Comparison of properties of residual lignins of pine pretreated with alkaline and further oxidative delignification and their detaching behavior during enzymatic hydrolysis.

The inhibition of lignin is important for affecting enzymatic hydrolysis efficiency. Insight into the inhibitory mechanisms is fundamental for effectively enhancing substrate enzymatic digestibility. This study compared the enzymatic saccharification of two pretreated pine wood. One was pretreated by alkaline with sodium sulfide (AS), and another was alkaline with sodium sulfide subject to acidic sodium chlorite (AS-AC). At a similar lignin content, the substrate enzymatic digestibility (SED) of sample AS-AC was much higher than that of AS. Moreover, elevated pH from 4.8 to 6.0, did not affect the SED of AS-AC, while the SED of AS was substantially decreased. The characterization of the isolated lignin from the two pretreated samples showed that the lignin in AS-AC had a smaller molecular weight and stronger hydrophilic properties, resulting in a weaker adsorption capacity to cellulase than the lignin in AS. Furthermore, the behavior of the residual lignin was monitored during enzymatic hydrolysis and revealed that lignin was detached from substrate AS-AC in large quantities into the enzymatic solution, but less from AS. Adding the two isolated lignins to the enzymatic systems showed a small negative effect, and was eliminated by the elevation of pH. It can be speculated that once the residual lignin detached from the substrate and into the enzymatic solution, its inhibitory effect on the enzymatic hydrolysis of lignocellulose will be weakened.