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The effect of pressure on diffusivity in binary or multicomponent systems such as gas−liquid or gas−solid systems has rarely been reported. The diffusivity of carbon dioxide in extruded gelatinized starch has been measured in this study at pressures of up to 117 bar (1700 psi). Such data are fundamentally not only useful in the understanding of the supercritical carbon dioxide (SC-CO2)/starch systems but also can be useful for the design and control of processes utilizing carbon dioxide injection or mixing in starch-based matrices. The methodology developed here was an improvement over a previously reported technique, enabling high-pressure data to be obtained. The diffusivity of carbon dioxide in the melt was found to be a strong function of pressure but not of moisture content in the range of 34.5−39% (w/w) studied. This diffusivity value decreased from 7.5 × 10-10 to 0.9 × 10-10 m2/s as pressure was increased from atmospheric to 115 bar. The low-pressure diffusivity value was only an order of magnitude lower than that reported for a carbon dioxide in water system and comparable to reported values of diffusivity of CO2 in softened polymers. These diffusivity values are also the same order of magnitude as the reported values of the diffusivity of water in starch, suggesting similar mechanisms of diffusion for carbon dioxide and water diffusivity in starch. The observed pressure dependency of the diffusivity may be due to the melt's high compressibility at these pressures. The solubility of carbon dioxide in the starch melt was proportional to the product of the solubility of carbon dioxide in water and the melt's moisture content.

The residence‐time distribution (RTD) studies are needed to characterize mixing conditions, flow patterns, and the extent of conversions and reactions of the biopolymers in any plasticating or cooking extruder. For the supercritical fluid extrusion process (SCFX), mixing of the fluid and starch‐based biopolymers is an important step affecting cell nucleation and growth. RTD was studied in the mixing zone of the extruder, where CO2 and gelatinized starch are brought together. Three levels of each of the process variables (screw speed and CO2 injection pressure) were investigated, using specially designed apparatus for color‐dye injection, and a die‐restrictor to change the pressure profile and hold up volume in the extruder. Control RTD plots were for no CO2 injection conditions at the same pressure, and the RTD of the full extruder section. Experimental RTDs were evaluated by: (1) the Wolf‐White model, (2) a cascade of continuously stirred reactors (CSTR) model and (3) a model based on plug‐flow in series with cascade of CSTRs. The last approach was most successful in capturing all aspects of the observed RTD curves including their tails. Injection of CO2 at the highest pressure (110.3 bar) increased the RTD compared to no CO2‐injection conditions. There were significant differences in the plug flow component of the RTD as well as differences in average residence time as a function of both screw speed and CO2 injection pressure conditions.

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