Morphology and mechanical behaviour of pea-based starch-protein composites obtained by extrusion

Starch-legume protein composites were obtained by extrusion of pea flour and pea starch-protein blend at various specific mechanical energies (100-2000 kJ/kg) and a temperature low enough to avoid expansion. The morphology of these composites displayed protein aggregates dispersed in a starch matrix, revealed by microscopy. Image analysis was used to determine the median width of protein aggregates (D-50), their total perimeter and surface, from which a protein/starch interface index (I-i) was derived. The mechanical properties of composites were determined by a three-point bending test. The pea flour composites had a higher interface index I-i (1.8-3.1) with lower median particle width D-50 (8-18 mu m) and a more brittle behaviour than the blend composites that had a lower I-i (1-1.1) and higher D-50 (22-31 mu m). For both materials, rupture stress and strain were negatively correlated with I-i. This result suggested that there was a poor interfacial adhesion between the pea starch and proteins.