Scholarly article on topic 'Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review'

Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Zahid Anwar, Muhammad Gulfraz, Muhammad Irshad

Abstract From the last several years, in serious consideration of the worldwide economic and environmental pollution issues there has been increasing research interest in the value of bio-sourced lignocellulosic biomass. Agro-industrial biomass comprised on lignocellulosic waste is an inexpensive, renewable, abundant and provides a unique natural resource for large-scale and cost-effective bio-energy collection. To expand the range of natural bio-resources the rapidly evolving tools of biotechnology can lower the conversion costs and also enhance target yield of the product of interest. In this background green biotechnology presents a promising approach to convert most of the solid agricultural wastes particularly lignocellulosic materials into liquid bio based energy-fuels. In fact, major advances have already been achieved to competitively position cellulosic ethanol with corn ethanol. The present summarized review work begins with an overview on the physico-chemical features and composition of agro-industrial biomass. The information is also given on the multi-step processing technologies of agro-industrial biomass to fuel ethanol followed by a brief summary of future considerations.

Academic research paper on topic "Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review"

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Agro-industrial lignocellulosic biomass a key to unlock the future bio-energy: A brief review

Zahid Anwar a'b'*, Muhammad Gulfrazb, Muhammad Irshada'

a Department of Biochemistry, NSMC, University of Gujrat, Pakistan b PMAS Arid Agriculture University Rawalpindi, Rawalpindi, Pakistan



Article history:

Received 21 December 2013 Received in revised form 2 February 2014 Accepted 7 February 2014


Lignicellulosic biomass Green biotechnology Environmental friendly Bio-energy Bio-ethanol Industrial enzyme

From the last several years, in serious consideration of the worldwide economic and environmental pollution issues there has been increasing research interest in the value of bio-sourced lignocellulosic biomass. Agro-industrial biomass comprised on lignocellulosic waste is an inexpensive, renewable, abundant and provides a unique natural resource for large-scale and cost-effective bio-energy collection. To expand the range of natural bio-resources the rapidly evolving tools of biotechnology can lower the conversion costs and also enhance target yield of the product of interest. In this background green biotechnology presents a promising approach to convert most of the solid agricultural wastes particularly lignocellulosic materials into liquid bio based energy-fuels. In fact, major advances have already been achieved to competitively position cellulosic ethanol with corn ethanol. The present summarized review work begins with an overview on the physico-chemical features and composition of agro-industrial biomass. The information is also given on the multi-step processing technologies of agro-industrial biomass to fuel ethanol followed by a brief summary of future considerations.

Copyright © 2014, The Egyptian Society of Radiation Sciences and Applications. Production

and hosting by Elsevier B.V. All rights reserved.

1. Introduction

Lignocellulosic materials are the most promising feedstock as natural and renewable resource essential to the functioning of modern industrial societies. A considerable amount of such materials as waste byproducts are being generated through

agricultural practices mainly from various agro based industries (Pérez, Muñoz-Dorado de la Rubia, & Martínez, 2002). Sadly, much of the lignocellulosic biomass is often disposed of by burning, which is not restricted to developing countries alone. Recently lignocellulosic biomasses have gained increasing research interests and special importance because of their renewable nature (Asgher, Ahmad, & Iqbal, 2013;

* Corresponding author. Department of Biochemistry, NSMC, University of Gujrat, Pakistan. Tel.: +92 345 5463838.

E-mail addresses: (Z. Anwar), (M. Irshad). 1 Tel.: +92 344 4931030.

Peer review under responsibility of The Egyptian Society of Radiation Sciences and Applications


1687-8507/Copyright © 2014, The Egyptian Society of Radiation Sciences and Applications. Production and hosting by Elsevier B.V. All rights reserved.

Ofori-Boateng & Lee, 2013). Therefore, the huge amounts of lignocellulosic biomass can potentially be converted into different high value products including bio-fuels, value added fine chemicals, and cheap energy sources for microbial fermentation and enzyme production (Asgher et al., 2013; Iqbal, Kyazze, & Keshavarz, 2013; Irshad et al., 2013; Isroi et al., 2011).

