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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This book covers the utilization of lignocellulosic biomass for biofuel production as well as other industrial applications such as in biotechnology, paper and pulp, chemical and bioplastics. Lignocellulosic materials such as agricultural residues (e.g., wheat straw, sugarcane bagasse, corn stover), forest products (hardwood and softwood), and crops such as switchgrass and salix, are becoming a potent source for generating valuable products. Lignocellulosic Biomass Production and Industrial Applications describes the utilization of lignocellulosic biomass for various applications. Although there have been numerous reports on lignocellulosic biomass for biofuel application, there have been very few other applications reported for lignocellulosic biomass-based biotechnology, chemicals and polymers. This book covers both application areas. Besides describing the various types of biofuel production, such as bioethanol, biobutanol, biodiesel and biogas from lignocellulosic biomass, it also presents various other lignocellulosic biomass biorefinery applications for the production of enzymes, chemicals, polymers, paper and bioplastics. In addition, there are chapters on valorization of lignocellulosic materials, alkali treatment to improve the physical, mechanical and chemical properties of lignocellulosic natural fibers, and a discussion of the major benefits, limitations and future prospects of the use of lignocellulosic biomass.
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Contents
Cover
Title page
Copyright page
Preface
Chapter 1: Valorization of Lignocellulosic Materials to Polyhydroxyalkanoates (PHAs)
1.1 Introduction
1.2 Lignocellulose: An Abundant Carbon Source for PHA Production
1.3 Lignocellulosic Pretreatment Techniques
1.4 Hydrolysis of Lingocellulose
1.5 Lignocellulose Biomass as Substrate for PHA Production
1.6 Conclusion
References
Chapter 2: Biological Gaseous Energy Recovery from Lignocellulosic Biomass
2.1 Introduction
2.2 Simple Sugars as Feedstock
2.3 Complex Substrates as Feedstock
2.4 Biomass Feedstock
2.5 Waste as Feedstock
2.6 Industrial Wastewater
2.7 Conclusion
Acknowledgments
References
Chapter 3: Alkali Treatment to Improve Physical, Mechanical and Chemical Properties of Lignocellulosic Natural Fibers for Use in Various Applications
3.1 Introduction
3.2 Alkali Treatment
3.3 Application of the Alkali-Steam-Treated Fibers
3.4 Summary
References
Chapter 4: Biodiesel Production from Lignocellulosic Biomass Using Oleaginous Microbes
4.1 Introduction
4.2 Lignocellulosics Distribution, Availability and Diversity
4.3 Prospective Oleaginous Microbes for Lipid Production
4.4 Technical Know-How for Biodiesel Production from LCBs
4.5 Fermentation
4.6 Transesterification for Biodiesel Production
4.7 Characteristics of Fatty Acid Methyl Esters
4.8 Conclusion
References
Chapter 5: Biopulping of Lignocellulose
5.1 Introduction
5.2 Composition of Lignocellulosic Biomass
5.3 Pulping and its Various Processes
5.4 Biopulping – Process Overview
5.5 Advantages and Disadvantages of Biopulping
5.6 Future Prospects
Acknowledgment
References
Chapter 6: Second Generation Bioethanol Production from Residual Biomass of the Rice Processing Industry
6.1 Introduction
6.2 Residual Biomass
6.3 Rice and Processing
6.4 Pretreatment Techniques
6.5 Hydrolysis
6.6 Fermentation
6.7 Bioethanol Production
6.8 Concluding Remarks
Acknowledgments
References
Chapter 7: Microbial Enzymes and Lignocellulosic Fuel Production
7.1 Introduction
7.2 Lignocellulosic Biomass as Sustainable Alternative for Fuel Production
7.3 Enzymes and Their Sources for Biofuel Generation
