If you’ve spent any time baking or even just browsing bread posts online, you’ve probably heard bakers talk about reading their crumb. It might seem like an abstract concept, but it’s actually one of the most valuable skills you can develop as a baker. In this article, we'll walk through different crumb structures and what they reveal about fermentation. The goal isn’t to chase a perfect crumb, but to help you start reading what your loaf is trying to tell you so you can refine your process with each bake.
Crumb is the interior of the loaf. It is what you see when you slice into baked bread and look at the structure inside. The size, shape, and feel of the holes can tell you a lot about how the dough fermented and how it was handled.
Every section of the loaf gives us clues about what is working and what still needs attention. Some loaves have a soft, pillowy interior that feels light without being full of holes. Others hold a tighter crumb but still show signs of strength and structure. Even a dense interior can be exactly what that dough was meant to become, especially considering the flours used.
Alveoli are the gas pockets inside the crumb, often what we call the “holes.” Their size, shape, and distribution can tell you a lot about how your dough fermented and how well the structure held through shaping and baking. In a well-fermented loaf, alveoli tend to vary in size and stay evenly dispersed. They should feel supported by a clear gluten network rather than collapsed or unstable. Extremely large pockets or tunnels can indicate shaping issues or fermentation that didn’t build strength evenly. A tight or overly uniform crumb might point to under-fermentation, over-fermentation or even too much applied pressure during shaping.
The shape of the baked loaf provides visual confirmation that supports what you see in the crumb. A loaf that is well-fermented usually stands tall, with a round or oval silhouette and strong shoulders. The score will bloom cleanly and open with upward lift. When fermentation is not complete, the dough may spread outward instead of rising, creating a flatter and more irregular loaf shape. Some bakers call this a UFO or pancake shape. A tight or shallow score that does not open well can suggest the dough lacked enough internal gas. When that is paired with a crumb that looks tight or compressed, the shape helps confirm that fermentation was cut short. On the other hand, a loaf that sinks at the shoulders or collapses slightly after scoring may point to fermentation that went too far and a structure that could not hold up during the bake.
A crumb that feels light and tender, with just enough holes to feel open without being hollow, can be a great result. Not every loaf needs wild, lacy, or dramatic holes to be successful. A denser crumb can still be perfectly fermented, especially with flours like rye or whole wheat. What matters most is whether the crumb feels balanced for the dough you made. Does it feel strong and structured? Does it hold together without being gummy or misshapen?
For this article, several loaves of sourdough bread were baked to capture different stages of fermentation. Some under fermented, some over over fermented, and some right where they needed to be. In the next section, we’ll walk through each one and share what the crumb might reveal about how it was handled and fermented.
The crumb shows some medium sized holes, but the areas between them are dense and tight. The alveoli look compressed rather than developed. There’s some structure, but not enough gas was produced or retained to allow the loaf to fully open in the oven. The larger bubbles near the top and middle appear slightly collapsed or stretched out, which can happen when the dough lacks fermentation strength. Shaping looks solid, but the dough did not expand into it. This suggests bulk fermentation was cut short.
This loaf is under-fermented, with a crumb that shows compressed and tight alveoli throughout. The bubbles are small and lack definition, and even the slightly larger pockets near the base and sides appear dense and collapsed rather than open and airy. The alveoli have a squashed look, as if the dough was trying to rise but didn’t have enough internal gas or fermentation strength to hold its shape. Too much pressure could have been placed in one area during shaping. Surface bubbles were trapped on the surface during shaping. This kind of compression suggests the dough was shaped too early, before enough fermentation had occurred to build volume and structure.
This loaf is also under-fermented, but it’s visually distinct from the others. The crumb is slightly more open, with a mix of hole sizes and a few larger pockets that stretch toward the crust. But the structure is still irregular and has signs of tunneling. The alveoli in the bottom half and around the larger pockets are compressed, showing the dough did not build enough internal gas to fully lift or stretch. Toward the top and middle, the bubbles become more erratic. Some are flat or collapsed, others form loose tunnels that drift sideways or downward. From a distance, this crumb gives the illusion of openness, but a closer look shows it never reached full expansion. It looks like it began to rise, then stalled before fermentation could finish shaping the interior. This dough needed a longer bulk rise and more rest time between shaping steps.
