Twin-screw extrusion at low temperature with carbon dioxide injection to assist expansion: extrudate characteristics Original Research Article

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 μm at 0.1 and 0.6 MPa while cell density was 6.395 × 107 and 1.048 × 107 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 μm range, while at 0.5 MPa, cells 10–30 μm diameter occupied only 36% of cross-sectional area while most cells were >30 μm.