Rheology and Expansion of Starch-Water-CO₂Mixtures with Controlled Gelatinization by Supercritical Fluid Extrusion

A slit die rheometer fitted to a twin-screw extruder was used to measure the viscosity and expansion characteristics of starch-based melts plasticized with water and supercritical CO2. Experiments were conducted for four intermediate moisture starch-water mixtures with varying degree of starch gelatinization (DG), achieved by blends of pregelatinized and native common corn starch. SC-CO2 was shown to be an effective plasticizer for starch-water mixture at 0.45 g SC-CO2/100 g sample, lowering the viscosity of the melt by an average of 14%. Apparent viscosity was measured for shear rate ranging from 100–200 s−1. The viscosity reduction factors based on DG and SC-CO2 indicated that the free volume added to starch-water mixtures with SC-CO2 is an effective mechanism for viscosity reduction. In addition, the higher water affinity of pregelatinized starch increased the water induced plasticization effect in the mixtures. SC-CO2 was a good blowing agent producing distinct expanded microcellular morphology for the starch-water mixtures with DG at or above 80%. The expansion and cellular characterization of these starch-based extrudates produced by SC-CO2 injection may be governed by the viscosity-dependent parameters including gas retention capability, CO2 diffusivity and the pressure drop rate.