Flavour & Fire
The Sourdough Journal
Understanding Fermentation
Too far and too little can look surprisingly alike — until you know what to look for.
Fermentation is one of those parts of sourdough that becomes easier to understand once you stop looking at it as a fixed amount of time. A recipe might tell you that the dough needs four hours, six hours or perhaps an overnight rest, but fermentation does not follow the clock in quite that way. The microorganisms in your starter respond to their surroundings, and those surroundings change from one kitchen, season and bake to the next.
Temperature, flour, hydration, starter activity and the amount of time available all influence what happens inside the dough. As fermentation progresses, the dough changes in ways that can be seen, felt and sometimes even smelled. Learning to recognise those changes gives you something far more useful than a fixed timetable: an understanding of where your dough is in the process.
What Is Fermentation?
At its simplest, fermentation is the process in which microorganisms use available food and produce new compounds as a result. In sourdough, yeasts and lactic acid bacteria living in the starter become part of the dough once the starter is mixed with flour and water.
The yeasts produce carbon dioxide as they metabolise available sugars, and that gas becomes part of the expanding dough. The lactic acid bacteria produce organic acids and other compounds that contribute to the flavour, aroma and changing character of the dough. Fermentation is therefore doing much more than simply making bread rise. It is one of the processes responsible for turning a mixture of flour and water into something with structure, aroma and depth.
Much of this activity happens beyond what the eye can see, which is why understanding the visible signs becomes so important.
Fermentation Begins Before the Dough Looks Different
Fermentation does not suddenly begin when the dough starts to rise noticeably. Microbial activity is already taking place while the dough may still look almost unchanged.
In the early stages, the microorganisms are becoming active in their new environment while the dough itself is developing alongside them. Sugars become available, gases are produced and acids begin to accumulate, while the gluten structure continues to change through mixing, folding and time.
This is one reason the first part of fermentation can be difficult to judge. A dough may appear quiet while considerable activity is already taking place beneath the surface. Later, those changes become easier to recognise because the dough begins to show the results of everything that has been happening inside it.
What Temperature Changes
Temperature is one of the most important factors influencing fermentation because microorganisms become more or less active depending on their surroundings.
A warmer dough will generally ferment more quickly than a cooler one, while a cooler dough slows the process down. This does not mean that warmer is automatically better or that there is one ideal temperature for every sourdough. It simply means that temperature changes the pace at which fermentation develops.
That is why the same recipe can behave very differently on a warm summer afternoon and a cool winter morning. The ingredients may be identical, yet the dough may reach the same stage at very different times.
Temperature is therefore not just another number to record. It is part of the context in which the fermentation is taking place.
What the Dough Is Telling You
As fermentation progresses, the dough gradually becomes different from the mixture you started with. It may become smoother, lighter and more elastic, while small pockets of gas begin to appear within the structure. Depending on the dough and the stage of fermentation, the surface may become slightly domed, the edges may show bubbles and the whole mass may feel more alive when you move it.
These signs do not all appear at exactly the same time, and they will not look identical in every dough. A highly hydrated dough may behave differently from a firm one, while wholegrain flour can produce a different texture and appearance from a finely milled white flour.
This is why fermentation is best read as a collection of signals rather than a single visual test. Volume matters, but so do elasticity, tension, bubbles, texture and the way the dough responds when it is gently moved.
The more often you bake, the easier it becomes to recognise the combination rather than searching for one perfect sign.
Gas Is Part of the Story
The carbon dioxide produced during fermentation needs somewhere to go, and this is where the developing structure of the dough becomes important.
A dough with sufficient structure can retain some of that gas within its network, allowing the dough to expand while remaining cohesive. As fermentation continues, those pockets of gas become part of what you eventually see in the dough and, later, in the crumb of the baked loaf.
This also explains why fermentation cannot be considered separately from dough development. A dough can be actively fermenting, but if its structure is unable to retain the gas being produced, it may spread rather than rise in the way you hoped.
Fermentation creates the gas. The dough’s structure determines how well it can hold on to it.
Why the Clock Can Mislead You
Time is useful because it gives you a framework for the process, but it cannot tell you exactly what is happening inside the dough.
If a recipe says that bulk fermentation should take five hours, that number only makes sense within the conditions for which it was given. Change the temperature of the dough, the maturity of the starter or the flour you are using, and the same five hours can lead to a different result.