7.4 Microbial Enzymes towards Lignocellulosic Biomass Degradation
7.5 Applications in Biofuel Production
7.6 Conclusion
References
Chapter 8: Sugarcane: A Potential Agricultural Crop for Bioeconomy through Biorefinery
8.1 Introduction
8.2 Present Status of Sugarcane Production and its Availability
8.3 Morphology of Sugarcane
8.4 Factors Involved in Sugarcane Production
8.5 Major Limitations of Sugarcane Production
8.6 An Overview of Biotechnological Developments for Sugarcane Improvement
8.7 By-Products of Sugarcane Processing
8.8 Applications of Sugarcane for Biorefinery Concept
8.9 Utilization of Sugarcane Residue for Bioethanol Production
8.10 Conclusion
References
Chapter 9: Lignocellulosic Biomass Availability Map: A GIS-Based Approach for Assessing Production Statistics of Lignocellulosics and its Application in Biorefinery
9.1 Introduction
9.2 Geographical Information System (GIS)
9.3 Application of GIS in Mapping Lignocellulosic Biomass
9.4 Biofuels from Lignocellulosics
9.5 Conclusion
References
Chapter 10: Lignocellulosic Biomass Utilization for the Production of Sustainable Chemicals and Polymers
10.1 Introduction
10.2 Lignocellulosic Biomass
10.3 Pretreatment Strategies
10.4 Value-Added Chemicals from Lignocellulosic Biomass
10.5 Sustainable Polymers from Lignocellulosic Biomass
10.6 Potential Challenges for a Sustainable Biorefinery
10.7 Environmental Effects of Biorefineries
10.8 Future Perspectives of Biorefineries and Their Products
10.9 Conclusion
References
Chapter 11: Utilization of Lignocellulosic Biomass for Biobutanol Production
11.1 Introduction
11.2 Bioconversion of Lignocellulosic Biomass to Biobutanol
11.3 Composition of Lignocellulosic Biomass
11.4 Structure of Lignocellulosic Biomass
11.5 Biobutanol Production from Lignocellulosic Biomass
11.6 Conclusion
References
Chapter 12: Application of Lignocellulosic Biomass in the Paper Industry
12.1 Introduction
12.2 Major Raw Materials Used in the Paper Industry
12.3 Pulp and Papermaking Process
12.4 Waste Generation
12.5 Waste to Value-Added Products
12.6 Conclusion
References
Index
End User License Agreement
Guide
Cover
Copyright
Contents
Begin Reading
List of Illustrations
Chapter 1
Figure 1.1
Flow diagram of polyhydroxyalkanoates (PHA) production from lignocellulose precursor molecules.
Figure 1.2
A process diagram of PHA production from lignocellulosic biomass.
Chapter 2
Figure 2.1
Potential feedstock for biohydrogen production.
Chapter 4
Figure 4.1
Comprehensive metabolic engineering pathways and targets for biodiesel production from lignocellulosic biomass.
Chapter 5
Figure 5.1
Overview of the steps involved in different pulping processes.
Figure 5.2
Degradation of wood by fungal hyphae.
Chapter 8
Figure 8.1
Biorefinery concept for sugarcane [39].
Chapter 9
Figure 9.1
Flowchart for assessing the biomass estimates using GIS.
Chapter 10
Figure 10.1
A generalized concept for production of value-added chemicals and polymers from lignocellulosic biomass.
List of Tables
Chapter 1
Table 1.1
Variations in cellulose, hemicellulose and lignin composition in different lignocellulosic materials.
Table 1.2
Glimpse of different physical and chemical methods for lignocellulose pretreatment.
Table 1.3
Microbial utilization of different lignocellulosic materials for polyhydroxyalkanoates production.
Chapter 2
Table 2.1
Feedstock used for hydrogen production.
Chapter 3
Table 3.1
Properties of engineering materials [2].
Table 3.2
Composition of different lignocellulosic fibers [2].
Table 3.3
Properties of natural and man-made fibers [10, 11].
Chapter 4
Table 4.1
Algal species and their respective lipid accumulation.
Table 4.2
Lipid content of oleaginous yeast and molds.
