Why Your Sourdough Crumb Looks Nothing Like the Photo (The Science of Gas, Gluten, and Oven Spring)

You follow the recipe. You do the folds, you nail the timing (or think you do), you score with confidence, and then you slice into the loaf and... it looks nothing like the picture. The crumb is dense in patches, or weirdly uneven, or there are those big tunnels near the top and a brick-like base. You know the feeling.

Crumb structure is one of those things that looks simple on the surface but is actually the result of several overlapping processes happening simultaneously inside your dough. Understanding what is actually going on, scientifically, makes it much easier to diagnose what went wrong and, more usefully, what to adjust next time. So here is what is actually happening inside your loaf, from bulk ferment right through to that moment in the oven when everything either comes together or does not.

Gas production is only half the story

Most people assume crumb structure is purely about fermentation , more gas equals bigger holes, right? Not quite. Gas production from your yeast and bacteria is essential, yes, but the crumb you end up with is really a product of two things working together: how much gas is being produced, and how well your gluten network can trap and hold that gas.

Think of it like a foam mattress versus a net. If your gluten is weak or underdeveloped, the gas bubbles escape or merge into fewer, larger pockets. If your gluten is strong and well-organised, it holds each bubble in place, stretching as the gas expands but not tearing. The result is a more even, open structure with a good distribution of irregular holes , the kind bakers get excited about.

What happens during bulk fermentation (properly explained)

Bulk fermentation is where most of the structural work happens. As the wild yeast in your starter consumes the sugars in the flour, it produces carbon dioxide and ethanol. The bacteria (mainly lactobacillus strains) produce lactic and acetic acids as byproducts of their own fermentation. Those acids aren't just responsible for flavour , they actively condition the gluten, making it more extensible and easier to work with.

Early in bulk, the dough feels dense and relatively stiff. As fermentation progresses, the gluten relaxes, the dough becomes more aerated, and you'll notice it starting to feel lighter, slightly domed, and alive. That progression from heavy to buoyant is your signal that things are working.

The stretch-and-fold sets you do during bulk are not just busywork. Each fold realigns the gluten strands, building strength and encouraging the network to develop in a way that can actually hold structure through shaping and proofing. Skip them, and you often end up with a dough that spreads sideways rather than rising upward.

Shaping and why it affects crumb more than people think

This is the part most home bakers underestimate. Shaping is not just about aesthetics or making the loaf look tidy before it goes in the banneton. The tension you build during shaping creates a kind of outer skin on the dough that helps it hold its form during the final proof and then direct its expansion in the oven.

A tight, well-shaped loaf has surface tension that channels oven spring upward and outward in a controlled way. A loosely shaped loaf, or one that was degassed too aggressively during shaping, loses the gas you worked hours to build and often bakes flat with a dense, patchy crumb.

The goal with shaping is to build tension without tearing the gluten network. You are not kneading here, you are dragging and folding the dough against the work surface to create a taut outer layer. Late spring is actually a lovely time to practise shaping , the kitchen is warm enough that the dough stays workable, but not so warm that it's already exhausted before you get to it.

Oven spring and the final transformation

Oven spring is the dramatic rise your loaf gets in the first 10-15 minutes of baking, before the crust sets. When cold (or room temperature) dough hits a very hot oven, several things happen almost simultaneously. The residual yeast activity surges one last time before the heat kills it off. The gases inside expand rapidly. The water in the dough converts to steam, which inflates the structure further. And the gluten sets, locking everything in place as the proteins denature and the starches gelatinise.

That's why the score you make before baking matters so much , it creates a controlled weak point in the crust that lets the loaf expand upward rather than tearing at the side or staying squat. A deep, confident score at a shallow angle gives you that classic ear. A tentative scratch barely helps at all.

The other critical factor here is steam in the early bake, which keeps the crust flexible long enough for oven spring to do its job fully. Baking in a Dutch oven traps the steam from the dough itself, which is why it's so effective. Without steam, the crust sets too quickly, and the loaf can't expand as much.

Putting it all together

Crumb structure is really a chain: starter health, gluten development, bulk fermentation, shaping, proofing, scoring, and oven setup. A weak link anywhere in that chain shows up in the slice. The good news is that once you understand which stage drives which outcome, troubleshooting becomes a lot more straightforward. It's a bit like debugging , if the output looks wrong, work backwards through the inputs until you find where things went off.

If you want someone to actually look at your specific bake and tell you what is going on, the DoughRise Coach does exactly that. Unlimited AI coaching messages, personalised bake plans, and proper technique guidance means you can describe your loaf (or send a photo of that crumb), and get a real diagnosis rather than just a generic checklist. It is genuinely useful for breaking out of whatever plateau you have been stuck on.

Keep baking, keep adjusting, and don't stress if this week's loaf is not perfect. The best ones are usually made after you've had a few that weren't.

Happy baking! Find everything you need at doughrise.store

Photo by Viktor SOLOMONIK on Unsplash