Cellulose from Lignocellulosic Waste
Bioconversion of renewable lignocellulosic biomass to biofuel and value-added products is globally gaining significant importance. Lignocellulosic wastes are the most promising feedstock considering its great availability and low cost. Biomass conversion process involves mainly two steps: hydrolysis of cellulose in the lignocellulosic biomass to produce reducing sugars and fermentation of the sugars to ethanol and other bioproducts. However, sugars necessary for fermentation are trapped inside the recalcitrant structure of the lignocellulose. Hence, pretreatment of lignocellulosic wastes is always necessary to alter and/or remove the surrounding matrix of lignin and hemicellulose in order to improve the hydrolysis of cellulose. These pretreatments cause physical and/or chemical changes in the plant biomass in order to achieve this result. Each pretreatment has a specific effect on the cellulose, hemicellulose, and lignin fraction. Thus, the pretreatment methods and conditions should be chosen according to the process configuration selected for the subsequent hydrolysis steps. In general, pretreatment methods can be classified into four categories, including physical, physicochemical, chemical, and biological pretreatment. This chapter addresses different pretreatment technologies envisaging enzymatic hydrolysis and microbial fermentation for cellulosic ethanol production and other bioproducts. It primarily covers the structure of lignocellulosic wastes; the characteristics of different pretreatment methods; enzymatic hydrolysis; fermentation and bioproducts; and future research challenges and trends.
KeywordsCellulose Lignocellulosic wastes Biorefinery Pretreatment Enzymatic hydrolysis Fermentation Bioproducts
Degree of polymerization
Food and Agriculture Organization of the United Nations
Generally recognized as safe
International Union of Biochemistry and Molecular Biology
Separate hydrolysis and fermentation
Simultaneous saccharification to co-fermentation
Simultaneous saccharification and fermentation
Semi-simultaneous saccharification and fermentation
Secondary wall inner layer
Secondary wall middle layer
Secondary wall outer layer
World Health Organization
Conventional petroleum refineries utilize physical and chemical processes to refine crude oil to different fractions that are used for the production of several products as fuels, chemicals, and materials. However, these products have one more thing in common besides the raw material; they are produced because of their economic value. On the other hand, bioprocessing and bioproducts have gained commercial interest because of the perceived “green” advantages of using biomass rather than fossil energy for the production of chemicals and industrial products.
There are several issues influencing the current global interest in the biorefining of biomass feedstocks, more specifically lignocellulosic wastes, rather than fossil reserves, to produce a wide variety of extracts, fuels, and chemicals: the fossil fuels have finite reserves and are nonrenewable, the higher increase of the global warming when fossil fuels are burned to provide energy, and the security of supply of fossil fuels as commodities may not be reliable in the future due to the world regions they originated (Charlton et al. 2009; Anwar et al. 2014). This has led to the concept of biorefinery, which, according to American National Renewable Energy Laboratory, is defined as “a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass” (www.nrel.gov/biomass/biorefinery.html). Thus, biorefinery concept is analogous to today’s petroleum refineries (Yang 2007; Taylor 2008; Cavka 2013).
Availability of resources seems not to be a short-term problem into biorefinery. A multitude of feedstock has been tested for biorefinery applications covering cultivated crops, agricultural wastes, forest resources, urban and industrial wastes, and algae (Kajaste 2014). Recent studies indicate that the demand for biomass to produce required biofuels and platform chemicals of the petrochemical industry can be covered without significant changes to the current agricultural land use. However, there is a clear shift from agricultural products to lignocellulosic feedstocks (plant biomass predominantly comprised of cellulose, hemicellulose, and lignin) for the production of chemicals, using different pretreatments for the achievement of specific fractionation results, depending on the desired solid or liquid by-product to obtain (Bos and Sanders 2013; Forster-Carneiro et al. 2013; García et al. 2014a).
Currently the most promising and abundant cellulosic feedstocks derived from plant residues in the USA, South America, Asia, and Europe are corn stover, sugarcane bagasse, rice, and wheat straws, respectively (Limayem and Ricke 2012; Phitsuwan et al. 2013), as they are abundant, low-cost, nonfood materials.
However, the expertise required to exploit biomass as a viable source of base commodities is also diverse and requires a multidisciplinary approach. Taking into account the emerging research trends, the concept of fractionating biomass into its core constituents, an important step in the development of biorefining technologies, has the potential to benefit a wide range of bioprocessing industries due to the ease and improved efficiency associated with working with less variable material feedstocks (FitzPatrick et al. 2010).
2 Structure of Lignocellulosic Wastes
Lignocellulosic wastes are the most abundant source of unutilized biomass. They are composed mainly of cellulose (30–50 %), hemicellulose (15–35 %), and lignin (10–20 %) (Mielenz 2001; Gírio et al. 2010). Celluloses and hemicelluloses correspond to approximately 70 % of the entire biomass and are tightly linked to the lignin through covalent and hydrogenic bonds that make the structure highly robust and resistant to any kind of hydrolysis (Knauf and Moniruzzaman 2004; Limayem and Ricke 2012). This highly recalcitrant nature is related to the presence of lignin, the degree of crystallinity of cellulose, the degree of polymerization of the polysaccharides, the available surface area, and the moisture content (Van Dyk and Pletschke 2012). In addition, other components can be found to a lesser extent, such as pectin, proteins, extractives, and ash (Jorgensen et al. 2007; Gírio et al. 2010).
Composition of some lignocellulosic wastes
Nabarlatz et al. 2007
Alves et al. 2010
John et al. 2006
Parajó et al. 2004
Nabarlatz et al. 2007
Garrote et al. 2002
Parajó et al. 2004
Nabarlatz et al. 2007
Garrote et al. 2007
Michelin et al. 2012
Jiménez et al. 2007
Parajó et al. 2004
Alves et al. 2010
Petersson et al. 2007
Kim et al. 2009
Kim et al. 2011
Petersson et al. 2007
Nabarlatz et al. 2007
Olive tree biomass
Cara et al. 2008
Vázquez et al. 2007
Song and Wei 2010
Alves et al. 2010
Jiménez et al. 1990
Parajó et al. 2004
Garrote et al. 2007
Nabarlatz et al. 2007
Kim et al. 2008
Jiménez et al. 2007
Jiménez et al. 1990
Saha et al. 2005
Nabarlatz et al. 2007
Ruiz et al. 2011a
Petersson et al. 2007
The most predominant component found in all plant cell walls is cellulose. It is a linear homopolymer of d-glucose monomers linked by β-1,4-glycosidic bonds that can contain over 10,000 glucose units (Cheng and Timilsina 2011). The cellulose chains are cross-linked by strong hydrogen bonds to form cellulose microfibrils. These microfibrils exhibit a crystalline region, which is very recalcitrant to degradation and a small part of amorphous cellulose, which is easier to degrade (Van Dyk and Pletschke 2012). The orientation of cellulose microfibrils in the secondary cell wall has a strong effect on the structural properties of various plant types (Abdul Khalil et al. 2012).
The cellulose microfibrils are present in the secondary cell wall usually embedded in an amorphous matrix of hemicelluloses and lignin (Cziple and Marques 2008; Huber et al. 2012). These microfibrils in the matrix are often associated in the form of bundles or macrofibrils (Menon and Rao 2012), and lignin and hemicellulose fill the spaces between cellulose microfibrils in primary and secondary cell walls, as well as the middle lamellae (Eriksson and Bermek 2009).
The high molecular weight and ordered tertiary structure makes natural cellulose insoluble in water. Although starchy materials require temperatures of only 60–70 °C to be converted from crystalline to amorphous form, cellulose requires 320 °C and a pressure of 25 MPa to change from a rigid crystalline structure to an amorphous structure in water (Deguchi et al. 2006; Limayem and Ricke 2012). Cotton, flax, and chemical pulp represent the purest sources of cellulose (up to 90 % cellulose), while wood contains approximately 50 % cellulose (Aitken 2012; Limayem and Ricke 2012).
