Extrusion, structure and mechanical properties of complex starchy foams

Wheat-based extruded foams were generated according to different conditions of barrel temperature (120, 140 and 160 °C), feed moisture content (20%, 22% and 24%), screw speed (400, 600 and 800 rpm), die opening (0.75, 1 and 1.25 mm) and addition of sucrose and sodium bicarbonate at different levels (0%, 3%, 6% and 0%, 0.2%, 0.4%, respectively). Their cellular structure, analyzed by micro-computed X-ray tomography, was linked to the bulk expansion properties and enabled to explain the discrepancies in stress at rupture values obtained with a 3-point bending test. Samples with a wide range of relative densities (0.09–0.48), mean cell sizes (240–2710 μm), mean cell wall thicknesses (195–396 μm) and cell densities (430–37670 cm−3) were obtained. Most of the samples showed an anisotropic expansion with a favoured radial expansion, resulting in cellular structures with larger and less numerous cells and shapes closer to ellipsoids than to spheres that would be expected for isotropic expansion. A good fit with the Gibson and Ashby model was observed for mechanical properties. For similar relative densities, slight but significant differences in the stress at rupture might be explained by differences in cellular structure.