Eudicot Nutshells: Cell-Wall Composition and Biofuel Feedstock Potential

Nutshells
are the recalcitrant and membranous layers that encompass
and provide protection to the seed of a fruit. Rich in lignin, these
are often waste products of the food industry and are a potential
energy-dense feedstock for biorefineries to convert to biofuels. Assessing
their potential requires an understanding of the relative amounts
of cellulose, hemicelluloses, and lignin, the composition and structure
of the lignin, and a saccharifiability assessment, all provided here.
In the context of understanding the general character of phylogenetically
diverse eudicot species, we provide a chemical analysis of the lignin
in the shells of commonly eaten nuts and detail the chemical composition
of their lignins. Particularly striking observations include the >40%
levels of Klason lignins in the nutshells from macadamia, pecan, and
Brazil nut and lignin compositions ranging from <1% syringyl (S)
in macadamia to 77% S in pistachio and essentially 0% p-hydroxyphenyl (H) in mango to as high as 7.5% H in walnut. Novel
units in the lignin polymer (such as the hydroxystilbenes recently
discovered in macaúba and coconut shell lignins) were not evident
in any of the samples, but the series provided such a range of compositions
that it aided more detailed assignment.