Twin-screw extrusion at low temperature with carbon dioxide injection to assist expansion: extrudate characteristics
2004
Jeong, H.S. | Toledo, R.T.
Low temperature extrusion may be a suitable process for encapsulating heat labile ingredients in a dry solid matrix. We tested the feasibility of twin-screw extrusion at low temperature using CO2 for expansion, and evaluated properties of extrudates. Pre-gelatinized rice flour extrudates produced at 40°C barrel temperature at the feed section (6 D), 50°C at the next 20 D section and 60°C at the die plate were highly porous when CO2 was injected into the melt at 13.5 D from the feed end of a 25 L/D extruder. When CO2 was injected at 0.1-0.5 MPa, extrudate expansion increased to a maximum at 0.5 MPa, but decreased at 0.6 MPa because of structure collapse after exiting the die. At CO2 injection pressure of 0.1-0.5 and 0.6 MPa respectively, expansion ratio was 7.39, 11.02, 8.50; bulk density was 0.951, 0.388, 0.620 g/cm3; water solubility index was 5.12%, 10.39%, 12.73% and specific mechanical energy was 40.69, 42.64, 43.07 kJ/kg, respectively. The average size of cells in extrudates was 28.74 and 65.58 micrometer at 0.1 and 0.6 MPa while cell density was 6.395 x10(7) and 1.048x10(7) cells/cm3, respectively. Ratio of area occupied by cells to total area was 40.13% at 0.1 MPa, 81.16% at 0.5 MPa, but decreased to 67.40 at 0.6 MPa. The pore size distribution at 0.1 MPa was about 80% in the 10-30 micrometer range, while at 0.5 MPa, cells 10-30 micrometer diameter occupied only 36% of cross-sectional area while most cells were >30 micrometer.
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