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Why walnuts are such tough nuts to crack

If you’ve ever had trouble breaking into a ripe, shelled walnut, don’t blame your fingers: Scientists have discovered that walnuts have a uniquely tough shell, made of an intricately shaped and previously unknown type of cell.

Almonds, macadamia nuts, and most other nuts have tough outer shells made of layers of thick-walled cells and separating fibers in a polygonal pattern. Walnut shells, however, are made of a dense tissue in which individual cells and their shapes are difficult to discern, even under high magnification.

To investigate further, researchers from Austria and Germany isolated individual walnut shell cells by soaking them in a solution designed to remove lignin, the complex organic polymer that makes plant cell walls sturdy. They then examined the cells using laser-scanning and electron microscopes.

The team found that walnut shells are composed of a previously unknown cell type—dubbed the “polylobate sclereid” cell—which sports irregular lobes with many different concave and convex contours. These fit together in an intricate and remarkably strong pattern, with each cell surrounded by an average of 14 neighbors. If the structure were a 3D wooden puzzle, it would be impossible to take apart without breaking it into pieces.

This property gives the walnut shell its higher tensile strength, making it much harder to crack, the researchers reported last month in Advanced Science. That’s because any crack passing through the walnut shell must cut through the cells themselves, rather than simply pulling them apart at the joints. The findings could inspire artificial materials built on the same principle, the researchers say, or even ways to reuse discarded walnut shells—a development that would be great news for recycling “nuts.”