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Lignocellulosic wastes refer to plant biomass wastes that are composed of cellulose, hemicellulose, and lignin. They may be grouped into different categories such as wood residues (including sawdust and paper mill discards), grasses, waste paper, agricultural residues (including straw, stover, peelings, cobs, stalks, nutshells, nonfood seeds, bagasse, domestic wastes (lignocellulose garbage and sewage), food industry residues, municipal solid wastes and the like (1). Currently, the second generation bio-products such as bioethanol, biodiesel, bio hydrogen and methane from lignocellulose biomass are increasingly been produced from wastes rather than from energy crops (jatropha, switch grass, hybrid poplar and willow) because the latter competes for land and water with food crops that are already in high demand. The use of food crops such as corn and sugarcane to produce biofuels is increasingly being discouraged due to the current worldwide rise in food prices. In order to minimize food-feed-fuel conflicts, it is necessary to integrate all kinds of bio waste into a biomass economy (2). Furthermore, the use of LCW offers a possibility of geographically distributed and greenhouse-gas-favorable sources of products (3).
The lignocellulosic biomass, which represent the largest renewable reservoir of potentially fermentable carbohydrates on earth (4), is mostly wasted in the form of pre-harvest and post-harvest agricultural losses and wastes of food processing industries. Due to their abundance and renewability, there has been a great deal of interest in utilizing LCW for the production and recovery of many value-added products (5). Among the main recovery products include enzymes, reducing sugars, furfural, ethanol, protein and amino acids, carbohydrates, lipids, organic acids, phenols, activated carbon, degradable plastic composites, cosmetics, biosorbent, resins, medicines, foods and feeds, methane, biopesticides, biopromoters, secondary metabolites, surfactants, fertilizer and other miscellaneous products (6). Alongside producing these products, the processes also remove wastes from the environment.
The barrier to the production and recovery of valuable materials from LCW is the structure of lignocellulose which has evolved to resist degradation due to crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages (7). Cellulose, hemicellulose and lignin form structures called micro fibrils, which are organized into micro fibrils that mediate structural stability in the plant cell (8). The main goal of any pretreatment, therefore, is to alter or remove structural and compositional impediments to hydrolysis and subsequent degradation processes in order to enhance digestibility, improve the rate of enzyme hydrolysis and increase yields of intended products (9). These methods cause mechanical, physical chemical or biological changes in the plant biomass in order to achieve the desired products.
Technology of LCW bioconversion has long been considered to be rather expensive.
However, recent increases in grain prices mean that the switch to second generation bio-products such as biofuels from LCW will reduce competition with grain for food and feed, and allow the utilization of materials like straw which would otherwise go to waste. Technologies that will allow cost effective conversion of biomass into fuels and chemicals consider economy of scale, low-cost pretreatment systems and highly effective and efficient biocatalysts (10).
This assignment reviews the recent developments in LCW pretreatment value addition products
Physical and chemical characteristics of lignocellulosic biomass
The term "lignocellulosic biomass" is used when referring to higher plants, softwood or hardwood. The main components of the lignocellulosic materials are cellulose, hemicellulose and lignin. Cellulose is a major structural component of cell walls, and it provides mechanical strength and chemical stability to plants. Solar energy is absorbed through the process of photosynthesis and stored in the form of cellulose. (Raven et al.,1992) Hemicellulose is a copolymer of different C5 and C6 sugars that also exist in the plant cell wall. Lignin is polymer of aromatic compounds produced through a biosynthetic process and forms a protective layer for the plant walls. In nature, the above substances grow and decay during the year. It has been estimated that around 7.5x1010 tonnes of cellulose are consumed and regenerated every year (Kirk-Otmer, 2001). It is thereby the most abundant organic compound in the world.
Apart from the three basic chemical compounds that lignocellulose consists of, water is also present in the complex. Furthermore, minor amounts of proteins, minerals and other components can be found in the lignocellulose composition as well.
The composition of lignocellulose highly depends on its source. There is a significant variation of the lignin and (hemi)cellulose content of lignocellulose depending on whether it is derived from hardwood, softwood, or grasses. Table 1 summarizes the composition of lignocellulose encountered in the most common sources of biomass.
Table 1: the composition of lignocellulose encountered in the most common sources of biomass.
Internal structure - physical properties
Lignocellulosic biomass has a complex internal structure. It is comprised of a number of major components that have, in their turn, also complex structures. To obtain a clear picture of the material, an analysis of the structure of each main component is made in this section, concluding with the description of the structure of lignocellulose itself. Also addressed are the physical properties of each of the components of lignocellulose, and how each of these components contributes to the behaviour of the complex structure as a whole. The study is oriented towards breaking down the complex of lignocellulose and utilizing the components to produce sugars, and possibly, lignin, as this is one of the main goals of pretreatment.
Cellulose is the Î²-1,4-polyacetal of cellobiose (4-O-Î²-D-glucopyranosyl-D-glucose). Cellulose is more commonly considered as a polymer of glucose because cellobiose consists of two molecules of glucose. The chemical formula of cellulose is (C6H10O5)n and the structure of one chain of the polymer is presented in Figure 1.
