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 (D₅₀), 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 D₅₀ (8-18 μm) and a more brittle behaviour than the blend composites that had a lower I_i (1-1.1) and higher D₅₀ (22-31 μm). 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.