Hemicellulose is another polysaccharide found in abundance in plant cell walls. It is a complex branched heteropolymer of sugars and sugar derivatives which form a highly branched network. It consists of about 100–200 sugar units typically made up of different sugars including hexoses (d-glucose, d-galactose, and d-mannose), pentoses (d-xylose and l-arabinose), and/or sugar acids (uronic acids), namely, d-glucuronic, d-galacturonic, and 4-O-methyl-D-glucuronic acids (Cheng and Timilsina 2011; Limayem and Ricke 2012). Other sugars such as α-l-rhamnose and α-l-fucose may also be present in small amounts, and the hydroxyl groups of sugars can be partially substituted with acetyl groups (Gírio et al. 2010).
Xylans are the main hemicellulose components of secondary cell walls, its backbone chain is primarily composed of d-xylose residues (nearly 90 %) linked by β-1,4-glycosidic bonds (Gírio et al. 2010; Limayem and Ricke 2012). Most xylans occur as heteropolysaccharides, containing different substituent groups in the backbone chain such as acetyl, arabinosyl, and glucuronosyl residues (Michelin et al. 2013). Branch frequencies vary depending on the nature and the source of feedstocks. The hemicelluloses of softwood are typically glucomannans, while hardwood hemicellulose is more frequently composed of xylans (McMillan 1994).
The primary role of the hemicelluloses has been proposed to act as a bonding agent between lignin and cellulose. Its covalent linkage to lignin and its noncovalent interaction with cellulose may be important in maintaining the integrity of the cellulose in situ and helping to protect the fibers against degradation by cellulases (Uffen 1997; Michelin et al. 2013).
Lignin is another highly polymerized molecule but is quite amorphous relative to cellulose and hemicellulose (Aitken 2012). It is a complex hydrophobic cross-linked aromatic biopolymer with a molecular weight of 10,000 Da (Mielenz 2001; Limayem and Ricke 2012), composed of three major phenolic components, namely, p-coumaryl, coniferyl, and sinapyl alcohol (Menon and Rao 2012).
Its ratio varies between different plants, wood tissues, and cell wall layers (Menon and Rao 2012). Older and more woody plants contain higher levels of lignin deposited in cell walls to provide rigidity and strength, conferring impermeability to cell walls and forming an effective physic–chemical barrier against microbial attack (Raven et al. 1999; Kumar et al. 2009; Van Dyk and Pletschke 2012) and oxidative stress. Therefore, it is the most recalcitrant component of lignocellulosic material to degradation (Himmel et al. 2007; Sánchez 2009; Michelin et al. 2013).
Lignin covalently bonds to hemicellulose and is responsible for much of the mechanical strength of wood (Aitken 2012). Forest woody biomass is primarily composed of cellulose and lignin polymers. Softwood barks have the highest level of lignin (30–60 %) followed by the hardwood barks (30–55 %), while grasses and agricultural residues contain the lowest level of lignin (10–30 % and 3–15 %, respectively) (Demirbas 2005).
3 Biorefinery Processing of Cellulose
Nowadays, the application of a biorefinery process based on cellulose has increased, for example, in the pulp and paper industry (Jahan et al. 2013). Moshkelani et al. (2013) mentioned that the incorporation of a biorefinery unit into an operating kraft pulping process has significant technological, economic, and social advantages over the construction of a grassroot biorefinery. Recently, Fornell et al. (2013) performed a techno-economic analysis of a kraft pulp mill-based biorefinery producing both ethanol and dimethyl ether and concluded that the biorefinery can be self-supported in terms of fuel if the process is well integrated. Pettersson and Harvey (2012) reported black liquor gasification as an alternative technology for energy and chemical recovery in kraft pulp mills by comparing the black liquor gasification with downstream production of dimethyl ether or electricity and with recovery boiler-based pulping biorefinery for different types of mills. In a recent work, Lundberg et al. (2014) investigated the conversion of an existing Swedish kraft pulp mill to the production of dissolving pulp, with export of electricity, lignin, and hemicellulose stream suitable for upgrading, and they concluded that the profitability of this kraft pulp mill biorefinery is very dependent on the particular mill and the specific investment needs. Moreover, there are few companies that operate under the concept of biorefinery. An overview of almost all the biorefinery demonstration and pilot plants has been published in www.iea-bioenergy.task42-biorefineries.com, and new plants are continuously being built around the world, the USA being one of the main operators of biorefineries and producer of biofuels and high value-added compounds.
3.1 Pretreatment Technologies
Due to the robust structure of lignocellulosic wastes, a pretreatment is required to alter its structure and chemical composition and promote fractionation (Fig. 3). According to Romaní et al. (2013a), the criteria for an effective pretreatment include: (1) avoiding size reduction, (2) preserving hemicellulose or lignin fraction, (3) limiting formation of inhibitors or sugar degradation, (4) minimizing energy input, and (5) being cost-effective. Yang and Wyman (2008) mentioned that the choice of pretreatment technology is not trivial and must take into account sugar production and solid concentrations for each pretreatment in conjunction with their compatibility with the overall process, feedstock, enzymes, and organisms to be applied and to meet the biorefinery concept. In the following sections, some different pretreatments used in second-generation processing of lignocellulosic wastes will be addressed.
3.1.1 Physical Pretreatment
Some of the important variables in the pretreatment process are residence time, temperature, and particle size. Normally, when larger chips are used, heat transfer problems lead to overcooking of the exterior (with associated formation of inhibitors) and incomplete hydrolysis of the interior. This problem can be overcome by reducing particle size before the application of the pretreatment. This size reduction process not only changes the particle size and shape, but also increases bulk density, improves flow properties, and increases porosity and surface area. This higher surface area increases the number of contact points for chemical reaction (Ruiz et al. 2011a). Also, reduction of the crystallinity of the cellulose is one of the important objectives of physical pretreatment. Chipping, grinding, and milling are among the different mechanical size reduction methods that have been used to enhance the digestibility of lignocellulosic wastes (Agbor et al. 2011). However, the power and energy requirement of this pretreatment is relatively high depending on the final particle size and the lignocellulosic waste characteristics. Delgenes et al. (2002) cited by Agbor et al. (2011) reported that the milling pretreatment increases biogas, biohydrogen, and bioethanol yields.
Extrusion pretreatment is a promising physical pretreatment method for lignocellulosic waste conversion to bioethanol production (Alvira et al. 2010). According to Lamsal and Brijwani (2010), the extrusion pretreatment can provide a unique continuous reactor environment for a combination of thermomechanical and chemical pretreatment of lignocellulosic wastes at higher yields. The materials are subjected to heating, mixing, and shearing, resulting in physical and chemical modifications during the process (Alvira et al. 2010). Lamsal et al. (2010) compared the particle size reduction and thermochemical extrusion pretreatment for the structural modification of wheat bran and soybean hull as raw material. They concluded that the use of chemical pretreatment in combination with extrusion did not result in improvement in hydrolysis of the lignocellulosic materials. Washing of thermomechanically treated samples was found to be very critical in maximizing the reducing sugar yields. Karunanithy et al. (2012) investigated the effect of biomass moisture content and extruder parameters such screw speed and barrel temperature on sugar recovery. They concluded that these variables were significant to the cellulose, hemicellulose, and total sugar recovery.
3.1.2 Physicochemical Pretreatment
18.104.22.168 Autohydrolysis Pretreatment
Autohydrolysis process is an environmentally friendly process in which the lignocellulosic wastes are pretreated with compressed hot water; it is based on the selective depolymerization of hemicellulose, which is catalyzed by hydronium ions generated in situ by water autoionization and by acetic acid from acetyl groups. Moreover, autohydrolysis pretreatment causes re-localization of lignin on the surface of lignocellulosic biomass. This process avoids difficult steps in chemical handling and recovery (e.g., sulfuric or hydrochloric acid) compared with dilute sulfuric acid or base pretreatment (Ruiz et al. 2012a, 2013a). Autohydrolysis process has been considered a cost-effective pretreatment, and in general, the major advantages that this process offers are as follows: (1) the process does not require the addition and recovery of chemicals different from water, (2) limited equipment corrosion problems, and (3) simple and economical operation. For that reason, autohydrolysis can be considered an environmentally friendly fractionation process.