Figure 1 Structure of single cellulose molecule
Many properties of cellulose depend on its degree of polymerization (DP), i.e. the number of glucose units that make up one polymer molecule. The DP of cellulose can extend to a value of 17000, even though more commonly a number of 800-10000 units is encountered (Kirk-Otmer, 2001). For instance, cellulose from wood pulp has a DP between 300 and 1700.
The nature of bond between the glucose molecules (Î²-1,4 glucosidic) allows the polymer to be arranged in long straight chains. The latter arrangement of the molecule, together with the fact that the hydroxides are evenly distributed on both sides of the monomers, allows for the formation of hydrogen bonds between the molecules of cellulose. The hydrogen bonds in turn result in the formation of a compound that is comprised of several parallel chains attached to each other (Faulon et al., 1994).
An illustration of the arrangement of the cellulose molecules in parallel chains and the accompanying hydrogen bonding is given in Figure 2.
Figure 2 Demonstration of the hydrogen bonding that allows the parallel arrangement of the cellulose polymer chains
Cellulose is found in both the crystalline and the non-crystalline structure. The coalescence of several polymer chains leads to the formation of microfibrils, which in turn are united to form fibres. In this way cellulose can obtain a crystalline structure. Figure 3 illustrates structure as well as the placement of cellulose in the cell wall.
Figure 3 Formation of micro- and macrofibrils (fibres) of cellulose and their position in the wall
Cellulose is a relatively hygroscopic material absorbing 8-14% water under normal atmospheric conditions (20 °C, 60% relative humidity). Nevertheless, it is insoluble in water, where it swells. Cellulose is also insoluble in dilute acid solutions at low temperature. The solubility of the polymer is strongly related to the degree of hydrolysis achieved. As a result, factors that affect the hydrolysis rate of cellulose also affect its solubility that takes place, however, with the molecule Being in a different form than the native one. At higher temperatures it becomes soluble, as the energy provided is enough to break the hydrogen bonds that hold the crystalline structure of the molecule. Cellulose is also soluble in concentrated acids, but severe degradation of the polymer by hydrolysis is caused. In alkaline solutions extensive swelling of cellulose takes place as well as dissolution of the low molecular weight fractions of the polymer (DP < 200) (Krassig and Schurz, 2002). Solvents of cellulose that have been applied in industrial or laboratory practice include uncommon and complex systems, such as cupriethylenediamine (cuen) hydroxide or the cadmium complex Cadoxen. Additionally, aqueous salt solutions, such as zinc chloride, dissolve limited amounts of cellulose (Kirk-Otmer, 2001). Cellulose does not melt with temperature, but its decomposition starts at 180oC (Thermowoodhandbook, 2003).
The term hemicellulose is a collective term. It is used to represent a family of polysaccharides such as arabino-xylans, gluco-mannans, galactans, and others that are found in the plant cell wall and have different composition and structure depending on their source and the extraction method.
The most common type of polymers that belongs to the hemicellulose family of polysaccharides is xylan. As shown in Figure 4, the molecule of a xylan involves 1->4 linkages of xylopyranosyl units with Î±-(4-O)-methyl-D-glucuronopyranosyl units attached to anhydroxylose units. The result is a branched polymer chain that is mainly composed of five carbon sugar monomers, xylose, and to a lesser extent six carbon sugar monomers such as glucose.
Important aspects of the structure and composition of hemicellulose are the lack of crystalline structure, mainly due to the highly branched structure, and the presence of acetyl groups connected to the polymer chain (Kirk-Otmer).
Figure 4 A schematic representation of the hemicellulose backbone of arborescent plants
Hemicellulose extracted from plants possesses a high degree of polydispersity, polydiversity and polymolecularity (a broad range of size, shape and mass characteristics). However, the degree of polymerization does not exceed the 200 units whereas the minimum limit can be around 150 monomers.
Hemicellulose is insoluble in water at low temperature. However, its hydrolysis starts at a temperature lower than that of cellulose, which renders it soluble at elevated temperatures (Thermowoodhandbook, 2003). The presence of acid highly improves the solubility of hemicellulose in water.
Lignin is the most complex natural polymer. It is an amorphous three-dimensional polymer with phenylpropane units as the predominant building blocks. More specifically, p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (Figure 5) are the ones most commonly encountered.
Figure 5 P-coumaryl- , coniferyl- and sinapyl alcohol: dominant building blocks of the three-dimensional polymer lignin
Dividing higher plants into two categories, hardwood (angiosperm) and softwood (gymnosperm), it has been identified that lignin from softwood is made up of more than 90% of coniferyl alcohol with the remaining being mainly p-coumaryl alcohol units. Contrary to softwoods, lignin contained in hardwood is made up of varying ratios of coniferyl and sinapyl alcohol type of units (Kirk-Otmer, 2001).
The property of polydispersity, just as with hemicellulose, characterizes lignin as well. Different branching and bonding in otherwise similar molecules are encountered (Lin and Lin, 2002). Figure 6 presents a model structure of lignin from spruce pine.
Figure 6 Model structure of spruce lignin
Lignin in wood behaves as an insoluble three-dimensional network. It plays an important role in the cell's endurance and development, as it affects the transport of water, nutrients and metabolites in the plant cell. It acts as binder between cells creating a composite material that has a remarkable resistance to impact, compression and bending.
