The increase of crumb firmness prepared from doughs enriched with water-soluble proteins and/or water-extractable pentosans was investigated at various aging times with DSC, to assess starch retrogradation, and with an Instron dynamometer, to determine the elastic modulus. The crumb enriched in soluble proteins became firmer, and that with extra pentosans remained softer than the standard recipe crumb. It was soon evident that neither starch retrogradation nor water loss (due to the crumb-to-crust migration) were affected by the presence of extra proteins and/or pentosans: thus, neither process could explain the observed differences in the trends of the elastic modulus. According to image analysis investigations, the protein-rich crumb had narrow and regular alveoli, which were well separated from one another, whereas a coarser structure was obtained by adding extra pentosans to the dough recipe. It was, therefore, concluded that differences of crumb firmness might be mainly due to the structure formed in the course of leavening and baking, rather than to starch retrogradation and moisture loss. These processes indeed contribute to significantly increase the rigidity of the alveolar walls, but the overall elastic modulus of a sponge-like system, like a bread crumb, largely depends on the manner in which imposed strains are spread through the alveolar structure. Accordingly, for a given rigidity of the alveolar walls, the elastic modulus of the crumb will depend on the size, shape and distribution of the alveoli.