Category Archives: Biotechnological Applications of Hemicellulosic Derived Sugars: State-of-the-Art

Expert Commentary and Five Year View

The Tactical Garbage to Energy Refinery (TGER) is a trailerable, skid-mounted device capable of converting waste products (paper, plastic, packaging and food waste) into electricity via a standard 60 kW diesel generator. Additionally, the sys­tem can utilize available local biomass as a feedstock. Waste materials are converted into bio-energetics which displaces the diesel fuel used to power the generator set. The system also co-produces excess thermal energy which can be further utilized via a “plug and play” heat exchanger to drive field sanitation, shower, laundry or cooling devices. With additional engineering, the TGER could include a small sub­system to recover water introduced with the wet waste and produce potable water to further reduce logistics overhead. The system requires a small “laundry packet” of enzymes, yeasts and industrial antibiotics to support the biocatalytic subsystem.

Feed materials (daily)-01 Aug 08

Garbage (gallons)

90 20% paper, 50% cardboard, 30% plastic

Garbage (lbs)


Food (gallons)


Diesel (gallons)


Energy content of feed

Heats of




comustion (btu/lb) LHV

Total energy (BTU)

Total energy (kWhr)












Plastic-polyethylene terephthalate










Diesel (DF2)




Total 5437352 1593.597

Electrical energy production

Total (kWh) 267.5

Offboard (kWh) 221.2

Total thermal-to-electrical energy conversion efficiency (% of energy content of feed) 16.8%

Offboard energy conversion efficiency (% of thermal energy content of feed)


Diesel fuel savings (gallons)


Energy delivery efficiency (% of electrical energy for offboard use)


% Contribution to feed energy Diesel 7.08%

Biofuels 92.92%

The residuals from waste conversion are environmentally benign including simple ash, which can be added to improve soil for agriculture, and carbon dioxide.

The TGER will deploy on a XM 1048 5-ton trailer and is designed to sup­port a 550 man Force Provider Unit (FPU), which produces approximately 2,200 pounds of waste daily. On a daily operational basis, this would conserve approx­imately 100 gal of diesel. The capability for such conversion would provide immediate and responsive energy requirements for expeditionary operations as well as yielding estimated cost savings of $2,905/day [10]. A projected fielding plan for the TGER involves identification of current Modified Table of Organization and Equipment (MTO&E) trailers associated with FPU kitchen support which would then be modified to include the waste conversion technology. This would avoid any changes to the MTO&E or prime mover designation. Estimations indicate that the additional tasks associated with maintenance support for the operator and mechanic would not exceed those standards for the assigned Military Occupational Specialty and Generator Mechanic. Higher order support may follow a Contractor Logistics Support or low density support plan similar to that for the reverse osmosis purification unit equipment.

Anticipated field employment of the system is such that the TGER would be pulled by the assigned 5-ton family of medium tactical vehicles assigned to accom­pany the FPU Containerized Kitchen. Upon occupation of the FPU site, the TGER would start up initially on diesel fuel alone. This would provide immediate power to the kitchen and begin to heat up/power the system components. As waste is devel­oped from the kitchen, it will be introduced to the TGER and the two energetic materials (synthetic gas and ethanol) will begin to displace the diesel fuel. By six to twelve hours (depending on the waste stream), the TGER will run on 98% waste energetics and is capable of running for 12 h with a one hour maintenance shut-down intervening.

Improvements for future models revolve around three subsystems: the gasifier, bioreactor and materials handling. The current downdraft gasifier equipment is too complicated and unreliable under desert conditions. However, modifications to the current design could reduce the complexity of the system and, with a thorough inspection, repair and evaluation by the manufacturer, we believe a number of alter­ations to the downdraft gasifier would mitigate its reliability problems. Ultimately, it would be advantageous to consider alternative thermo-chemical approaches.

The issues with the bioreactor are much less complex and more easily addressed, as the system was custom built by Purdue University and several supporting subcontractors. Repairing and upgrading this system will primarily involve replac­ing and upgrading the two heat exchangers, modifying the system software to accommodate the changed thermo-dynamics and thermal management, and adjust­ing the “plumbing” of the ethanol collection and delivery system.

