Melt expansion by extrusion-cooking was investigated when processing corn grits in a twin-screw extruder. A growth phase followed by a shrinkage phase were experimentally observed through an image processing method, and analysed by use of classical expansion indices: sectional, longitudinal and volumetric expansion indices, for both growth and shrinkage phases. It was shown that the expansion phenomenon was strongly dependent on the geometrical characteristics of the die insert, and particularly on die insert diameter. Melt growth developed more in the radial direction which revealed an important structural anisotropy of the expanding melt, due probably to the elastic properties of the biopolymer-based melt. Melt shrinkage which occurred further, contributed to a significant decrease in the bulk density of the expanding melt, and could arise from either surface tension forces, or from the recovery of melt elasticity. (C) 2003 Elsevier Ltd. All rights reserved.