This is why experienced bakers often talk about fermentation in terms of the state of the dough rather than simply the number of hours that have passed. The clock tells you how long the dough has been fermenting. The dough tells you what that time has actually done.
Underfermentation
An underfermented dough has not had enough time for fermentation to develop to the stage required by the recipe and the dough’s structure.
It may still feel relatively dense and resistant, with limited visible gas and little of the lightness that develops as fermentation progresses. When baked, an underfermented loaf can have a tighter crumb and may show uneven or more concentrated areas of larger holes rather than the more developed structure you were expecting.
The important point is that underfermentation is not simply about needing a certain number of additional minutes. The dough needs more fermentation under its particular conditions, and those conditions determine how quickly it gets there.
Overfermentation
At the other end of the process, fermentation can continue beyond the point where the dough can comfortably support the structure being created.
An overfermented dough may become very soft, fragile or difficult to handle. It can lose some of its ability to hold tension and may spread easily when turned out or shaped. Instead of feeling elastic and responsive, it can begin to feel slack and delicate, as though the structure has passed the point where it can support further expansion.
This is one reason overfermentation can sometimes be confused with underfermentation. Both can produce disappointing loaves, and neither is always obvious from one visual sign alone. The difference becomes clearer when you consider the whole history of the dough: how it felt earlier, how quickly it changed, how much gas developed and how its structure responded along the way.
Why Under and Over Can Look Surprisingly Similar
A dough that has not developed enough and a dough that has gone too far can both fail to produce the loaf you expected, but they arrive there from opposite directions.
One has not developed enough fermentation and structure; the other has continued beyond the point at which the structure can comfortably contain what fermentation has produced. In both cases, the dough may fail to hold its shape properly or behave differently from what the recipe suggests.
This is why a single test rarely tells the whole story.
If you only look at volume, for example, you can miss what is happening to the texture and strength of the dough. If you only judge by the clock, you can miss the effect of temperature. If you only look for bubbles, you may overlook how the dough feels when handled.
Fermentation becomes much easier to read when these clues are considered together.
The Role of Your Starter
The activity of your starter also influences the pace and character of fermentation.
A mature, active starter contains a living community of yeasts and bacteria whose activity changes according to feeding, temperature, flour and time. A starter used close to its peak of activity may behave differently from one that has been sitting for a long period after feeding.
This does not mean that every bake requires a starter at exactly the same moment of maturity. It means that the starter is part of the conditions surrounding the dough, and its state can help explain why two otherwise identical recipes do not always ferment at the same speed.
Understanding your starter therefore makes understanding your dough easier.
Fermentation Is a Moving Process
One of the most useful shifts in sourdough baking is learning to see fermentation as a process rather than a destination.
The dough does not move neatly from one stage to another and then stop. Activity continues, structures change and the balance between strength, gas production and acidity develops over time. The moment you choose to shape, refrigerate or bake is therefore a decision made within that moving process.
There is no single moment when the dough suddenly becomes fermented. There is simply a point at which it has developed far enough for the next stage of your recipe.
That point may be slightly different from one bake to another, even when you are aiming for the same loaf.
Learn the Dough, Not Just the Recipe
Recipes are useful starting points because they give you ratios, temperatures and approximate timings from which to work. But the more you understand fermentation, the less dependent you become on treating those numbers as absolute instructions.
You begin to notice that a dough which reaches a certain temperature behaves differently from one that is several degrees cooler. You recognise the difference between a dough that is gaining strength and one that is beginning to lose it. You start to see bubbles not simply as proof that fermentation exists, but as part of a larger picture of what is happening inside the dough.
That is where sourdough becomes less about following instructions and more about learning a process.
The recipe still gives you the starting point. Fermentation tells you when the dough has travelled far enough.
From Fermentation to the Next Stage
By the time fermentation has developed sufficiently, the dough should have changed in ways that give you information about what comes next. It may feel lighter and more cohesive, hold some tension while still remaining extensible, and show signs of gas and activity throughout its structure.
From here, the process moves towards shaping, where the structure you have developed needs to be handled carefully rather than rebuilt from scratch.
Knowing when that moment has arrived is not something that can be reduced to one number or one visual test. It comes from bringing together everything you have observed: time, temperature, starter activity, dough strength, volume, texture and the way the dough responds to your hands.
The more you learn to read those signals, the less mysterious fermentation becomes.
Fermentation is not something you simply wait through. It is something you learn to recognise.
← Previous
You are here
Understanding Fermentation
Too far and too little can look surprisingly alike — until you know what to look for.