Structural characterization of lignin and lignin-carbohydrate complex (LCC) of sesame hull

Abstract

In the present study, lignin and lignin-carbohydrate complex (LCC) constituting the cell wall structure of sesame hulls were investigated to explore novel techniques of dehulling. Milled wood lignin (MWL), Björkman LCC, and acid-soluble LCC (LCC-AcOH) were extracted from sesame hulls and characterized by carbohydrate composition analysis, molecular weight analysis, UV–vis spectroscopy, FT-IR, thermal analysis, Py-GC/MS, 2D HSQC NMR, and 31P NMR. The results showed that rhamnose accounted for the largest proportion of the lignin and LCC fractions, followed by glucose. Björkman LCC had the largest molecular weight, MWL had the smallest molecular weight, and LCC-AcOH had the largest polydispersity index. The lignin of sesame hulls consisted of syringyl (S), guaiacyl (G), p-hydroxyphenyl (H), and caffeyl alcohol (C) units. The most abundant monomer was guaiacyl (G), followed by caffeyl alcohol (C). C-type lignin is a new type of lignin that is different from the three traditional lignin monomers. The major lignin-linked bonds in the MWL and LCC-AcOH were β-O-4′ and β-β′, and β-5′ bonds were present in the Björkman LCC. The major LCC chemical bonds in the three fractions were PhGly. These findings will provide the factual basis for exploring different dehulling methods to enhance the quality of sesame products.