3D microstructure of supercritical fluid extrudates I: Melt rheology and microstructure formation
2009
Cho, K.Y. | Rizvi, S.S.H.
The influence of melt rheology and processing conditions on the expansion and 3D microstructure of biopolymeric foams produced by supercritical fluid extrusion (SCFX) were investigated. Starch-based SCFX extrudates with five whey protein isolate (WPI) concentrations (0-18 wt%) and four SC-CO2 levels (0-0.75 wt%) were produced. Melt rheology was studied with an online slit die rheometer. The 3D microstructure of foams was determined using X-ray microtomography. The starch-based melt showed shear-thinning behavior, with a lower consistency coefficient and higher flow behavior index with the addition of SC-CO2. Whey protein acted as a diluent, which resulted in reduced melt viscosity. SC-CO2 increased the expansion of whey protein added starch-based extrudates. However, structural collapse was observed at the 0.75 wt% SC-CO2 level during post-extrusion drying at 85 °C. The cross-sectional expansion ratio of SCFX extrudates decreased by 48.9% with the addition of 18 wt% WPI. The cell number densities per unit volume and average cell size of SCFX extrudates were 1.4 x 103-1.9 x 104 cells/cm3 and 310.0-724.4 μm, respectively, depending on WPI and SC-CO2 levels. A decrease in melt viscosity due to the addition of whey protein might be responsible for the lower cell number density and related decrease in expansion. Processing parameters and whey protein levels were critical to controlling the microstructure of starch-based SCFX extrudates.
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