Solvents that have been identified to significantly dissolve lignin include low molecular alcohols, dioxane, acetone, pyridine, and dimethyl sulfoxide. Furthermore, it has been observed that at elevated temperatures, thermal softening of lignin takes place, which allows depolymeristation reactions of acidic or alkaline nature to accelerate (O'Connor et al., 2007).
As mentioned above, lignocellulose is a class of biomass that consists of three major compounds cellulose, hemicellulose and lignin. It also includes water and a small amount of proteins and other compounds, which do not participate significantly in forming the structure of the material (Raven et al., 1992). Inside the lignocellulose complex, cellulose retains the crystalline fibrous structure and it appears to be the core of the complex. Hemicellulose is positioned both between the micro- and the macrofibrils of cellulose. Lignin provides a structural role of the matrix in which cellulose and hemicellulose is embedded (Faulon et al., 1994).
Considering that cellulose is the main material of the plant cell walls, most of the lignin is found in the interfibrous area, whereas a smaller part can also be located on the cell surface (Kirk-Otmer, 2001).
Chemical interaction between components
There are four main types of bonds identified in the lignocellulose complex. Those are ether type of bonds, ester bonds, carbon-to-carbon bonds and hydrogen bonds. These four bonds are the main types of bonds that provide linkages within the individual components of lignocellulose (intrapolymer linkages), and connect the different components to form the complex (interpolymer linkages). The position and bonding function of the latter linkages is summarized in Table 2 (Faulon et al, 1994).
Table 2: Overview of linkages between the monomer units that form the individual polymers lignin, cellulose and hemicellulose, and between the polymers to form lignocellulose
Pretreatment is a crucial process step for the biochemical conversion of lignocellulosic biomass into e.g. bioethanol. It is required to alter the structure of cellulosic biomass to make cellulose more accessible to the enzymes that convert the carbohydrate polymers into fermentable sugars (Mosier et al., 2005). Pretreatment has been recognised as one of the most expensive processing steps in cellulosic biomass-to-fermentable sugars conversion and several recent review articles provide a general overview of the field (Alvira et al. 2009; Carvalheiro et al., 2008; Hendriks and Zeeman, 2008; Taherzadeh and Karimi, 2008).
Figure 7 Schematic presentation of effects of pretreatment on lignocellulosic biomass (Hsu et al, 1980)
Pretreatment involves the alteration of biomass so that (enzymatic) hydrolysis of cellulose and hemicellulose can be achieved more rapidly and with greater yields. Possible goals include the removal of lignin and disruption of the crystalline structure of cellulose (Figure 7). The following criteria lead to an improvement in (enzymatic) hydrolysis of lignocellulosic material:
â€¢ Increasing of the surface area and porosity
â€¢ Modification of lignin structure
â€¢ Removal of lignin
â€¢ (Partial) depolymerization of hemicellulose
â€¢ Removal of hemicellulose
â€¢ Reducing the crystallinity of cellulose
In an ideal case the pretreatment employed leads to a limited formation of degradation products that inhibit enzymatic hydrolysis and fermentation, and is also cost effective. However, these are actually the most important challenges of current pretreatment technologies. In the following sections the most common pretreatment techniques of biomass are described.
PRETREATMENT TECHNOLOGIES FOR LIGNOCELLULOSIC WASTES
Reduction of particle size is often needed to make material handling easier and to increase surface/volume ratio. This can be done by chipping, milling or grinding. Mechanical pretreatment is usually carried out before a following processing step, and the desired particle size is dependent on these subsequent steps. For mechanical pretreatment factors like capital costs, operating costs, scale-up possibilities and depreciation of equipment are very important.
Reduction of biomass size below #20 sieves shows the best mechanical performance (11).
Mechanical pretreatment technologies increase the digestibility of cellulose and hemicellulose in the lignocellulosic biomass. The use of mechanical chopping (12); hammer milling (13); grind milling (14); roll milling (15 vibratory milling (16) and ball milling (17) have proved success as a low cost pretreatment strategy. The pulverized materials with increased surface area have been found to facilitate the subsequent physicochemical and biochemical pretreatments of corn stover, barley straw sugar cane baggase, wheat straw, wood waste and municipal solid waste. They result to improved digestibility of cellulose and hemi-cellulose to glucan and xylan, respectively; they further enhance enzymatic digestibility with lower enzyme loads. Mechanical pretreatment also result to substantial lignin DE polymerization via the cleavage of uncondensed-aryl ether linkages (18). Solubility and fermentation efficiency of the natural lignocellulosic residues is also substantially increased by mechano-physicochemical pretreatment, leading to value-added utilization of these residues (19).
Elevated temperatures and irradiation are the most successful physical treatments in the processing of LCW. Thermogravimetric treatment of wood waste under both inert and oxidant atmospheres from room temperature up to 1100 K leads to moisture loss; hemicellulose, cellulose and lignin decomposition (Lapuerta et al., 2004). On the other hand, pyrolysis of nutshells, straws, sawdust and municipal solid wastes at temperatures of 600 - 1200 K result to yields of char, liquid and gaseous products of up to 55% of the original LSW (Puértolas et al., 2001; Demirbas, 2002; Bonelli, 2003; Chen et al., 2003; Álvarez et al., 2005; Phan et al., 2008; Zabaniotou et al., 2008).
