Behaviors of controlled starch gelatinization in supercritical fluid extrusion

Supercritical fluid extrusion (SCFX) is an innovative food processing technology that offers sub-steam expansion of extrudates using supercritical carbon dioxide (SC-CO2) under low-temperature, low-shear conditions. In this study, blends of pregelatinized and native corn starches were used as model systems to create controlled degree of gelatinization (DG) into gas holding matrices, necessary for microcellular expansion. This strategy minimizes heat-shear inputs needed for gelatinization and avoids unnecessary reduction of SC-CO2 solubility which is critical for driving expansion.; This research has established that controlled gelatinization of starch during SCFX process was a key determinant in governing the SC-CO2 sorption and rheological properties of the starch-based matrices. CO 2 sorption behavior was determined for the pressure range from sub-critical (4 MPa) up to supercritical (16 MPa) at 50°C using a high pressure decay sorption system. The CO2 solubilities in starch-water mixtures, after accounting for the estimated swollen volume by Sanchez-Lacombe equation of state, were found to increase almost linearly with pressure, but were independent of the DG. The solubility of CO2 above 5 MPa was higher than what could be accounted for by the water fraction alone in the starch-water mixture. The diffusion coefficient of CO2 increased with CO2 concentration and decreased with increasing DG from 50 to 100%. Apparent viscosity and expansion of starch-water-CO2 mixtures with controlled DG were evaluated in the pressure range of 1 to 4 MPa using a slit die rheometer fitted to a twin-screw extruder. Starch-water mixtures with DG at or above 80% proved necessary to produce good expanded microcellular morphology. The viscosity of starch-water mixtures with 60-100% DG and 0.45wt% CO2 showed a 14-54% reduction. Both expansion and cellular characterization may be governed by the viscosity-dependent parameters including gas holding capability, diffusivity and pressure drop rate.; Controlled gelatinization in SCFX process was then integrated into the manufacture of an expanded masa-based chip, demonstrating its versatility in offering commercially viable solution for future snack novelty. SCFX extrudates with over 90% DG produced controlled microcellular expansion, responsible for eliminating hard, glassy texture of typical tortilla products and the costly sheeting and equilibration steps of the conventional commercial process.