Anuj K. Chandel, Om V. Singh, and L. Venkateswar Rao

Abstract Hemicellulose is the second most abundant polysaccharide in nature, after cellulose. As a substrate, it is readily available for the production of value-added products with industrial significance, such as ethanol, xylitol, and 2, 3-butanediol. Hemicellulose is a heterogeneous carbohydrate polymer with a xylose-linked backbone connecting to glucose, galactose, mannose, and sugar acids. In general, it represents about 35% of lignocellulosic biomass. It is estimated that the annual production of plant biomass in nature, of which over 90% is lignocellu — lose, amounts to about 200 x 109 tons per year, where about 8-20 x 109 tons of the primary biomass remains potentially accessible. Hemicellulose, which is generally 20-35% of lignocellulose amounts to nearly ~70 x 109 tons per year. Continuous efforts by researchers in the last two decades have led the way for the successful conversion of hemicellulose into fermentable constituents by developed candidate pretreatment technologies and engineered hemicellulase enzymes. A major chal­lenge is the isolation of microbes with the ability to ferment a broad range of sugars and withstand fermentative inhibitors that are usually present in hemicel — lulosic sugar syrup. This chapter aims to explore and review the potential sources of hemicellulose and their degradation into fermentable sugars, as well as advocating their conversion into value-added products like ethanol, xylitol, and 2, 3-butanediol.

Keywords Hemicellulose ■ Ethanol ■ Xylitol ■ 2, 3-Butanediol ■ Hydrolysis ■ Fermentation

1 Introduction

Biomass in the form of cellulose, hemicellulose, and lignin provides a means of collecting and storing solar energy, and hence represents an important energy and material resource [1-3]. After cellulose, hemicellulose is the principal fraction of the

L. V. Rao (B)

Department of Microbiology, Osmania University, Hyderabad-500 007 (A. P), India e-mail: vrlinga@gmail. com

O. V. Singh, S. P. Harvey (eds.), Sustainable Biotechnology,

DOI 10.1007/978-90-481-3295-9_4, © Springer Science+Business Media B. V. 2010 plant cell wall that could serve as a potential substrate for the production of value — added products under optimized conditions [4]. In general, the secondary cell walls of plants contain cellulose (40-80%), hemicellulose (10-40%), and lignin (5-25%). The arrangement of these components allows cellulose microfibrils to be embedded in lignin, much as steel rods are embedded in concrete to form reinforced concrete [5]. The composition of hemicellulosic fractions from different natural sources is summarized in Table 1.

The carbohydrate fraction of the plant cell wall can be converted into fermentable monomeric sugars through acidic and enzymatic (hemicellulase/cellulase) reactions, which have been exploited to produce ethanol, xylitol, and 2, 3-butanediol via microbial fermentation processes [1, 4, 12]. In the hemicellulosic fraction of the plant cell wall, xylan is the major backbone, linking compounds like arabinose, glucose, mannose, and other sugars through an acetyl chain [4]. They can be char­acterized as galactomannans, arabinoglucuronoxylans, or glucomannans based on their linkage with the main xylan backbone [13].

Thermal, chemical, and enzyme-mediated processes and combinations thereof are being explored in order to obtain monomeric components of hemicellulose with maximum yield and purity. The depolymerization of hemicellulose by chemical or enzyme-mediated processes yields xylose as the major fraction and arabinose, mannose, galactose, and glucose in smaller fractions [12]. This sugar syrup can be converted into ethanol; xylitol; 2, 3-butanediol (2, 3-BD); and other compounds [4]. The use of hemicellulose sugar as a primary substrate for the production of multiple compounds of industrial significance is summarized in Fig. 1.

A wide variety of microorganisms are required for the production of metabo­lites from hemicellulosic-derived sugar syrup. The ability to ferment pentoses is not widespread among microorganisms and the process is not yet well-established in

Lignocellul

Fig. 1 Mechanistic steps involved in hemicellulose bioconversion into ethanol, xylitol and 2, 3-butanediol industry. However, several yeast species have the basic ability to carry out these processes, i. e., Candida shehatae, Pichia stipitis, and Pachysolen tannophilus for ethanol production; C. utilis, C. intermedia, and C. gulliermondii for xylitol pro­duction; and Klebsiella oxytoca ATCC 8724, Bacillus subtilis (Ford strain), and Aeromonas hydrophilia for 2,3-butanediol production [4]. This chapter presents sig­nificant advancements in hemicellulose biotechnology, with an emphasis on acidic and enzymatic hydrolysis and the conversion of hemicellulose hydrolysates into commercial products like ethanol, xylitol, and 2, 3-BD.