What causes frozen french or Italian bread to bust open while baking?

When bread dough freezes, the water within the dough expands and forms ice crystals. These crystals can puncture the cell walls of the dough and cause it to become weak and crumbly. When the dough is baked, the trapped water vapor turns to steam and causes the bread to burst open.

To prevent frozen bread from bursting, it is important to thaw the dough completely before baking. This can be done by placing the dough in the refrigerator overnight or by microwaving it on low power for a few minutes.