What Actually Happens to Your Dough During Bulk Fermentation (And Why It Matters)

⏱ 7 min read

Bulk fermentation is the part of sourdough baking that most people either rush through or obsess over to the point of paralysis. And honestly, I get it. You mix your dough, you add your starter, you cover the bowl , and then what? You just wait? What is actually happening in there?

This time of year is a good moment to think about it properly. Spring is arriving, kitchen temperatures are finally creeping up after months of sluggish winter ferments, and your dough is going to start behaving differently. Understanding what bulk fermentation is actually doing at a biological and chemical level makes you a much better baker, because you stop guessing and start reading your dough with genuine confidence.

The short version: your dough is alive

When you mix flour, water and your sourdough starter together, you are not just combining ingredients. You are creating a living ecosystem. Your starter introduces two key players: wild yeast and lactic acid bacteria. They work alongside each other throughout the bulk ferment, and each one is doing something distinct.

The wild yeast is primarily responsible for producing carbon dioxide gas. That gas gets trapped inside the gluten network you have built in the dough, and it is what makes your loaf rise. The lactic acid bacteria, meanwhile, are producing acids , mainly lactic acid and acetic acid. These are what give sourdough its flavour and its keeping quality.

Neither of these processes is instant. Both take time, and both are sensitive to temperature. That is the fundamental reason bulk fermentation is not something you can rush without consequences.

What gluten is actually doing during this time

Gluten often gets talked about like it is just a structural thing , the scaffolding that holds the bread together. But during bulk fermentation, gluten is actively developing and changing in ways that matter a lot to the final loaf.

When you first mix your dough, the gluten strands are relatively disorganised. They become more extensible and elastic over time through two processes happening at once. The first is mechanical development , the stretch and folds you do during bulk fermentation are physically aligning and strengthening the gluten network. The second is enzymatic activity , enzymes naturally present in the flour (proteases in particular) are gently breaking down some protein bonds, which makes the dough more extensible without losing strength.

The balance between these two forces is part of what makes sourdough feel so different from commercial yeast doughs. A long, cool bulk ferment gives the enzymes more time to work, which results in that characteristic open, slightly irregular crumb structure. A short, warm ferment means less enzymatic activity, and you often get a tighter, more uniform crumb.

The acid side of things (and why flavour is complicated)

Here is where it gets genuinely interesting. Lactic acid and acetic acid are not just flavour compounds , they have a real effect on the gluten network too. Acetic acid (the sharper, more vinegary one) tends to tighten gluten structure. Lactic acid (the milder, more yoghurt-like one) has a slightly softening effect. This is part of why sourdough baked at different temperatures or with different hydrations can taste and feel so different even if the process looks similar on paper.

The balance between these two acids is affected by hydration, temperature and fermentation time. Wetter doughs at warmer temperatures tend to favour lactic acid production, giving a milder flavour. Stiffer doughs at cooler temperatures tend to push towards acetic acid, which means more tang. Neither is better , it just depends what you are going for.

Spring baking tends to naturally push things towards the lactic end because ambient temperatures are rising, which is part of why spring loaves often have a slightly softer, more rounded flavour compared to winter ones. Worth knowing if you are expecting your usual tang and wondering where it has gone.

How to actually know when bulk fermentation is done

This is the question I get more than any other. And the honest answer is: it is not just about time. Time is a starting point, not a destination.

The signs to look for are a combination of visual and tactile cues. The dough should have grown noticeably , somewhere between 50% and 75% increase in volume is a common target for most sourdoughs, though this varies with hydration and flour type. The surface should look domed and slightly domed at the edges of the bowl, with visible bubbles just under the surface. When you do a final stretch, the dough should feel airy and slightly jiggly rather than dense and resistant.

One tip that took me a while to internalise: a well-fermented dough feels almost pillowy and light when you go to shape it. If it feels tight and heavy, it probably needs more time. If it feels almost too soft and tears easily, you have likely over-fermented it and the gluten has started to degrade. Both are useful data points. Think of it a bit like debugging , if the output is wrong, check what inputs have changed.

A clear straight-sided container (like a large Kilner jar or a square dough tub) makes this so much easier because you can actually see the rise rather than trying to guess from the top of an opaque bowl.

Why this all connects to pizza too

Everything above applies just as much to sourdough pizza as it does to bread. The bulk ferment on a sourdough pizza dough is arguably even more important, because you do not have the oven spring of a covered bake to rescue you , the dough goes straight onto a hot surface and needs to be in exactly the right state. A properly bulk-fermented pizza dough is extensible, flavourful and bakes to a light, airy base with genuine chew. An under-fermented one fights you during stretching and comes out dense and bland.

If you are getting into sourdough pizza this spring (which, honestly, you should be , there is nothing better on a Friday evening with mates coming round), the DoughRise Pizza Making Kit gives you the right tools to actually do the process justice, from the dough stage through to the bake. Getting the equipment right removes one more variable and lets you focus on understanding the fermentation itself.

Happy baking! Find everything you need at doughrise.store

Photo by Stephen Han on Unsplash