Abstract
In order to investigate the effects of carbon dioxide assisted low temperature extrusion on the extrudate characteristics and to develop highly porous extrudate, extrusion was carried out using pre-gelatinized rice flour with different carbon dioxide injection pressure. Screw configuration using short pitch and reverse screw elements improved mixing, allowed extrudate expansion, and prevented back-flow of injected gas towards the feed port. Carbon dioxide injection pressure greater than 0.5 MPa produced highly dense extrudate, with irregular pore size and structure col1apse after exiting the die. Depending on the $CO_2$ injection pressure, expansion ratio was ranged between 7.39 and 11.02, but that value at 0.6 MPa decreased to 8.50. Bulk density was decreased from 0.95 $g/cm^3$ at 0.1 MPa to 0.39 $g/cm^3$ at 0.5 MPa. WSI was increased markedly from 5.12 to 12.73% according to increase of $CO_2$ injection pressure. The SME varied between 40.69 and 43.07 kJ/kg as an increasing $CO_2$ injection pressure. The average cel1 size of various extrudates ranged between 28.74 and 65.58/lm. Cell density decreased from $6.40{\times}10^7\;cells/cm^3$ at 0.2 MPa to $1.05{\times}10&7\;cells/cm^3$ at 0.6 MPa. Cell area ratios were increased from 40.13% to 81.16% as $CO_2$ injection pressure increased to 0.5 MPa, but it was decreased 67.40 at 0.6 MPa. The cells size at 0.1 MPa distributed to 10-30 ${\mu}m$ size about 80%. As increasing $CO_2$ injection pressure cell number of 10-30 ${\mu}m$ size decreased but large cell number increased. Cells size of 10-30${\mu}m$ in 0.5 MPa occupied about 36%.