Table 4.3
Studies on lipid production by metabolic engineering strategies.
Table 4.4
Lignocellulose pretreatment methods for biodiesel production.
Chapter 5
Table 5.1
Major biochemical compositions of various types of lignocellulosic biomass.
Table 5.2
Reaction mechanisms of some ligninolytic enzymes (adapted from [40]).
Chapter 7
Table 7.1
Compositional analysis of lignocellulosic biomass.
Table 7.2
Sources of enzymes/proteins used in lignocellulosic biomass conversion to biofuels.
Table 7.3
Production of microbial enzymes from lignocellulosic biomass.
Table 7.4
Group I, II and III enzymes used in lignocellulosic ethanol production.
Chapter 8
Table 8.1
Different varieties of sugarcane grown in different states of India [13].
Table 8.2
Sugarcane varieties and their salient features.
Table 8.3
Few examples of disease-causing organisms, symptoms and management of sugarcane crop [17].
Table 8.4
Use of sugarcane for different biofuels and biobased products in a biorefinery concept.
Chapter 9
Table 9.1
Surplus biomass available in different states of India.
Table 9.2
Availability of specific crops in India.
Table 9.3
Biomass residue available in different states of the USA.
Chapter 10
Table 10.1
Value-added chemicals and polymers obtained from lignocellulose-based substrate and their applications.
Chapter 11
Table 11.1
Reducing sugar yield from several types of lignocellulosic biomass.
Table 11.2
Biobutanol production from several types of lignocellulosic biomass.
Chapter 12
Table 12.1
Differences between mechanical and chemical pulping.
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Lignocellulosic Biomass Production and Industrial Applications
Edited by
Arindam Kuila
Vinay Sharma
This edition first published 2017 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2017 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication DataNames: Kuila, Arindam, editor. | Sharma, Vinay, editor.Title: Lignocellulosic biomass production and industrial applications / edited by Arindam Kuila and Vinay Sharma.Description: Beverly, MA : Scrivener Publishing ; Hoboken, NJ : John Wiley & Sons, 2017. | Includes index. |Identifiers: LCCN 2017010812 (print) | LCCN 2017013368 (ebook) | ISBN 9781119323853 (pdf) | ISBN 9781119323877 (epub) | ISBN 9781119323600 (cloth)Subjects: LCSH: Lignocellulose–Biotechnology. | Biomass–Industrial applications.Classification: LCC TP248.65.L54 (ebook) | LCC TP248.65.L54 L5383 2017 (print) | DDC 662/.88–dc23LC record available at https://lccn.loc.gov/2017010812
Preface
Lignocellulosic materials such as agricultural residues (e.g., wheat straw, sugarcane bagasse, corn stover), forest products (hardwood and softwood), and crops such as switchgrass and salix, are becoming a potent source for generating different valuable products. Lignocellulosic biomass is mainly composed of cellulose, hemicellulose and lignin, along with smaller amounts of pectin, protein and extractives (soluble nonstructural materials such as nonstructural sugars, nitrogenous material, chlorophyll and waxes). Cellulose and hemicellulose are the main constituents of lignocellulosic biomass, occupying a major portion of the fibrous structure of plant cell walls. This book, entitled Lignocellulosic Biomass Production and Industrial Applications, describes the utilization of lignocellulosic biomass for different possible applications. Although there have been numerous reports on lignocellulosic biomass for biofuel application, there have been very few other applications reported for lignocellulosic biomass-based chemicals, polymers, etc. Therefore, this book covers all of the possible lignocellulosic biomass applications. It describes the different types of biofuel production, such as bioethanol, biobutanol, biodiesel and biogas, from lignocellulosic biomass. Also presented are various other lignocellulosic biomass biorefinery applications for the production of chemicals, polymers, paper and bioplastics. In addition, there is a discussion of the major benefits, limitations and future prospects of the use of lignocellulosic biomass.
