Sugar yields from sunflower stalks treated by hydrothermolysis and subsequent enzymatic hydrolysis
Introduction
Lignocellulosic biomass is considered one of the major sustainable resources for producing biofuels and value-added chemicals as an alternative to dwindling fossil fuels, and as a part of a carbon dioxide-neutral industry. Cellulose, one of the main components of lignocellulosic biomass, is a partly crystalline polymer that can be hydrolyzed to glucose, thereby becoming a major carbon source for industrial microbes for fermenting liquid biofuels and a variety of bio-based chemicals. However, cellulose is closely merged and bonded chemically with hemicelluloses and lignin (Wyman, 1996). Therefore, for cellulose to be susceptible to enzymatic hydrolysis, lignocellulosic biomass must be pretreated either thermochemically or biologically.
Pretreatment processes enhance the susceptibility of cellulose-containing materials to hydrolytic enzymes by removing lignin or hemicelluloses, and/or altering the crystallinity of the cellulose (Hendriks and Zeeman, 2009). Among pretreatment methods widely studied to date, hydrothermal treatment using steam or liquid hot water has been shown to be effective in solubilizing hemicelluloses, even in the absence of additives (Wyman et al., 2011). In the hydrothermal treatment of lignocellulosic biomass, hemicellulosic sugars can be extracted at lower temperature (170–180 °C) to avoid their further degradation. Afterward, the remaining solids may be treated at higher temperature (190–200 °C) to increase the enzymatic convertibility of cellulose to glucose (Thomsen et al., 2009). A one-step hydrothermolysis at temperatures between 180 °C and 200 °C, as an alternative simplified process, shows higher enzymatic cellulose convertibility than at temperatures below 180 °C. However, further degradation of xylose to 2-furfural, a potent fermentation inhibitor, is unavoidable.
Many hydrolytic enzyme inhibitors are released or produced during the pretreatment of biomass and the subsequent enzymatic hydrolysis. These inhibitors include hemicellulosic sugars containing xylose, galactose, mannose, and arabinose as a feedback inhibitor of hydrolytic enzyme (Xiao et al., 2004), xylooligosaccharides, water-soluble lignins, lignin-degradation products (Kim et al., 2006, Zhu et al., 2006, Kumar and Wyman, 2009a), and inorganic salts (Bin and Hongzhang, 2010).
Washing the pretreated solids with hot water after solid–liquid separation of pretreatment residue is generally carried out for improving the enzymatic digestibility of the substrates (Ruiz et al., 2008, Caparros et al., 2008, Merino and Cherry, 2007). In practical application, however, additional processes are required for solid–liquid separation, washing with hot water, and wastewater treatment. To date, many trials have been carried out to enhance the susceptibility of pretreatment residue to cellulase enzyme, including the addition of organic solvents in the pretreatment (Araque et al., 2008), surfactants (Qing et al., 2010a), polyethylene glycol (Borjesson et al., 2007), and protein (Kumar and Wyman, 2009b) in enzymatic hydrolysis.
Sunflower is the fourth largest oil-seed source worldwide, with more than 25 million hectares being cultivated (Diaz et al., 2011). After seed harvesting, sunflower stalks can be considered a raw material for paper pulp production (Caparros et al., 2008) and bioethanol production (Vaithanomsat et al., 2009). Glucose yields by acid-catalyzed pretreatment and enzymatic hydrolysis of sunflower stalks have varied from 72% to 90% (Diaz et al., 2011, Sharma et al., 2002, Ruiz et al., 2008, Vaithanomsat et al., 2009).
The overall objective of this study is to investigate the variables of hydrothermal pretreatment and the enzymatic hydrolysis of sunflower stalks in order to increase sugar yields via a one-step hydrothermal treatment at the optimum temperature for maximizing the yield of hemicellulosic sugars, with minimal formation of potent microbial inhibitors such as 2-furfural and 5-hydroxymethyl-2-furaldehyde (HMF).
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Materials
Sunflower stalks as a feedstock were generously provided by Chungbuk Agricultural Research and Extension Services (Ochang, Chungcheongbuk-Do, Korea). They had been planted in June 2009 and harvested in October of that year. The feedstock was dried under sunlight in a greenhouse, and milled to smaller than 20 mesh with a knife mill and a sieve. One part of the feedstock was stored at room temperature for daily experiments and the other at −20 °C. The compositions of the sunflower stalks were
Composition of feedstock
The composition of the sunflower stalks is summarized in Table 1. The water extractives content was over 20% (w/w), but 0.7% (w/w) glucose was detected upon the acid hydrolysis of water extractives with sulfuric acid. The cellulose content as glucose (35.8 g/100 g dry biomass) was similar to that reported by Diaz et al. (2011).
Hydrothermal treatment of sunflower stalks and the effect of washing the pretreated residue on the sugar yield
All the densities of Cellic CTec2 and Cellic HTec2 were 1.20 g/ml, and the cellulase activities of Cellic CTec2, Cellic HTec2, and their mixture (9 + 1, v/v) were 87 FPU/ml, 23
Conclusions
The one-step hydrothermal treatment of sunflower stalks at the pretreatment temperature that maximizes the hemicellulosic sugar yield is a very promising pretreatment method for converting raw feedstock to a substrate susceptible to hydrolytic enzymes. However, to increase the glucose yield by enzymatic hydrolysis, the partial removal of the LF from the pretreated slurry, which thus eliminates most of the enzyme inhibitors, is mandatory.
Acknowledgements
The authors wish to thank Dr. Sang-Young Nam of Chungbuk Agricultural Research and Extension Services (South Korea) for providing the sunflower stalks.
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