According to Ruiz et al. (2013a), after autohydrolysis pretreatment, the cellulose shows degradation at temperatures >230 °C. It has been reported that the cellulose started to degrade in hexoses and oligosaccharides above 230 °C, and almost all cellulose was decomposed at 295 °C (Sakaki et al. 2002). An application of the solid residue (cellulose + lignin) after autohydrolysis pretreatment is used as raw material for pulp and paper marking (Ruiz et al. 2011b). Alfaro et al. (2010) reported that the cellulose pulp autohydrolysis reduces Kappa number and viscosity and decreases paper strength. Vila et al. (2011) studied the susceptibility of autohydrolysis solids to kraft processing pulp, being obtained cellulose pulps with low Kappa numbers and highly susceptible to alkaline oxygen bleaching.
22.214.171.124 Microwave Pretreatment
Microwaves as an alternative heating source have been successfully applied for extraction of numerous biologically active compounds from a wide variety of natural resources, because it is characterized as a selective, efficient, and environmental friendly process. This technique consists in a rapid delivery of energy to the total volume and subsequent rapid heating of the material structure accelerating the solubilization of compounds. Polar solvents have permanent dipole moment and can absorb microwave radiation, and water as a polar compound can absorb the microwave energy and transfer it to the sample, having as advantage over the organic solvents being a secure and ecologic reagent (Rodriguez-Jasso et al. 2011).
Budarin et al. (2010) reported the interaction between cellulose with microwave irradiation with high quality fuels produced from biomass. The substrate pretreated with microwave process has also been used for cellulose production. Zhao et al. (2010) reported that when some substrates were pretreated with microwave irradiation, the reducing sugar content and carboxymethylcellulase production were increased, and most cellulase was produced by the substrates pretreated at 450-W microwave for 3 min. Moreover, the microwave pretreatment has been used for enhancing enzymatic hydrolysis of cellulose (Peng et al. 2013).
126.96.36.199 Steam Explosion Pretreatment
This process is also considered as a hydrothermal process and has almost the same fundament of autohydrolysis. However, in this process, the high pressure steaming is followed by a rapid decompression. Steam explosion of lignocellulosic wastes has been largely studied in the last 15–20 years. This physicochemical method modifies lignocellulose material to allow the fractionation of the three polymer streams: hemicellulose into the liquor, and lignin and cellulose remaining in the insoluble fraction. Steam pretreatment of the lignocellulosic materials at temperatures of 160–220 °C generates acetic acid (from hemicellulose), which catalyzes the hydrolysis of hemicellulose resulting in its solubilization. When the process reaction is performed under high pressures followed by explosive decompression, the resulting residue is rendered highly degradable by enzymes and microorganisms.
Deepa et al. (2011) extracted cellulose nanofibers from banana fibers by steam explosion pretreatment; these nanofibers had better thermal properties over the untreated fibers. Romaní et al. (2013b) used steam exploded pretreated solids (Eucalyptus globulus as raw material) subjected to simultaneous saccharification and fermentation at high solids loading and reached an ethanol concentration of 51 g/L with a 91 % yield. Oliveira et al. (2013) reported the use of an industrial-scale steam explosion pretreatment of sugarcane straw for the production of bioethanol and high value-added chemicals.
188.8.131.52 Ultrasound Pretreatment
The ultrasound pretreatment is a new emerging technology that has potential as an alternative pretreatment technology. According to Yunus et al. (2010), the ultrasound pretreatment does not hydrolyze the biomass to soluble sugars. The function is to generate a pretreated substrate that is more easily hydrolyzed via increasing the accessible surface area and affecting the crystallinity. This process has been recently reviewed by Bussemaker and Zhang (2013) for biorefinery and biofuel applications, and they commented that the ultrasonic waves create pressure differences within a solution for the enhancement of physical (mechano-acoustic) and chemical (sonochemical) processes; this ultrasonic phenomenon is generated by either piezoelectric or magnetostrictive transducers, where the transducer is attached to a vessel filled with a sonication solution and the mechanical vibrations of the piezoelectric material creates a pressure wave through the solution. This effect of ultrasound on lignocellulosic wastes has been employed to improve the extractability of hemicelluloses, cellulose, and lignin or to get clean cellulosic fiber from used paper; however, only a few attempts to improve the susceptibility of lignocellulosic materials according to biorefinery concept have been made (Bussemaker et al. 2013).
Yunus et al. (2010) studied the effect of ultrasonic pretreatment on oil palm empty fruit bunch fiber prior to acid hydrolysis, and they concluded that the exposition of this material to ultrasonication power has a marked effect on the efficiency of low temperature acid hydrolysis. García et al. (2011) reported the ultrasound pretreatment of lignocellulosic waste and showed that the application of this pretreatment improved the effectiveness of the classic pretreatments, obtaining higher yield and selectivity of the products. Nikolic et al. (2011) reported the utilization of microwave and ultrasound pretreatments in the production of bioethanol using corn as raw material, having concluded that ultrasonic and microwave pretreatments effectively increased the glucose concentration obtained after liquefaction and consequently improved the ethanol yield and productivity during the SSF process.
3.1.3 Chemical Pretreatment
Several chemical pretreatments have been studied in order to remove the hemicellulosic fraction, to cleave the bindings between the lignin and the polysaccharides, and to distort the arrangement of the cellulose crystallinity, improving the accessibility of the cellulose component to the action of hydrolytic enzymes so that an efficient hydrolysis of carbohydrates to fermentable sugars occurs (Sun and Cheng 2002; Mosier et al. 2005). Moreover, the combination of sequential chemical pretreatments is an interesting alternative for the lignocellulosic wastes being in accordance with the biorefinery concept (Ruiz et al. 2011b).
184.108.40.206 Alkaline Pretreatment
The alkaline pretreatment is one of the most studied and makes use of several reagents as sodium hydroxide, calcium hydroxide, potassium hydroxide, aqueous ammonia, and ammonia hydroxide, sometimes in mixture with hydrogen peroxide (Chen et al. 2013; Gonçalves et al. 2014). According to Chen et al. (2013) and García et al. (2014a), alkali reagent is believed to cleave hydrolyzable linkages in lignin and glycosidic bonds of polysaccharides causing a reduction in the degree of polymerization, increasing the internal surface area and decreasing crystallinity of the cellulose and disruption of the lignin structure which increases the reactivity of the remaining polysaccharides as delignification occurs (Pedersen and Meyer 2010). Also, the alkali pretreatment is effective for agricultural residues and herbaceous crops, due to the smaller amount of lignin present in these types of materials (Galbe and Zacchi 2012). The alkaline pretreatment has as advantage the low energetic demand due to the severity of the alkaline media, where high reaction temperatures are usually not required (below 140 °C) and as disadvantage the alkali price and the difficulty of its recuperation that still involves prohibitive costs (Sun and Cheng 2002; Hamelink et al. 2005).
In a recent work, Gonçalves et al. (2014) studied the production of bioethanol using a sequential alkali pretreatment with hydrogen peroxide and sodium hydroxide, obtaining a high susceptibility of pretreated materials to enzymatic action. Selig et al. (2009) pretreated corn stover with alkaline peroxide at pH 11.5 resulting in reduction of lignin content in the pretreated solids and improvement of sugar production by cellulases. Prinsen et al. (2013) studied the modification of the lignin structure of a eucalyptus feedstock during alkaline delignification by kraft, soda–anthraquinone, and soda-O2, indicating that soda-O2 process produced higher lignin degradation and provides results as a pretreatment for the deconstruction of eucalyptus feedstock for subsequent use in lignocellulose biorefineries.