Arindam KuilaVinay SharmaBanasthali, IndiaFebruary 2017
Chapter 1Valorization of Lignocellulosic Materials to Polyhydroxyalkanoates (PHAs)
Arpan Das
Department of Microbiology, Maulana Azad College, Kolkata, West Bengal, India
Corresponding author: [email protected]
Abstract
Biobased products have generated great interest since sustainable development policies are expanding along with decreasing fossil fuel reserves and growing environmental concerns. Among the petrochemical products, synthetic plastic plays an important role in human daily life, but its recalcitrant properties cause pervasive environmental pollution. In this regard, Polyhydroxyalkanoates (PHAs) are very encouraging resources that might serve as an eco-friendly alternative to petrochemical plastics. But the main obstacle is the cost of that polymer material, which is used as a carbon source during the production of PHAs. Lignocellulosic biomasses represent a very promising substrate for PHA production as they are cheap, abundant and do not compete with the human food chain. Lignocellulosic hydrolysates with a wide range of sugars and organic acids can extensively influence the overall yield of PHAs. This chapter provides a glimpse into the current research focusing on the production of PHAs using lignocellulosic materials as main carbon source.
Keywords: Polyhydroxyalkanoates, agrowastes, lignocellulose, cellulose, hemicellulose, lignin
1.1 Introduction
The accumulation of petrochemical polymers in our surroundings and growing awareness of environmental pollution throughout the world has triggered the search for new biocompatible products for a safe environment. Currently, most polymer products are designed and prepared synthetically and very limited consideration is given to their ultimate disposal. However, these nondegradable plastics are building up in the environment at the rate of 25 million tons per year, which may persist for hundreds of years. Under these circumstances it is worth designing and developing appropriate biodegradable materials whose disposal ensures a better environment and ecosystem. Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible plastics that have been identified as an alternative to petroleum-based synthetic plastics. This type of polyester polymer is produced by many bacteria, archaea as well as some fungi. It accumulates as discrete granules to levels as high as 90% of cell dry weight as a response to environmental stress and nutrient imbalance (when the carbon substrate is in excess of other nutrients such as nitrogen, sulfur, phosphorus or oxygen [1]), and plays a role as a sink for intracellular energy and carbon storage. These water insoluble storage polymers are biodegradable, exhibit thermoplastic properties and can be produced from different renewable carbon sources. PHAs are high molecular mass polymers with properties similar to conventional plastics such as polypropylene. Therefore, they have a wide range of applications such as in the manufacture of bottles, packaging materials, films for agriculture and also in medical applications [2, 3]. The main advantage is that the biodegradable polymers are completely degraded to water, carbon dioxide and methane by anaerobic microorganisms in various environments such as soil, sea, lake water and sewage and, hence, are easily disposable without harm to the environment. However, the high cost of PHA production compared to cheap petrochemical polymers, prevents their use on an industrial scale. Continuous efforts are being made and several studies are going on to develop a cost-effective strategy by using inexpensive substrates as a carbon source, which can significantly affect the production of PHA and has become an important objective for the commercialization of bioplastics. Since about 45% of the total cost of PHA production are attributed to carbon sources, such as refined glucose or sucrose [4], cheap wastes from agriculture and the food industry are used as inexpensive carbon substrates, thus improving the economic feasibility of PHA production. Moreover, lignocellulosic biomasses are considered to be very promising renewable sources for the biotechnological production of fuels and chemicals, including PHA. Lignocellulose hydrolysate is a potentially inexpensive and renewable feedstock that can be processed through different physical, chemical or enzymatic processes to fermentable sugars such as glucose, galactose, xylose, and mannose. However, during the process to produce fermentable sugars, other by-products like acetic acid, 5-hydroxymethyl furfural, formic acid, phenolic compounds, etc., are released during the treatment of hemicellulose and lignin. These compounds are exceedingly toxic to microorganisms during subsequent fermentation processes. In order to increase the fermentability of the hydrolysate, a number of detoxifcation methods are also required to remove potential inhibitors. Overliming [5], activated charcoal [6], membrane filtration [7], ion exchange resins [8], and biological treatments [9] are among the most frequently used treatments.
