The most common reason bread ends up hollow just under the crust is that the dough over-proofed before it went into the oven. Gas in an over-proofed dough grows unevenly, and once the oven's heat hits it, the crust sets quickly while the trapped air collects into one large pocket right beneath it.
How the dough is shaped plays a direct role too. If the dough isn't pulled tight enough while shaping, large uneven air pockets are left inside; a well-shaped dough ends up with smaller, more evenly spread gas bubbles instead.
How does oven temperature affect the hollow spot?
Putting the dough into an oven that hasn't fully preheated can also leave a hollow center; a slow start at low heat delays crust formation, and the gas inside keeps expanding during that time, growing the pocket further. Preheating the oven fully first cuts down on this risk.
Steam matters here too; without enough steam in the first few minutes, the crust sets earlier than it should, and the gas still expanding inside gets trapped, forming one big pocket.
How do you avoid bread turning out hollow?
Checking the dough with the finger-poke test during final proofing helps: press lightly, and if the dent springs back slowly, the dough is ready; if it doesn't spring back at all, the dough has over-proofed and needs to be gently knocked down and reshaped before baking.
Pulling the dough tight while shaping and keeping the surface taut helps the gas that forms during baking split into smaller, more even bubbles, cutting the risk of a hollow center considerably.
Does the type of flour affect the hollow-center problem?
High-protein bread flours hold gas inside a stronger gluten network than low-protein flours, which helps the dough form smaller, more evenly spread gas bubbles. Using a low-protein flour raises the risk of a hollow center somewhat.
Adding too much water to the dough can also make the problem worse; an overly wet dough can't be pulled tight enough while shaping and is more prone to leaving large, uneven gaps as it bakes. Sticking to the water ratio given in the recipe reduces this risk.
Slicing bread right out of the oven can also make the hollow spot look more dramatic than it actually is; letting the loaf cool for at least twenty to thirty minutes lets the internal structure settle and shows the real proportion of empty space more accurately.
Long, thin loaves like baguettes can develop a hollow-seeming interior for a slightly different reason; steam disperses faster in a thin shape, which can let the crust set early, and that can be mistaken for the large, open crumb structure baguettes are known for. In these loaves, big holes are actually a desired trait, not a flaw.
In compact loaves like sandwich bread, on the other hand, small and even holes are expected; that's why a hollow-seeming interior needs to be judged differently depending on the type of bread.
Dusting the shaped dough lightly with flour and covering it with a damp cloth during final proofing keeps the surface from drying out and helps the crust form more evenly during baking.