Crumb is the interior structure of the loaf — the network of holes and walls — while the crust is the outer baked shell. An "open" crumb has large irregular holes from strong gluten and sufficient fermentation; a "tight" crumb is dense, often from under-fermentation or over-degassing (degassing means releasing gas from the dough, though in sourdough this is usually gentle rather than the full degassing done in baguette dough). The Italian and French term for those gas pockets is alveoli, so "open alveolation" simply means holey crumb. The ideal crumb has even, glossy holes with thin cell walls, is moist but not gummy, with a slight chew and a balanced sweet-sour-malty flavor. Tunnel-like holes come from over-shaping or aggressive degassing before final proof — the gentler the final shape, the more large pockets survive. To get more open crumb generally, bakers reach for higher hydration, stronger flour, fewer but larger folds, longer bulk fermentation, and careful final shaping that preserves large gas pockets.
The poke test confirms proofing status: a slow rebound means ready to bake, a quick snap-back means under-proofed, and a dent that stays with no rebound means over-proofed. Under-proofed crumb is tight, gummy, and dense with a pale ear; over-proofed crumb has large uneven holes, a flat loaf, sour and raw-tasting interior, and tends to collapse on cooling. Dense or gummy crumb usually means under-proofing or under-fermentation, or slicing too warm — bread should rest at least an hour before cutting, ideally two to four, as starches continue to gel and crumb bloom (the slight opening of structure as the loaf cools) develops. Cutting too early causes a gummy interior; even overnight develops flavor.
Storage depends on how soon the bread will be eaten. Cut sourdough stays fresh for one to two days when stored cut-side down on a board or in a paper bag that keeps the crust crisp; plastic softens the crust quickly. Longer storage means freezing: slice the whole loaf, wrap tightly in plastic and foil, and freeze up to three months, then toast slices straight from the freezer. To refresh slightly stale bread, mist it lightly with water and bake at 200°C for 5–8 minutes to restore crust crunch, or cube it for croutons and panzanella.
Other common problems have specific fixes. Pale crust means the oven is too cool or the bake too short — Maillard reactions need 230°C-plus for color even when the internal temperature is done. Overly sour bread comes from long fermentation with abundant acetic acid; reduce bulk time, feed starter more often, or lower starter hydration to tame it. Flat dough that won't rise points to a weak starter, over-fermented collapsed gluten, or bulk temperatures that are too cold. When dough is over-fermented and unrecoverable for shape, pour it into an oiled pan, dimple with fingers, top with olive oil and salt, and bake at 230°C for an excellent focaccia. Dough sticking to the banneton traces to insufficient flour, dough that is too wet, or a proof that has run too long. Crust that is too thick indicates over-baking or excess steam that kept gelatinizing the surface, so total bake time should be shortened and steam vented earlier. A rubbery crust means under-baking or insufficient rest; bake until deep brown and cool fully so moisture redistributes and the crust crisps.