Damage protection in fruits: Comparative analysis of the functional morphology of the fruit peels of five Citrus species via quasi-static compression tests

Abstract

Due to their special peel tissue, comprising a dense flavedo (exocarp), a less dense albedo (mesocarp), and a thin endocarp, most citrus fruits can withstand the drop from a tree or high shrub (relatively) undamaged. While most citrus fruit peels share this basic morphological setup, they differ in various structural and mechanical properties. This study analyzes how various properties in citrus peels of the pomelo, citron, lemon, grapefruit, and orange affect their compression behavior. We compare the structural and biomechanical properties (e.g., density, stress, Young’s modulus, Poisson’s ratio) of these peels and analyze which properties they share. Therefore, the peels were quasi-statically compressed to 50% compression and analyzed with manual and digital image correlation methods. Furthermore, local deformations were visualized, illustrating the inhomogeneous local strain patterns of the peels. The lateral strain of the peels was characterized by strain ratios and the Poisson’s ratio, which were close to zero or slightly negative for nearly all tested peels. Our findings prove that—despite significant differences in stress, magnitude, distribution, and thickness - the tested peels share a low Poisson’s ratio meaning that the general peel structures of citrus species offer a promising inspiration for the development of energy dissipating cellular structure that can be used for damage protection.

Analysis of the peel structure of different Citrus spp. via light microscopy, SEM and μCT with manual and automatic segmentation

“…An auxetic material behavior or at least a very low lateral expansion has been demonstrated in various citrus fruit peels. 7…”

“…The peels of citrus fruits, and especially of the pomelo, are characterized by specific mechanical properties allowing for high energy dissipation during impact. Whether it is high energy dissipation, [1][2][3][4] (nearly auxetic) lateral compression behavior 3,[5][6][7] or other mechanical properties 3,4,6,8 that indicate their excellent damping properties. Therefore, the peels of citrus fruits can generally act as an inspiration for biologically inspired technical material systems and lightweight structures.…”

Digital image correlation techniques for motion analysis and biomechanical characterization of plants

“…However, these devices deliver only limited local information of the structure under investigation. Larger strain fields can also be calculated by applying markers, e.g., dots, on the deforming structure and by subsequent computer-based manual or automated tracking (e.g., Jentzsch et al., 2022a ).…”

Stereo Camera Setup for 360° Digital Image Correlation to Reveal Smart Structures of Hakea Fruits

“…Its first applications were in materials science, but over the years, DIC has been utilized more and more in other research fields, such as aerospace, including large-scale composite structures [ 6 ], civil engineering, such as bridge monitoring [ 12 , 13 ], human and animal biomechanics to analyse a wide variety of organs and tissues [ 14 , 15 ] and wood research, from entire trees to processed composites [ 16 ]. In recent years, DIC has also been used in plant analysis, namely to analyse plant tissue strains under tensile loading (e.g., mistletoe–host interface [ 17 ] or the branch–branch connections of cacti [ 18 ]) to analyse compression (e.g., citrus peels [ 19 ]), and to characterise plant movements, such as the snap-buckling closure [ 20 ] and reopening [ 21 ] of the Venus flytrap or the desiccation-driven motion of pine cone scales [ 22 , 23 ]. This has led to a better understanding of the functional principles of the plant material systems, which is a prerequisite for successful transfer to bioinspired materials.…”