The alleviation of lignin inhibition on enzymatic hydrolysis of cellulose by changing its ultrastructure

Lignin is regarded as the primary factor that inhibits enzymatic hydrolysis of cellulose. However, the impact of cellulose ultrastructure on the inhibition of lignin during enzymatic hydrolysis is still unclear. In this work, the addition of lignin during enzymatic hydrolysis of cellulose with different ultrastructure was investigated. Furthermore, the influences of changing cellulose ultrastructure on cellulose conversion of pine after hydrothermal treatment (HT) without and with additives (2-naphthol (2 N) and phloroglucinol (PL)) were explored in detail. The results showed that enzymatic hydrolyzability of allomorphic cellulose was strongly inhibited by lignin. In contrast, a small inhibition degree of lignin was observed for amorphous cellulose. Surprisingly, the cellulose conversion of HT treated pine was dramatically decreased (18.45–6.25%) after ball-milling treatment (BM), while it was significantly increased (18.45–74.49%) after concentrated phosphoric acid treatment (CP). In addition, the addition of 2 N or PL during HT could promote or inhibit cellulose conversion due to the changes in the condensation degree of lignin. These results indicated that the physiochemical structure of lignin has a significant effect on cellulose hydrolysis. More importantly, the increasing degree of cellulose conversion was enhanced in sequence from HT+PL (6.19–35.00%), HT (18.45–74.49%), to HT+ 2 N (21.12–92.30%) after CP, which was attributed to the reducing lignin condensation and changing cellulose ultrastructure. Overall, this work demonstrated that the inhibition degree of lignin was highly dependent on the cellulose ultrastructure. • The effect of lignin on hydrolysis of cellulose with different structures was studied. • Enzymatic hydrolysis of crystalline cellulose was strongly inhibited by lignin. • Enzymatic hydrolysis of amorphous cellulose was slightly inhibited by lignin. • A new insight to alleviate the lignin inhibition on enzymatic hydrolysis of cellulose.