1.1.1 What is PHA?
Polyhydroxyalkanoates (PHAs) are storage compounds that are widely produced by many microorganisms under nutrient-limited growth conditions, such as nitrogen, phosphorous or oxygen starvation, and when an excess of carbon source is present [10]. These storage materials serve as the carbon and energy reserves of the producing microorganisms. Generally, PHAs are considered as an alternative to petrochemical-based synthetic polymers. Based on the chain length of the fatty acid monomers, PHAs can be classified into three categories: short-chain-length (scl) PHAs with 3 to 5 carbon atoms, medium-chain-length (mcl) PHAs with 6 to 14 carbon atoms and long-chain-length (lcl) PHAs with more than 14 carbon atoms [11]. The difference in length and/or chemical structure of the alkyl side chain of the PHAs influences the material properties of the polymers to a great extent [12]. In general, the scl-PHAs are more crystalline than the mcl-PHAs. As such, scl-PHAs usually exhibit thermoplastic-like properties, while mcl-PHAs and lcl-PHAs behave like elastomers or adhesives. Due to their physical characteristics, scl-PHAs can be used for manufacturing items for packaging or everyday plastics commodities. However, PHAs are disadvantaged due to their significantly higher production costs, while a major portion of the final cost is represented by the price of carbon substrate (28–50%). Therefore, research has focused on inexpensive fermentable raw materials as substrates for biotechnological PHA production.
1.1.2 Mechanism of PHA Production
Polyhydroxybutyrate is the intracellular granule, synthesized by bacteria, and acts as an energy storage facility. In some Bacillus sp., it provides energy for sporulation [13]. The low molecular weight P(3HB) is a part of bacterial Ca2+ channels [14]. These granules are synthesized by the microorganisms in a limited concentration of O, N, P, S, or trace elements, e.g., Mg, Ca, Fe and high carbon concentration in the medium [15]. Generally these nutrient sources are used for the synthesis of proteins essential for the growth in bacteria. But, nitrogen source depletion leads to the cessation of protein synthesis, which in turn leads to the inhibition of tricarboxylic acid cycle (TCA cycle) enzymes, such as citrate synthase and isocitrate dehydrogenase, and consequently slows down the TCA cycle [16]. As a result, the acetyl-CoA routes to P(3HB) biosynthesis. Both the shortening of external nutrients and internal sources, such as RNA or enzymes, facilitate the PHA synthesis. Figure 1.1 is a schematic representation of glucose and xylose metabolism for PHA production. Xylose is assimilated in bacteria by the pentose phosphate pathway through isomerization to d-xylulose by xylose isomerase, followed by a phosphorylation by xylulokinase that produces d-xylulose 5-phosphate, finally yielding glucose 6-phosphate. It has been noticed that in some bacteria like Pseudomonas putida, the enzymes responsible for converting xylose to the entry intermediate xylulose 5-phosphate of PP pathway are missing. By introducing the relevant enzymes XylA and XylB, P. putida KT2440 was able to utilize xylose [11].
Figure 1.1 Flow diagram of polyhydroxyalkanoates (PHA) production from lignocellulose precursor molecules.
From Figure 1.1 it can also be seen that PHB formation and the TCA cycle share the same precursor, acetyl-coenzyme A (acetyl-coA), indicating that when synthesizing PHAs using aerobic bacteria, the role of oxygen becomes crucial. It is also reported that when dissolved oxygen (DO) is limited to a certain degree (30–60%), the PHA production quantity changes. The best DO level for optimal PHA production has been found to be 30%. The mechanism behind this is that, under limited DO conditions, an influx of acetyl-coA will move towards PHA production and away from the TCA cycle [17].