Irradiation can cause significant breakdown of the structure of LSW. Microwave irradiation at a power of up to 700 W at various exposure times resulted to weight loss due to degradation of cellulose, hemicellulose and lignin, and the degradation rates are significantly enhanced by the presence of alkali (Zhu et al., 2005a, 2005b, 2006). In addition, gamma radiation has been shown by Yang et al. (2008) to cause significant
breakdown of the structure of powder of 140 mesh wheat straw, leading to weight loss and glucose yield of 13.40% at 500 kGy.
Combined chemical and physical treatment systems are of importance in dissolving hemicellulose and alteration of lignin structure, providing an improved accessibility of the cellulose for hydrolytic enzymes (20). The most successful physicochemical preatments include thermochemical treatments such as steam explosion or (steam disruption), liquid hot water (LHW), ammonia fiber explosion (AFEX) and CO2 explosion (21). In these processes, chipped biomass is treated with high-pressure saturated steam, liquid ammonia or CO2 and then the pressure is swiftly reduced, making the materials to undergo an explosive decompression.
Steam explosion is typically initiated at a temperature of 160 - 260°C (corresponding pressure of 0.69 - 4.83 MPa) for several seconds to a few minutes before the material is exposed to atmospheric pressure. The processes cause hemicellulose degradation and lignin transformation due to high temperature, thus increasing the potential of cellulose hydrolysis. Addition of H2SO4 (or SO2) or CO2 in steam explosion of LCW can effectively improve enzymatic hydrolysis, decrease the production of inhibitory compounds, and lead to more complete liquefaction of hemicellulose, glucan, xylan, mannan, galactan, and arabinan (22). Such pretreatments also lead to higher digestion efficiencies during production of monosaccharide's, oligosaccharides, lactic acid, antibacterial violet pigments and methane gas (23). Wet oxidation pretreatment at 200 - 210°C in the presence of alkali or Na2CO3 leads to LCW solubilization and better enzymatic convertibility to value-added products (25).
Liquid hot water (LHW) pretreatment utilizes pressurized hot water at pressure less than 5 Mpa and temperature range of 170 - 230°C for several minutes followed by decompression up to atmospheric pressure. Bagasse, corn stalk and straws of wheat, rice and barley pretreated by LHW have been reported to effect 80 - 100% hemicellulose hydrolysis, resulting to 45 - 65% xylose (26).
On the other hand, in AFEX treatment, the dosage of liquid ammonia ranging from 1 - 2 kg ammonia/kg dry biomass, temperature 90°C, and residence time of 30 min can significantly improve the saccharification rates (27). On CO2 explosion, 75% of the theoretical glucose released during 24 h of the enzymatic hydrolysis has been reported (28). Ethanol yield of up to 83% of the theoretical value has been achieved for LCW subjected to physicochemical treatment (29).
Chemicals ranging from oxidizing agents, alkali, acids and salts can be used to degrade lignin, hemicellulose and cellulose from LCW. Powerful oxidizing agents such as ozone and H2O2 effectively remove lignin; does not produce toxic residues for the downstream processes; and the reactions are carried out at room temperature and pressure (30). Alkali (NaOH, Ca(OH)2, NaOH urea, Na2CO3) hydrolyses of rice straw (31); spruce wood waste (Zhao et al., 2007); sugarcane, cassava and peanuts wastes (32); corn cob (33); organic fraction of municipal solid waste (34) have been investigated. When these pretreatments are performed by using 0.5 - 2 M alkali at 120 - 200°C, they substantially facilitate saccharification and improve enzymic hydrolysis of LCW.
Dilute and concentrated acids at high temperature are suited for hydrolysis of LCW. Studies by del Campo et al. (2006) and Karimi et al. (2006) have established that 0.5% H2SO4 is optimal for treatment of wastes from vegetables tables and rice straw, respectively. More concentrated H2SO4 (up to 2.5 M) has been shown to be able not only to hydrolyse cellulose and hemicellulose, but also in separating lignin and other organic components from LCW (34). SO2 and fly ash in flare gas; HNO3, HCl and polyhydric alcohol in the presence of sulfuric acid are also useful in LCW pretreatment (35): Recent studies have shown that when acids are combined with alkali, they play a more effective role in LCW pretreatment than acids and alkalis alone (36).
Organic acids such as oxalic, acetylsalicylic and salicylic acid can be used as catalysts in the organosolv process whereby an organic or aqueous organic solvent mixture with inorganic acids (HCl or H2SO4) are used to break the internal lignin and hemicellulose bonds. The organic solvents used in the process include methanol, ethanol, acetone, ethylene glycol, triethylene glycol and tetra hydro furfural alcohol (37). The use of a carboxylic acid catalyst, maleic acid, for hemicellulose hydrolysis in corn stover overcomes the technical and economic hurdle of hemicellulose hydrolysis (Lu and Mosier, 2007).
Biological treatment involves the use of whole organisms or enzymes in pretreatment of LCW. Both fungi and bacteria are used for bio treatment of LCW. Commercial preparations of fungal and bacterial hydrolytic and oxidative enzymes are also widely used instead of these microorganisms.
