Residence time distribution and flow pattern of reduced-gluten wheat-based formulations in a twin–screw extruder

Abstract

Residence time distribution (RTD) and flow pattern in a co-rotating twin screw extruder were evaluated using a color tracer method. A reduced-gluten formulation was prepared by mixing wheat flour, sesame protein concentrates and Tef flour in the proportion of 50:20:30, respectively. The effects of screw speed (150, 180 and 220 rpm), feed moisture content (17, 19 and 21 g/100 g wet basis) and feed rate (40 and 90 g/min) on RTD and flow pattern were investigated. All tested conditions were found to have a significant (P < 0.05) effect on the mean residence time and other RTD parameters. Higher screw speed, higher moisture content and higher feed rate resulted in shorter mean residence times and lower RTD spread due to enhanced barrel fill, mass flow rate and system pressure. A dispersion number was used to define the level of axial mixing in the extruder and, as defined, the axial mixing was more influenced by the feed rate than screw speed. Increasing feed rate resulted in the mixing pattern to become more like plug flow, whereas, mixed flow condition achieved when the screw speed was increased. Feed moisture had no significant effect on mixing pattern.