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UGA BCMB 8020 - Saha2003

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REVIEW PAPERHemicellulose bioconversionReceived: 17 December 2002 / Accepted: 17 February 2003 / Published online: 16 April 2003Ó Society for Industrial Microbiology 2003Abstract Various agricultural residues, such as corn fi-ber, corn stover, wheat straw, rice straw, and sugarcanebagasse, contain about 20–40% hemicellulose, the sec-ond most abundant polysaccharide in nature. The con-version of hemicellulose to fuels and chemicals isproblematic. In this paper, various pretreatment optionsas well as enzymatic saccharification of lignocellulosicbiomass to fermentable sugars is reviewed. Our researchdealing wi th the pretrea tment and enzymatic sacc hari-fication of corn fiber and development of novel andimproved enzymes such as endo-xylanase, b-xylosidase,and a-L-arabinofuranosidase for hemicellulose biocon-version is described. The barriers, progress, and pros-pects of developing an environmentally benignbioprocess for large-scale conversion of hemicellulose tofuel ethanol, xylitol, 2,3-butanediol, and other value-added fermentation products are highlighted.Keywords Hemicellulose Æ Arabinoxylan ÆBioconversion Æ Hemicellulase Æ Xylanolytic enzymesIntroductionHemicelluloses, the second most common polysaccha-rides in nature, represent about 20–35% of lignocellu-losic biomass. Xylans are the most abundanthemicelluloses. In recent years, bioconversion of hemi-cellulose has received much attention because of itspractical applications in various agro-industrial pro-cesses, such as efficient conversion of hemicellulosicbiomass to fuels and chemicals, delignification of paperpulp, digestibility enhancement of animal feedstock,clarification of juices, and improvement in the consis-tency of beer [134, 139, 144]. Enzymes that degrade, orhelp to degrad e, hemicellulose are of great interest to thepaper and pulp industry due to their bleach-boostingproperties (biobleaching of pulp), which reduces envi-ronmentally unfriendly chlorine consum ption [91, 135].Cellulase-free xylanase can facilitate lignin removal frompaper pulp without any harmful effect. The utilization ofhemicellulosic sugars is essential for efficient conversionof lignocellulosic materials to fuel ethanol and othervalue-added fermentation products. Xylan-degradingenzymes hold great promise in saccharifying variouspretreated agricultural and forestry residues to fer-mentable sugars. Other potential applications of hemi-cellulases include biopulping of wood, coffee processing,fruit and vegetable maceration, and preparation of highfiber baked goods [19]. In addition, xylan-degradingenzymes play a great role in elucidating the structures ofcomplex xylans. In this article, a brief review on thebioconversion of hemicellulose—particularly arabin-oxylans present in various agricultural residues—to fuelethanol, xylitol and 2,3-butanediol, is presented.Structure of hemicelluloseHemicelluloses are heterogeneous polymers of pentoses(xylose, arabinose), hexoses (mannose, glucose, galac-tose), and sugar acids. Unlike cellulose, hemicellulosesare not chemically homogeneo us. Hardwood hemicel-luloses contain mostly xylans, whereas softwood he-micelluloses contain mostly glucomannans [84]. Xylansof many plant materials are heteropolysaccharides withhomopolymeric backbone chains of 1,4-linked b-D-xylopyranose units. Besides xylose, xylans may containarabinose, glucuronic acid or its 4-O-methyl ether, andacetic, ferulic, and p-coumaric acids. The frequency andcomposition of branches are dependent on the sourceof xylan [1]. The backbone consists of O-acetyl, a-L-J Ind Microbiol Biotechnol (2003) 30: 279–291DOI 10.1007/s10295-003-0049-xBadal C. SahaB. C. Saha (&)Fermentation Biotechnology Research Unit, National Center forAgricultural Utilization Research, Agricultural Research Service,U. S. Department of Agriculture, 1815 North University Street,Peoria, IL 61604, USAE-mail: [email protected].: +1-309-6816276Fax: +1-309-6816427arabinofuranosyl, a-1,2-linked glucuronic or 4-O-methylglucuronic acid substituents. However, unsub-stituted linear xylans have also been isolated from guarseed husk, esparto grass, and tobacco stalks [35]. Xy-lans can thus be categorized as linear homoxylan,arabinoxylan, glucuronoxylan, and glucuronoarabin-oxylan.Xylans from different sources, such as grasses, cere-als, softwood, and hardwood, differ 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, and1.1% anhydrou ronic acid [126]. Wheat arabinoxylancontains 65.8% xylose, 33.5% arabinose, 0.1% man-nose, 0.1% galactose, and 0.3% glucose [51]. Corn fiberxylan is one of the complex heteroxylans containing b-(1,4)-linked xylose residues [117]. It contains 48–54%xylose, 33–35% arabinose, 5–11% galacto se, and 3–6%glucuronic acid [31]. About 80% of the xylan backboneis highly substituted with monomeric side-chains ofarabinose or glucuronic acid linked to O-2 and/or O-3 ofxylose residues, and also by oligomeric side chainscontaining arabinose, xylose, and sometimes galactoseresidues (Fig. 1) [122]. A model for the corn fiber cellwall is shown in Fig. 2 [121]. The heteroxylans, whichare highly cross-linked by diferulic bridges, constitute anetwork in which the cellulose microfibrils may beimbedded. Structural wall proteins might be cross-linkedtogether by isodityrosine bridges and with feruloylatedheteroxylans, thus forming an insoluble network [60]. Insoftwood heteroxylans, arabinofuranosyl residues areesterified with p-coumaric acids and ferulic acids [88]. Inhardwood xylans, 60–70% of the xylose residues areacetylated [131]. The degree of polymerization of hard-wood xylans (150–200) is higher than that of softwoods(70–130).Pretreatment of hemicelluloseLignocellulosic biomass includes various agriculturalresidues (straws, hulls, stems, stalks), deciduous andconiferous woods, municipal solid wastes, waste fromthe pulp and paper industry, and herbaceous energycrops. The compositions of these materials vary. Themajor component is cellulose (35–50%), followed byhemicellulose (20–35%) and lignin (10–25%). Table 1gives the composition of some lignocellulosics. Pro-teins, oils, and ash make up the remaining fraction oflignocellulosic biomass [140]. The structure of thesematerials is very complex, and native biomass is gen-erally resistant to an enzymatic hydrolysis .


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