Flow, mixing and residence time distribution of maize starch within a twin-screw extruder with a longitudinally-split barrel

Abstract

A longitudinally-split barrel was used to study the internal modifications of maize starch during extrusion-cooking in a Creusot-Loire BC-72 twin-screw extruder. After steady state conditions were obtained, the extruder barrel was dismantled and samples were taken at points along the screws in order to study changes in starch granular structure and macromolecular levels in the different phase transition zones. Mass transport is the predominant process in the extruder before the reverse flight section where comminution is the major event. In this reverse flight zone, starch granules are progressively sheared and melted. Shear along the barrel surface causes rapid cooking, with the formation of a glassy phase. The results show that the combined effect of shear, together with heat and pressure, are mainly responsible for starch modifications. The residence time distribution is a function of extruder size.