X-ray tomography study of the cellular structure of extruded starches and its relations with expansion phenomenon and foam mechanical properties

The cellular structure of extruded starch foams, with different amylose content (0–70%) has been determined by X-ray tomography (10 and 40 μm), leading to highly contrasted 3D images. These foam structures were analysed using a 3D granulometric approach. Their cellular features were determined for relative density (D∗) values in the interval [0.11, 0.34]. Their mean cell size (MCS) and mean cell wall thickness (MWT) varied in the range [0.2, 5 mm] and [75, 630 μm], respectively, and were strongly correlated. For the same value of D∗, the finest structures (MCS < 2 mm and MWT < 300 μm) were found the most resistant to rupture. These finest cellular structures were mainly obtained for larger moisture content (24%) during extrusion. The influence of the extrusion variables on the phenomena occurring at die expansion, such as nucleation, bubble growth and coalescence, was discussed. Finally, the influence of amylose content was attributed to elongational viscosity, in relation with the storage modulus in the rubbery domain.