2. Physico-chemical characteristics of lignocellulosic biomass

All plant materials are mostly composed of three major units i.e., cellulose, hemicellulose and lignin. Lignocellulosic materials including agricultural wastes, forestry residues, grasses and woody materials have great potential for bio-fuel production. Typically, most of the agricultural lignocellulosic biomass is comprised of about 10-25% lignin, 20-30% hemicellulose, and 40-50% cellulose (Iqbal, Ahmed, Zia, & Irfan, 2011; Kumar, Barrett, Delwiche, & Stroeve, 2009; Malherbe & Cloete, 2002). Cellulose is a major structural component of plant cell walls, which is responsible for mechanical strength while, hemicellulose macromolecules are often repeated polymers of pentoses and hexoses. Lignin contains three aromatic alcohols (coniferyl alcohol, sinapyl alcohol and p-coumaryl alcohol) produced through a biosynthetic process and forms a protective seal around the other two components i.e., cellulose and hemicelluloses (Fig. 1) (Calvo-Flores & Dobado, 2010; Jiang, Nowakowski, & Bridgwater, 2010; Menon & Rao, 2012). In general the composition of lignocel-lulose highly depends on its source whether it is derived from the hardwood, softwood, or grasses. Table 1 shows the typical chemical compositions of all these three components in various lignocellulosic materials that vary in composition due to the genetic variability among different sources (Bertero, de la Puente, & Sedran, 2012; Iqbal et al., 2013; John, Monsalve,

Medina, & Ruiz, 2006; Kumar et al., 2009; Malherbe & Cloete, 2002; Prassad, Singh, & Joshi, 2007). To obtain a clear picture of the material, an analysis of the structure of each main component is made in the following section.

2.1. Physical and structural properties of cellulose

Cellulose is a highly stable polymer consisting of glucose and attached with linear chains up to 12,000 residues. It is majorly composed of (1,4)-D-glucopyranose units, which are attached by b-1,4 linkages with an average molecular weight of around 100,000 (Himmel et al., 2007). Plant biomass contain 40-50% of cellulose molecules which are held together by intermolecular hydrogen bonds in native state, but they have a strong tendency to form intra-molecular and intermolecular hydrogen bonds and this tendency increases the rigidity of cellulose and make highly insoluble and highly resistant to most organic solvents. Naturally cellulose molecules are exists as bundles which aggregated together in the form of micro-fibrils order i.e., crystalline and amorphous regions (Iqbal et al., 2011; Taherzadeh & Karimi, 2008). The chemical formula of cellulose is (C6H10O5)n and the structure of one chain of the polymer is presented in Fig. 1.

2.2. Physical and structural properties of hemicelluloses

Hemicellulose is the second most abundant heterogeneous polymers that mainly consist of glucuronoxylan, gluco-mannan and trace amounts of other polysaccharides. Grasses and straws contain arabinan, galactan and xylan, while mannan is a component of hardwood and softwood hemicel-lulose (Brigham, Adney, & Himmel, 1996). They are catalogued with sugar as a backbone, i.e., xylans, mannans and glucans, with xylans and mannans being the most common (Wyman et al., 2005). Galactans, arabinans and arabinogalactans are included in the hemicellulose group; however, they do not

Fig. 1 - Diagrammatic illustration of the framework of lignocellulose; cellulose; hemicellulose and lignin. Adapted with permission from, Menon & Rao, 2012.

Table 1 - Percent composition of lignocellulose components in various lignocellulosic materials.

Lignocellulosic material Lignin (%) Hemicellulose (%) Cellulose (%) Referencea

Sugar cane bagasse 20 25 42 Kim and Day (2011)

Sweet sorghum 21 27 45 Kim and Day (2011)

Hardwood 18-25 24-40 40-55 Malherbe and Cloete (2002)

Softwood 25-35 25-35 45-50 Malherbe and Cloete (2002)

Corn cobs 15 35 45 Prassad et al. (2007)

Corn stover 19 26 38 Zhu, Lee, and Elander (2005)

Rice straw 18 24 32.1 Prassad et al. (2007)

Nut shells 30-40 25-30 25-30 Howard, Abotsi, Van Rensburg,

and Howard (2003)

Newspaper 18-30 25-40 40-55 Howard et al. (2003)

Grasses 10-30 25-50 25-40 Malherbe and Cloete (2002)

Wheat straw 16-21 26-32 29-35 McKendry (2002)

Banana waste 14 14.8 13.2 John et al. (2006)

Bagasse 23.33 16.52 54.87 Guimaraes, Frollini, Da Silva,

Wypych, and Satyanarayana (2009)

