| The main organic component of straw resources, cellulose and hemicellulose, can be hydrolyzed into pentose enriched in xylose and hexose enriched in glucose, which are the substancial feedstock in the fermentation and chemical industry. Thus, hydrolysis is the key process in the conversion of straw resources.In the present study, we chosen corn stover as the representative of straw resources and studied the influence of the five key factors including cellulase loading, substrate concentration, temperature, pH and reaction time on enzymatic hydrolysis of corn stover. Based on the results of one-factor-at-a-time(OFAT) experiment, central composite rotatable design(CCRD) according to the principles of response surface methodology(RSM) was carried out to optimize the enzymatic hydrolysis process. A quadratic polynomial model assessing the effects of the five independent variables on the reducing sugar yield had been developed. According to the analysis of variance(ANOVA),the model was significant and there was no lack of fit. Model analysis was performed with the help of SAS8.2 software and the optimum values of the five factors were obtained which was cellulase loading of 57.50 FPU/g of substrate, substrate concentration of 64.72g/L (w/v), temperature of 47.94°C, pH of 4.78 and reaction time of 49.05 h. And a reducing sugar yield of 46.34g/100g of substrate was achieved. Compared with the reducing sugar yield obtained under optimum conditions of OFAT experiment, there was a 4.37% increase, while the cellulase loading decreased 2.5 FPU/g of substrate, substrate concentration increased 14.72g/L and reaction time dropped 22.95h.In order to improve the enzymatic hydrolysis efficiency of corn stover, pretreatment had been performed. The dilute acid treatment which was described as one of the most effective methods in solubilizing hemicellulose sugars and increasing the enzymatic digestibility of cellulose was selected for corn stover pretreatment. The optimum conditions of dilute sulfuric acid pretreatment under three different temperatures were determined aiming at solubilizing the sugars from the hemicellulose fraction and let the cellulose the most unattacked as possible. The optimum condition of dilute sulfuric acid pretreatment under 80℃was sulfuric acid concentration of 4.2% and reaction time of 177min.One-factor-at-a-time(OFAT) experiment was carried out for the corn stover after dilute sulfuric acid pretreatment at 120℃. Based on the results, central composite rotatable design(CCRD) according to the principles of response surface methodology(RSM) was performed to optimize the enzymatic hydrolysis process of the corn stover after dilute sulfuric acid pretreated at 80℃,100℃,and 120℃respectively. A quadratic polynomial model assessing the effects of the five independent variables on the reducing sugar yield had been developed for the three different pretreated corn stover, respectively. According to the analysis of variance(ANOVA),the model was significant and there was no lack of fit. Model analysis was performed with the help of SAS8.2 software and the optimum values of the five factors were obtained.The optimum condition for the enzymatic hydrolysis of corn stover after dilute sulfuric acid pretreatment at 80℃was cellulase loading of 22.33 FPU/g of substrate, substrate concentration of 76.46g/L (w/v), temperature of 48.28°C, pH of 4.79 and reaction time of 36.81 h and a reducing sugar yield of 46.34g/100g of substrate was achieved. The optimum condition for the enzymatic hydrolysis of corn stover after dilute sulfuric acid pretreatment at 100℃was cellulase loading of 22.24 FPU/g of substrate, substrate concentration of 78.68g/L (w/v), temperature of 49.12°C, pH of 4.78 and reaction time of 36.9 h and a reducing sugar yield of 40.84g/100g of substrate was achieved. The optimum condition for the enzymatic hydrolysis of corn stover after dilute sulfuric acid pretreatment at 121℃was cellulase loading of 21.95 FPU/g of substrate, substrate concentration of 77.04g/L (w/v), temperature of 48.93°C, pH of 4.78 and reaction time of 37.72 h and a reducing sugar yield of 41.22g/100g of substrate was achieved. Compared with the reducing sugar yield obtained under optimum conditions of OFAT experiment of the enzymatic hydrolysis of corn stover after dilute sulfuric acid pretreatment at 121℃, there was a 4.52% increase, while the cellulase loading decreased 3.05 FPU/g of substrate, substrate concentration increased 17.04g/L and reaction time dropped 22.28h.The xylose selectivety of corn stover dilute sulfuric acid pretreatment at 100℃was good. Compared with the reducing sugar yield obtained under optimum condition of enzymatic hydrolysis for original corn stover, there was a 7.03% increase with a total reducing sugar yield of 53.37 g/100g of substrate combined the pretreatment and the following enzymatic hydrolysis while the cellulase loading decreased 35.26 FPU/g of substrate, substrate concentration increased 13.96g/L and reaction time dropped 12.15h. The xylose selectivety of corn stover dilute sulfuric acid pretreatment at 121℃was best. Compared with the reducing sugar yield obtained under optimum condition of enzymatic hydrolysis for original corn stover, there was a 6.11% increase with a total reducing sugar yield of 52.45 g/100g of substrate combined the pretreatment and the following enzymatic hydrolysis while the cellulase loading decreased 35.55 FPU/g of substrate, substrate concentration increased 12.32g/L and reaction time dropped 11.33h. The xylose selectivety of corn stover dilute sulfuric acid pretreatment at 80℃was not very good. Compared with the reducing sugar yield obtained under optimum condition of enzymatic hydrolysis for original corn stover, there was no apparent increase of the total reducing sugar yield combined the pretreatment and the following enzymatic hydrolysis, however, the cellulase loading decreased 35.17 FPU/g of substrate, substrate concentration increased 11.74g/L and reaction time dropped 12.24h. The above results demonstrated that dilute sulfuric acid pretreatment can effectively reduce the reaction time, decrease the cellulase loading and improve the utilization of substrate for the following enzymatic hydrolysis.Response surface methodology was proved to be reasonable and efficient and well operational. The optimum conditions obtained in the study was useful for the corn stover conversion into reducing sugar.
|
PDF Full Text Request