Process dynamics and material properties for supercritical fluid extrusion of starch-based microcellular foams

This study was undertaken to understand the complex mechanisms involved in the production of starch-based microcellular foams using supercritical fluid extrusion (SCFX) processing, to explore strategies for enhancing extrudate expansion, and to characterize the flow behavior of starch-based doughs.; SCFX process dynamics and post-extrusion drying mechanism were described using a mathematical model for bubble growth. Bubble growth in the biopolymeric melt was assumed to be driven by diffusion of CO₂ (injected into the melt in supercritical phase) into nucleated pores during extrusion and by water vapor pressure during oven drying. The model was written in Visual Basic, and experimental data for pregelatinized corn and potato starch based SCFX extrudates with 4–7% whey protein concentrate was used for validation. Predicted bubble radius (R) was 50–220 microns, expansion ratio (ER) 4–11 and open cell fraction (f_o) 0.05–0.27, as the drying temperature varied from 70 to 95°C. Moreover, the model predicted decrease in extrudate collapse and open cell fraction with increase in yield and failure stresses of the melt, respectively. Simulation results were comparable to the experimental values of R and f_o.; High effective diffusivity (D_eff) of CO₂ in porous SCFX extrudates causes large proportion of injected CO₂ to escape to the environment. Strategies for decreasing D_eff and thus enhancing expansion of extrudates were evaluated. D_eff was reduced by either increasing the bubble density (N_bubble) from 0.27 × 10 6 to 1.14 × 106 cm−3 or by reducing the product temperature from 60 to 40°C. Variation in N _bubble was achieved by changing the nozzle diameter to obtain different pressure drop rates, while product temperature was varied by introducing a cooling zone prior to entry of melt into the nozzle. These strategies resulted in increase of expansion ratio up to 160%.; Rheology of dough is an important material property that affects the process dynamics of extrusion processes. Rheology of intermediate moisture (50–80% dry basis) doughs prepared from blends of raw and pregelatinized wheat starch was characterized using offline (capillary rheometer) and online (slit-die extrusion) measurements. In the case of capillary rheometer, apparent viscosity of blends decreased by up to 50% as pregel starch concentration increased from 5 to 45%, whereas for slit-die extrusion apparent viscosity had a minimum at 60% pregel concentration and it decreased by as much as 70% as pregel concentration increased from 0 to 60%. The trends were explained in terms of volume fraction of starch, additional conversion of starch during extrusion, and greater affinity for water of pregel starch as compared to raw starch.