| Lignocellulosic material like corn stover is very abundant in China, but its effective availability is significantly low. Pretreatment of lignocellulose can disrupt its inherent biological recalcitrance, which will improve the substrate enzymatic degradation efficiency. The effects of different pretreatments by sodium hydroxide（Na OH）, alkaline sodium sulfite（Na2SO3） and alkaline hydrogen peroxide（H2O2） on compositional contents, substrate accessibility（the amount of adsorbed endoglucanase） and enzymatic hydrolysis efficiency were investigated using corn stover as raw material in this paper. Meanwhile, the correlation between substrate accessibility and the efficiency of enzymatic hydrolysis was studied. Finally, combined with industrial production, microwave assisted alkaline sodium sulfite pretreatment and its operation process were investigated in order to improve substrate enzymatic hydrolysis based on the retention of available carbohydrates as much as possible. The main research contents were as follows:At first, the effects of different pretreatments by NaOH, Na2SO3 and H2O2 on compositional contents, substrate accessibility（the amount of adsorbed endoglucanase） and enzymatic hydrolysis efficiency were investigated. The results showed the lignin content of substrate obtained by Na OH pretreatment was the lowest, which was 7.40 g/100 g, while that of Na2SO3 pretreatment was 14.51 g/100 g, and that of H2O2 pretreatment was 17.96 g/100 g. Due to the removal of lignin, the relative content of glucan and xylan both increased. Scanning electron microscope also showed that the structure of pretreated substrates was destroyed to varying degrees. At 4°C, with the initial cellulase loading of 6.50 mg/g glucan, the amount of adsorbed enzyme for NaOH pretreated substrate was 4.70 mg/g glucan, increased by 88.00% compared with that of raw material. Through changing the initial amount of cellulase added, the adsorption of cellulase on different substrates met the Langmuir equation（R2>0.95）, and the substrate after NaOH pretreatment had the maximum adsorption capacity（Wmax） and adsorption constant（K）. The enzymatic hydrolysis of all pretreated substrates was improved, and NaOH pretreated substrate with the lowest lignin content had the highest enzymatic digestibility（18.90 g/100 g） while that of alkaline Na2SO3 pretreatment and alkaline H2O2 pretreatment was followed respectively. For all substrates, enzymatic hydrolysis correlated well with substrate accessibility, indicating that the accessibility of substrate was a key factor to determine the efficiency of enzymatic hydrolysis.Then the effect of different lignin removal rates on substrate accessibility and enzymatic hydrolysis was studied by NaOH pretreatment with different mass concentrations. The results indicated: in a certain range, with the increasing concentrations for NaOH pretreatment, the removal rate of lignin increased, and the amount of adsorbed cellulase and enzymatic hydrolysis efficiency also increased. The obtained substrate by 2.00 g/100 g NaOH pretreatment had the highest amount of adsorbed cellulase（4.70 mg/g glucan） and enzymatic hydrolysis（18.90 g/100 g）. However, with the further increasing concentrations for NaOH pretreatment, as large amounts of lignin was removed, more non-crystalline cellulose and hemicellulose were also removed（the retention rate of carbohydrates was very low）. And the accessibility and enzymatic hydrolysis efficiency of substrate began to decrease instead, indicating again the important role of substrate accessibility in hydrolysis.Finally, on the basis of trying to improve the retention rate of carbohydrates, microwave assisted alkaline sodium sulfite pretreatment for corn stover was utilized and optimized. The optimal process conditions were as follows: microwave power 500 W, total microwave time 30 min, Na2SO3 mass concentration 4.00 g/100 g, and NaOH mass concentration 0.10 g/100 g. Substrate obtained by microwave assisted alkaline sodium sulfite pretreatment had a higher cellulase adsorption capacity（increased by 8.88%） and enzymatic hydrolysis（increased by 15.85%） compared with conventional chemical pretreatment with the same chemical reagent dosage and boiling temperature. Through observing and analyzing the property and structure of substrates before and after pretreatment with scanning electron microscope（SEM）, thermal gravimetric analysis（TGA）, X-ray diffraction（XRD）, fourier-transform infrared spectroscopy（FTIR）, confocal laser scanning microscope（CLSM）, the results showed that microwave assisted alkaline sodium sulfite pretreatment could break the chemical bonds between lignin and hemicellulose, remove some of the key functional groups, and destroy the structure of lignin, which was in favor of removing more lignin, destroying the resistant layer structure of lignocellulose and reducing the effect of lignin on the steric hindrance and ineffective adsorption of cellulase. Then the accessibility and enzymatic hydrolysis of pretreated substrate would be improved. |
