Comparison of Wheat Straw Lignin Preparations. II. Straw Lignin Solubilisation in Alkali

In order to understand the characteristic solubility of Gramineae ligins in alkali, three lignin fractions were extracted from wheat straw: an alkali-soluble lignin LA, a milled-straw lignin LM and an enzyme lignin LE. Molecular size distributions were compared by size exclusion chromatography on mu-Bondagel, and acidic groups (carboxylic and phenolic groups) were estimated by potentiometric titration. The molecular size distribution of alkali lignin LA was intermediate between those of LM and LE. Solubilisation of wheat straw lignin by alkali treatments may thus result from depolymerisation to molecular weights of the same order as those obtained by ultragrinding. Alkali lignin was found to be significantly richer in acidic groups than LM or LE. It is concluded that Gramineae lignins differ from hardwood and softwood lignins by a relatively higher content of alkali-labile linkages within the lignin network or between lignin and polysaccharides.