Sponge gourd fibres 15.46 17.44 66.59 Guimaraes et al. (2009)

a For detailed references please see Iqbal et al. (2013). Adapted with permission from Iqbal et al., 2013.

share the equatorial b-1,4 linked backbone structure. In hardwoods, glucuronoxylan (O-acetyl-4-O-methyl-glucurono-b-D-xylan) is the predominant component. Xylospyranose is the backbone of the polymer and connected with b-1,4 linkages. Hemicellulosic biomass contains 25-35% of hemicellulose, with an average molecular weight of <30,000. Cellulose and hemicellulose binds tightly with non-covalent attractions to the surface of each cellulose micro-fibril. Hemicelluloses were originally believed to be intermediates in the biosynthesis of cellulose (Vercoe, Stack, Blackman, & Richardson, 2005).

2.3. Physical and structural properties of lignin

Lignin is generally the most complex and smallest fraction, representing about 10-25% of the biomass by weight. It has a long-chain, heterogeneous polymer composed largely of phenyl-propane units most commonly linked by ether bonds. Lignin acts like a glue by filling the gap between and around the cellulose and hemicellulose complexion with the polymers. It is present in all plant biomass; therefore, it is considered byproduct or as a residue in bio-ethanol production process. Lignin is comprised of complex and large polymer of phenyl-propane, methoxy groups and non-carbohydrate poly phenolic substance, which bind cell walls component together (Hamelinck, Hooijdonk, & Faaij, 2005). Phenyl-propanes (3 carbons attached with 6 carbon atom rings) are main block of lignin. These phenyl-propanes denoted as 0, I, II methoxyl groups attached to rings give special structure I, II and III. These groups depend on the plant source which they are obtained. Structure I exist in plants (grasses) and structure II found in the wood (conifers) while structure III present in deciduous wood.

3. Biotechnological importance of lignocellulosic biomass

From the biotechnological point of view a wide variety of lignocellulosic biomass resources are available as potential

candidate that are also convert able into high value bio-products like bio-ethanol/bio-fuels. From the last several years a considerable improvement from the green biotechnology related to lignocellulose biomass has appeared. The ever increasing costs of fossil fuels and their greenhouse effects are creating a core demand to explore alternative cheaper and eco-friendly bio-fuels resources as a strategy for reducing global warming (Asgher et al., 2013; Iqbal et al., 2013). Environmental pollution, global warming, and the future of oil production are among major causes of public and private interests in natural bio based resources as an alternative or substitute for fossil fuel oil. One potential method for the low-cost production of bio-ethanol is to utilize the lignocellulosic or agro-industrial biomass because they contain carbohydrates that must be first converted into simple sugars (glucose) and then fermented into ethanol (Alonso, P^rez, Morcuende, & Martinez-Carrasco, 2008; Balat & Balat, 2009; Lin & Tanaka, 2006). Given this reality, nations around the world are investing in alternative sources of energy, including bio-ethanol. The conversion of lignocellulosic biomass into higher value added products like fine chemicals or bio-fuel production normally requires a multi-step processing that include (i) pre-treatment (mechanical, chemical, or biological etc) (ii) enzymatic hydrolysis (iii) fermentation process (Wyman, 1999; Xiao, Wang, Xia, & Ma, 2012). Fig. 2 illustrating a thermo-mechanical and biochemical processing of ligno-cellulosic biomass into various values added biotechnological products.

4. Role of pre-treatment

Pre-treatment is an important step for the recovery of cellu-losic content from lignin based biomass as compare to the starchy materials. While dealing with lignocellulosic biomasses, pre-treatment is also required to break down the lignin barrier to recover cellulose, which is further subjected to enzymatic hydrolysis to convert into fermentable sugars. During the past few decades, several pre-treatment

Fig. 2 - Thermo-mechanical and biochemical processing of lignocellulosic biomass into various values added biotechnological products. Adapted with permission from, Menon & Rao, 2012.

approaches have been developed for generating cost-effective fermentable sugar from most of the agricultural cellulose and hemicellulose containing lignocellulosic materials (Yang & Wyman, 2008). In this background, there are a number of reports on pre-treatment technologies for a variety of feedstocks. Some of the most promising pre-treatment categories have already been commercialized for the productions of bio-energy are summarized in the Table 2. An effective pre-treatment is characterized by several criteria: preserving hemicellulose fractions, to yield maximum fermentable sugar contents, limiting the loss of carbohydrate, to minimize the formation of inhibitors due to degradation products, minimizing energy input, and the process is economically efficient as well as cost-effective. While, comparing various pre-treatment options, all of the above mentioned criteria should be comprehensively considered as a basis to achieve maximal end product of interest. Hydrolysis of biomass can be done by different ways mainly including physico-chemically, chemically or biologically (Asgher et al., 2013; Hamzeh, Ashori, Khorasani, Abdulkhani, & Abyaz, 2013; Rohowsky et al., 2013; Yang, Dai, Ding, & Wyman, 2011; Yang & Wyman, 2008).

