Structural properties of protein-stabilized starch-based supercritical fluid extrudates

Supercritical fluid extrusion, a low temperature and low shear process, was used to produce pre-gelatinized corn and potato starch-based extrudates, containing 4–10% thermosetting egg white (EW) or whey protein concentrate (WPC-34), and dried at 22–100°C. Addition of proteins reduced shrinkage of high-moisture extrudates, as indicated by increases in expansion ratio by up to 140 and 341% when the drying temperatures were 22 and 100°C, respectively. Products containing 7% EW or WPC-34 and dried at 85°C expanded best while maintaining an intact structure, with expansion ratio (∼12) and bulk density (∼0.10 g/cm3) comparable to steam extrudates. The products had a unique composite and uniform microcellular structure, with average cell sizes in the range of 50-250 μm and cell density of the order of 106 cells/cm3. The classical nucleation theory and a qualitative model for cell growth and shrinkage based on glass transition temperature were used to explain the microcellular structure.