The secret of the hardness of walnut and pistachio shells

Two group of researchers study on the micro- and nano scale walnut and pistachio shells to understand the secret of it hardness. One is the Department of Nanobiotechnology, University of Natural Resources and Life Sciences Vienna (BOKU), in Austria, and the other, the Department of Biomaterials, Max-Planck Institute of Colloids and Interfaces Science Park Potsdam-Golm, Germany. The knowledge about the structure of walnut and pistachio shells is of interest for several reasons, from when it comes to cracking of nuts for kernel extraction, as well as using the nutshell waste in future bioeconomic approaches. About walnut The outer protective shells of nuts can have remarkable toughness and strength, which are typically achieved by a layered arrangement of sclerenchyma cells and fibers with a polygonal form. Here, the tissue structure of walnut shells is analyzed in depth, revealing that the shells consist of a single, never reported cell type: the polylobate sclereid cells. These irregularly lobed cells with concave and convex parts are on average interlocked with 14 neighboring cells. The result is an intricate arrangement that cannot be disassembled when conceived as a 3D puzzle. Mechanical testing reveals a significantly higher ultimate tensile strength of the interlocked walnut cell tissue compared to the sclerenchyma tissue of a pine seed coat lacking the lobed cell structure. The higher strength value of the walnut shell is explained by the observation that the crack cannot simply detach intact cells but has to cut through the lobes due to the interlocking. Understanding the identified nutshell structure and its development will inspire biomimetic material design and packaging concepts. Furthermore, these unique unit cells might be of special interest for utilizing nutshells in terms of food waste valorization, considering that walnuts are the most widespread tree nuts in the world. ? About pistachio An article published in Nutfruit explains, based on research of the same team, that pistachio shells also count on remakable mechanical propierties. The reason behind is that it also count on a unique shape of sclerenchyma cells with numerous lobes, like the walnut. The two species grow 3D-jigsaw puzzles, in which each cell interlocks on average with fourteen neighbouring cells. Pistachio brings this interlocking to perfection by balljoint-like structures. These connections keep the shell tissue assembled even when lignin, an aromatic glue between the cells, is removed. Lignin plays a role in gluing the puzzle cells together, but also within the cell wall material between the cellulose microfibrils. It adds hydrophobicity to the shell and contributes to the shell mechanics. Pistachio shells come along with three times less lignin than walnut shells. Through the optimisation of cell shape and interlocking pistachio seems not to need such a strong interface and can save lignin. On the nano-level the cell wall is also optimized for high mechanical performance by a layered arrangement. This is achieved by twisting the orientation of the cellulose microfibrils from layer by layer. By changing the angle of the twist and the thickness of the layers mechanical performance an also be tuned.? Pictures 1 - 3D reconstruction of the segmented cells, showing the interlocking between neighboring cells (part of figure 2 of the original paper, Antreich et al.)2 - Comparison of the structure of walnut and pine shell tissue. The latter does not have polylobed cells but rather densely arranged cells (it is figure 1 of the article by Antreich et al.)? Sources The Puzzle of the Walnut Shell: A Novel Cell Type with Interlocked Packing Sebastian J. Antreich, Nannan Xiao, Jessica C. Huss, Nils Horbelt, Michaela Eder, Richard Weinkamer, and Notburga Gierlinger Adv. Sci. 2019, 6, 1900644 DOI: 10.1002/advs.201900644 Twist and Lock: Nutshell Structures for High Strength and Energy Absorption Nannan Xiao, Sebastian J. Antreich, Martin Felhofer & Notburga Gierlinger Nutfruit, March 2022, p. 28?