220.127.116.11 Acid Pretreatment
The use of acid pretreatment to catalyze the hydrolysis of lignocellulosic wastes in their sugar constituents is well known and effective. Acid pretreatment has been considered as a suitable technology for industrial-scale bioethanol production from glucose and hemicellulosic sugars (Ruiz et al. 2013c). Different types of acids have been used in these pretreatments such as: phosphoric, sulfuric, and organic acid (oxalic, citric, tartaric and acetic) (Qin et al. 2012; Avci et al. 2013; Ruiz et al. 2013c).
Castro et al. (2014) optimized the phosphoric acid pretreatment of Eucalyptus benthamii wood chips and produced bioethanol from this pretreated material with an ethanol yield of 240 g ethanol/kg of raw material. García et al. (2014b) produced bioethanol from the shells of Jatropha curcas using dilute sulfuric acid pretreatment, the cellulose conversion being above 80 %.
18.104.22.168 Organosolv Pretreatment
As alternative to conventional chemical pulping, processes utilizing aqueous organic solvents, known as organosolv, have been studied in the last 30 years. This pretreatment involves the use of organic solvents or their aqueous solutions for extracting lignin, based on the fact that lignin can be solubilized in certain solvents in a wide range of temperatures (100–250 °C) (González Alriols et al. 2009; Agbor et al. 2011). During the organosolv process, the lignin structure is broken into smaller parts and dissolved from the raw material and separated in the form of a liquor rich in phenolic compounds that represents the process effluent. This lignin can be isolated and has the advantage of being a relatively pure product with excellent properties that may be used as a precursor in the production of various commercial products according to the biorefinery concept (Ruiz et al. 2011b). The organosolv pretreatment has been investigated due to the effective results for hemicelluloses/lignin depolymerization, increasing the cellulosic fraction digestibility (Geng et al. 2012). However, most of the used organic solvents need to be recovered for economic and environmental reasons (Alvira et al. 2010; Galbe and Zacchi 2012). The organosolv process has been developed as part of a commercial lignocellulose biorefinery technology known as the Lignol process. Lignol is a pilot plant that obtained several high value products in a cost-effective process where solvents were recovered and recycled at the end of the process (Pan et al. 2005).
Torre et al. (2013) reported that the cellulose pulp with the organosolv process is especially attractive because it can be used in boiler combustion chambers. Li et al. (2012) studied the fractionation of organosolv lignin with organic solvents and reported that this fractionation provides a way to prepare lignin with homogeneous structure and good functional properties for several potential applications. Kautto et al. (2013) reported that the organosolv pulping can be used as a pretreatment step in bioethanol production allowing, in complement to the production of bioethanol, the production of a pure lignin and other coproducts according to biorefinery concept.
22.214.171.124 Ozonolysis Pretreatment
Ozonolysis pretreatment includes using ozone gas, a powerful oxidant and soluble in water, in order to breakdown lignin and hemicelluloses and increase cellulose biodegradability; soluble compounds of smaller molecular weight such as acetic and formic acid (Balat 2011) may also be released. The lack of degradation by-products and operation at ambient conditions constitute the advantages of this pretreatment (Garcia-Cubero et al. 2009), while the disadvantages reside in the cost of ozone (Sun and Cheng 2002).
Travaini et al. (2013) pretreated sugarcane bagasse in a fixed bed reactor with ozone and studied the effect of ozone concentration and sample moisture, and they concluded that ozonolysis process is a promising pretreatment to obtain a high glucose conversion from cellulose.
126.96.36.199 Ionic Liquid Pretreatment
Ionic liquids are organic salts that exist as liquids at low temperatures, with tunable physicochemical properties, low vapor pressures, good thermal stability, and different combination of anions and cations to their synthesis (Fort et al. 2007; Lee et al. 2008). Recent studies of particular interest have indicated that both cellulose and lignin can be dissolved in a variety of ionic liquids and, perhaps more important, easily regenerated from these solutions. Thus, studies have shown ionic liquids with potential to be used as an environmentally benign pretreatment of lignocellulosic wastes (Fort et al. 2007; Zhu 2008; FitzPatrick et al. 2010).
Yuan et al. (2013) mentioned that anionic liquid-based biorefining strategy could integrate biofuel production into a biorefinery scheme in which the major components of poplar wood can be converted into value-added products. Shafiei et al. (2013) produced bioethanol from spruce wood chips using ionic liquid as pretreatment; they reported that ethanol yield was between 66.8 % and 81.5 %. Labbé et al. (2012) investigated three ionic liquids as potential media to act on biomass and make the cellulose component more accessible to hydrolytic enzymes. The ionic liquid [emim][OAc] was the most efficient in the fractionation of lignocellulosic waste. In general terms, the treatment with ionic liquids is a useful technology for the development of the biorefinery concept (Stark 2011).
3.1.4 Biological Pretreatment
The biological or microbial pretreatment involves the use of microorganisms or enzymes. In addition, in biological pretreatment, particle size, moisture content, resident time, and temperature, besides the microbial agents used, could affect lignin degradation and enzymatic hydrolysis yield (Patel et al. 2007; Wan and Li 2010).
Unlike most of the chemical and physicochemical pretreatment methods, biological pretreatment offers as advantages low energy consumption and no chemical requirement, in addition to mild operational conditions and likely ease of integration into a consolidated bioprocessing setup (Sun and Cheng 2002; Yang and Wyman 2008; FitzPatrick et al. 2010). The disadvantages include long-time process, large space requirement, and the need for continuous monitoring of microorganism growth (Wyman et al. 2005; Taherzadeh and Karimi 2008).
Research on fungal pretreatment is mainly focused on evaluating fungi that selectively degrade lignin and hemicellulose, while utilizing little cellulose (Chen et al. 1995; Singh et al. 2008; Shi et al. 2009; Wan and Li 2010). Cellulose is more recalcitrant to fungal attack than other components. On the other hand, microbial consortium pretreatment is conducted by microbes screened from natural environment’s typically rotten lignocellulosic biomass. In contrast to fungal pretreatment, which is usually conducted under sterilized conditions, in most cases, sterilization of lignocellulosic feedstocks is not necessary when using a microbial consortium for pretreatment, which is an advantage over fungal pretreatment. In enzymatic pretreatment, the most commonly used enzymes are cellulases and hemicellulases with the disadvantages of the cost of enzymes to be high and therefore its application is limited.
3.1.5 Integrated Pretreatment
Other pretreatment processes, using integrated methods, have been a target of some researches in order to improve the efficiency of fractionating, decrease the formation of inhibitors, and shorten process time (Mood et al. 2013; Zheng et al. 2014).
Thus, in literature, there are several studies with combination of alkaline and dilute acid pretreatments resulting at more effective delignification and less carbohydrate degradation, in comparison with alkali and acid pretreatment solely (Lu et al. 2009), and combinations of dilute acid and microwave pretreatments resulting in highest biomass fragmentation and swelling as well as complete hemicellulose degradation (Chen et al. 2011). Combination of biological and steam explosion pretreatment reduced the pretreatment time significantly (Taniguchi et al. 2010), while the combination of biological and dilute acid pretreatments led to enhanced enzymatic hydrolysis (Na et al. 2010). The combination of dilute acid and steam explosion pretreatment revealed a high xylose yield, a low level of inhibitors, and an enhanced saccharification efficiency (Sun and Cheng 2002). The use of microwave-based heating, instead of the conventional heating, in alkali pretreatment, removed more lignin and hemicelluloses in shorter pretreatment time (Yuanxin et al. 2005, 2006), while the synergic effect of the combination of ultrasonic pretreatment, instead of the conventional heating pretreatment, and different ionic liquids also enhanced the saccharification ratio (Ninomiya et al. 2010).
