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 (fo) 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 fo.; High effective diffusivity (Deff) of CO2 in porous SCFX extrudates causes large proportion of injected CO2 to escape to the environment. Strategies for decreasing Deff and thus enhancing expansion of extrudates were evaluated. Deff was reduced by either increasing the bubble density (Nbubble) 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.
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机译：进行这项研究是为了了解使用超临界流体挤出（SCFX）加工生产淀粉基微孔泡沫所涉及的复杂机制，探索增强挤出物膨胀的策略，并表征淀粉基面团的流动行为。使用用于气泡生长的数学模型描述了SCFX的过程动力学和挤出后的干燥机理。假定生物聚合物熔体中气泡的生长是由挤压过程中CO 2 （以超临界相注入熔体中）扩散到成核孔中以及烤箱干燥过程中的水蒸气压驱动的。该模型用Visual Basic编写，并使用预糊化玉米和马铃薯淀粉的SCFX挤出物（含4–7％乳清蛋白浓缩物）的实验数据进行验证。随着干燥温度在70至95°C之间变化，预计气泡半径（R）为50-220微米，膨胀比（ER）为4-11，开孔率（f o ）为0.05-0.27 。此外，该模型预测，随着熔体的屈服应力和破坏应力的增加，挤出物塌陷和开孔率将分别下降。仿真结果与R和f o 的实验值相当。 CO 2 在多孔SCFX挤出物中的高有效扩散率（D eff ）导致大量注入的CO 2 逸出到环境中。评价了降低D 从而增强挤出物膨胀的策略。通过将气泡密度（N bubble ）从0.27×10 6 增加到1.14×10 6 ，可以降低D eff super> cm −3 或将产品温度从60°C降低到40°C。通过改变喷嘴直径以获得不同的压降率，可以实现N <气泡>的变化，而通过在熔体进入喷嘴之前引入冷却区来改变产品温度。这些策略使扩展率提高了160％。面团的流变学是重要的材料特性，会影响挤出过程的过程动力学。使用离线（毛细管流变仪）和在线（狭缝模头挤出）测量来表征由原始和预胶化小麦淀粉的混合物制备的中等水分（干基含量为50-80％）面团的流变性。在毛细管流变仪的情况下，当预凝胶淀粉的浓度从5增加到45％时，混合物的表观粘度最多降低50％，而对于狭缝模头挤压，在60％的预凝胶浓度下，表观粘度最小，并且下降幅度最大。当预凝胶浓度从0增加到60％时为70％。用淀粉的体积分数，在挤出过程中淀粉的额外转化以及与原料淀粉相比对预凝胶淀粉的水更大的亲和力来解释这种趋势。
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