Fungal pretreatment of agricultural residues is a new method for improvement of digestibility (38). White-, brown- and soft-rot fungi are used to degrade lignin and hemicellulose in waste materials whereby brown rots mainly attack cellulose, while white and soft rots attack both cellulose and lignin. White-rot fungi are the most effective basidiomycetes for biological pretreatment of lignocellulosic materials (39). Recent studies have shown that Aspergillus terreus (40); Trichoderma spp (41); Cyathus stercoreus (42); Lentinus squarrosulus (43); Lentinus edodes (44); Trametes pubescens (45); Pleurotus spp (46); Penicillium camemberti (47), Phanerochaete chrysosporium (48) grown at 25 - 35°C for 3 - 22 days resulted to 45 - 75% and 65 - 80% holocellulose and lignin degradation, respectively. The postreatement by anaerobic biopro cesses of LCW effluents that have been pretreated with fungi can lead to higher biogas than the original effluents (49). Recombinant strains of Saccharomyces cerevisiae have been genetically engineered to carry out simultaneous saccharification and fermentation (SSF) to produce extracellular endoglucanase and _-glucosidase that are able to ferment cellulose and hemicellulose to 6-carbon and 5-carbon sugars and subsequent fermentation to ethanol (50). In bioorganosolv process, fungal (Ceriporiopsis subvermispora) pretreatment of wood waste for 2 - 8 weeks followed by organic solvent treatment at 140 - 200°C for 2 h has achieved considerable energy efficient delignification and hemicellulose hydrolysis (51).
Bacterial pretreatment of LCW involves both anaerobic and aerobic systems. Anaerobic degradation utilizes mainly mesophillic, rumen derived bacteria (52). Aerobic-anaerobic systems have an upper hand when it comes to degradation of LCW richer in lignin content (53) while in aerobic system alone, actinomycete Streptomyces griseus is able to produce high levels of extracellular hydrolytic enzyme that degrade lignocellulose (54). Escherichia coli and Klebsiella oxytoca strains have been genetically engineered to produce microbial biocatalysts that produce bioethanol from lignocellulosic materials (55).
Enzymatic pretreatment of LCW utilize hydrolytic and oxidative enzymes which are mainly derived from fungi and bacteria. Cellulases are usually a mixture of several enzymes. At least three major groups of cellulases are involved in the hydrolysis process: (1) endoglucanase (56) which attacks regions of low crystallinity in the cellulose fiber, creating free chain ends; (2) exoglucanase or cellobiohydrolase (CBH) (1,4-_-glucan cellobiohydrolase) which degrades the molecule further by removing cellobiose units from the free chainends and (3) _-glucosidase which hydrolyzes cellobiose to produce glucose (57). In addition, there are also a number of ancillary enzymes that attack hemicellulose, such as glucuronidase, acetylesterase, feruloylesterase, xylanase, _-xylosidase, galactomannanase and glucomannanase (58). During the enzymatic hydrolysis, cellulose is degraded by cellulases to reducing sugars that can be fermented by yeasts or bacteria to ethanol.
Ligninolytic enzymes are primarily involved in lignin degradation in oxidative reactions that are mainly free radical driven in the presence (or sometimes absence) of mediators. The main enzymes involved are lignin pero- xidase, manganese peroxidase and laccase (59). The hydrolytic and oxidative enzymatic reactions are mainly carried out at 30 - 45°C with low enzyme loading rate at reaction time of 6 - 26 h. All the pretreatment methods discussed above are summarized in Figure 8
Figure 8: A summary of various methods used in the pretreatment of lignocellulosic wastes
VALUE-ADDED PRODUCTS FROM LIGNOCELLULOSIC WASTES
Fermentable sugars comes first in the value chain of processed LCW with glucose, xylose, xylitol, cellobiose, arabinose, pentose and galactose being the main reduced sugars produced (60). In these sugar producing processes, hydrolysable sugars yield of up to 83.3% has been achieved at the reaction temperatures of 37 - 50°C for 6 -179 h at pH 5 - 6. The size of substrate added determines the amount of the saccharification products (61). In the enzymatic hydrolysis step using celluclast® supplemented with novozym®, a degree of saccharification of 100% has been achieved (62). Some transgenic plant residues have been reported to yield nearly twice as much sugar from cell walls compared to wild-types (63). Glucose seems to be the major monosaccharide product from LCW. The challenge facing depolymerization of hemicellulose into fermentable sugars is the requirement for a consortium of enzymes to complete the hemicellulose hydrolysis, leading to high enzyme costs. Efforts to overcome the problem include process improvement and the use of modified microorganisms that produce the required hemicellulose enzymes (64).
Lignocellulosic enzymes, mainly from fungi and bacteria, are important commercial products of LCW bioprocessing used in many industrial applications including chemicals, fuel, food, brewery and wine, animal feed, textile and laundry, pulp and paper and agriculture (65). Overall, extracellular enzymes are secondary metabolic products released in the presence of inducers at N-limited media (66). They include hydrolytic enzymes such as celluloses'; hemicellulose's and pectinases; degradative enzymes like amylases, proteases; and ligninolytic enzymes like laccases, peroxidases and oxidases. Celluloses production from LCW has been extensively studied (67). Phytases, mannanases and amylases are also produced by microorganisms using LCW as the main feedstock (68).
