaping and Final Proof (5-16+ hours)

After bulk fermentation, you shape the dough and begin the final proof. During this phase, fermentation continues but at a slower pace because the microbial population has already consumed much of the easily available food. The purpose of the final proof is to allow the dough to relax after shaping (which degasses it somewhat) and to accumulate enough gas for a good oven spring.

Many bakers do a cold retard at this stage, placing the shaped dough in the fridge for 10-16 hours overnight. The cold temperature dramatically slows yeast activity but does not stop bacterial acid production entirely. This is why cold-retarded loaves tend to have more flavor complexity and a more pronounced tang than loaves proofed entirely at room temperature. The extended time also allows enzymes to continue breaking down starches and proteins, which contributes to better crust color (more sugars for caramelization) and flavor development.

Stage 5: Oven Spring (first 10-15 minutes of baking)

Fermentation does not stop the instant the dough enters the oven. For the first several minutes, until the internal temperature reaches about 140°F, yeast activity actually increases due to the warmth. This final burst of gas production, combined with the expansion of existing gas bubbles and the conversion of water to steam, produces oven spring. The bread rises dramatically during this window.

Once the internal temperature exceeds 140°F, the yeast dies and fermentation officially ends. The remaining rise comes purely from thermal expansion of gases and steam generation. By 180°F, the proteins have coagulated and the starches have gelatinized, setting the crumb structure permanently. From this point on, baking is about crust development, browning (Maillard reaction and caramelization), and driving out excess moisture.

How Fermentation Affects Flavor

Beyond the basic lactic acid versus acetic acid balance, fermentation produces hundreds of flavor compounds that contribute to the complex taste of sourdough bread. These include organic acids beyond the big two, alcohols, aldehydes, and esters, each contributing subtle notes that make sourdough so much more interesting than commercial yeast bread.

The flour you use also interacts with fermentation to produce different flavors. Whole wheat flour contains more minerals and enzymes, which accelerate fermentation and tend to produce a stronger, more assertive flavor. White bread flour ferments more slowly and produces a milder, more neutral flavor that lets the tangy sourdough character shine through. Rye flour is an entirely different beast, fermenting rapidly and producing distinctive earthy, almost fruity notes.

Your starter’s particular microbial population also influences flavor. Every starter is unique, harboring its own specific community of yeast and bacteria species. This is why two bakers using the same flour, same water, same technique, and same timeline can produce bread that tastes subtly different. Your starter adapts to your flour, your water, your feeding schedule, and your kitchen environment over time, developing a microbial fingerprint that is genuinely yours. If you are curious about starting your own or revitalizing a sluggish one, our starter guide and rescue guide walk through the process.

Practical Takeaways

All of this science boils down to a handful of practical principles that will make you a better baker immediately.

Temperature is your master variable. Track your dough temperature and adjust water temperature to control it. A consistent dough temperature at the start of bulk fermentation is the single most powerful tool for producing consistent results. Want faster fermentation? Use warmer water. Want slower fermentation with more flavor development? Use cooler water. It really is that direct.

Time and temperature are a team. Never follow a recipe’s timing blindly. If the recipe says four hours at 78°F and your kitchen is 68°F, you need significantly more time. Watch the dough, not the clock. Volume increase, bubble activity, and texture changes are your real indicators. The windowpane and poke tests we cover in our dough readiness guide give you objective checkpoints.

Acidity is both friend and foe. Moderate acid development strengthens gluten, improves keeping quality, and adds flavor. Excessive acid development weakens gluten, makes the bread taste aggressively sour, and can produce a gummy texture. The key is catching fermentation at the right point, which is why the 50-75% volume increase guideline exists. Push past that and you risk crossing from pleasantly fermented to over-fermented.

Fermentation is what separates bread from crackers. It is what takes a simple mixture of flour, water, and salt and transforms it into something with depth, complexity, and soul. You do not need to memorize Latin species names or understand enzyme kinetics to bake great sourdough. But understanding the broad strokes of what is happening inside your dough, what the yeast is doing, what the bacteria are doing, and how temperature drives everything, gives you the ability to troubleshoot problems, make intentional adjustments, and produce bread that is not just edible but genuinely excellent. And that, if you ask me, is worth getting a little nerdy about.