An overview of instrumented indentation technique for the study of micromechanical properties in food: A case study on bean seed coat

Food texture features depend on the structure–property-functionality relationship and mechanical properties. This work describes the application of the instrumented indentation technique (IIT) to determine the micromechanical properties of the bean seed coat evaluating different indentation loads. The bean seed coat's multilayer structure is composed of a cuticle (C), palisade cells (PAL), subepidermal pillar cells (SE), and spongy parenchyma cells (SP). Values of R_a and R_q revealed a rough surface due to the waxes and minerals present in the C. In the indentation tests, the C was indented at loads below 20 mN exhibiting values of hardness (H) and elastic modulus (E) of 383 ± 58 MPa and 11 ± 5 GPa, respectively. For PAL, when the load increased to 400 mN, H and E decreased to 198 ± 29 MPa and 3 ± 0.8 GPa, indenting 10% of the total thickness of the seed coat (103 ± 9 μm). The values of H and E decreased as the applied indentation load increased. This variability is influenced by the composition (waxes, lignin, condensed tannins, and CaOx crystals) and the effect of the layer-substrate system of multilayer structure of the seed coat.