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 settings of 100, 120, 140 and 160 degrees C. High performance size exclusion chromatography was used to determine the extent of degradation of starch 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 the twin-screw extruder and improved by including a more accurate description 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 stress, 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.