Pulse legumes are an excellent source of protein, starch and dietary fibers. Thus, they can contribute to the development of extruded snacks in complement with cereals. The critical sensory attribute of these solid foams is their texture, which is governed by density, cellular structure, and mechanical properties of intrinsic material (cell wall). The intrinsic material can be envisioned as a dense starch-protein composite. In addition to its composition, its mechanical properties depend on the morphology created during extrusion. The precise knowledge of constitutive laws of this intrinsic material is primordial to predict the texture of solid foams. The aim of this study is to determine the relationship between morphological features and mechanical properties of pea composites by laboratory experiment and numerical simulation.

Pea flour and blends of pea starch-pea protein isolates PPI (SP blends), having protein content of 0.5–88% (dry basis), were processed by a twin-screw extruder at die temperature low enough to avoid expansion. The dense composites were obtained at various specific mechanical energy SME (100-2000 J/g), in order to obtain a wide range of starch-protein morphologies. The blends with 0.5% and 88% protein content refer to 100% pea starch and 100% PPI, respectively. The morphology was observed by confocal scanning laser microscopy (CLSM). Image analysis was used to determine the total perimeter and area of protein aggregates, from which a protein/starch interface index, Ii, was derived. The mechanical properties were determined by three-point bending experiments and compared with those of numerical simulation using finite element method (FEM).

Generally, the composites displayed protein aggregates in an amorphous starch matrix. High protein amount (> 45% db) and low SME (200 kJ/kg) resulted in a bi-continuous blend. Mechanical tests revealed that pea flour composites exhibited brittle behaviour with rupture in the elastic domain, whereas SP blend composites displayed higher breaking stress and strain with rupture in the plasticity region. The interface index Ii explained the variation of mechanical properties, regardless of the composite formulation. Maximum stress and strain of composites decreased with increasing Ii between the phases, whatever the formulation, which may be attributed to the poor compatibility between starch and proteins. The flexural modulus and maximum stress and strain of SP blend composites were correlated negatively with protein content. Using FEM simulation based on morphology data, the elastoplastic model parameters were computed in function of protein phase content, aggregates orientation, and Ii and interface stiffness by taking into account imperfect interface with voids. CLSM images were meshed, and interface element was introduced at each location where starch/protein transition was detected. Contact element was used to take into account interfacial crack. The failure possibility was assessed using maximum strain criterion. The FEM results showed that it is possible to modulate the mechanical properties by tuning the composite morphology through extrusion variables and composition. The obtained constitutive laws of starch-protein composites will be integrated into multi-scale numerical models to predict the mechanical properties of legume based extruded snacks.