| Due to the increasingly serious energy crisis and environment problem, the development of bio-based industrial products from lignocellulosic biomass is in great demand, for example, cellulosic ethanol. For production of chemicals from biomass, enzymatic hydrolysis of cellulose and hemicellulose to fermentable sugars is generally employed. However, the conversion efficiency of natural materials is usually very poor, low efficiency of sugar molecule releasing. Biomass recalcitrance, a general designation of substrate-related factors, is responsible for the efficiency of sugar molecule releasing. Many factors contributing to high recalcitrance of biomass mainly include structural heterogeneity and complexity of cell wall, precisely arranged crystalline cellulose, low accessibility of cellulose, lignification and the heterogeneous reaction of saccharification. Of these characteristics above, which are the most important still remains unclear. Pretreatment is one of the key points to enhance the efficiency of biomass utilization and relieve recalcitrance. Due to low energy-consuming and environment friendly, biological pretreatment has received extensive attention. White rot fungi, most belonging to basidiomycetes, are the most promising microorganisms for biological pretreatment due to their capacity of degrading lignin to water and carbon dioxide. Therefore, biomass degradation system by white rot fungi was firstly established, and then the relationship between recalcitrance change and biodegradation was investigated in this paper.To establish a high efficient degradation system, 33 strains of basidiomycetes, obtained from Shennongjia Nature Reserve in China, were employed to treat three kinds of biomass. After enzymatic hydrolysis of pretreated biomass, transformation rates and saccharification rates were used to evaluate the effect of different pretreatments. The result indicated that the effect of different degradation system varied a lot, with the suitable strains for pretreatment of corn stover, wheat straw and bamboo were strain CD2, Pleurotus sp. No.4 and Coriolus versicolor, respectively. The effect of biological pretreatment could be optimized by altering the cultivation conditions and medium for biodegradation by basidiomycetes. Strain CD2 showed the greatest superiority in promoting enzymatic hydrolysis, with the highest saccharification rate reaching 62.0%, which was 2.32 times of untreated corn stover. Therefore, the best biodegradation system of "strain CD2-corn stover", which was the most promising biopretreatment for cellulosic ethanol production, was chosen for further study. The strain CD2, identified as Irpex lacteus by ITS sequence analysis, is a white rot fungus. The optimum growth temperature was respectively 30℃and 28℃on PDA and corn stover medium. Corn stover pretreated with I. lacteus CD2 was brickie and turned white from its original color of brown.Corn stover was decayed by different mode with I. lacteus CD2 at varying durations. Hemicellulose and lignin was respectively selectively degraded at the initial and middle stage of biodegradation, with the content of acid soluble lignin being significantly enhanced. Cellulose, hemicellulose and lignin were simultaneously degraded at the last stage. During biodegradation and modification, ether hydrolysis, demethylation and cleavage of aromatic rings were the main changes on lignocellulose function groups. No extracellular ligninolytic enzymes were detected during 120 days of cultivation on corn stover medium except for low LiP activity after 25 days. In PDB medium, low level of mycelium-associated laccase, intracellular MnP and LiP could be detected. FeRA could be detected in the extracellular fermentation extracts all along the decay periods. The results suggested that lignin degradation by I. lacteus CD2 followed a special mechanism independent on extracellular ligninases.To find the key characteristics influencing saccharification rate, studies on kinetics of enzymatic hydrolysis under different enzyme loading for varying reaction time were carried out. The results indicated initial velocity could not represent the final efficient of enzymatic hydrolysis. Transformation rates of corn stover samples versus enzyme loading followed the same curve proposed by Bailey. Based on this, biomass recalcitrance to enzyme hydrolysis was quantified and calculated as the percentage of holocellulose not hydrolyzed to reducing sugars during enzymatic hydrolysis. The quantified recalcitrance was proved to be in negative correlation with saccharification rate, which illustrated that this parameter could scientifically estimate the general characteristic of biomass for enzymatic hydrolysis.To make certain the chemical and physical mechanism of the reduced recalcitrance by I. lacteus CD2, further research mainly focused on the changes of lignin, hemicellulose, crystallinity index, surface area, pore size distribution, and surface structure, which were developed by means of components' content determination, XRD analysis, nitrogen adsorption method and ESEM analysis. The results indicated that many factors contributed to the reduced recalcitrance to enzymatic hydrolysis, including lignin degradation and modification, hemicellulose degradation, increased surface area and porosity, mycelium penetration. Acid insoluble lignin and acid soluble lignin were respectively in positive and negative correlation with recalcitrance, which demonstrated that the reduced recalcitrance by I. lacteus CD2 was mainly determined by the extent of lignin degradation and modification. The crystallinity index of cellulose didn't significantly influence recalcitrance. This thesis not only proclaimed the inherent correlation between reduced recalcitrance and biomass quality altering by I. lacteus CD2, but also provided theoretical and technical foundation for biological pretreatment of biomass with white rot fungi.
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