Guide
Reading Your Sourdough Crumb: How to Diagnose Under-Fermented, Over-Fermented, and Fool's Crumb Loaves
The cut is the report card. Height, ear, and Instagram holes lie. Density, hole placement, wall thickness, and smell tell you whether bulk was short, long, or just uneven.
Most “open crumb” photos that disappoint at the table are fool’s crumb: a few caverns under the crust and a tight, under-fermented core. That is not high hydration succeeding. It is gas that never got distributed, then bolted for the top in the oven.
The calculator won't score your loaf. It will tell you if the mix could hold a skin, and when bulk should have ended for that temperature and starter amount. Read the crumb first. Then change one number.
What you're looking for
- Weigh it in your hand. Light for its size usually fermented; dense usually did not — holes at the crown do not count.
- Look at hole placement, not hole size. Evenly scattered holes mean bulk did its job. A cave under the crust with a tight floor is fool's crumb.
- Check the walls. Dull and thick: under. Thin, shiny, greasy: over. Soft but set: on time.
- Smell the center, not the crust. Floury is short. Vinegar and alcohol are long. Nutty and faintly tangy is the window.
Match the crumb to the problem
Match symptoms to a primary defect. Change the root cause on the next bake — not your scoring pattern.
| Symptoms | Primary defect | Root cause | Recipe fix |
|---|---|---|---|
| Tight even crumb, heavy loaf, thick pale crust, little ear, floury smell. | Under-fermented | Bulk ended early for that dough temp and starter amount. Cold kitchen, young starter, or 20% starter treated like a 5-hour mix at 18°C. | Give bulk the hours the calculator calls for. Raise dough temp or starter amount — do not add water to “open it up.” |
| Shiny thin walls, greasy look, loaf spread, weak or no ear, vinegar or alcohol. | Over-fermented | Past the bulk window: too warm, too much starter, a past-peak starter, or a long bulk that ignored the temperature. | Shorten bulk toward the table's center. Cooler dough temp, less starter, peaked starter. Fridge proof if the kitchen runs hot. |
| Caves clustered under the crust, dense tight floor. Looks open in a photo, eats like sandwich bread with holes. | Fool’s crumb | Gas never distributed. Short or uneven bulk, then aggressive shaping parked a few bubbles at the top. Under-proofed core, big oven spring in the wrong places. | Finish bulk so the mass is aerated throughout. Gentle coil folds. Proof to a slow spring-back. Then score. |
| Uniform small holes, light loaf, decent ear. Sandwich crumb. | Tight even crumb | Hydration below the flour’s sweet spot, or a thorough degas at shape. Not under-fermentation if the loaf is light. | If you want more open crumb, raise true hydration toward the sweet spot without crossing the ceiling. Keep bulk on time. |
| Knife smears hours after cooling. Shiny collapsed walls. | Gummy / wet | Sliced hot; underbaked (core under 206°F); or true hydration at or over the flour ceiling, so the net never set. | Cool 2 hours. Bake to 206–210°F in the center. If the mix was a puddle, drop hydration or use Rescue. |
| Melted on the stone, no height, ragged skin, maybe a burst side. | Pancake / spread | No gluten skin: ceiling crossed, bulk run long, or both. Weak flour (AP, 00, einkorn) at country-loaf water. | Check the hydration bands first, then the bulk window. Shape only after the mix can hold a skin. |
Gummy crumb isn't always about bulk
A gummy crumb is often blamed on bulk when the starch never set. Wheat starch sets in the loaf's core around 206–210°F (96–99°C). Probe the center, not the crust. If you pull at 200°F, the walls look baked and the middle stays paste.
Core temperature
206–210°F
Hold until the probe in the thickest part reads this band. Lid off if the crust is already dark.
Cooling rule
2 hours
Steam is still migrating. Slice sooner and the knife smears — that is unset crumb, not a recipe failure.
If the loaf hit 208°F and cooled two hours and still smears, look at hydration and bulk — the ceiling and the window — not the oven.
The three loaves people confuse
Under-fermented
The gluten net is intact and unused. You can still save the next bake by giving bulk the hours the temperature table calls for — or by warming the dough so those hours fit a weekday. Do not add water to “open it up.” Under-fermented high hydration is how you get fool’s crumb.
Over-fermented
The net ate itself. Acid and time thinned the walls; oven spring has nothing to push against. Shorter bulk, cooler dough temp, or less starter. A past-peak starter already spent part of the window in the jar — the calculator's late-starter factor (0.88×) exists for this.
Fool’s crumb
The loaf is under-fermented in the middle and over-sprung at the skin. Big holes are a leak, not a goal. Finish bulk until the dough is airy all the way through, fold to stack layers without knocking the gas out, and proof until a floured finger springs back slowly. Then the ear is structure, not a blister over a cave.
Change one number
If the crumb is tight and the mix was stiff, hydration was the lever — see the flour hydration ceiling. If the crumb is tight and the mix was slack enough, bulk was short — see the bulk guide.
Fool’s crumb is almost always the second case wearing the first case’s photography. Hit the bulk window at a hydration the flour can hold. The open crumb is a side effect of an even ferment, not a hole you score into existence.