The walnut shell is a hard and protective layer that provides an essential barrier between seed and environment. The shell is based on only one unit cell type: the polylobate sclerenchyma cell. For a better understanding of the interlocked walnut shell tissue, we investigate the microstructural and compositional changes during the development of the shell from the soft to the hard state by Confocal Raman microscopy.<br/>Walnut shell development takes place beyond the outer green husk, which protects and delivers components during the development of the walnut. Our results show that in the developing shell (July) the cells towards this outer green husk are the thickest and most lignified cell walls and across the shell a gradient in cell wall thickness and lignin content pictures the ongoing development, but pectin distribution shows opposite pattern. Raman imaging also shows that lignin is deposited first into the pectin network between the cells and cell corners, at the very beginning when the secondary cell wall is added onto the primary cell wall. Furthermore, Raman imaging of fluorescence visualized numerous pit channel within every cell, connecting all the interlocked polylobate walnut shells with each other and enabling transport during development. In the final mature stage (October) thick-walled highly lignified cells were found across the entire shell and fluorescence was increasing throughout the cell wall, but also a remarkable fluorescent layer towards the lumen was detected in the inner part. This final sealing of the micro-spaces with aromatic components reminds on heartwood formation of trees and is suggested to improve protection properties of the mature walnut shell.<br/>Our results clearly indicate the potential of Confocal Raman spectroscopy to gain new insights into chemical changes of the walnut shell microstructure of different development stages. In the long term, it will inspire biomimetic material design and packaging concepts, but is also important 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

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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