4.1. Chemical pre-treatments

To date chemical pre-treatment is the most studied technique among various pre-treatment categories that was originally developed and therefore has extensively been used for delignification of cellulosic materials. Chemical hydrolysis is an important treatment method for recovery of sugar

monomers from cellulose and hemicellulose polymers from lignocellulosic biomass by optimizing chemical reagents. The most commonly used chemical pre-treatments include: acid and alkali based hydrolysis approaches.

4.1.1. Acid based hydrolysis

Chemical treatment of cellulosic biomass with concentrated hydrochloric acid or sulphuric acid is conventional procedure. The entire process of pre-treatment can be operating at very low temperature as compared to dilute-acid pre-treatment. On the other hand one of the possible drawbacks of this process is that it's required in higher concentration (30-70%), therefore cause high level of corrosive reaction. In this background the whole process needs further expenditure in the form of specialized non-metallic or non corrosive material such as ceramic or carbon-brick lining. In comparison to the other pre-treatment procedures particularly dilute-acid hydrolysis the environmental hazards and high operating cost involved in concentrated-acid hydrolysis reduce the interest on industrial scale (Katzen, Madson, & Monceaux, 1995; Wyman, 1999). Dilute-acid pre-treatment has some advantages over concentrated-acid hydrolysis to solve the issues like acid recovery, toxicity, acid and special maintenance against corrosion materials (Sivers & Zacchi, 1995; Sun & Chen, 2007). Acid pre-treatment has been applied on several biomass feed-stocks like herbaceous material (grass), hardwoods and agricultural wastes. Most of the substrates give better results by solubilizing the hemicellulose (Liao, Liu, Wen, Frear, & Chen, 2007; Wyman et al., 2005). Other two factors including temperature and incubation time had also

Table 2 - Most promising pre-treatment technologies.

» p. -> >

h-i pu

«S. w

m p-ro n> o

Method of Sugar Inhibitor Byproduct Reuse of Applicability Equipment

pre-treatment yield formation generation chemicals to different cost


Mechanical L Nil No No Yes H

Mineral acids H H H Yes Yes H

Alkali H L H Yes Yes Nil

Liquid hot H H L No

Organosolv H H H Yes Yes H

Wet oxidation H or L Nil L No - H

Ozonolysis H L H No — H

C02 explosion H L L No - H

Steam explosion H H L — Yes H

AFXE H L - Yes H

Ionic liquids H/L L — Yes Yes —

L = low; H = high.

Adapted with permission from, Menon & Rao, 2012.

Success at pilot scale


Limitations & disadvantages

Yes Reduce cellulose crystallinity High power consumption than inherent biomass energy

Yes Hydrolysis of cellulose and hemicellulose. alters lignin structure Hazardous, toxic and corrosive

Yes Removal of lignin and hemicellulose, Long residence time,

increases accessible surface area irrecoverable salts formed

Yes Removal of hemicellulose making Long residence time, less

enzymes accessible to cellulose lignin removal

Yes Hydrolyze lignin and hemicellulose Solvents needs to drained, evaporated, condensed and reused

Removal of lignin, dissolves hemicellulose and causes cellulose decrystallization

No Reduces lignin content, no toxic Large amount of ozone

residues required

Hemicellulose removal, cellulose decrystallization, cost-effective Does not modify lignin

Yes Hemicellulose removal and Incomplete destruction of

alteration in ligninecarbohydrate matrix

lignin structure

- Removal of lignin and Not efficient for biomass with

hemicellulose high lignin content

Dissolution of cellulose, increased amenability to cellulase Still in initial stages

important impact on alteration the structure of biomass. Major disadvantage of this process is the formation of secondary products which can lower the yield of sugars due to conversion of products in to furfural and hydroxyl-methyl furfural compounds and these compounds interfere in bio-ethanol fermentation process.

