Jing Dai and Armando G. McDonald

Abstract In this study, the target product was the generation of sugars from woody biomass that can be the substrate for conversion into value-added chemicals, such as polyhydroxyalkanoates, lactic acid, succinic acid, etc. In order to release sugars from wood economically, wood needs to be pretreated to enhance the enzymatic hydroly­sis of cellulose and hemicelluloses. The primary goal of this study was to determine the optimal condition to obtain fermentable monosaccharides from hydrolysates of hybrid poplar by a hot-water pretreatment (150-210 °C, 0-30 min). The pretreat­ment conditions were optimized using a response surface methodology (RSM) on a 23 full central composites design was performed by varying on temperature, reac­tion time, and solid loading. After pretreatment, the solid residue was subsequently treated with a cellulase preparation, and released sugars were quantified by HPLC. The total sugar yield was applied as response variable to the RSM. The optimal pretreatment condition for producing sugars was 200 °C, 18 min, and 20 % solid loading.

Keywords Hybrid poplar ■ Hot-water pretreatment ■ Enzymatic hydrolysis ■ Sugars ■ Response surface methodology

9.1 Introduction

Lignocellulosic biomass includes a wide range of carbon-rich resources, which can be utilized as feedstock for production of many industrial products ranging from lumber, paper, chemicals, biofuels, and value-added biodegradable polymers [1]. The technique for conversion of lignocellulosic biomass to fuel ethanol has been well developed. Cellulosic ethanol, however, has economic barriers to overcome to be economically competitive [2, 3]. Therefore, upgrading the conversion of cel — lulosic biomass to higher value products such as polyhydroxyalkanoates (PHA)

A. G. McDonald (H) ■ J. Dai

Renewable Materials Program, Department of Forest, Rangeland and Fire Sciences, University of Idaho, Moscow, Idaho 83844-1132, USA e-mail: armandm@uidaho. edu

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_9, © Springer-Verlag Berlin Heidelberg 2013 would gain better commercial value compared to cellulosic ethanol. The cost of the carbon substrate reportedly contributes more than 40 % of the production cost of PHA [4-6]. The use of inexpensive renewable agricultural materials such as woody biomass as feed stocks could be a tremendous advantage to the economics of PHAs production.

Hybrid poplar, as a short rotation fast growing wood species with low lignin content, has been highlighted as a good biomass resource for fuel and chemi­cal production [7, 8]. Xylose is the main constituent of hardwood hemicellulose (acetyl-4-omethylglucuronoxylan). Recently, studies showed that xylose can be obtained via a pretreatment process using dilute sulfuric acid (H2SO4) [9, 10]. Cellulose in wood is present as a semi-crystalline polymer and is comprised of glucose building blocks linked by P-o-4 linkages which can be cleaved by acids or enzymes. The major proportion of cellulose exists in the crystalline form. How­ever, cellulose is more susceptible to degradation in its amorphous form [11]. Thus, breaking down cellulose crystalline structure to make it more accessible to cellulase enzymes usually requires pretreatment with heat, long reaction time, and addition of catalysts [8]. Lignin as a bonding component in wood is an in­hibitor for hydrolysis or further fermentation process [11]. A pretreatment process can not only depolymerize lignin structure, but also remove some lignin in wood [12, 13]. Enzymatic hydrolysis is the most common method for converting woody biomass to sugars. Compared with acid hydrolysis, enzymatic hydrolysis yields no fermentation inhibitors such as furfural and it does not need neutralization and detoxification [2]. The only disadvantage of enzymatic hydrolysis is longer reaction times required for releasing the sugars. However, enzymatic hydrolysis is a better choice if further fermentation or bioconversions are required to produce value-added chemicals.

Hot-water pretreatment with controlled pH has been shown to improve enzymatic digestibility of lignocellulosic biomass [2,14]. Acetic acid and other organic acids are released from the hemicelluloses, which help autocatalyze hemicellulose hydrolysis and disrupt cellulose and lignin structure. Unlike the anaerobic fermentation for cellulosic ethanol production, organic acids, such as acetic acid, are not considered as inhibitors during PHA biosynthesis but used as carbon source for PHA production

[6] . The pH of the pretreatment liquor needs to be between 4 and 7 to minimize decomposition of sugars [2]. For the purpose of scale-up or industrial production, determining the optimal pretreatment conditions by using statistical approach is important. The experimental design works for variety of species, chemical reagents, temperature, and reactor features.

The aim of this study was to find optimal conditions to obtain total sugars (mainly glucose and xylose) by enzymatic hydrolysis via a hot-water pretreat­ment. A response surface methodology (RSM) was chosen to determine the optimal pretreatment conditions for sugar concentrations in enzymatic hydrolysates. Re­action time, temperature, and solid loading were the three variables tested in this design.