Comparison of characteristics of soluble lignins from untreated and ammonia-treated wheat straw

Abstract

Wheat straw ( Triticum aestivum, cultivar ‘Hachimankomugi’) was treated with aqueous ammonia (ammonia level: 30 g kg −1 straw) at 25 °C for 36 days and then air dried. Soluble lignins were isolated with 90% aqueous dioxane from ether-extracted residues of the ammonia-treated and untreated wheat straw samples without ultragrinding and their chemical and physical properties were compared. The ammonia-treated wheat straw showed a higher dry matter digestibility (determined by a nylon bag technique) and yielded a larger quantity of dioxane-soluble lignin than the untreated straw. IR spectra indicated the presence of guaiacyl- and syringylpropane units in the two lignins. The two soluble lignins showed similar molecular weight distributions. The lignin preparation from the ammonia-treated straw contained less p-coumaric acid and much more ferulic acid than that from the untreated straw. Most of the p-coumaric acid in both soluble lignins was present in the ester-linked form, whereas ferulic acid was predominantly in the ether-linked form. The lignin from the ammonia-treated straw was much more abundant in ether-linked ferulic acid than that from the untreated straw. The results provided evidence of the solubilization of the ferulic acid ether-rich lignin fraction from wheat straw by ammonia treatment. Mechanisms of the solubilization of wheat straw lignin and improvement of biodegradability of wheat straw by ammonia treatment were discussed in relation to possible ferulic acid cross-linkages in the cell walls.