Fractional characterization of alkali-labile lignin and alkali-insoluble lignin from wheat straw

Wheat straw was treated with sodium hydroxide at different temperatures (0, 20, 40, 60, 80 degrees C) and various concentrations (0.5, 1.5, 3.0, 5.0, 10.0%) for 6 h, respectively. Soluble lignins and free-phenolic monomers in each of the alkaline treatment hydrolysates were isolated. The tightly bound phenolic acids and aldehydes were determined by nitrobenzene oxidation of alkali-insoluble lignin in the treated straw residues. The predominant components of alkali-labile free-phenolic monomers were found to be ferulic and p-coumaric acids, which together comprised about 80% of the total. Ferulic acid was released faster than p-coumaric acid, while p-coumaric acid appeared to be more effect on dissolution of lignin than ferulic acid during the alkaline treatments. About 90% of p-coumaric acid in wheat straw was present in the ester-linked form to lignin, while more than 60% of ferulic acid was ether-linked to lignin. The results of alkaline nitrobenzene oxidation showed that a slightly high guaiacyl content in the original wheat straw and relatively high syringyl content in alkali treated wheat straw.