J Ind Microbiol Biotechnol 2003 30 279 291 DOI 10 1007 s10295 003 0049 x R EV IE W P A PE R Badal C Saha Hemicellulose bioconversion Received 17 December 2002 Accepted 17 February 2003 Published online 16 April 2003 Society for Industrial Microbiology 2003 Abstract Various agricultural residues such as corn ber corn stover wheat straw rice straw and sugarcane bagasse contain about 20 40 hemicellulose the second most abundant polysaccharide in nature The conversion of hemicellulose to fuels and chemicals is problematic In this paper various pretreatment options as well as enzymatic sacchari cation of lignocellulosic biomass to fermentable sugars is reviewed Our research dealing with the pretreatment and enzymatic sacchari cation of corn ber and development of novel and improved enzymes such as endo xylanase b xylosidase and a L arabinofuranosidase for hemicellulose bioconversion is described The barriers progress and prospects of developing an environmentally benign bioprocess for large scale conversion of hemicellulose to fuel ethanol xylitol 2 3 butanediol and other valueadded fermentation products are highlighted Keywords Hemicellulose Arabinoxylan Bioconversion Hemicellulase Xylanolytic enzymes Introduction Hemicelluloses the second most common polysaccharides in nature represent about 20 35 of lignocellulosic biomass Xylans are the most abundant hemicelluloses In recent years bioconversion of hemicellulose has received much attention because of its practical applications in various agro industrial processes such as e cient conversion of hemicellulosic B C Saha Fermentation Biotechnology Research Unit National Center for Agricultural Utilization Research Agricultural Research Service U S Department of Agriculture 1815 North University Street Peoria IL 61604 USA E mail sahabc ncaur usda gov Tel 1 309 6816276 Fax 1 309 6816427 biomass to fuels and chemicals deligni cation of paper pulp digestibility enhancement of animal feedstock clari cation of juices and improvement in the consistency of beer 134 139 144 Enzymes that degrade or help to degrade hemicellulose are of great interest to the paper and pulp industry due to their bleach boosting properties biobleaching of pulp which reduces environmentally unfriendly chlorine consumption 91 135 Cellulase free xylanase can facilitate lignin removal from paper pulp without any harmful e ect The utilization of hemicellulosic sugars is essential for e cient conversion of lignocellulosic materials to fuel ethanol and other value added fermentation products Xylan degrading enzymes hold great promise in saccharifying various pretreated agricultural and forestry residues to fermentable sugars Other potential applications of hemicellulases include biopulping of wood co ee processing fruit and vegetable maceration and preparation of high ber baked goods 19 In addition xylan degrading enzymes play a great role in elucidating the structures of complex xylans In this article a brief review on the bioconversion of hemicellulose particularly arabinoxylans present in various agricultural residues to fuel ethanol xylitol and 2 3 butanediol is presented Structure of hemicellulose Hemicelluloses are heterogeneous polymers of pentoses xylose arabinose hexoses mannose glucose galactose and sugar acids Unlike cellulose hemicelluloses are not chemically homogeneous Hardwood hemicelluloses contain mostly xylans whereas softwood hemicelluloses contain mostly glucomannans 84 Xylans of many plant materials are heteropolysaccharides with homopolymeric backbone chains of 1 4 linked b Dxylopyranose units Besides xylose xylans may contain arabinose glucuronic acid or its 4 O methyl ether and acetic ferulic and p coumaric acids The frequency and composition of branches are dependent on the source of xylan 1 The backbone consists of O acetyl a L 280 arabinofuranosyl a 1 2 linked glucuronic or 4 Omethylglucuronic acid substituents However unsubstituted linear xylans have also been isolated from guar seed husk esparto grass and tobacco stalks 35 Xylans can thus be categorized as linear homoxylan arabinoxylan glucuronoxylan and glucuronoarabinoxylan Xylans from di erent sources such as grasses cereals softwood and hardwood di er in composition Birch wood Roth xylan contains 89 3 xylose 1 arabinose 1 4 glucose and 8 3 anhydrouronic acid 68 Rice bran neutral xylan contains 46 xylose 44 9 arabinose 6 1 galactose 1 9 glucose and 1 1 anhydrouronic acid 126 Wheat arabinoxylan contains 65 8 xylose 33 5 arabinose 0 1 mannose 0 1 galactose and 0 3 glucose 51 Corn ber xylan is one of the complex heteroxylans containing b 1 4 linked xylose residues 117 It contains 48 54 xylose 33 35 arabinose 5 11 galactose and 3 6 glucuronic acid 31 About 80 of the xylan backbone is highly substituted with monomeric side chains of arabinose or glucuronic acid linked to O 2 and or O 3 of xylose residues and also by oligomeric side chains containing arabinose xylose and sometimes galactose residues Fig 1 122 A model for the corn ber cell wall is shown in Fig 2 121 The heteroxylans which are highly cross linked by diferulic bridges constitute a network in which the cellulose micro brils may be imbedded Structural wall proteins might be cross linked together by isodityrosine bridges and with feruloylated heteroxylans thus forming an insoluble network 60 In softwood heteroxylans arabinofuranosyl residues are esteri ed with p coumaric acids and ferulic acids 88 In hardwood xylans 60 70 of the xylose residues are acetylated 131 The degree of polymerization of hardwood xylans 150 200 is higher than that of softwoods 70 130 Fig 1 Schematic structure of corn ber heteroxylan Reprinted from 122 with permission from Elsevier Amsterdam Fig 2 Model for corn ber cell walls Reprinted from 121 with permission from John Wiley Sons on behalf of SCI Pretreatment of hemicellulose Lignocellulosic biomass includes various agricultural residues straws hulls stems stalks deciduous and coniferous woods municipal solid wastes waste from the pulp and paper industry and herbaceous energy crops The compositions of these materials vary The major component is cellulose 35 50 followed by hemicellulose 20 35 and lignin 10 25 Table 1 gives the composition of some lignocellulosics Proteins oils and ash make up the remaining fraction of lignocellulosic biomass 140 The structure of these materials is very complex and native biomass is generally resistant to an enzymatic hydrolysis In the current model of the structure of