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The effects of three types of screw elements, forward, mixing disc, and pin-mixing element, on residence time distributions of glutinous rice flour in a single screw extruder with different die opening areas have been investigated. Both mixing disc and pin-mixing element yield longer residence time, higher specific mechanical energy (SME), and higher extrudate temperature compared with those for the forward screw element. The variation in die diameter did not significantly affect the mean residence time when the forward element was used. Nevertheless, increasing die diameter from 20 to 40 mm significantly reduced the mean residence time when mixing disc was used. Based on SME ranging from 3.6 to 20 kJ/kg, the extruder used in this study was classified as a low shear extruder. Due to the high moisture content (45%) and large die opening area, the die pressure was lower than 11.5 kPa. The utilization of mixing disc was found to raise the die pressure. The degree of starch gelatinization in extrudates was a function of the mean residence time. Three models were employed to fit the RTD data. The flow pattern in different screw profiles was discussed based on the model analysis. |
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