The effect of a mixing element in single- and twin-screw extruders on the amount of phosphorus incorporated into starch was investigated for chemical modification of starch that has applications in the food and paper industries. Starch was mixed with sodium hydroxide and sodium trimetaphosphate and then the mixture was extruded. Screw speeds, which result in approximately the same residence time in both single- and twin-screw extruders with and without a mixing element, were determined. Screw speeds of 220 rpm in a single-screw extruder with a mixing element, 180 rpm in a single-screw extruder without a mixing element, and 160 rpm in a twin-screw extruder with and without a mixing element resulted in similar residence times. The presence of a mixing element did not have an effect on vessel dispersion number and flow pattern in the single-screw extruder but it affected vessel dispersion number and changed the flow pattern in the twin-screw extruder. The amount of phosphorus bound to starch did not increase with the mixing element in single- and twin-screw extruders for a constant mean residence time. (C) 2003 Elsevier Science Ltd. All rights reserved.