Isolation and Characterization of a Guaiacyl Lignin from Saponified Cork of Quercus suber L.

Extractive-free cork from Quercus suber L. has been submitted to depolymerisation by sodium methoxide in methanol (transesterification) from which a saponified cork (CORK sap ) in a yield of 34.4% was obtained. From CORK sap a milled cork lignin (MCL sap ), an ether soluble part, and a lignin carbohydrate complex (LCC sap ) were isolated, with the respective yields of 11.9%, 0.54%, and 0.79% (based on CORK sap , corresponding to 4.1%, 0.19% and 0.27% based on original cork). The isolates were characterized by FTIR spectroscopy, analytical pyrolysis (Py-GC-FID), elemental analysis, OMe determination, molecular weight determination, and acid hydrolysis followed by monomeric sugar analysis. The elimination of aliphatic suberinic acids of cork before MCL isolation was helpful to obtain a pure sample. MCL sap was essentially free of suberinic acids and its carbohydrate content was around 5%. It was demonstrated for the first time that milled cork lignin belongs to a guaiacyl-type lignin which is very similar to spruce lignin containing up to 98% guaiacyl units, 1% 4-hydroxy-phenylpropane units, and approx. 1-2% syringyl units. The total hydrolysis of LCC sap followed by sugar analysis yielded 8.3 % arabinose and 0.7% xylose.