3.2 Enzymatic Hydrolysis
After the pretreatment, the cellulose is more susceptible to enzymatic attack (Cheng and Timilsina 2011). Therefore, the enzymatic hydrolysis is the second step in the biorefinery processing of cellulose from lignocellulosic wastes. It involves cleaving the cellulose polymers to soluble monomeric sugars using a class of enzymes known as cellulases.
The cellulases are highly specific, and the enzymatic hydrolysis of cellulose is usually carried out under mild conditions of pressure, temperature, and pH (Binod et al. 2010). Most cellulases show an optimum activity at temperatures and pH in the range of 45–55 °C and 4–5, respectively (Duff and Murray 1996; Galbe and Zacchi 2002; Talebnia et al. 2010; Cheng and Timilsina 2011). Enzymes tolerant to high temperature and low pH are preferred for the enzymatic hydrolysis due to most current pretreatments making use of acid and heat. Besides, thermostable enzymes have several advantages including higher specific activity and higher stability, which improves the overall enzymatic performance.
Ultimately, improvement in catalytic efficiencies of enzymes reduces the cost of process by enabling lower enzyme dosages (Dashtban et al. 2009). A cellulase dosage of 10–30 FPU/g cellulose is often used in laboratory studies because it results in an efficient hydrolysis with high glucose yield in a reasonable time (48–72 h). However, enzymes loading may vary depending on the pretreatment, type, and concentration of raw materials (Talebnia et al. 2010). Although the cost of enzyme production is still high, a reduction in the costs may be obtained in the case less noble materials are used (Castro and Pereira Jr 2010).
Fungi and bacteria can produce cellulases for the hydrolysis of lignocellulosic wastes, and these enzymes have been mainly produced by species of Trichoderma, Aspergillus, Schizophyllum, and Penicillium. Of all these fungal genera, Trichoderma has been most extensively studied for cellulase production. Bacteria belonging to Clostridium, Cellulomonas, Bacillus, Thermomonospora, and Streptomyces can produce cellulases. Although many cellulolytic bacteria, particularly the cellulolytic anaerobes such as Clostridium thermocellum and Bacteroides cellulosolvens, produce cellulases with high specific activity, they do not produce high enzyme titers (Sun and Cheng 2002).
Therefore, most research for commercial cellulase production has been focused on fungi (Talebnia et al. 2010). Besides, due to the promising thermostability and acidic tolerance of thermophilic fungal enzymes, they have good potential to be used for hydrolysis of lignocellulosic wastes at industrial scale (Dashtban et al. 2009).
The use of a mixture of cellulases from different microorganisms or a mixture of cellulases and other enzymes has been extensively studied since this can raise the rate of enzymatic hydrolysis of lignocellulosic biomass. The combination of enzymes such as cellulase, xylanases, and pectinases exhibits a significant increase in the extent of cellulose conversion (Sun and Cheng 2002; Binod et al. 2010). Tabka et al. (2006) observed that the addition of accessory enzymes, such as xylanases, feruloyl esterase, and laccase, on pretreated wheat straw could act in synergistically, improving the enzymatic hydrolysis of cellulose on pretreated material.
Currently, the use of enzymes from a cellulolytic complex in the hydrolysis of biomass is one of the more emerging applications. The lignocellulosic feedstocks contain from 20 % to 60 % of cellulose, which may be fully converted to glucose by enzymatic action. In subsequent steps, the monosaccharide can be used as a building block for obtaining a vast range of products, which range from biofuels to polymers. These technologies fall under the definition of cellulosic biorefineries, which aim to the integrated and integral use of agro-industrial wastes generated in a given production chain with value addition (Castro and Pereira Jr 2010).
The three enzymes involved in hydrolysis of cellulose to glucose by synergistic action include:
3.2.1 Endoglucanase (EG)
EG (Endo-1,4-β-d-glucanohydrolase; EC 188.8.131.52) cleaves the β-1,4-glucosidic linkages in the interior of cellulose molecule to produce cellooligosaccharides with free chain ends. It is responsible for starting cellulose hydrolysis by attacking randomly regions of low crystallinity in the cellulose fiber and making it more accessible for cellobiohydrolases (Talebnia et al. 2010; Cheng and Timilsina 2011). The EG is the cellulolytic enzyme responsible for the rapid solubilization of cellulose due to their fragmentation in oligosaccharides. It is also referred as carboxymethylcellulase (CMCase) because of the use of carboxymethylcellulose (CMC) as substrate to measure its activity (Michelin et al. 2013).
Studies have shown that many fungi produce multiple EGs. For example, T. reesei produces at least five EGs (EGI/Cel7B, EGII/Cel5A, EGIII/Cel12A, EGIV/Cel61A and EGV/Cel45A), whereas three EGs were isolated from white-rot fungus Phanerochaete chrysosporium (EG28, EG34 and EG44) (Dashtban et al. 2009).
3.2.2 Cellobiohydrolase (CBH)
CBH (1,4-β-d-glucan cellobiohydrolase; EC 184.108.40.206) is an exoglucanase that preferentially hydrolyzes β-1,4-glycosidic bonds of the cellulose, releasing cellobiose units from chain ends (Michelin et al. 2013). Cellobiose, the end product of CBHs, acts as an inhibitor, which can limit the ability of the enzymes to degrade cellulose (Dashtban et al. 2009). Microcrystalline cellulose (Avicel) has been used as substrate to measure its activity (Michelin et al. 2013).
Although IUBMB (International Union of Biochemistry and Molecular Biology) defines the CBH as a catalyst of the hydrolysis of only the nonreducing ends of the cellulosic fiber and oligosaccharides with degree of polymerization (DP) higher than three in cellobiose, studies have shown that some CBHs can act from the reducing ends of the cellulosic chains, which increases the synergy between opposite-acting enzymes (Lynd et al. 2002; Michelin et al. 2013). For example, T. reesei has shown to have two CBHs acting from nonreducing (CBHII/Cel6A) and reducing (CBHI/Cel7A) ends (Zhang and Lynd 2004; Dashtban et al. 2009).
3.2.3 β-Glucosidase (BGL)
BGL (1,4-β-d-glucosidase glucanohydrolase; EC 220.127.116.11) cleaves cellobiose and other cellodextrins with a DP up to six releasing glucose units. The hydrolysis rates decrease markedly as the substrate DPs increase (Zhang et al. 2006). BGL is also inhibited by its end product (glucose).
BGLs are very amenable to a wide range of simple sensitive assay methods, based on colored or fluorescent products released from some synthetic substrates, such as p-nitrophenyl β-d-1,4-glucopyranoside. Also, BGL activities can be measured using cellobiose, which is not hydrolyzed by endoglucanases and exoglucanases (Ghose 1987; Zhang and Lynd 2004; Zhang et al. 2006).
BGLs have been isolated from many different fungal species including Ascomycetes such as T. reesei and Basidiomycetes such as white-rot and brown-rot fungi. In T. reesei, two β-glucosidases (BGL I/Cel3A & BGL II/Cel1A) have been isolated from culture supernatant, but the enzymes were found to be primarily bound to the cell wall. However, BGL production in T. reesei is very low compared to other fungi such as A. niger (Dashtban et al. 2009).
3.3 Fermentation and Bioproducts
Fermentation can be defined as a process performed mainly by microorganisms such as bacteria, yeast, and fungi in order to obtain a bioproduct from a suitable nutrient source. This process can be performed by solid-state or submerged cultures. Solid-state fermentation (SSF) is a complex heterogeneous three-phase (gas–liquid–solid) process defined as the growth of microorganisms, often fungi, on the surface of a porous and moist solid substrate particle in which enough moisture is present to maintain microbial growth and metabolism; submerged fermentation has been defined as fermentation in a liquid medium (Pandey 2003; Ruiz et al. 2012b).