On the other hand, hemi-cellulolytic enzymes, mainly xylanases, are produced from a wide range of LCW biomass (69). Pectinases such as endopolygalacturonase (endo-PG), exo-polygalacturonase (exo-PG) and pectin liase are mainly produced from solid state fermentation processes utilizing agricultural residues (70), while protease has been produced by Penicillium janthinellum in submerged cultures (71). Among the ligninases produced from LCW, laccases are the mostly studied (72), followed by Manganese peroxidase and lignin peroxidase (73).
Very high enzyme activities (31,786 U/L) have been reported when the experiments are carried out under optimal conditions (pH 5.5 - 6: temperature 30 - 45°C) (74). Recovery of pure enzymes is achieved through 50 - 80% (NH4)2SO4 saturation followed by chromatographicall purification techniques (75). Several efforts have been made to increase the production of enzymes through strain improvement by mutagenesis and recombinant DNA technology. Cloning and sequencing of the various genes of interest could economize the enzymes production processes (76).
Worldwide, there is a growing concern over the fossil oil prices increase, the security of the oil supply and the negative impact of fossil fuels on the environment, particularly greenhouse gas emissions (77). Conversion of LCW to biofuels provides the best economically feasible and conflict-free second generation renewable alternatives (78). Significant advances have been made towards bioconversion of plant biomass wastes into bioethanol, biodiesel, biohydrogen, biogas (methane).
Production of ethanol from sugars or starch from sugarcane and cereals, respectively, impacts negatively on the economics of the process, thus making ethanol more expensive compared with fossil fuels. Hence, the technology development focus for the production of ethanol has shifted towards the utilization of residual
lignocellulosic materials to lower production costs (79). Currently, research and development of saccharification and fermentation technologies that convert LCW to reducing sugars and ethanol, respectively, in eco-friendly and profitable manner have picked tempo with breakthrough results being reported (80). Ethanol yield of 6 - 21% has been obtained through fermentation of agricultural and municipal residues (81). While microaeration enhances productivity of bioethanol from LCW using ethanologenic E. coli (82), simultaneous saccharification and fermentation (SSF) using recombinant Saccharomyces cereviasiae result to as high as 62% of the theoretical value (83). The principal benefits of performing the enzymatic hydrolysis together with the fermentation, instead of in a separate step after the hydrolysis, are the cofermentation of both hexoses and pentoses during SSF, reduced end-product inhibition of the enzymatic hydrolysis and the reduced investment costs (84). The long-term benefits of using waste residues as lignocellulosic feedstocks will be to introduce a sustainable solid waste management strategy for a number of lignocellulosic waste materials; contribute to the mitigation in greenhouse gases through sustained carbon and nutrient recycling; reduce the potential for water, air, and soil contamination associated with the land application of organic waste materials; and to broaden the feedstock source of raw materials for the bio-ethanol production industry (85).
Biodiesel is a renewable fuel conventionally prepared by transesterification of pre-extracted vegetable oils and animal fats of all resources with methanol, catalyzed by strong acids or bases (86). They are fatty acid methyl or ethyl esters used as fuel in diesel engines and heating systems (87). Production of biodiesel from lignocellulosic residues such as olive oil wastes has been a subject of researchtowards improving the thermal waste treatment systems and cleaner energy production (Arvanitoyannis et al., 2007a, 2007b). Since the current supplies from LCW based oil crops and animal fats account for only approximately 0.3%, biodiesel from algae is widely regarded as one of the most efficient ways of generating biofuels and also appears to represent the only current renewable source of oil that could meet the global demand for transport fuels (Schenk et al., 2008).
Hydrogen has been considered a potential fuel for the future since it is carbon-free and oxidized to water as a combustion product (88). While conventional burning or composting seem to be the most cost-effective hydrogen production methods, bacteria such as Enterobacter aerogenes and Clostridium sp isolates can convert saccharified LCW biomass into biohydrogen (89). Biohydrogen production from agricultural residues such as olive husk pyrolysis (90); conversion of wheat straw wastes into biohydrogen gas by cow dung compost (Fan et al., 2006); bagasse fermentation for hydrogen production (91) generate up to 70.6% gas yields. System optimization for accessibility of polysaccharides in LCW and the use of genetically efficient bacterial strains for agrowaste-based hydrogen production seem to be the ideal option for clean energy generation. Hydrogen gereration from inexpensive abundant renewable biomass can produce cheaper hydrogen and achieve zero net greenhouse emissions (92).
Biogas production from lignocellulosic materials is a steady anaerobic process where methane rich biogas comes mostly from hemicellulose and cellulose. Anaerobic biomethane production is an effective process for conversion of a broad variety of agricultural residues to methane to substitute natural gas and medium calorific value gases (Demirbas and Ozturk, 2005). Biogas containing 55 - 65% methane can be produced from jute caddis - a lignocellulosic waste of jute mills by anaerobic fermentation, using cattle dung as sole source of inoculum (93). Anaerobic digestion of poultry droppings, cow dung and corn stalk can give up to 137.16 L of biogas from 0.28 m3 digester (94). Mesophilic aerobic pretreatment to delignify sisal pulp waste prior to its anaerobic digestion has been shown to improve methane yields (Mshandete et al., 2005, 2008).
Overall, the success of biofuels production from LCW is dependent on the optimal performance and cost effectiveness of pretreatment and product generation processes.
