Optimization of fed-batch enzymatic hydrolysis from alkali-pretreated sugarcane bagasse for high-concentration sugar production
Introduction
Over the last few years, several studies have begun to investigate the effects of high-solid loading (>15% solids, w/w) on different unit operations within the process stream (Hodge et al., 2008; Jørgensen et al., 2007; Kristensen et al., 2009, Lu et al., 2010, Zhang et al., 2010) as a means of improving the process economics of the lignocellulose to ethanol conversion. It has been suggested that using high-solids loading, especially the combination of a high-solids pretreatment followed by high-solids hydrolysis has great potential at improving the process efficiency (Modenbach and Nokes, 2012). The conversion process is more environmentally friendly, as less water is consumed and the production cost is due to reduced size of equipment (tanks and distillation column etc.) and reduced energy utilization for distillation (Mohagheghi and Schell, 2010, Stenberg et al., 1998).
However, a high content of lignocellulosic substrate might cause poor mass transfer and high viscosity, which would make the mixing difficult, the heat transfer efficiency lower and raise the power consumption in the stirred tank reactors (Fan et al., 2003, Jørgensen et al., 2007). To decrease these negative effects and maximize the end product concentration, fed-batch hydrolysis has been proposed as a feasible method (Laopaiboon et al., 2007, Rudolf et al., 2005, Varga et al., 2004). Yang et al., 2010 reported that 30% solids could be hydrolyzed with sugar concentration reaching to nearly 220 g/L. However, the pretreatment conditions were complex and time-consuming since both steam explosion and alkaline hydrogen-peroxide pretreatments were applied, which inevitably increased the energy consumption and capital cost. A fed-batch process was used to increase the solids loading to 30% by Wang et al. (2012), however, only about 100 g/L of reducing sugar and 50% glucan rate were obtained with a cellulase loading of 20 FPU/g substrate. Therefore, choosing a substrate pretreated by a low-energy-consuming method and optimization of fed-batch process were necessary for high concentration sugar and subsequent bio-ethanol production.
The high-solids alkali pretreatment has been clarified to be a more efficient, and water-saving method than liquid hot water and HCl pretreatment in our precious report (Zhang et al., 2013). In this study, for the purpose of cheap and efficient processes for the bio-ethanol production, the raw sugarcane bagasse (SCB) was pretreated with diluted NaOH at a S/L ratio of 1:6. Taking the alkali-pretreated SCB as substrate, the optimal condition of fed-batch hydrolysis was investigated, aiming to improve the overall solids and sugar concentrations under low enzyme loading of 10 FPU/g substrate.
Section snippets
Materials
Sugarcane bagasse (SCB) was provided by Guangxi Fenghao Group Co. Ltd. (Pingxiang, China). It was premilled and screened, with the fraction between 20 and 80 meshes used for these experiments. A cellulase mixture namely Cellic CTec2, was provided from Novozymes A/S (Bagsaevrd, Denmark). The cellulase activity was 310 FPU/ml (FPU is the activity unit of cellulase when filter paper is used as the enzymatic substrate), assayed by the description of IUPAC (Ghose, 1987). The β-glucosidase activity
Compositional changes before and after alkali pretreatment
In order to benefit lignocellulosic enzymes accessing the recalcitrant structure of cellulose for maximum recovery of sugars, a pretreatment process should be adapted to remove lignin or hemicellulose. This would decrease the crystallinity of cellulose, and increase the biomass surface area (Balat et al., 2008, Jørgensen et al., 2007). In this work, the sugarcane bagasse was pretreated with sodium hydroxide at a high-solids loading of 16.67%. The chemical composition of raw SCB was 41.95% of
Conclusions
Fed-batch enzymatic hydrolysis process from alkali-pretreated sugarcane bagasse was investigated to increase solids loading, produce high-concentration fermentable sugar and finally to reduce the cost of the production process. The hydrolysis system was initiated with 12% (w/v) in flasks, and finally reached 33% (w/v) solids loading. After the enzymatic hydrolysis for 120 h, the maximum concentrations of cellobiose, glucose and xylose obtained were 9.376 g/L,129.50 g/L,56.03 g/L, respectively, and
Acknowledgements
This work was funded by the National High-tech R&D Program (2013AA065803), National Key Technology R&D Program (2011BAD22B01), Program of National Natural Science Foundation of China (21176237, 21211140237), and Cooperation Project between Chinese Academy of Sciences and Guangxi Academy of Sciences, and Science & Technology Project of Guangzhou (2013J4300026).
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