1.2 Lignocellulose: An Abundant Carbon Source for PHA Production
Chemically, lignocellulose, the most abundant raw material on earth, is composed of two linear polymers, cellulose and hemicellulose with a nonlinear lignin polymer [18]. In addition, small amounts of other materials, such as ash, protein, pectin, etc., are present in different degrees based on the source. Lignocellulose is physically hard, dense and recalcitrant towards degradation. However, it is an extremely rich and abundant source of carbon and chemical energy, therefore, the recycling of carbon involving lignocelluloses is essential to maintain the global carbon cycle. Although the composition of lignocellulose strongly depends on the type and origin of the particular plant biomass (Table 1.1), the average proportions (w/w) are as follows: 35–50% cellulose, 20–40% hemicellulose, and 5–30% lignin [19, 20].
Table 1.1 Variations in cellulose, hemicellulose and lignin composition in different lignocellulosic materials.
Lignocellulose
Sugar cane bagasse
Rice straw
Wheat straw
Newspaper
Agricultural residues
Hardwood
Softwood
Grasses
1.2.1 Cellulose
Cellulose, the most widespread organic material in the world, is the primary product of photosynthesis in terrestrial environments. Its regeneration occurs rapidly, and it does not represent a direct food resource for humans [21]. Cellulose naturally occurs in wood, hemp and other plant-based materials and serves as the dominant reinforcing material in plant structures. This biopolymer is also synthesized by algae, tunicates, some fungi, invertebrates and certain bacteria belonging to the genera Acetobacter, Agrobacterium, Alcaligenes, Pseudomonas, Rhizobium or Sarcina. Even some amoeba (protozoa, for example, Dictyostelium discoideum) can synthesize cellulose [22, 23]. Since its discovery in 1838 by Payen, the chemical and physical properties of cellulose have been extensively investigated. A number of efforts of scientists from very different fields have been dedicated to understanding and controlling its biosynthesis, assembly and structural features. It is a linear condensation polymer consisting of D-anhydroglucopyranose joined together by β-1,4-glycosidic bonds with a degree of polymerization (DP) from 100 to 20,000 [24]. It also has a technical name, 1,4-β-polyanhydroglucopyranose. Every d-glucose unit is corkscrewed at 180° with respect to its neighbors, and the repeated segment is frequently treated as a dimer of glucose, known as cellobiose. Each cellulose chain possesses a directional chemical asymmetry with respect to the terminus of its molecular axis: one end is a chemical reducing functionality (hemiacetal unit) and the other is a hydroxyl group, known as the non-reducing end. Coupling of adjacent cellulose chains and sheets of cellulose by hydrogen bonds and van der Waals forces results in a parallel alignment and a crystalline structure with straight, stable supramolecular fibers of great tensile strength and low accessibility, which is known as cellulose microfibril. Due to the fact that cellulose possesses a substantial degree of crystallinity, it functions as a rigid, load-bearing component of the cell wall. The individual chains in these fibrils are associated in various degrees of parallelism. Regions containing highly oriented chains are called crystallites; those in which the chains are more randomly oriented are termed amorphous. In naturally occurring cellulose, the degree of crystallinity varies between 40% and 90% and the rest of the cellulose is amorphous. The amorphous regions are the target site for enzymatic hydrolysis and these regions facilitate the penetration and adsorption of enzyme. The resistance of celluloses to enzymatic breakdown is a function of their degree of crystallinity. Furthermore, the rigidity of the cellulose microfibril is strengthened within a matrix of hemicellulose lignin and pectin.
1.2.2 Hemicelluloses
Hemicellulose is the second most abundant component of lignocellulosic biomass. The dominant sugars in hemicelluloses are mannose in softwoods and xylose in hardwoods and agriculture residues. Furthermore, these heteropolymers contain galactose, glucose, arabinose, and small amounts of rhamnose, glucuronic acid, methyl glucuronic acid, and galacturonic acid [25]. The average degree of polymerization of hemicelluloses is in the range of 80–200. They are usually associated with various other cell wall components such as cellulose, cell wall proteins, lignin, and other phenolic compounds by covalent and hydrogen bonding, and by ionic and hydrophobic interactions [26]. In contrast to cellulose, which is crystalline and strong, hemicellulose have a random, amorphous, and branched structure with little resistance to hydrolysis, and they are more easily hydrolyzed by acids to their monomer components. Composition of hemicelluloses is very variable in nature and depends on the plant source.