Organic acids are some of the products of ligninolytic residues fermentations via environmentally friendly integrated processes. Volatile fatty acids including acetic acid, propionic acids and butyric acid are produced from a wide range of LSW such as cereal hulls (95); bagasse residues (96); food wastes (97) and sisal leaf decortications residues (98). In addition, lactic acid is produced from waste sisal stems (99), sugarcane bagasse (100) and kitchen waste (Ohkouchi and Inoue, 2007) by using Lactobacillus isolates. Furthermore, formic acid, levulinic acid, citric acid, valeric acid, caproic acid and vanillinic acid are obtainable from bioprocessing of LCW (101). Overall, organic acids production requires batch or continuous incubation conditions, the average reaction parameters being 35°C, pH 6.0, hydraulic retention time (HRT) of up to 8 days and organic loading rates of 9 g/l d. Product yields of up to 39.5 g/l have been reported (102).
Compost, a nutrient-rich, organic fertilizer and soil conditioner, is a product of humification of organic matter. This process is aided by a combination of living organisms including bacteria, fungi and worms which transform and enhance lignocellulosic waste into humic-like substances (103). Vermicomposting is the bio-oxidation and stabilization of organic matter involving the joint action of earthworms and microorganisms, thereby turning wastes into a valuable soil amendment called vermicomposting (104). Substrates suitable for making humus-rich compost include cereal straw and bran (105); urban wastes (106); water hyacinth (107); lemon tree pruning's, cotton waste and brewery waste (108); horticultural wastes (109); olive, palm and grape wastes (110). While bacteria inoculants such as Bacillus shackletonni, Streptomyces thermovulgaris and Ureibacillus thermosphaericus are used to improve the composting process (111), ligno-cellulolytic fungi inocula (e.g. Trichurus spiralis) may also be used in a pretreatment process
before composting in order to reduce the resistance of the substrate to biodegradation (Hart et al., 2003; Vargas-García et al., 2007). A new earthworm strain of Perionyx sansibaricus is able to humify a substrate combination of guar gum industrial waste, cow dung and saw dust (112). Composting can, therefore, be considered as a low-cost technology to convert agro industrial LCW into value-added biofertilizers.
Biodegradable polymers constitute a loosely defined family of polymers that are designed to degrade through the action of living organisms. Such commercially available biodegradable polymers are polycaprolactone, poly (lactic acid), polyhydroxyalkanoates, poly (ethylene
glycol), and aliphatic polyesters like poly (butylene succinate) (PBS) and poly (butylene succinate-co-butylene adipate) (113). Lignocellulosic material-thermoplastic polymer composites are among the emerging products of LCW. In most cases, lignocellulosic biomass flour is used as the reinforcing filler and polypropylene as the thermoplastic matrix polymer to manufacture particle-reinforced composites (114). Natural fibres from LCW are considered to be of low-cost by-products, environmentally friendly and practically sustainable raw materials (115). Evaluations of LCW fiber plastic composites utilizing wood fibre wastes (116); wheat and rice straw (117); jute/cotton, sisal/cotton and ramie/cotton hybrid fabrics (118); non-wood plant fibres (119); waste newsprint paper (120); flax and hemp (121); oil palm wastes (122); cotton gin waste (123); banana fibres (124); cereal husks (Yang et al., 2004b, 2007; López et al., 2007); tissue paper wastes and corn peels (125); bagasse (126) and nanofibers from the agricultural residues (127) have shown that such composites are suitable for making products that have improved biodegradability, mechanical strength, thermal stability, electrical conductivity and recyclability.
Treated LCW wastes are also used in the construction industry for manufacturing of light-weight agro-gypsum panels (128) and lightweight sand concretes (129) with improved structural and thermal properties. Biocomposites are very promising in producing sustainable current and future green materials to achieve durability without using toxic chemicals. The challenge facing the biocomposite industry is to make materials that have better rubber/fiber interface, improved wettability and compatibility.
Food and feed
Bioconversion of lignocellulosic agro-residues through mushroom cultivation and single cell protein (SCP) production offer the potential for converting these residues into protein-rich palatable food and reduction of the environmental impact of the wastes.
Mushrooom cultivation provides an economically acceptable alternative for the production of food of superior taste and quality which does not need isolation and purification (130). Cultivation of edible mushrooms such as Lentinus spp, Lentinula spp, Leonotis spp, Pleurotus spp, Agaricus spp, Agrocybe spp, Volvariella spp, Lentinus spp and Grifola spp is achievable on a wide range of LCW substrates such as wood waste, corncob meal, wheat straw, barley straw, soybean straw, cereal bran, cotton waste, sorghum stalk, banana pseudostem, hazelnut husks, waste tea leaves, dry weed plants, peanut shells, waste paper and olive mill wastewater (131). Mushrooms with increased number of fruit bodies and high contents of protein and total carbohydrates are obtained when LCW substrates are used in combination.