This loaf looks good but is a little over-fermented. The crumb has thinned out across the top, the crust has started to separate in areas and the alveoli look slack and stretched. You can see how the bubbles have expanded past their peak and lost tension, which gives the crumb a soft, almost deflated texture. Some of the walls between the holes have collapsed inward, which happens when the dough starts to weaken before the bake. There’s still structure, as seen by larger holes in the center but they are starting to lose their shape. Toward the bottom, the crumb tightens and looks slightly compressed, which can happen when the dough loses strength and can’t push upward as effectively during oven spring. The overall impression is a loaf that held on just long enough to bake, but had already started to fall past its prime before hitting the oven.
This loaf shows classic signs of over-fermentation. The crumb has a texture with a fragile, custard-like feel, and many of the bubbles have started to collapse. Instead of rising evenly with strong vertical lift, the interior has slumped inward. The shoulders of the loaf are deflated and there is no bloom or spring near the score. The crust is starting to separate in places. You can see how the structure tried to hold but lost tension late in the process. The lack of vertical stretch combined with the glossy, gelatinized interior suggests that the dough was pushed beyond its limit before baking. It no longer had the strength to maintain shape, and instead sank into itself. This is the kind of loaf that shows what happens when fermentation is extended just a bit too far.
This loaf is over-fermented, but in a more subtle way than the others. The honeycomb pattern dominates the crumb, with tightly packed, gelatinized cells that suggest a breakdown in structure. Rather than stretching vertically, the bubbles have expanded outward, creating a flat, spongy interior with little upward lift. The largest bubbles have lost their definition and the shoulders have collapsed slightly. There’s no bloom at the score, just a gentle spreading outward. The overall texture is glossy and custardy, but lacks fullness.
This is a well fermented loaf with structure that feels balanced from top to bottom. The crumb has a mix of small and medium alveoli that are evenly distributed without any signs of collapse or tunneling. The walls are thin and slightly glossy, which points to strong gluten development and a dough that was given enough time to ferment without being pushed too far. The top has good bloom and the shoulders are rounded. You can see that the dough was handled gently but with purpose, and the fermentation supported that shaping rather than fighting against it.
This is a well fermented loaf with inclusions, which can make crumb reads a little trickier. The distribution of bubbles is slightly irregular in places but there is still clear vertical lift and strength in the structure following the direction of the shaping of the dough. You can see thin walls and a range of medium to small alveoli throughout, with no signs of collapse or compression at the base. Some larger pockets near the outside edges may be a result of shaping or steam interacting with the inclusions. Even with the irregular zones from the inclusions, the crumb shows strength. The structure holds throughout the slice, with no signs of collapse or weakness.
This is a properly fermented loaf with solid structure and gentle openness throughout. The crumb has good vertical lift and balanced development from top to bottom. You can see a variety of bubble sizes, with a few larger pockets scattered near the top and sides. Some of these look like shaping bubbles that were trapped and expanded in the oven. They sit just under the surface without disrupting the overall crumb. The interior is light and elastic, with thin cell walls and no signs of collapse or thick tunnels. While not as uniform as some of the others, this loaf still shows clear strength. Fermentation but could have been pushed a little further. A longer rest period between shaping and refrigeration might have allowed for even more openness.
Every loaf is different, and many factors influence the final texture and structure. Some bakers prefer a tight, uniform crumb while others aim for large, wild open holes. Both are valid depending on your goals. Reading is nuanced and it takes practice. The examples provided within this blog are quick reads based on crumb alone. They do not include any details about flour choices, starter maintenance, methods, or timing and that context matters when reading crumb. Compare your own results over time or ask for second opinions in sourdough groups or fellow baker friend. Take photos. Keep notes. The more you bake and observe, the more consistent your results will become.
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