Elucidation on structural changes of lignin-carbohydrate complexes during kraft cooking

To understand the structure of lignin-carbohydrate complexes (LCCs) in the process of kraft cooking, we injected C-13-enriched lignin precursor into intemode cavities of the stalk of growing rice plant to obtain C-13-enriched rice stalks. According to the procedure of Bjorkman et al, labeled lignin-carbohydrate complexes were separated and determined by C-13-NMR. The result of analysis by C-13-NMR spectra of C-13-enriched LCC fractions from rice plant showed that lignin was linked to carbohydrate by benzyl ether bond, ester bond and alpha-ketal linkage. It was also proved that beta-O-4 was main substructures of lignin in LCC. Then, the obtained LCC and labeled LCC were cooked by kraft method respectively. Lignin-carbohydrate complex-kraft lignins (LCC-KLs) were isolated from black liquor. The overall structures of the obtained LCC-KLs were determined by C-13-NMR and FT-IR spectra. Marked increase in the signal intensities of alpha-carbons was observed when alpha-C-13-labelled sample was applied. The result showed that most of benzyl ether bonds and ketal linkage between lignin and carbohydrate were broken in pulping, but minor content of ketal linkage was observed. Main substructures of LCC-KL Were found to be beta-O-4 units, with low content of phenylcoumaran units and pinoresinol units.