Improved enzymatic hydrolysis yield of rice straw using electron beam irradiation pretreatment
Introduction
Cellulose is the most abundant renewable biomass on earth (Klemm et al., 2005). In addition, the high price of oil and global warming have emphasized the importance of biofuels such as bioethanol (Farrell et al., 2006). Therefore, the production of biofuels from lignocellulosic resources has become an important area in the development of alternative energy sources (Lynd et al., 2008, Orts et al., 2008, Waltz, 2008). Recently, the selection of raw materials for biofuel production has become a critical issue due to the conflict between food and energy resources for fuel production (Cassman and Liska, 2007). Accordingly, non-food resources are now being considered as feedstocks for biofuel production. For example, the US Department of Energy set a goal to replace 30% of the current gasoline used in the US with cellulosic ethanol by 2030 (Sherwood, 2006). To achieve such an aggressive goal, significant improvement in the production cost of cellulosic biofuels is required.
Lignocellulose, which is a complex of cellulose, hemicelluloses and lignin, only renders approximately 20% of its theoretical glucose yield upon subjection to enzymatic hydrolysis due to its recalcitrance (Kim et al., 2006, Koullas et al., 1992). Therefore, lignocellulose needs to be pretreated to enable the cellulose to be more accessible to cellulolytic enzymes. Accordingly, many physicochemical pretreatment processes for lignocellulose have been evaluated in the last few decades, including a variety of physical and chemical pretreatments (Gharpuwy et al., 1983, Jeoh et al., 2007, Lee and Kim, 1983). These studies have led to the development of highly effective thermochemical processes, such as acid or alkaline pretreatment, which enable 80–90% of the theoretical enzyme digestibility of cellulase to be attained (Kim et al., 2002, Merino and Cherry, 2007, Zhao et al., 2008). However, these thermochemical pretreatment processes often result in the generation of byproducts such as furfural, hydroxymethylfurfural (HMF) and acetic acid, which significantly inhibit enzymatic hydrolysis and fermentation (Haagensen et al., 2008). Therefore, it is necessary to develop enzymes and microorganism that are resistant to such inhibitory substances or to employ additional steps to remove the inhibitors. However, both of these methods poses further cost burdens on the production of biofuel from lignocellulose. Therefore, pretreatment and enzymatic hydrolysis steps to achieve fermentable sugar are currently known to have much more room for reducing processing cost than other processes (Lynd et al., 2008).
The milling process, which is a physical pretreatment method known to increase the surface area of biomass, was studied during the early stages of lignccellulose pretreatment. However, the high energy consumption and low effectiveness of this method prevented its application (Gharpuwy et al., 1983, Lee and Kim, 1983). Alternatively, γ-ray (Beardmorel et al., 1980, Khan et al., 2006) and electron beam irradiations (Kamakura and Kaetsu, 1978, Khan, 1986) were considered as physical pretreatment processes. Because these methods do not involve the use of extreme temperatures, the generation of inhibitory substances produced during acid or alkali pretreatment can be either avoided or minimized. For example, when electron beam irradiation (EBI) was applied to lignocellulosic feedstocks such as rice straw, sawdust and softwood spruce (Kamakura and Kaetsu, 1978, Khan, 1986), it was found to increase the cellulase digestibility of the biomass (Khan, 1986). However, when compared to the vast amount of information available regarding thermochemical pretreatment, there is very little information available regarding the effectiveness and mechanisms involved in EBI pretreatment.
Therefore, this study was conducted to verify the effectiveness and feasibility of EBI as a lignocellulose biomass pretreatment method for the enhanced enzymatic hydrolysis of cellulose for biofuel production. To accomplish this, rice straw was pretreated with EBI, after which its impact was evaluated based on the enzymatic digestibility (with cellulase and β-glucosidase) and various physical characteristics of the pretreated rice straw.
Section snippets
Biomass feedstock
Rice straw harvested from Korea University Farm (Deokso, Korea) in 2006 was air-dried at ambient temperature. The dried rice straw was then milled using a cutting mill (MF 10, IKA, Staufen, Germany), sieved using two sieves with mesh sizes of 425 and 710 μm (Chung Gye Sang Gong Sa, Seoul, Korea), and then dried in a vacuum drying oven (SH-45S, BioFree, Seoul, Korea) at 45 °C for 5 days. The resulting solid content of the milled and dried rice straw measured at 105 °C was 96.6% (w/w). The rice straw
Effect of EBI current
After the rice straw samples were pretreated by EBI at various currents at fixed doses and strengths, they were hydrolyzed by the addition of cellulase at a concentration of 60 FPU per g of glucan for 120 h. Fig. 1 shows the yields of glucose from glucan (calculated based on% theoretical maximum as in Eq. (1) in samples that were taken at the specified reaction time intervals. As the hydrolysis reaction time increased, the accumulated glucose yield, which indicates the enzymatic digestibility of
Conclusion
Rice straw was irradiated to increase the enzyme digestibility by cellulase using an electron beam at a current of 0.03–0.24 mA, a dose of 7.6–90 kGy and 1–2 MeV. Enzymatic hydrolysis of untreated rice straw with 60 FPU of cellulase and 30 CBU of β-glucosidase, resulted in a yield of 5.1% and 22.6% of theoretical maximum after hydrolysis for 24 and 132 h, respectively. However, enzymatic hydrolysis of rice straw that was pretreated by EBI at 80 kGy at 0.12 mA and 1 MeV under the same conditions produced
Acknowledgements
This work was supported by the National Nuclear R&D Program (Grant No. M2 08B020028010) of MEST/KOSEF and by a Grant (20070301-034-013) from BioGreen 21 Program, Rural Development Administration, Republic of Korea.
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