The Puzzle of the Walnut Shell: A Novel Cell Type with Interlocked Packing

Abstract

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.

From the Soft to the Hard: Changes in Microchemistry During Cell Wall Maturation of Walnut Shells

“…Finally the much denser (brighter on the scans) and dry nutshell protects the seed, which is mostly surrounded by an empty cavity ( Figure 1A, Supplementary Movie 5). Zooming into the nutshell via x-ray tomography showed that the entire shell is composed of polylobate cells, each of which tightly interlocked with on average 14 neighbors (Antreich et al, 2019). While other nutshells, like Macadamia build up their cells with different cell types, i.e., fibers and sclerenchyma cells (Schuler et al, 2014), walnut relies only on the puzzle cell type (Antreich et al, 2019).…”

“…Although wood and nutshell have different mechanical functions, which are realized by isotropic puzzle cells (Antreich et al, 2019) for high compression forces and anisotropic fiber arrangement for high tensile strength respectively (Gibson, 2012), the underlying secondary cell wall and its maturation show common features.…”

“…For novel applications and material development, fundamental knowledge of the structure as well as the chemistry of the shell is needed. Recently, a polylobate cell shape with interlocked packing was found to enable the superior mechanical properties of walnut shells (Antreich et al, 2019). All cells are connected via numerous pits (Reis et al, 1992;Antreich et al, 2019), which maintain symplastic connection by a recess of the cell wall (Reis et al, 1992).…”

“…Recently, a polylobate cell shape with interlocked packing was found to enable the superior mechanical properties of walnut shells (Antreich et al, 2019). All cells are connected via numerous pits (Reis et al, 1992;Antreich et al, 2019), which maintain symplastic connection by a recess of the cell wall (Reis et al, 1992). Chemical studies of the cell wall have shown that lignin is a main component (above 50%), followed by cellulose (25%), and hemicelluloses (22%) (Demirbas, 2005).…”

Topological Interlocking and Geometric Stiffening as Complementary Strategies for Strong Plant Shells

“…[6] During development on the parental plant, individual layers of the pericarp or the testa may harden substantially via extensive secondary cell wall formation and lignification. [7,8] Cracking these shells after they have reached maturity requires remarkably high compression forces in some species, ranging from 300 to ≈700 N for pecans, walnuts, and hazelnuts; and up to ≈4000 N for macadamia shells. [9] In the natural environment, the encapsulated seed, known as the dispersal unit or diaspore, is typically dispersed by granivorous and scatter-hoarding animals.…”