Advances in Pseudo-Lignin Deposition and Its Effects on Enzymatic Hydrolysis of Cellulose

Lignocellulosic biomass is one of the most abundant renewable resources on earth. In order to improve the conversion rate of lignocellulosic biomass, increase the enzymatic hydrolysis efficiency of cellulose and the yield of fermentable sugars and to reduce the usage of cellulase and the conversion cost of biomass, it is essential to obtain the pretreated lignocellulosic biomass. However, the natural barrier between lignin, cellulose and hemicellulose limits the availability of cellulase to the cellulose components encapsulated inside. The main pretreatment methods of lignocellulose covered physical method, chemical method, physical chemical method and biological method. At present, the more adopted pretreatment methods include the dilute acid(hydrochloric acid, sulfuric acid, nitric acid, etc.)with the mass fraction less than 4% at the temperature of 120-210℃, high temperature hot water method, steam explosion method and liquid hydrothermal method. The dissolution and removal of lignin or most of the hemicellulose by various pretreatment methods is beneficial to improve cellulase accessibility. At present, a large number of studies have shown that lignin can significantly inhibit the enzymatic hydrolysis of cellulose, pretreatment can reduce lignin content and improve the efficiency of cellulose hydrolysis. In recent years, studies have shown that the relative content of Klason lignin would increase after the pretreatment of lignocellulose under severe conditions by using dilute acid and high temperature water. During the pretreatment processes of lignocellulosic biomass, lignin droplets may be deposited on the surface of the cellulose in the form of pseudo-lignin, so that it can inhibit cellulase hydrolysis more than natural lignin. However, no relevant review paper was found in the published articles. In this paper, the formation processes of lignin droplets and pseudo-lignin during the pretreatment of biomass were described. The possible mechanisms of pseudo-lignin production were put forward, and its composition and properties were reviewed. On this basis, the effects of lignin droplets and pseudo-lignin on the enzymatic hydrolysis of lignocellulose were summarized. Finally, the effects of lignin droplet and pseudo-lignin on the enzymatic hydrolysis of cellulose were discussed, and then the regulation strategy of pseudo-lignin formation was summarized. The authors believe that it is of great significance to study the mechanisms, structure characteristics, regulation mechanisms of the production of pseudo-lignin in the process of biorefinery. The formation of pseudo-lignin is a heterogeneous reaction process. At the same time, the formation mechanisms of pseudo-lignin can be studied from the mesoscale behavior due to the influence of mass transfer(molecular level)and the flow(macroscopic statistical level), and at the same time, the related models and theories should be established to realize their scientific quantitative descriptions and regulation. Moreover, it is beneficial to the development of lignocellulosic biomass refining processes and also it is conducive to promote the formation and development of an interdisciplinary scale of science. © 2020, Editorial Department of Scientia Silvae Sinicae. All right reserved.