4.1.2. Alkali based hydrolysis

Alkali based pre-treatment involves the use of bases, such as sodium and ammonium hydroxide, for the pre-treatment of agricultural lignocellulosic feed-stocks. Alkaline hydrolysis causes various structural alterations inside the lignocellulosic material during treatment process such as the depletion of lignin barrier, cellulose swelling, and partial decrystallization and solvation of cellulose and hemicelluloses, respectively (Cheng et al., 2010; Ibrahim, El-Zawawy, Abdel-Fattah, Soliman, & Agblevor, 2011; Sills & Gossett, 2011; Zhu, Wan, & Li, 2010). Lignocellulosic feed-stocks that have been shown to benefit from the method of alkaline pre-treatment are corn stover, switch-grass, bagasse, wheat, and rice straw (Hu, Wang, & Wen, 2008; Zhao, Wang, Zhu, Ragauskas, & Deng, 2008; Zhu, Sheng, Yan, Qiao, & Lv, 2012). Zhao et al. (2008) had showed the effectiveness of sodium hydroxide pre-treatment for hardwoods, wheat straw, switch-grass, and soft-woods with less than 26% lignin content.

4.2. Biological pre-treatment

Biological pre-treatment employs wood degrading microorganisms, including white rot fungi (WRF), brown or soft-rot fungi, and bacteria to modify the chemical composition and/ or structure of the lignocellulosic biomass. Bio-delignification is useful for pre-treatment purposes because it replaces or supplements the chemical-based pre-treatments, which include mechanical treatment with acid, alkali, and steam explosion (Iqbal et al., 2013). In spite of this biological pre-treatments are more effective, economical, eco-friendly and less health hazardous as compare to the physico-chemical or chemical-based pre-treatment approaches. Therefore, from the last few years research scientists are directing their interests towards biological delignification. Recent advances in the characterization of ligninolytic enzymes involving the degradation of lignin have given new impetus to the research in this area, which has now become amenable to biotechno-logical exploitation (Asgher, Iqbal, & Asad, 2012). Bioconversions of lignocellulosic materials to useful products normally require multi-step processes that include pre-treatment, enzymatic hydrolysis, and fermentation (Xiao et al., 2012), so that the modified or pre-treated biomass is more amenable to enzyme digestion. Increasing understanding of termites and fungal systems has provided insights for developing more effective pre-treatment technologies to realize the above mentioned advantages or benefits of biological pre-treatment over some others. However, biological pre-treatment is a very slow process that also requires careful control of growth conditions and large amount of space to perform treatment. In addition to this most of the lignolytic microorganisms solubilize/consume not only lignin but also hemicellulose and cellulose. Because of these drawbacks/ limitations the biological pre-treatment faces techno-

economic barriers and therefore is less attractive commercially (Eggeman & Elander, 2005).

5. Enzymatic hydrolysis

Enzymatic hydrolysis is an effective and economical method to achieve fermentable sugars under mild and eco-friendly reaction conditions from the pre-treated cellulosic biomass (Wyman et al., 2005). The entire process of enzymatic hydrolysis critically depends on variety of factors viz., pH, time, temperature substrates and enzyme activities, etc. Enzymatic saccharification is done separately from fermentation known as separate hydrolysis and fermentation (SHF). When cellulose hydrolysis and fermentation are carried out simultaneously the phenomenon is known as simultaneous saccharification and fermentation (SSF). Now a days this process of simultaneous saccharification of both cellulose and hemicellulose is achieved by co-fermentation of both hexoses and pentoses sugars (SSCF) with the help of genetically engineered microbes that ferment xylose and glucose in the same medium where both enzymes for cellulose and hemi-celluloses are available. The major advantage of this technology is that SSF and SSCF can be performed in the same tank which makes the entire process cheap, feasible and cost-effective (reduce the capital and operational investment). Biological, physical and chemical methods have been employed for detoxification (removal of inhibitory compounds in fermentation) of lignocellulosic hydrolyzates (Olsson & Hahn-Hagerdal, 1996). Lignocellulosic materials have different degree of inhibition and tolerance levels vary according to different microbial strains. Degree of tolerance varies with different strains of Saccharomyces cerevisiae, so inhibitory compounds are detoxified by changing the substrate concentration and altering the pH of media (Linden, Peetre, & Hahn-Hagerdal, 1992; Palmqvist, Galbe, & Hahn-Hagerdal, 1998).