On the other hand, SCP production from LCW offers a potential substrate for conversion of low-quality biomass into an improved animal feed and human food. SCP is the protein extracted from cultivated microbial biomass. It can be used for protein supplementation of a staple diet by replacing costly conventional sources like soymeal and fishmeal to alleviate the problem of protein scarcity. Moreover, bioconversion of agricultural and industrial wastes to protein-rich food and fodder stocks has an additional benefit of making the final product cheaper (Anupama and Ravindra, 2000). Removal of nucleic acids and toxins from SCP is key to ensure the safety of food and feed. Among the SCP obtained from LCW using agricultural wastes as the main growth media, Saccharomyces cerevisiae, Trichoderma reesei and Kluyveromyces marxianus top the list (132). SCP yield of 51 and 39.4% efficiency of conversion of beet-pulp into protein has been reported from the above strains. Solid state fermentation of LCW seems to be the most preferred culturing method, while cloning is being considered as a suitable technique for improvement of SCP production (133).
LCW provides a suitable growth environment for mushrooms that comprise a vast source of powerful new pharmaceutical products. In particular, Lentinula edodes, Tremella fuciformis and Ganoderma lucidum contain bioactive compounds such as anti-tumor, anti-inflammatory, anti-virus and anti-bacterial polysaccharides. Moroever, they contain substances with immunomodulating properties, as well as active substances that lower choresterol (134). Future prospects for research on bioactive compounds from fungi grown on such cheap and ubiquitous substrates look bright and could lead to breakthroughs in the search for antibacterial, antiviral and anticancer chemotherapies.
Adsorbents obtained from plant wastes are feasible replacements for costly conventional methods of removing pollutants such as heavy metals ions, dyes, ammonia and nitrates from the environment. The use of lignocellulosic agrowastes is a very useful approach because of their high adsorption properties, which results from their ion-exchange capabilities. Agricultural wastes can be made into good sorbents for the removal of many metals, which would add to their value, help reduce the cost of waste disposal, and provide a potentially cheap alternative to existing commercial carbons (135). Chemically modified plant wastes such as rice husks/rice hulls, spent grain, sugarcane bagasse/fly ash, sawdust, wheat bran, corncobs, wheat and soybean straws, corn stalks, weeds, fruit/vegetable wastes, cassava waste fibres, tree barks, azolla (water fern), alfalfa biomass, coirpith carbon, cotton seed hulls, citrus waste and soybean hulls show good adsorption capacities for Cd, Cu, Pb, Zn and Ni (136). They are usually modified with formaldehyde in acidic medium, NaOH, KOH/K2CO3 and CO2, or acid solution or just washed with warm water (137). Scanning electron micrographs with energy spectra shows that heavy metals are immobilized via two possible routes: adsorption and cation exchange on hypha, and the chelation by fungal metabolite (138).
LCW have also been shown to be able to adsorb dyes from aqueous solutions. Adsorption of reactive dyes by sawdust char and activated carbon (139); ethylene blue by waste Rosa canina sp. seeds (140); anionic dyes by hexadecyltrimethylammoniummodified coir pith (141); and methylene red by acid-hydrolysed beech sawdust (142) have been reported. Ammonia and nitrate removal by using agricultural waste materials as adsorbents or ion exchangers have also been studied (143). Prehydrolysis enhances the adsorption properties of the original LCW material due to the removal of the hemicelluloses during sulphuric acid treatment, resulting in the 'opening' of the lignocellulosic matrix's structure, the increasing of the surface area and the activation of the material's surface owing to an increase in the number of dye binding sites (144). The main value-added products from LCW are generally summarized in Figure 9.
Figure 9 : The main value-added products from lignocellulosic wastes (SSF=simultaneous fermentation and saccharification,
VFAs = volatile fatty acids).
Pretreatment of lignocellulosics aims to decrease crystallinity of cellulose, increase biomass surface area, remove hemicellulose, and break the lignin barrior. Pretreatment makes cellulose more accessible to hydrolytic enzymes to facilitate conversion of carbohydrate polymers into fermentable sugars in a rapid way with the concomitant more yield. Therefor it is an very important process which can
Hydrolysis of non-pretreated materials is slow, and results in low product yield. Some pretreatment methods increase the pore size and reduce the crystallinity of cellulose. Pretreatment also makes cellulose more accessible to the cellulolytic enzymes, which in return reduces enzyme requirements and thus the cost therefore it is essential to have pretreatment of lignocellulosic residues
Features of promising technologies for pretreatment of lignocellulosic biomass.
Mosier N, Wyman C, Dale B, Elander R, Lee YY, Holtzapple M, Ladisch M.
Laboratory of Renewable Resources Engineering, Department of Agricultural and Biological Engineering, Purdue University, Potter Engineering Center, 500 Central Drive, West Lafayette, IN 47907-2022, USA.
Cellulosic plant material represents an as-of-yet untapped source of fermentable sugars for significant industrial use. Many physio-chemical structural and compositional factors hinder the enzymatic digestibility of cellulose present in lignocellulosic biomass. The goal of any pretreatment technology is to alter or remove structural and compositional impediments to hydrolysis in order to improve the rate of enzyme hydrolysis and increase yields of fermentable sugars from cellulose or hemicellulose. These methods cause physical and/or chemical changes in the plant biomass in order to achieve this result. Experimental investigation of physical changes and chemical reactions that occur during pretreatment is required for the development of effective and mechanistic models that can be used for the rational design of pretreatment processes. Furthermore, pretreatment processing conditions must be tailored to the specific chemical and structural composition of the various, and variable, sources of lignocellulosic biomass. This paper reviews process parameters and their fundamental modes of action for promising pretreatment methods.