Currently, the use of lignocellulosic wastes has been employed as nutrient source in fermentative process, and special attention has been given to cellulose present in these materials. Due to the complexity of the cellulose, several forms to lead the fermentative process were developed in order to obtain a high efficiency. For example, for the conversion of the sugars from cellulose in molecules of interest, such as ethanol, a step of enzymatic hydrolysis of the cellulose prior to submerged fermentation is necessary, and both steps can be performed separately (SHF – separate hydrolysis and fermentation), simultaneously (SSF – simultaneous saccharification and fermentation), and semi-simultaneously (SSSF– semi-simultaneous saccharification and fermentation) (Dashtban et al. 2009; Castro and Pereira Jr 2010; Gonçalves et al. 2014).
In SHF, the hydrolysis of cellulose occurs in a separate step and after that, the sugars released are fermented to ethanol. The advantage of this method is that both steps can be carried out in each optimum condition (e.g., optimum temperature for cellulase hydrolysis is around 50 °C and for fermentation 30 °C). Besides, in this process, the cells can be recycled, since there is no raw material in suspension during fermentation. The main drawback is the accumulation of intermediate sugars (cellobiose and glucose) during the hydrolysis, which can cause inhibition of the cellulase enzymes and a reduction in the final conversion to glucose. This makes the process inefficient, and the costly addition of β-glucosidase is needed in order to promote a reduction of the inhibition of endo- and exoglucanase by its hydrolysis products (particularly cellobiose) and increase the final conversion of substrate to glucose that will be used for fermentation (Philippidis et al. 1993; Kádár et al. 2004).
In SSF, the enzymes are less susceptible to inhibition by hydrolysis products because the released glucose is fermented simultaneously. Therefore, addition of high amounts of β-glucosidase is not necessary, which reduces the costs of the process (Dashtban et al. 2009). The maintenance of low glucose concentration in the medium also promotes the continuous hydrolysis reaction and reduces the risk of contamination of the system. Besides, this process contributes to lower cost of investment at the plant since the two steps are performed in the same reactor (Castro and Pereira Jr 2010). The main drawback of SSF is the need to lead the process in suboptimal conditions of temperature and pH. The development of recombinant yeast strains (i.e., improved thermotolerance) is expected to enhance the performance of SSF (Galbe and Zacchi 2002). The semi-simultaneous saccharification and fermentation (SSSF) is a good alternative that includes a short presaccharification period before the SSF process (Gonçalves et al. 2014).
Considering the overall conversion of lignocellulosic wastes into ethanol, another alternative has been described. In this process, known as co-fermentation (CF), the fermentation of pentoses (mainly xylose from hemicellulosic fraction of the lignocellulosic wastes) and hexoses (glucose from cellulose) occurs in one reactor. The conduction of co-fermentation with concomitant hydrolysis of cellulose and/or hemicellulose fractions is called SSCF (simultaneous saccharification to co-fermentation) (Castro and Pereira Jr 2010).
Further process integration can be achieved by a process known as consolidated bioprocessing (CBP) which aims to minimize all bioconversion steps into one step using one or more microorganisms (enzyme production, hydrolysis of cellulose and hemicellulose, and fermentation of glucose and xylose happen in one reactor) and, thus, reduce product inhibition and operation costs (Limayem and Ricke 2012).
These processes are integrated within the concept of “biorefinery”-industrial installations designed to produce a wide range of bioproducts from conversion of biomass (Guo et al. 2010). A number of high value bioproducts can be obtained from lignocellulosic wastes such as bioethanol, organic acids, biohydrogen, enzymes, packaging materials, biocomposites, and others. In the sequence, some bioproducts that can be obtained from lignocellulosic wastes will be described.
In the past few years, much research has been done to find a viable alternative for ethanol production from lignocellulosic wastes (known as bioethanol, or cellulosic ethanol or second-generation ethanol) in view of fast depletion of fossil fuels and food shortages.
Ethanol is either used as fuel (pure or as an additive to gasoline) or a chemical feedstock and can be obtained from: (1) sucrose-containing feedstocks (e.g., sugar cane, sweet sorghum and sugar beet), (2) starchy materials (e.g., corn, wheat, and barley), and (3) lignocellulosic biomass (e.g., sugar cane bagasse, corncob, wood and straw) (Balat et al. 2008). The ethanol production from lignocellulose wastes has several advantages, such as: is a renewable energy source, abundant, of low cost, and noncompetitive with food crops. The use of ethanol fuel can significantly reduce the use of petroleum and reduce greenhouse gas emission. Currently, ethanol production from lignocellulosic wastes is one of the most studied and promising alternatives for reuse of these feedstocks, due to the large incentive that has been given to use of biofuels in replacement of gasoline (Mussato and Teixeira 2010).
Brazil and the USA produce ethanol from the fermentation of sucrose from sugarcane juice and starch from corn, respectively, generating wastes as the sugarcane bagasse and straw and corncob and corn straw. Therefore, a variety of widely available sugar feedstocks can be used (Sánchez 2009).
The production of fuel ethanol from lignocellulosic wastes includes an initial pretreatment (e.g., steam explosion or diluted acid) to render cellulose more accessible to the subsequent step of enzymatic hydrolysis (with cellulases), which breaks down cellulose to fermentable sugars, and finally the fermentation of the sugars to ethanol by yeast Saccharomyces cerevisiae. This conventional strain presents optimum temperature at 30 °C and tolerates a high osmotic pressure, low pH levels, and inhibitory products (Limayem and Ricke 2012).
Hsu et al. (2011) investigated the ethanol production by S. cerevisiae BCRC 21812 from reducing sugar released by hydrolysis of corncob material with cellulases (CMCase, Avicelase, and β-glucosidase) from Streptomyces sp. strain. Sukumaran et al. (2009) studied the saccharification of three different feedstocks, i.e., sugarcane bagasse, rice straw, and water hyacinth biomass (using cellulase and β-glucosidase) for ethanol production by Saccharomyces cerevisiae. The highest yield of reducing sugars was obtained from rice straw followed by sugarcane bagasse.
Intensive efforts have been done in the last years to (1) develop efficient technologies for the pretreatment of lignocellulosic wastes, since several factors have been described to affect the hydrolysis of cellulose, such as porosity (accessible surface area) of the lignocellulosic waste, crystallinity of cellulose fiber, and lignin and hemicellulose contents (McMillan 1994); (2) develop enzymes for enhanced cellulose/hemicellulose saccharification; and (3) develop suitable technologies for the fermentation of both hexose and pentose sugars (Soccol et al. 2010).
Hydrogen is considered as a clean fuel, forming water (instead of greenhouse gases) as the only combustion product. It has high energy content and can be used directly as fuel for transportation or, after purification, to produce electricity (Guo et al. 2010). Therefore, the development of renewable and cost-effective process for its production will contribute to increase the energy production and to reduce greenhouse effect (Kapdan and Kaegi 2006). Other applications of hydrogen include the use as chemical reactant in the production of fertilizers, for refining diesel and for the industrial synthesis of ammonia (Guo et al. 2010).
The use of the hydrogen as energy resource has been restricted in large part due to the high production costs, technical storage requirements, and distribution methods (Dunn 2002). Nowadays, the most part (around 88 %) of the hydrogen production derives from fossil fuels (natural gas, heavy oils or coal) (Nath and Das 2003), and up to 4 % of hydrogen production derives from water electrolysis. However, all such processes consume high energy (Guo et al. 2010).
Recently, biohydrogen gas production from lignocellulosic biomass, mainly agricultural wastes, has received more and more attention. It seems economically viable and technically feasible to produce biohydrogen from lignocellulose biomass by an integrated process involving pretreatment steps for the raw material and enzymatic hydrolysis for the yield of fermentable reducing sugars. After, the biohydrogen production can be performed by anaerobic fermentation process (Chen et al. 2008; Lo et al. 2008b, 2009b).
According to Magnusson et al. (2008), although there are several different methods of hydrogen production, biohydrogen production by dark fermentation, compared to alternative methods such as biophotolysis of water or photofermentation, is advantageous due to its higher rate of production. Besides, nonbiological methods of hydrogen production such as electrolysis and steam reformation of methane require extensive amounts of energy and also are sources of polluting emissions, such as CO2, CO, NOx, and SOx.