During the intervening 18 months since the TGER fabrication, the commercial field of biomass fuel processing has greatly expanded. There are a number of new options for third party equipment such as improved shredders, pelletizers and pellet drying systems which did not exist previously.

2 Conclusion

Throughout the course of the 15 month program the TGER underwent testing in a variety of conditions and environments. Performance characteristics of the TGER varied in each environment and provided valuable information as to how to improve

Diesel Ethanol Solid waste Liquid

consump- consump — Ethanol processing waste Total waste

Power Power tion tion production rate (pellet processing processing Diesel

output efficiency rate rate rate production) rate rate Savings

54 kW 90%

1 gph 1 gph

1 gph

60 lb/h

13 lb/h

1,752lb/day3.6 gph

Table 6

Power vs. Fuel Consumption Table Recorded at Purdue University

Power Idle

25 kW

35 kW

45 kW

55 kW




1.3 gph

1.0 gph

1.2 gph

1.0 gph

Fuel gas

0 scmh

57 scmh

65 scmh

60 scmh

65 scmh


0 gph

0 gph

0 gph

0.5 gph

1 gph

Table 7 TGER performance data set recorded at VBC

Average TGER performance data at victory base camp

Solid waste processing

Power Diesel Pellet (pellet Liquid waste Total waste

efficiency consumption consumption production) processing processing Diesel saved

~80% 2 gal/h** 60 lb/h 54 lb/h 13 lb/h 1,752 lb/day 2.6lb/h the overall design of the TGER in order to achieve what we believe to be the optimal theoretical performance characteristics shown in Table 5.

Prior to the deployment to Victory Base Camp, the TGER underwent testing in a controlled environment at Purdue University. The fuel consumption of all three fuels (syngas, ethanol and diesel) was measured at varying loads using digital flow rate sensors as seen in Table 6.

Although the TGER did not perform as well in Iraq as it had when in a con­trolled environment at Purdue University, it did demonstrate the ability to conserve fuel and remediate waste in a forward deployed operational environment. Table 7 shows the TGER’s performance characteristics when it was running under optimal conditions at Victory Base Camp. With improved engineering and further devel­opment all of these performance characteristics can be improved, maximizing the TGER’s potential as a viable portable power generation system.

Acknowledgements The authors gratefully acknowledge the funding support of the US Army’s Rapid Equipping Force, the Small Business Technology Transfer Research program and the Research Development and Engineering Command. We also thank the many forward deployed personnel at Victory Base Camp, Iraq for their support on the ground. Special thanks to Ms. Donna

Hoffman for pre-deployment and deployment support and for extensive editing and preparation of the manuscript.

Limitations in Fermentative H2 Production

In spite of striking advantages, the main challenge encountered with fermentative H2 production processes are low substrate conversion efficiency and residual substrate present in acid-rich wastewater generated from the acidogenic process. Anaerobic bacteria have a theoretical maximum yield of 4 mol H2/mole glucose [3]. In prac­tice, yields are lower, as the NADH oxidation by NFOR is inhibited under standard conditions and only proceeds at very low partial pressures of H2 [11]. Up to 4 moles of H2 can theoretically be produced per mole of glucose through the known fermentative pathways [109]. However, various biological limitations such as H2- end-product inhibition and waste-acid and solvent accumulation limit the molar yield to around 2 moles per mole glucose consumed. Typical H2 yields range from 1 to 2 mol H2/mol glucose and result in 80-90% of the initial carbon remaining in the wastewater [7, 23, 25, 51, 109, 76, 110, 111]. Even under optimum conditions about 60-70% of the original organic matter remains as residue in the wastew­ater. Also a maximum yield of 4 mol H2/mole glucose is still low for practical applications [3].

