Microstructure and mechanical properties of hard Acrocomia mexicana fruit shell

Abstract

Fruit and nut shells can exhibit high hardness and toughness. In the peninsula of Yucatan, Mexico, the fruit of the Cocoyol palm tree (Acrocomia mexicana) is well known to be very difficult to break. Its hardness has been documented since the 1500 s, and is even mentioned in the popular Maya legend The Dwarf of Uxmal. However, until now, no scientific studies quantifying the mechanical performance of the Cocoyol endocarp has been found in the literature to prove or disprove that this fruit shell is indeed “very hard”. Here we report the mechanical properties, microstructure and hardness of this material. The mechanical measurements showed compressive strength values of up to ~150 and ~250 MPa under quasi-static and high strain rate loading conditions, respectively, and microhardness of up to ~0.36 GPa. Our findings reveal a complex hierarchical structure showing that the Cocoyol shell is a functionally graded material with distinctive layers along the radial directions. These findings demonstrate that structure-property relationships make this material hard and tough. The mechanical results and the microstructure presented herein encourage designing new types of bioinspired superior synthetic materials.

Hierarchical levels of organization of the Brazil nut mesocarp

“…Several nutshells are composed of fibers and sclereids 12,13,[22][23][24] . However, an interesting feature in the mesocarp of Brazil nut is how the sclereids are arranged, filling the gaps between the fiber bundles 1 .…”

“…in stiffness), and they improve the cohesion of tissues with different properties and avoid stress concentrations 27 . For example, the endocarp of the cocoyol palm tree (Acrocomia mexicana) 24 , has a harder external layer to protect seeds against predators trying to penetrate the shell and an inner layer with mechanisms to enhance shell toughness. Nature employs a variety of strategies to achieve such properties gradients, like adjusting the degree of lignification, as in the stem of Washingtonia robusta 28 , differential cell wall thickening, as in Phyllostachys pubescens 29 , changes in cell geometry, as seen in the endocarp of the cocoyol 24 , or by introducing density gradients by different pore sizes, as in Citrus maxima 30,31 .…”

Functional packaging of seeds

“…2b), the cell walls also appear to be heavily impregnated (presumably with some flavonoids, in addition to stilbenolignins; del Rio et al ., 2017). A similar tissue arrangement (Table 1) along with a high lignin content occurs in many other (tropical) species (Landucci et al ., 2020); for example, in the seed coat of macadamia ( Macadamia integrifolia ; Schüler et al ., 2014), the endocarp of the cocoyol fruit ( Acrocomia mexicana ; Flores‐Johnson et al ., 2018), and in the mesocarp of the Brazil nut ( B. excelsa ; Sonego et al ., 2019), though fibres are more bundled and form network‐like structures in the latter. There are also nonfibrous, mechanically strong, static encapsulation tissues.…”

Morphological characterization and productivity estimates of Acrocomia totai Mart. (Arecaceae) – a sustainable alternative of extractivism and cultivation

“…The fruit pulp, nutritionally rich (AMARAL et al 2019;MUNHOZ et al 2018 a b ;VIANNA et al 2015;RAMOS et al 2008), is consumed fresh or to prepare ice creams and juices, or flour for several local food products such as cakes and biscuits, jams, candies or as a food industry ingredient (SILVA et al 2020;AGUILERA et al 2018;MARCONDES et al 2018;SILVA et al 2018). The endocarp can be used as crushed rock or as activated charcoal (FLORES-JOHNSON et al 2018). The nut is consumed fresh or toasted, ground for sweets, and its excellent quality oil is edible or for cosmetics (MUNHOZ et al 2018;POTT;POTT 1994).…”