Under-proofed bread can explode and often has a hard, dense crumb. The reason it explodes is as the dough is not matured, often called under-fermented. This means that the gluten structure is weak and irregular

Bread fact 11
Under-proofed bread can explode and often has a hard, dense crumb. The reason it explodes is as the dough is not matured, often called under-fermented. This means that the gluten structure is weak and irregular