| Abstract: | High‐amylose starch acetate (DS 2) was processed in a Brabender twin‐screw extruder with ethanol and isopropanol as blowing agents at concentrations of 0, 2, 5, 10, 15, and 25%. A constant temperature of 150°C, a constant screw speed of 140 rpm, and a die nozzle with diameter of 4.0 mm and length of 16.2 mm were used to study the role of blowing agents on the expansion of the extrudates. Extrudates without blowing agent shrunk considerably after exiting the die as the cells collapsed drastically after expansion. Stable radial expansion of the extrudates increased with increase in the ethanol concentration to an optimum value of 18.0 at 5% (db) ethanol concentration and decreased with further increase in the ethanol concentration. Stable radial expansion increased to a maximum of 17.0 as the concentration of isopropanol was increased to 25% (db), though the rate of increase in expansion decreased with the increase in isopropanol concentration >10%. Flashing off of blowing agents aided in removing the heat generated during extrusion. The faster the extrudate cooled, the less likely it was to shrink. SEM were used to observe the effects of concentration of blowing agents on cell morphology. Various phenomena involved during the expansion are discussed. To obtain an extrudate with high expansion and low density, isopropanol at 15–25% (db) was found most suitable in this study. |
