DEGRADATION OF WHEAT-STARCH IN A TWIN-SCREW EXTRUDER

Wheat starch was processed in a Baker-Perkins MPF-50D co-rotating twin -screw extruder, at moisture contents of 25% and 30%, screw speeds of 200 rpm and 300 rpm, feed rate of 30 kg/h, and barrel temperature sett ings of 100, 120, 140 and 160 degrees C. High performance size exclusi on chromatography was used to determine the extent of degradation of s tarch along the extruder channel. Starch degradation was found to take place in the cooking zone. The first-order mechano-degradation models , which were developed for a single-screw extruder were extended to th e twin-screw extruder and improved by including a more accurate descri ption of gelatinization in the model. The degradation is described by the equation: chi = 1 - {1 - exp[-k(0) exp (- (Delta E(0) - beta tau)/ (RT)) t]} {1 - exp(-k tau t + B)} where tau is the average shear stres s, t is the mean residence time of the melt in the active zone of the screw channel, Delta E(0) is the activation energy, R is the ideal gas constant, beta is the activation volume, k and k(0) are experimental constants.