The generation of biohydrogen from lignocellulosic wastes by using dark fermentation usually requires a step of pretreatment of substrate, which increases the production cost. Some investigators demonstrated a two-stage process (i.e., hydrolysis and hydrogen production) where cellulose hydrolysis can be performed by using mixed or pure microbial culture, and after, the hydrolysates (rich in reducing sugars) are used for sequential biohydrogen production by an efficient hydrogen producer. In this way, the hydrogen yield could be increased, and thus, the process becomes more advantageous in practical applications due to a higher economical feasibility and less energy consumption (Lo et al. 2008a, 2009a).
Although pure cultures have been intensively investigated over the past years, involving species such as Bacillus coagulans (Kotay and Das 2007), Thermoanaerobacterium spp. (O-Thong et al. 2008), and Clostridium butyricum (Chen et al. 2005), few studies refer to the characterization of mixed cultures. In relation to mixed cultures producing biohydrogen, a wide range of species has been studied. In relation to mesophilic microorganisms, species from the genera Clostridium (C. pasteurianum, C. saccharobutylicum, C. butyricum), Enterobacter (E. aerogenes), and Bacillus can be cited, and in relation to thermophilic or extremophilic microorganisms, species include the genera Thermoanaerobacterium (T. thermosaccharolyticum), Caldicellulosiruptor (C. saccharolyticus), Clostridium (C. thermocellum), and Bacillus (B. thermozeamaize) (Guo et al. 2010).
Clostridium thermocellum is a thermophilic, acetogenic, anaerobic bacterium that degrades cellulose directly synthesizing a mixed product, such as acetate, hydrogen, and carbon dioxide, as well as lactate and ethanol under different growth conditions (Ng et al. 1977; Lynd et al. 1989). This bacterium expresses a suite of cellulolytic enzymes that degrades the cellulose to glucose and cellulodextrans. Of all known cellulose degrading microorganisms, C. thermocellum displays the highest rate of cellulose degradation (Lynd et al. 1989). Because of this characteristic and propensity to synthesize hydrogen, carbon dioxide, and acetate, C. thermocellum offers the potential for directly producing biohydrogen from cellulosic wastes. The high optimum growth temperature of C. thermocellum (60 °C) also reduces the chance of contamination by precluding the growth of predominant mesophilic microorganisms and allows for C. thermocellum-enriched cultures to be maintained. Besides, as the solubility of gases decreases with higher temperatures, the higher growth temperature also facilitates the efficient removal of product gases such as hydrogen and carbon dioxide.
Chen et al. (2005) reported Clostridium butyricum CGS5 as an efficient microorganism in converting sugars (e.g., glucose, xylose, sucrose) into hydrogen via dark fermentation. Unfortunately, C. butyricum CGS5, like most hydrogen-producing strains, cannot directly utilize cellulose or hemicellulose as carbon source for hydrogen production, and therefore, pretreatment and hydrolysis steps of the cellulosic feedstock were required to enable efficient cellulosic biohydrogen production.
18.104.22.168 Organic Acids
Some organic acids including citric and lactic acids can be produced by fermentation using hydrolysates rich in glucose obtained from cellulose of lignocellulosic wastes.
Currently, citric acid is produced mainly by fermentation (a small part is yet extracted from citrus fruits in Mexico and South America) submerged (SmF), in surface and in solid-state (SSF), using starch- or sucrose-based media (Jianlong 2000; Vandenberghe et al. 2000). The microorganism used is the filamentous fungus A. niger, which can accumulate citric acid in media rich in carbohydrate but deficient in phosphate and trace elements like Fe+2 and Mn+2.
Kumar et al. (2003) used pineapple, mixed fruit, and maosmi wastes as substrates to produce citric acid by solid-state fermentation using Aspergillus niger DS 1. Dhillon et al. (2011) also used different agro-industrial wastes, such as apple pomace, brewers’ spent grain, citrus waste, and sphagnum peat moss to evaluate their suitability for the production of citric acid through solid-state and submerged fermentation by A. niger NRRL 567 and NRRL 2001.
Citric acid is used in several industrial sectors such as the food, beverage, and pharmaceutical industries. It is mainly used as an additive (antioxidant and acidulant) in the production of soft drinks, desserts, jellies, candies, and wines. It is also used as flavor enhancer and plasticizer. In pharmaceutical industry, it is used as anticoagulant (blood transfusion) and in the production of effervescent products. It is also used to adjust the pH of astringent lotions, as sequestering agent, and hair fixatives.
The food industry consumes about 70 % of the total production of citric acid (Rohr et al. 1983), due to some characteristics such as its pleasant acidic taste and its high solubility in water. Besides, it is worldwide accepted as “GRAS” (generally recognized as safe), approved by the joint FAO/WHO Expert Committee on Food Additives. The pharmaceutical industry consumes 12 %, and the rest 18 % has market for other applications (Penna 2001).
Lactic acid is another important organic acid that has attracted great attention because of its wide applications in food, chemical, and pharmaceutical industries. It is widely used as acidulant and preservative in the food industry, being used as taste-enhancing additive in soft drinks, jellies, syrups, and fruit juices. In the pharmaceutical industry, it is used for the adjustment of pH of pharmaceutical preparations and topical wart preparations. Other applications include blood coagulant and dietary calcium source. Lactic acid has been used in the manufacture of cellophane, resins, and some herbicides and pesticides. Another important application of lactic acid is in textile and tanning industries (Penna 2001).
Lactic acid has two enantiomers: d-(−) and l-(+) lactic acid. The production of optically pure d- or l-lactic acid can be obtained by fermentation when the appropriate microorganism is used (Abdel-Rahman et al. 2013).
Lactic acid is produced by fermentation and by chemical synthesis. Fermentative production offers some advantages, such as the use of cheap and renewable substrates, low production temperature, and low energy consumption. Currently, most of lactic acid produced worldwide is obtained from fermentative pathway (Abdel-Rahman et al. 2011). A racemic mixture of DL-lactic acid is usually produced by the chemical pathway.
Lactic acid can be produced from renewable materials by various microorganism species, including bacteria, fungi, yeast, microalgae, and cyanobacteria. Strains of Lactobacillus sp. have been common among the bacterial cultures (John et al. 2007a) and that of Rhizopus sp. among the fungal cultures (Tay and Yang 2002; Koutinas et al. 2007).
A major concern in lactic acid production by fermentation is to reduce the process costs with respect to substrate costs and production efficiency. Inexpensive and renewable materials can be used as substrates, including by-products of agricultural industries, food industries, and natural unutilized biomass such as starchy and lignocellulosic wastes (Nguyen et al. 2013).
Recently, lactic acid production has been studied with increased interest because of its application in the synthesis of biodegradable and biocompatible polylactic acid (PLA) polymers (Koutinas et al. 2007). l-(+) lactic acid can be polymerized to form polylactic acid (PLA). This polymer can be used in the manufacture of new biodegradable plastics. In comparison with petrochemical plastics, PLA production is considered a relatively immature technology at the industrial scale. This is mainly due to the high production cost of lactic acid – the feedstock for PLA (Abdel-Rahman et al. 2013).
Production of lactic acid from fermentation aiming the production of polylactic acid has increased, due to decreased in petrochemical resources and the problems of environmental pollution caused by the petrochemical industry. The pure polymers of poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) are relatively heat sensitive, while stereo complexes of polylactic acid produced by blending PLLA and PDLA have a melting point approximately 50 °C higher than their respective pure polymers and are more biodegradable. The ratio of l- and d-lactic acid influences the properties and the degradability of the resulting polylactic acid (Datta and Henry 2006; John et al. 2007b).
