Nano- and micro-mechanical properties of wheat grain by atomic force microscopy (AFM) and nano-indentation (IIT) and their relationship with the mechanical properties evaluated by uniaxial compression test

The mechanical properties of hard and soft wheat cultivars at different structural-levels were evaluated at 10% and 15% moisture content by AFM, IIT and compression test. Nano and micro-indentation tests revealed that outer-pericarp cells of soft cultivar were stiffer than hard cultivar at 10% moisture. Bran-layers stiffness was reduced as a consequence of higher moisture levels in both cultivars. Additionally, endosperm hardness and stiffness were similar in both cultivars at 10% moisture, although soft-wheat endosperm was softer and less stiff than hard wheat at 15% moisture. Regarding compression test, soft cultivar showed lower fracture force than hard cultivar at the same moisture level. Moreover, stiffness, fracture force and deformation were reduced by higher moisture levels in both cultivars. The relationship between the nano-, micro- and macro-mechanical properties of wheat kernels indicated that soft cultivar at 10% and 15% was associated with stiffer outer-pericarp cells and higher fracture deformation, respectively. However, hard cultivar at 10% and 15% was related to higher endosperm hardness and fracture force and higher bran layers hardness, respectively.