6. Fermentation strategy

Ethanol production from biomass is mainly categorized into three steps process (1) achieve a fermentable sugars (2) conversion of fermentable sugars into ethanol and (3) ethanol separation and purification through distillation (Asgher, Shahid, Kamal, & Iqbal, 2014; Demirbas, 2005). Difference between lignocelluloses or starch ethanol production is the step for obtaining sugars before fermentation. Sugar crops or starchy crops need milling and grinding for recovery of sugars by extraction and fermentation becoming a relatively simple process that requires no hydrolysis or pre-treatment steps for obtaining sugars and transformation into ethanol (Icoz, Tugrul, Saral, & Icoz, 2009). Bio-ethanol production is mainly done by fed-batch process and low ethanol produce by multistage continuous fermentation. Basic steps for the conversion of lignocellulosic biomass are: (1) pre-treatment process which can reduce the lignin content and render cellulose and hemicellulose content for enzymatic hydrolysis (2) steps to convert enzymatic hydrolysis to break down polysaccharide to simple sugars (3) conversions of sugars (hexoses and

Fig. 3 - Generalized schematic representation of lignocellulosic materials bio-conversion into ethanol. Adapted with permission from, Asgher et al., 2014.

Table 3 - List of various lignocellulosic materials used for the production of different microbial enzymes.

Lignocellulosic Pre-treatment Microbial culture Enzymes produced Reference3

material type

Sugar cane bagasse Biological/ P. chrysosporium; T. versicolor; MnP, LiP, laccase, El-Nasser, Helmy, and El-Gammal, 1997;

chemical Trichoderma viride; cellulases xylanase El-Gammal, Kamel, Adeeb, and Helmy, 1998;

P. Sanguineus; Trichoderma Kansoh, Essam, and Zeinat 1999;

viride Irshad, Anwar, and Afroz, 2012;

Irshad, Bahadur, et al., 2012;

Yoon et al., 2012; Irshad

et al., 2013

Orange peel waste Chemical Trichoderma viride Endoglucanase, Irshad, Anwar, et al., 2012;

exoglucanase, Irshad et al., 2013


Corn cobs Biological Trametes versicolor; MnP, LiP, laccase, El-Nasser et al., 1997; Ahmed,

P. chrysosporium; protease, xylanase Zia, Iftikhar, and Iqbal, 2011;

Aspergillus niger Iqbal et al., 2011; Asgher

and Iqbal, 2011; Asgher, Iqbal,

and Asad, 2012; Asgher, Iqbal,

and Irshad, 2012

Corn stover Biological/ P. chrysosporium; T. versicolor; MnP, LiP, laccase, El-Nasser et al., 1997; Yang,

chemical Penicillium decumbens cellulase xylanase, Chen, Gao, and Li, 2001; Iqbal

et al., 2011; Asgher et al., 2011;

Rice Straw Biological/ P. chrysosporium; T. versicolor; MnP, LiP, laccase, cellulase Eun et al., 2006; Iqbal et al.,

chemical Trichoderma reesei 2011; Asgher et al., 2011

Penut shells Biological G. leucidum; P. chrysosporium Laccase, xylanase El-Nasser et al., 1997; Irshad,

Bahadur, et al., 2012

Newspaper Chemical Trichoderma viride Endoglucanase, Irshad et al., 2013

exoglucanase, ß-glucosidase

Wheat straw Biological/ P. chrysosporium; T. versicolor; MnP, LiP, laccase, El-Nasser et al., 1997;

chemical T. viride; F. trogii; L. edodes; cellulases xylanase, Kachlishvili, Penninckx, Tsiklauri,

P. dryinus; P. tuberregium and Elisashvili, 2006;

Elisashvili, Kachlishvili, and

Penninckx, 2008; Iqbal

et al., 2011; Asgher et al.,

2011; Irshad et al., 2013

Banana stalk Biological S. commune; P. chrysosporium; MnP, LiP, laccase, xylanase, Reddy, Ravindra Babu, Komaraiah,

T. versicolor; P. ostreatus endoglucanase Roy, and Kothari, 2003; Irshad,

Asgher, Scheikh, and Nawaz,

2011; Iqbal, Asgher, and Bhatti, 2011;

