The utilization of soybean straw III: Isolation and characterization of lignin from soybean straw

•CEL isolating efficiency was improved by combining dissolution-regeneration with enzymatic hydrolysis.•The yield of lignin was greatly improved from 26.0% to 82.5%.•There was not too much structural alternation induced by this superior and promising isolating method.

To improve the yield of cellulolytic enzyme lignin (CEL) and to minimize the structural changes induced by isolation, the dissolution-regeneration of ball-milled soybean stem in a lithium chloride (LiCl)/dimethyl sulfoxide (DMSO) solvent was applied prior to enzymatic hydrolysis. The regenerated stem was enzymatically hydrolyzed for 72 h to remove the carbohydrates and then extracted with 1, 4-dioxane to obtain the cellulolytic enzyme lignin (named as DR-CEL). As expected, the yield of DR-CEL was much higher than that of CEL. With 2 h of ball-milling, the yield of DR-CEL was 40.1% (based on the total lignin in the raw stem). As the ball-milling time increased to 4 h, the yield of DR-CEL improved up to 82.4%, which was much higher than that of CEL with a yield of 26.0%. All the isolated lignin preparations were characterized using FT-IR, 1H NMR and 1H-13C HSQC NMR. The results showed that the main structural characteristics of these two kinds of lignin (CEL & DR-CEL) were similar, except for a slightly higher amount of carbohydrates in DR-CEL. As a result, the combination of the dissolution–regeneration and enzyme hydrolysis of ball-milled sample has a great potential in isolating lignin with higher yield and minimal structural changes.