Individual Contribution of Grain Outer Layers and Their Cell Wall Structure to the Mechanical Properties of Wheat Bran

The mechanical properties of wheat bran and the contribution of each constitutive tissue on overall bran properties were determined on a hard wheat (cv. Baroudeur) and a soft wheat (cv. Scipion). Manual dissection allowed three different layers to be separated from wheat bran, according to radial and longitudinal grain orientations, which were identified by confocal laser scanning microscopy as outer pericarp, an intermediate strip (comprising inner pericarp, testa, and nucellar tissue), and aleurone layer, respectively. Tissue microstructure and cell wall composition were determined. Submitted to traction tests, whole bran, intermediate, and aleurone layers demonstrated elastoplastic behavior, whereas pericarp exhibited elastic behavior. By longitudinal orientation, pericarp governed 50% bran elasticity (elastic strength and rigidity), whereas, in the opposite orientation, bran elastic properties were mostly influenced by the other tissues. Regardless of test orientation, the linear force required to bran rupture corresponded to the sum of intermediate and aleurone layer strengths. According to radial orientation, the intermediate strip governed bran extensibility, but according to longitudinal orientation, all tissues contributed until bran disruption. Tissues from both wheat cultivars behaved similarly. A structural model of wheat bran layers illustrated the detachment of pericarp from intermediate layer within radial bran strips.