Flow-history engineering of starch-based edible foams through restriction–diverging die design

Starch-water systems can be structured by extrusion into expanded edible matrices, where water acts simultaneously as a plasticizer, controlling matrix mobility, and as an internal blowing agent, driving vapor-induced expansion at the die exit. The resulting cellular architecture governs density, cell connectivity, and texture-relevant mechanical response. Therefore, controlling the coupling between flow history and depressurization is central to the design of starch-based expanded food structures. Here, we propose a process-oriented die design strategy to regulate deformation history and pressure release during starch foam extrusion. A restriction-diverging die was designed to impose controlled extensional deformation upstream of the exit while allowing partial relaxation of the deformation history before final expansion. A restriction-only control die, obtained by removing the downstream diverging section so that the exit coincided with the restricted throat, was also tested to isolate the role of the diverging compensation region. Generalized Newtonian CFD simulations were used to quantify velocity fields, pressure losses, and depressurization rates, whereas die swell was evaluated experimentally as an indirect manifestation of residual elastic recovery. The restriction-diverging die strongly reduced anisotropic die swell while preserving global foam density. At comparable density, the modified die produced a finer and more homogeneous cellular morphology, lower open-cell content, and higher compressive stress. The restriction-only control die exhibited much larger transverse swelling and a cellular morphology similar in size to the conventional die but more elongated in the thickness direction, supporting the contribution of the downstream diverging section to swell reduction under the tested conditions. These results show that flow-history engineering can be used as a practical route to tune the structure and texture-relevant properties of starch-based expanded food matrices.