The generation and accumulation of soluble acid metabolites causes a sharp drop in the system pH and inhibits H2 production. H2 yield is lower when more reduced organic compounds, such as lactic acid, propionic acid, and ethanol, are produced as fermentation products, because these represent end products of metabolic path­ways that bypass the major H2-producing reaction [11]. The undissociated soluble metabolites can permeate the cell membrane of H2-producing bacteria and then dissociate in the cell leading to physiological balance disruption [91]. Thus, some maintenance energy should be used to restore the physiological balance in the cell, which reduces the energy used for bacteria growth and inhibit the bacterial growth on the other hand. If the dissociated soluble metabolites is present in the system at a high concentration, the ionic strength will increase, which may result in cell lysis [91]. High concentrations of soluble metabolites can inhibit H2-producing bacterial growth thereby reducing H2 production [91, 78, 112]. The fermentation metabolic end-products and the resultant H2 yields vary based on the environmental conditions even within the same bacterium [3, 86].

H2 production is limited by the thermodynamics of the hydrogenase reaction, which involves the enzyme-catalyzed transfer of e — from an intracellular electron carrier molecule to H+ [11]. The partial pressure of H2 is one of the important fac­tors, as the pressure increases, H2 production decreases [7]. H2 production becomes thermodynamically unfavourable at H2 partial pressures greater than 60 Pa [11].

Operating bioreactors at low H2 partial pressure by stripping H2 from the solu­tion is as it is generated [57, 102], accomplishes both efforts simultaneously [11]. Conceptually, efforts are to be made in optimizing operational conditions to prevent consumption of H2 by propionic acid-producing bacteria, ethanol-producing bacte­ria and homoacetogens and those that channel more reducing equivalents towards reduction of H+ by hydrogenases to maximize H2 production [11]. The physiolog­ical and physicochemical conditions under which the microorganisms give optimal H2 yields is important and needs to be established. Optimization of process param­eters is one of the vital steps as to enhance H2 yield as well as to enhance substrate degradation efficiency and assumes significance prior to up-scaling the process.

Background Research

1.1 Natural Resource Limitation and Economic Security

Although the potential adverse environmental effects of CO2 emission is a major factor pressuring governments to steer their energy policy away from fossil fuels, the global decline of fossil fuel reserves is also a major driver for public and private organizations around the world to develop technologies to use renewable energy sources. Various estimates exist for the current proved reserves (Rp), and the Rp:consumption ratio (Rp:c), with units of years. For example, the global Rp:c of coal, oil, and natural gas have been estimated as 140, 40, and 60 [3, 13]. Using the widely-recognized global energy database provided in the British Petroleum (BP) energy report [14], we calculated Rp:c for coal, oil and natural gas as 133, 35, and 60, respectively. For coal and natural gas, the Rp:c value is similar to the previ­ously published estimates and indicates that issues may arise later in this century. However, for oil, our Rp:c value of 35 (years) is even less than that published pre­viously, indicating a serious situation with near-term pressure building to replace oil reserves either with new discoveries (perhaps some, but unlikely to be major) or with new alternatives (biofuels can play a role).

The earliest fuel ethanol production from lignocellulose biomass began in Germany, in 1920s [15], using sulfuric acid to hydrolyze wood. The ethanol yield was low at approximately 75-130 L (20-34 gallons) of ethanol per ton of wood hydrolyzed. From 1945 to1960s, several acid-hydrolysis ethanol plants were built in Europe, the USA, and the former Soviet Union. The capacities of these plants ranged from 10,000 to 45,000 tons of wood materials a year. Ethanol yield reached 190-200 L (50-53 gallons) per ton of wood. Subsequently, almost all of these wood — based ethanol plants were closed due to competition from the rapid development of the petroleum industry and relatively inexpensive crude oil feedstock.

The first gasification of biomass can be dated back to the 1800s, when wood was gasified to generate “town gas” for lighting and cooking. Although there are around 140 large gasification facilities in operation around the world today [16], these gasifiers are basically used to generate heat and/or electricity from coal (55% of total 140 large gasification facilities), oil, or natural gas, with a few plants using residues from the wood/pulp industry. The current main products generated from gasifier syngas are power (18%), chemicals (44%), and FT fuel (38%) [16]. To — date, there are no commercial scale gasification or pyrolysis facilities dedicated for biofuels production from lignocellulosic biomass. However, many research units have been built to investigate the mechanism, kinetics, and economical feasibility of biofuel production via syngas from biomass gasification.

