The sweetpotato flours were extruded in the Prism Eurolab KX 16 twin screw extrude,
using a slit die (15 2mm2) at three levels each of moisture (30, 35,40 g/100 g) and screw speed (150, 220, 300 rpm), maximum barrel temperature of 120 C, and feed rate of 1.5 kg/h. These conditions were obtained from preliminary studies in our laboratories on stable
sweetpotato extrusion as evidenced by no die jetting, continuous run and minimum torque fluctuations.The extruder set-up is detailed in Yong, Chan, Garcia, & Sopade. The extrudates were cooled overnight at room temperaturebefore their expansion characteristics were assessed. They
were then freeze-dried and cryo-milled as before
prior to further analysis. The non-extruded sweetpotato dried chips
were also cryo-milled, and all the cryo-milled samples averaged
160 mm particle size. Due to a limited sample size, cultivars Snow
White and L49 were extruded at only one condition (35 g/100 g
moisture, 300 rpm screw speed). The melt (die) temperature was
measured using a thermocouple attached to the die, and the specific
mechanical energy (SME)was calculated as in Eq because the noload
torque was negligible.