Sugar yields from sunflower stalks treated by hydrothermolysis and subsequent enzymatic hydrolysis

Highlights

• Sunflower stalks was subjected to batch-type hydrothermal treatments.

• Glucose yield reached 52–90% of theoretical yield by enzymatic hydrolysis.

• Removal of liquid fraction from pretreatment slurry remarkably increased the glucose yield.

• Hemicellulosic sugars may be the strongest enzyme inhibitor followed by xylooligomers.

Abstract

To produce fermentable sugar with fewer microbial inhibitors via few processes, batch-type hydrothermal treatments of sunflower stalks were performed, followed by enzymatic hydrolysis of pretreatment slurries with Cellic CTec2 and Cellic HTec2 (9:1, v/v, 0.1 ml/g dry biomass, 8.1 FPU). The yields of hemicellulosic sugars were maximized at the pretreatment condition of 180 °C for 30 min, while the furfural and 5-hydroxymethyl-2-furaldehyde (HMF) concentrations remained low. The glucose yield, however, was only 67.0% of the theoretical glucose yield (TGY). Either the treatment of raw biomass prior to pretreatment or the post-treatment of pretreated residue prior to enzymatic hydrolysis increased the glucose yield as follows: washing the pretreated solid with solvents (90% TGY) > partial removal of liquid fraction from the pretreatment slurry (PS, 83%) > removal of hot-water extractives from the biomass prior to pretreatment (77%) > prewetting of the biomass (70%).

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).