Drivers for Commerciallmplementation of AD

The drivers that stimulate commercial interest to implement AD include a complex set of economic, business, energy, environmental, and sociopolitical factors that are interactive and may be weighted differently for each AD implementation opportu­nity. The economic and business drivers relate to those that directly contribute to the profitability of an AD project through the rate of return on the investment. These drivers include (1) the revenues that can be realized by the production of biogas and other byproducts (e. g., fertilizer), (2) the cost savings derived from reduced waste disposal, (3) governmental credits (e. g., renewable energy credits, environmental credits, and carbon credits) that are earned by implementation of a AD project, and (4) potential business growth that results from overcoming the limitations posed by storage and disposal of the wastes generated from core business operations. Firstly, earned revenue from an AD project can be gained from sale of the bio­gas as fuel or energy produced therefrom. Additional revenue can be generated by receiving wastes from other factories or farms. A spillover benefit of such “service” is enhanced AD efficiency and process stability resulting from co-digestion of two or more types of biomass wastes. Secondly, AD is a proven technology to reduce pollution, and thus its implementation can reduce or eliminate the fees paid to the government for waste discharge or disposal. Depending on the nature and amounts of wastes, this saving can be substantial. Thirdly, methane biogas produced from AD of biomass wastes can replace fossil fuels, therefore implementation of AD should earn environmental and carbon credits as well as renewable energy credits that can be sold for additional revenues. Finally, for many factories or farms that produce large amounts of biomass wastes, the enterprise may be prevented from business growth by the inability to dispose of the wastes. AD can help overcome such waste disposal limitation by reducing overall waste output.

Sociopolitical factors can also drive implementation of AD, but are situation dependent and variable in type and impact. Examples include reduced odor impact on surrounding communities from handling or disposing of biomass wastes (e. g., livestock and poultry manure disposal by land application), and better perception of and public opinion on the business operation. In some circumstances, sociopo­litical factors may become a major driver to implement an AD project, superseding even the economic factors, especially in situations where the enterprise’s ability to continue its operation is threatened by public opposition to its waste storage and disposal.

Denaturing Gradient Gel Electrophoresis (DGGE)

Denaturing gradient gel electrophoresis (DGGE) is a gel electrophoresis method used to separate DNA fragments of the same length, but containing different base — pair sequences; it is used to determine the presence and abundance of different microbial species in a mixed population [12]. It is based on the principle that increasing the denaturant concentration will melt double-stranded DNA in distinct domains. When the melting temperature (Tm) of the lowest domain is reached, the DNA will partially melt, creating branched molecules with reduced mobility in a polyacrylamide gel [13]. DGGE analysis is able to compare many samples at the same time and to analyze them more easily than clone analysis and is thus suitable to reveal microbial community succession.

2, 3-Butanediol

2, 3-BD is the 2R, 3R isomer of 1, 4-butanediol, a potential bulk chemical that can be produced by a variety of microorganisms through microbial fermentation [55]. It has been utilized for the production of various chemical feedstocks and liquid fuels, including the formation of the liquid fuel additive methyl ethyl ketone by dehydration [56]. The esters of butanediol and suitable monobasic acids may find uses as effective plasticizers for thermoplastic polymers, such as cellulose nitrate and cellulose triacetates [55].

Soluble Metabolic Acid Intermediates

The soluble acid metabolites or volatile fatty acids (VFA) formed during the aci — dogenic process help in understanding the metabolic pathway [18]. The following equations show variable soluble acid metabolites generation during acidogenic fermentation.