Asgher et al., 2011

Rice bran Biological Aspergillus niger Protease Ahmed et al., 2011

Wheat bran Biological Aspergillus niger; Morchella Protease, endoglucanase, Papinutti and Forchiassin,

sculenta; ß-glucosidase, laccase, MnP 2007; Papinutti and Lechner,

F. sclerodermeus; Trametes 2008; Stoilova et al., 2010;

versicolor Ahmed et al., 2011

Apple pomace Chemical Trichoderma viride Cellulases Irshad et al., 2013

Oil palm empty Chemical Thermobifida fusca; CMCase. FPase, Shahriarinour et al., 2011;

fruit bunch fibre Aspergillus terreus ß-glucosidase Harun et al., 2013

Beech tree leaves Biological F. trogii; L. edodes; Laccase, CMCase. FPase, Kachlishvili et al., 2006;

Pleurotus dryinus; MnP, xylanase Elisashvili et al., 2008

P. tuberregium

Eucalyptus residue Biological Lentinula edodes Xylanase, cellulase, Silva, Machuca, and Milagres, 2005

MnP, laccase

a For detailed references please see Iqbal et al. (2013). Adapted with permission from, Iqbal et al., 2013.

pentoses) for ethanol production through microorganisms (4) production of ethanol from pentose sugars. Fig. 3 illustrating a step by step procedure to convert lignocellulosic biomass into ethanol. Several fungal species belonging with genera Fusa-rium, Rhizopus, Monilia, Neurospora and Paecilomyces have been found potential for fermenting glucose as well as xylose (Singh, Kumar, & Schugerl, 1992). Production of bio-ethanol

from cellulose is mostly conducted by using fermentative organism, but the conversion rate is very low due to byproducts formation. Filamentous fungus Fusarium oxysporum is also known for the production of bio-ethanol through SSF by direct utilizing the cellulose, but their conversion rate is low due to production of acetic acid as a byproduct (Panagiotou, VillasBoas, Christakopoulos, Nielsen, & Olsson, 2005).

7. Enzymes production

To date, the production of various ligninolytic enzymes including LiP, MnP, versatile peroxidise (VP), and laccases and other lignocellulolytic mainly endoglucanases (EC, cellobiohydrolases (EC and b-glucosidases (EC have been widely studied in submerged and solid culture processes in the laboratory, ranging from flask shake to large scale (Elisashvili, Penninckx, Kachlishvili, Asatiani, & Kvestiadze, 2006; Moldes, Bustos, Torrado, & Dominguez, 2007; Xia & Len, 1999). There are large numbers of microorganisms capable for degrading cellulose (Jungebloud et al., 2007). Trichoderma, Aspergillus, Penicillium and Fusarium species are commonly used for cellulases production (Iqbal et al.,

2011). Selections of desired fungal strains depend on several factors and selection of substrate for optimizing the cellulase producing conditions (Shazia, Bajwa, & Shafique, 2007). Lig-ninolytic, cellulases and hemicellulases are important industrial enzymes having numerous applications and biotechnological potential for various industries including chemicals, fuel, food, brewery and wine, animal feed, textile and laundry, pulp and paper and agriculture (Asgher, Ahmed, & Iqbal, 2011; Asgher & Iqbal, 2011; Eun, Beauchemin, Hong, & Bauer, 2006; Iqbal & Asgher, 2013; Iqbal et al., 2011; Irshad et al., 2013; Oberoi, Chavan, Bansal, & Dhillon, 2010; Papinutti & Forchiassin, 2007; Papinutti & Lechner, 2008; Stoilova, Krastanov, & Stanchev, 2010; Yoon, Ngoh, & Chua,

2012). A range of different lignocellulosic materials that has successfully been adopted for the production of different enzymes having industrial importance are summarized in Table 3.

8. Concluded remarks and future outlook

The energy and environmental crises which the modern world is experiencing is forcing to re-evaluate the efficient utilization or finding alternative uses for natural, renewable resources, using clean technologies. In this regard, lignocel-lulosic biomass holds considerable potential to meet the current energy demand of the modern world. This is also essential in order to overcome the excessive dependence on petroleum for liquid fuels. Further advanced biotechnologies are crucial for discovery, characterization of new enzymes, and production in homologous or heterologous systems and ultimately lead to low-cost conversion of lignocellulosic biomasses into bio-fuels and bio-chemicals. In current scenario future trends are being directed to lignocellulose biotechnology and genetic engineering for improved processes and products. To overcome the current energy problems it is envisaged that lignocellulosic biomass in addition of green biotechnology will be the main focus of the future research.


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