Mechanism Study On Improvement Of Enzymatic Hydrolysis Of Wheat Straw By Pretreatment With White-rot Fungus Pleurotus Ostreatus BP3

The complex structure of lignocellulosic feedstock is the main limitation of the conversion efficiency of cellulosic ethanol. The use of white-rot fungal pretreatment, which is a low energy comsuption and environmental friendly technology, can significantly reduce the content of lignin, thus reducing the resistance of hydrolysis of lignocellulosic feedstocks and enchance the efficiency of conversion. However, the mechanism that how enzymatic hydrolysis accelerated by white-rot fungal pretreatment is not clear. In this study, a simple and efficient bio-conversion technology of bioethanol from wheat straw was established assisted with an efficient pretreatment strain Pleurotus ostreatus BP3. On this basis, the structural changes during the bio-pretreatment of wheat straw were analysed by different instrumental analysis methods, which reveals the biological modification mechanism of wheat straw lignin by white-rot fungal pretreatment and the mechanism of efficient pretreatment for improved enzymatic hydrolysis.Firstly, the study found Mn²⁺ can significantly improve the efficiency of pretreatment with Pleurotus ostreatus BP3: the yield of glucose and ethanol from the pretreated wheat straw were 342.09mg/g and 118.22 mg/g respectively with additive of 0.025mM/g Mn²⁺ in raw substrates for biological pretreatment of 35 days. The correlation analysis of the composition changes of wheat straw and the final glucose and ethanol conversion rate under different pretreatment methods shows that the improvement of digestion efficiency is related to the degradation of lignin,so as the degradation of hemicellulose.Further structural characteristics of wheat straw lignocellulose, lignin and lignincarbohydrate complex?LCC? with different pretreatment conditions were evaluated by the technologies of FTIR, Py-GC/MS and NMR, etc. The results showed that in the process of white-rot fungal pretreatment, the wheat straw lignin demethylation reaction occurs, resulting the S/G ratio decreased by 24.21%; the linkages between lignin structural units?such as ?-O-4,?-?,?-5,etc.? break down, and some relatively difficult degradable C-C bonds can be almost completely degraded; the obvious benzene ring-opening reaction brings the destruction of lignin basic skeleton and the disintegration of the space network structure; particularly importantly, as a main bridge between hemicellulose and lignin, ferulate was degraded by 61.36% in the pretreatment process, leading to the destruction of LCC structure. All the above structural changes were promoted by the addition of Mn²⁺ into the medium of pretreatment. So the degradation of lignin and changes in the proportion of basic structural units, as well as the disintegration of LCC structure caused by degradation of ferulic acid are the key factors to enhance enzymatic hydrolysis with white-rot fungal pretreatment.Finally, the changes of wheat straw lignocellulose crystallinity, surface microstructure, pore size distribution and the surface area, cellulose accessibility of substrate were evaluated for biological pretreatment with Mn²⁺ present or absent conditions. An apparent increase of the mid-level holes number was observed, while the analysis results indicated that the specific surface area enlarged by 11.60%, cellulose accessibility increased by 14.98-fold, and the adsorption of cellulase to substrate enhanced by 3.33 times. These mentioned variations were also promoted by the addition of Mn²⁺. Thus, the surface and internal structure of wheat straw was disrupted by white-rot fungal pretreatment, resulting in the increase of porosity and specific surface area, which greatly relieves hydrolysis resistance barrier leading to the significant enhancement of adsorption between cellulase and substrate, and achieved the improvement in digestion efficiency.The paper reveals the importance and mechanism that how hydrolysis efficiency improved by biological degradation and modification of wheat straw lignin and LCC. It provides some theoretical directions for the development of bio-ethanol conversion and may help to improve lignocellulose bio-refinery.