2CH3- COOH + 2CO2 + 4H2 CH3-CH2-CH2-TOOH + 2CO2 + 2H2 2CH3-CH2-COOH + 2H2O COOHCH2CH2OCOOH + CO2 CH3-CH2OH + CO2

Depending on the pathway used by the microorganism and the corresponding end-products, H2 yields are variable. Products formed from pyruvate such as acetate, butyrate, butanol, acetone, lactate or ethanol determine the theoretical yield of H2 [3]. In obligate anaerobes, pyruvate is converted to H2 from the reduced Fd by the action of hydrogenase resulting in maximum yield of 2 mol H2/mole glucose. Two additional moles of H2 can be produced from NADH produced during glycoly­sis, where NADH is oxidized by Fd reduction by NADH:ferredoxin oxidoreductase (NFOR) [3]. Further, H2 can be produced from the reduced Fd by hydrogenase. The highest theoretical yield of 4 mol H2/mole glucose can be obtained when acetate or acetone is the fermentation end-product. Two molecules of formate are produced from two pyruvate molecules where a theoretical maximum yield of 2 mol H2/mole of glucose can be obtained. In the case of butyrate as the fermentation end-product, the maximum theoretical yield is 2 mol H2/mole glucose. When alcohols are the end-products, lower yields of H2 are obtained as alcohols contain additional H2 atoms that have not been converted to H2 gas [3]. The presence of higher concentra­tions of propionic acid or solventogenesis is generally not considered to be feasible for H2 production.

Production of Methane Biogas as Fuel Through Anaerobic Digestion

Zhongtang Yu and Floyd L. Schanbacher

Abstract Anaerobic digestion (AD) is a biotechnology by which biomass is con­verted by microbes to methane (CH4) biogas, which can then be utilized as a renewable fuel to generate heat and electricity. A genetically and metabolically diverse community of microbes (mainly bacteria and methanogens) drives the AD process through a series of complex microbiological processes in the absence of oxygen. During AD, bacteria hydrolyze the polymeric components (e. g., polysac­charides, proteins, and lipids) present in the feedstock and further ferment the resulting hydrolysis products to short chain fatty acids (SCFA), H2 and CO2, which are ultimately converted to methane biogas (a mixture of CH4 and CO2) by archaeal methanogens. Various biomass wastes (e. g., livestock manure, crop residues, food wastes, food-processing wastes, municipal sludge, and municipal solid wastes) are especially suitable for AD. As one of the few technologies that can both cost — effectively generate bioenergy and reduce environmental pollution, AD has been increasingly implemented in different sectors to convert otherwise wasted biomass to bioenergy. AD technologies can be categorized in many different ways. Each AD technology has its own advantages and disadvantages that make it suitable for particular feedstocks or objectives (i. e., production of energy or stabilization and treatment of wastewaters). Both drivers and barriers exist for commercial imple­mentation of AD projects, with the former stimulating, enabling, or facilitating AD implementation, while the latter function in opposite direction. This chapter will provide an overview of the microbiology underpinning the AD process, and discuss the characteristics of the biomass wastes suitable for AD and the AD technologies appropriate for each type of these feedstocks. The drivers and barriers for AD as well as the AD technology gaps and future research needs will also be discussed.

Z. Yu (b)

Department of Animal Sciences and Environmental Science Graduate Program,

The Ohio Agricultural Research and Development Center, The Ohio State University, Columbus, OH 43210, USA e-mail: yu.226@osu. edu

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

DOI 10.1007/978-90-481-3295-9_6, © Springer Science+Business Media B. V. 2010

Keywords Anaerobic digestion ■ Biomethanation ■ Methanogens ■ Methane biogas ■ Digesters ■ Biomass wastes ■ Feedstocks



anaerobic digestion


biochemical methane potential


biological oxygen demand


confined animal feeding operation


completely mixed contact reactor


chemical oxygen demand


continuously stirred tank reactor


dry anaerobic combustion


expanded granular sludge bed


hydraulic retention time


mixed plug-flow loop reactor


municipal solid wastes


organic fraction of municipal solid wastes


organic loading rate


ribosomal database project


short chain fatty acids


solid retention time


suspended solid


temperature phased anaerobic digestion


total solid


upflow anaerobic sludge blanket


volatile solid

1 Introduction

Anaerobic digestion (AD) is underpinned by a series of bioconversion processes that transform organic compounds, especially biomass wastes, to methane biogas (a mixture of approx. 60% CH4 and 40% CO2). Although it has been used for more than a century in treatment of municipal sludge and high-strength organic wastew­aters from industries, the main objectives have been to stabilize and sanitize the sludge and to remove the organic pollutants from the influents, with relatively little focus on biogas production. Recently, AD received tremendous renewed interest as the demand for and price of fuels continue to rise. AD is looked upon to be an impor­tant biotechnology to help build a sustainable society by simultaneously producing

Disclaimer: Mention of trade names or specific vendors is for informational purposes only and does not imply an endorsement or recommendation by the authors over other products that may also be suitable.

