The time-delayed expansion profile of supercritical fluid extrudates

3D microstructure of supercritical fluid extrudates I: Melt rheology and microstructure formation

“…The pressure drop rate was calculated using Matlab programming software (Mathworks Inc.) (Cho & Rizvi, 2008). For a nonNewtonian power law fluid such as a starch melt, the pressure drop across a cylindrical die is given by the following equation:…”

“…A higher moisture content (30 -45 wt%) in the extruder barrel is utilized to keep the product temperature low and to maximize SC-CO 2 solubilization in the melt. Because the SCFX process is conducted at low shear conditions and temperatures lower than 100°C, it enables the use of temperature-and shear-sensitive ingredients in product formulations (Cho & Rizvi, 2008). The SCFX process for starch-based foams consists of the following major steps: (1) development of a melt with gas-holding properties by the gelatinization and mixing of the feed in the extruder barrel, (2) injection of SC-CO 2 into the melt and mixing in the extruder barrel to create a melt-CO 2 solution, (3) pressure drop in the die leading to a thermodynamic instability and consequent nucleation of cells, and (4) expansion caused by diffusion of CO 2 into the nucleated cells as the extrudate proceeds through the die and immediately after its exit from the die.…”

3D Microstructure of supercritical fluid extrudates. II: Cell anisotropy and the mechanical properties

“…Piece density (PD) (kg/m 3 ), defined as the ratio of the mass of the sample to its total volume including the voids, was measured using the geometrical method (Cho & Rizvi, 2008). The volume of the extrudate was calculated by multiplying the cross-sectional area by the length, assuming the extrudate is a straight cylinder.…”

“…The extrusion process has been widely utilized in expanded cellular solid foods (Cho & Rizvi, 2008). Expanded products using conventional steam-based extrusion usually exhibit non-uniform cellular structure.…”

“…A novel technology, supercritical fluid extrusion (SCFX) utilizes supercritical CO 2 (SC-CO 2 ) as a blowing agent instead of steam and allows researchers to produce expanded products with a polydispersity index as high as 0.96 (Alavi et al, 1999). It is also possible to control the macro-and microstructure of SCFX extrudates, such as piece density, cell size, and cell size distribution, by manipulating process conditions, such as pressure drop rate in the die, the ratio of SC-CO 2 to the feed, and SC-CO 2 residence time (Cho & Rizvi, 2008). Although SCFX extrudates have a relatively uniform cell size distribution, they are expected to show a certain degree of anisotropy in microstructure because SCFX extrudates expand more in the cross-sectional direction compared to the longitudinal direction (Sokhey et al, 1996).…”