The hole gives heat a shorter route
In a thick lump of dough, the outside meets hot oil first. It can brown and turn crisp while heat is still working its way toward the middle, leaving the center undercooked. Cutting out the middle changes the shape: no part of the dough sits far from an exposed surface, so the interior cooks through sooner.
The hole also creates more exposed edge. That means more dough is in direct contact with the hot oil at once, helping the ring cook more evenly rather than relying on the browned exterior as a guide to what is happening inside.
A shape built around the fryer
That tradeoff matters because frying moves heat inward from the surface. A thick center slows the process; extending the frying time risks overcooking the outside before the middle is ready. Making a ring reduces that bottleneck, letting bakers aim for a crisp exterior and a cooked interior in the same batch.
The familiar doughnut shape is therefore more than decoration: its empty center solves a practical problem of getting a piece of dough to cook evenly in hot oil.