renewable bioenergy and protecting the environment. Indeed, a diverse range of feedstocks (e. g., municipal sludge, food-processing wastes and wastewaters, live­stock manures, the organic fraction of municipal solid wastes (OFMSW), crop residues, and some energy crops) are being diverted to AD for increasing biogas production [4]. Although AD is a relatively slow process and its operation and per­formance are sometimes unstable, the methane biogas derived from biomass wastes has become competitive, in both efficiency and cost, with heat (via burning), steam, and ethanol production [31]. In this chapter, the microbiological underpinning of the AD process as well as the recent understanding of the microbial communities driving AD will be discussed from a biotechnological perspective. This chapter will also provide an overview of the common characteristics of feedstocks that have great biogas potentials and the AD technologies suitable for each of these types of feed­stocks. The drivers and barriers for commercial AD implementation as well as the AD technology gaps and the research needs will also be discussed.

Strategies to Enhance Process Efficiency

1.2 Process Integration Approach

Utilization of remaining carbon present in wastewater from acidogenic H2 produc­tion (an organic acid rich effluent) for additional biogas (H2 or CH4) generation is one way to sustain the process. Integration of an acidogenic process with a terminal photo-fermentative process (for additional H2 production) [6, 7, 110] or acidogenic process (for additional H2 production) [86] or methanogenic process (for methane production) [23] were reported along with enhanced substrate degra­dation (Fig. 5). Soluble metabolites formed during methanogenic or from acidogenic processes could be utilized by photosynthetic bacteria [6, 7] or acidogenic cultures

Fig. 5 Biogas generation and substrate degradation pattern during integration of acidogenic H2 production (acidophilic) process with methaogenic (neutral) process [23]

[23] to produce additional H2. Photosynthetic bacteria can produce H2 by consum­ing organic acids which are abundant in the effluents generated from acidogenic H2 fermentation processes [4, 6, 110]. Theoretically, the maximum H2 yield may be obtained when glucose is converted to acetate as the terminal product through dark fermentation, then subsequently converted into H2 through photo-fermentation [113]. Integrated systems showed higher H2 yields compared to single-step fermen­tation [6, 13, 23, 73]. A two-stage process has been envisioned to obtain yields closer to the theoretical stoichiometric yield of 12 mol H2/mole glucose [86, 113]. However, the efficiency of both H2 production and substrate degradation were found to depend on the process used in the first stage along with the composition of the substrate [23]. The effluent from the first stage of operation generally contains ammonia, which inhibits the second stage process. This can be restricted by dilu­tion and neutralization (to adjust the pH to 7) prior to feeding [10]. Integration of an acidogenic H2 production process followed by a methanogenic anaerobic digestion for CH4 production facilitated an enhanced energy yield along with higher substrate removal efficiency [23, 75, 114, 115]. Integration of the acidogenic process with a photo-fermentation process showed a more positive influence over the correspond­ing methanogenic process integration (Table 4). This might be due to the presence of a relatively higher concentration of VFA bound residual carbon corresponding to the methanogenic process. Multi-stage process was often used to maximize H2 production. Initially, the process consisted of two stages, dark fermentation followed by photo fermentation [10] but three or even four stages have since been proposed in different configurations [109]. The acid-rich organic effluent generated from the initial process of dark fermentation was sent to photo-fermentative process followed by direct photolysis finally using microbial electrolysis cells to produce H2 at fourth stage.

Biotechnological Applications of Hemicellulosic Derived Sugars: State-of-the-Art

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

Table 1 Cell wall composition among various lignocellulosic sources considered for biofuel (% of dry material)

Lignocellulosic source










Sugarcane bagasse








Wheat straw








Corn stover








Switch grass








Pine wood








Aspen wood








Spruce wood















Birch wood








Douglas fir wood








* Total hemicellulose amount present in lignocellulosics on the basis of % of dry material — Sugarcane bagasse, 27.5; Switch grass, 30; Corn stover, 26.8; Wheat straw, 50; Pine, 26; Aspen, 29; Spruce, 26; Birch wood, 23; Salix wood, 21.7; Douglas fir wood, 20.3.


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.