Cellulases, enzymes involved in cellulose hydrolysis, represent currently the third largest sale in the world market for enzymes (Singhania et al. 2010), due to their wide applications in several industry sectors, such as in extraction and clarification of fruit juices; as animal feed additive to enhance the absorption of nutrients, improving the feed value and performance of animals (Bhat 2000; Kuhad et al. 2011); in the textile industry for cotton softening and denim finishing; in the detergent industry to improve the appearance and color brightness, besides softening the garment and removing dirt particles trapped within the microfibril network (Bhat 2000); and in paper recycling (Singhania et al. 2010). However, cellulases may become the largest volume industrial enzyme, if ethanol fuel production from lignocellulosic biomass becomes a reality, since in this process, a step of enzymatic hydrolysis using cellulases is necessary.
Cellulases are relatively costly enzymes, and a significant reduction in production cost will be important for their commercial use in biorefineries. Strategies that will make the use of cellulases in biorefinery as a more economical process include the increase of volumetric productivity for commercial enzyme production, the production of enzymes using cheaper substrates, the production of enzyme preparations with greater stability for specific processes, and the production of cellulases with higher specific activity on solid substrates (Zhang et al. 2006).
Agro-industrial wastes such as wheat straw, corncobs, and sugarcane bagasse (pretreated or not) have been widely included in the nutrient media composition as the main strategy in microbial cellulase biosynthesis and other enzymes that degrade biomass, due to their high cellulose content, wide availability, and low cost (Michelin et al. 2013).
Currently, Genencor International and Novozymes Biotech companies are the two largest producers of cellulases. Both companies have played a significant role in bringing down the cost of cellulase through active research and are continuing to bring down the cost by adopting novel technologies (Singhania et al. 2010). They reported the development of technology that has reduced the cellulase cost for the process of bioethanol production from US$5.40 per gallon of ethanol to approximately 20 cents per gallon of ethanol (Moreira 2005; Zhang et al. 2006).
A wide variety of microorganisms including fungi and bacteria have been reported to degrade cellulose by synthesizing enzymes of the cellulolytic complex. Filamentous fungi are especially interesting since they secrete these enzymes into the medium, with levels superior to those found in yeasts and bacteria. These microorganisms can degrade lignocellulosic wastes better than other microbes as they closely resemble their natural habitat.
One of the most extensively studied cellulase producers is the Trichoderma reesei fungus, which is capable of hydrolyzing native cellulose. It produces two cellobiohydrolases (CBHI and CBHII) and two endoglucanases (EG1 and EG2), in a rough proportion of 60:20:10:10, which together can make up to 90 % of the enzyme cocktail, while seven β-glucosidases – BGL I–VII – secreted by this fungus typically make up less than 1 % (Lynd et al. 2002; Aro et al. 2005; Herpoël-Gimbert et al. 2008). Other fungi that produce cellulases include Aspergillus, Humicola, and Penicillium (Singhania 2009).
Most commercial cellulases (including β-glucosidase) are produced by Trichoderma species and Aspergillus species (Zhang et al. 2006). Recently, Genencor Company has launched Accellerase®1500, a cellulase complex intended specifically for lignocellulosic biomass processing industries, which is produced with a genetically modified strain of T. reesei. This enzyme preparation is claimed to contain higher levels of β-glucosidase activity than all other commercial cellulases available today, to ensure almost complete conversion of cellobiose to glucose (Singhania et al. 2010).
The large market potential and the important role that cellulases play in the bioenergy and bio-based products industries provide a great motivation to develop better cellulase preparations for cellulose hydrolysis of lignocellulosic wastes.
Currently, there is a wide variety of wood and non-wood resources suitable for the production of cellulose nanostructures. The use of non-wood resources such as agricultural/agro-industrial wastes is particularly interestingly due to their renewable nature (Alemdar and Sain, 2008; Lavoine et al. 2012). Besides, the shortage of natural resources such as wood and the concerns over the environmental impact of other widely used materials have encouraged researches with alternative materials (Ghaderi et al. 2014). Other advantage of non-wood materials is that the cellulose microfibrils from wall of agricultural wastes are easier to break down than wood wall, and the fibrillation of this pulp demands less energy.
Microfibrillated cellulose (MFC) is being increasingly studied from a number of different cellulosic sources (Lavoine et al. 2012). Due to its nanometer scale, its high surface energy and its ability to form a nanoporous network, MFC has been studied for use in nanocomposites as a mechanical reinforcement (Siqueira et al. 2010) and as a dispersion stabilizer (Andresen et al. 2006).
All-cellulose nanocomposites are classified as biocomposite, a class of materials that has been recognized and studied during the past decade (Nishino et al. 2004). The matrix and reinforcement phases in these biocomposite materials are noncrystalline and undissolved cellulose, respectively (Soykeabkaew et al. 2009). These nanocomposites have promising properties, as they are a completely bio-based material and fully biodegradable. Besides, they also have remarkably high mechanical performance and transparency (Nishino et al. 2004; Gindl and Keckes 2005). These characteristics give composite materials the potential for many applications.
The use of MFC suspensions as coating slurries has several applications, such as in printing industry and, more recently, in the food packaging sector. One further possibility is the use of MFC in drug release applications (Lavoine et al. 2014). In the literature, there are diverse agricultural/agro-industrial sources, such as wheat straw and soy hulls (Alemdar and Sain 2008) and bagasse (Bhattacharya et al. 2008), that are being used to produce MFC.
4 Future Challenges and Trends of Cellulose
Recent advances in understanding the biochemical complex in cell wall structure and their biochemical characteristics offer new avenues for the development of new biological-based processes for biomass conversion to different bioproducts at industrial scale. In that sense, a huge potential exists in upgrading fuel and energy producing pathways into biorefineries in order to improve its financial performance and long-term sustainability. However, high production costs remain the bottleneck for large-scale development of this pathway. Another important issue is how extensively can biomass be utilized without causing significant and irreversible harmful environmental and social impacts.
One of the main problems during the pretreatment and hydrolysis of biomass is the variability in the content of lignin and hemicellulose. This variability depends on factors as the type of plant from which the biomass is obtained, crop age, method of harvesting, and others. This makes that no one of the pretreatment methods can be applied in a generic way for many different feedstocks (Claassen et al. 1999). The future trends for improving the pretreatment of lignocellulosic feedstocks also include the production of genetically modified plant materials with higher carbohydrate content or modified plant structure to facilitate pretreatment (Sanchez and Cardona 2008).
Therefore, the success and efficiency of the applied pretreatment must be accomplished by the proper selection of treatment conditions for raw materials that present different recalcitrance. A single pretreatment method does not provide efficient results due to its limited specificity. For this, the proper combination of pretreatment steps (firstly, for hemicelluloses solubilization and, secondly, for effective delignification) should be applied in order, not only to improve the effectiveness of the following biorefinery processing stages but also to avoid high contaminated hemicellulosic or lignin containing streams. These fractionation by-products could be properly exploited avoiding further purification steps, which are usually more expensive and require greater energy and amount of chemicals (García et al. 2014a). Thus, a number of lignocellulosic by-products and residues can be the focus of a biorefinery. The integration of the different processes outlined will certainly have a positive effect on the scientific, technological, economic, environmental, and social areas, not only for lignocellulosic materials but also for a wide range of coproducts (biofuels, chemicals, and others compounds).
Other challenges also need to be addressed including optimization of the fermentation technology in order to produce a range of bioproducts by selective fermentation of different chemicals, characterization of new enzymes or new enzymatic systems in order to lead to low-cost conversion of lignocellulosic biomass into biofuels and biochemicals, as well as logistics considerations and careful choice of raw material for the biorefinery.
Still in the context of future challenges and trends of cellulose, it is clear that different countries and regions have potential access to the dry matter as lignocellulosic wastes, for use in a commercial scale biorefinery. However, there are clearly a number of challenges that need to be addressed to the success of these processes based on renewable technologies that are expected to improve the environment and enhance the quality of life in rural areas through the diversification of the rural economy. Overall, these biorefineries will contribute to the establishment of a link between economic and social development and environmental protection.
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