1.2.3 Lignin
Lignin, the third main heterogeneous polymer in lignocellulosic residues, is a very complex molecule constructed of aromatic alcohols, including coniferyl alcohol, sinapyl and p-coumaryl units linked in a three-dimensional structure [27]. It is present in the middle lamella and acts as cement between the plant cells. It is also located in the layers of the cell walls, forming, together with hemicelluloses, an amorphous matrix in which cellulose fibrils are embedded and protected against biodegradation. Lignin acts as a binder of the lignocellulosic constituents, giving the plant structural support, impermeability, and resistance against microbial attack and oxidative stress. Not surprisingly, lignin is the most recalcitrant component of the plant cell wall, and the higher the proportion of lignin, the higher the resistance to chemical and enzymatic degradation [28]. Generally, softwoods contain more lignin than hardwoods and most of the agriculture residues. There are chemical bonds between lignin and hemicellulose and even cellulose. Lignin is one of the drawbacks of using lignocellulosic materials in fermentation, as it makes lignocellulose resistant to chemical and biological degradation.
1.2.4 Pectin
Pectins are polymers of d-galactopyranosyluronic acids joined by α-d-(1→4) glycosidic linkages. The main chain can be modified in various ways (ramification with neutral sugars, esterification, acetylation) [29]. Pectin is an acidic cell wall polysaccharide that functions as a sol-like matrix, providing water and ion retention, support and facilitation of cell wall modifying enzymes, cell wall porosity, cell-to-cell adhesion, cell expansion, cell signaling, developmental regulation, and defense [30].
1.3 Lignocellulosic Pretreatment Techniques
The structure of cellulose imparts tightly packed arrangements that are water insoluble and resistant to depolymerization [31]. Thus, it is imperative that a pretreatment regime alter the structure of biomass to make the cellulose more accessible to hydrolysis. A glimpse of different pretreatment processes is shown in Table 1.2. An effective pretreatment must meet the following requirements: (1) increase the accessible cellulose surface area, (2) disrupt the lignin barrier as well as cellulose crystallinity to allow proper enzymatic attack, (3) limit the formation of toxic degradation products that are inhibitory towards the enzymes or fermentative microorganisms, (4) reduce the loss of sugar components (cellulose and hemicellulose) and (5) minimize the capital and operating costs. Wide spectrums of pretreatment protocols have been investigated for hydrolysis and a few of them have been developed sufficiently to be called technologies. Pretreatment approaches can be broadly classified into four categories: (1) physical; (2) chemical; (3) physicochemical and (4) biological.
Table 1.2 Glimpse of different physical and chemical methods for lignocellulose pretreatment.
Pretreatment method
Processes
Possible changes in biomass
Ref.
Physical pretreatments
Milling:
Ball milling, Two-roll milling, Hammer milling, Colloid milling
Increase in accessible surface area and pore size
Irradiation:
Gamma-ray irradiation, Electron-beam irradiation, Microwave irradiation
decrease the degree of polymerization of cellulose
Others:
Hydrothermal, High pressure Steaming, Expansion, Extrusion, Pyrolysis
Decrease in degrees of polymerization
Chemical and physicochemical pretreatments
Explosion:
Steam explosion, Ammonia fiber explosion (AFEX), CO
2
explosion, SO
2
explosion
Remove hemicellulose; swell the plant material
Alkali:
Sodium hydroxide, Ammonia, NaOH/urea
Remove most of lignin and hemicellulose; swell the cellulose fibers, disrupt the connections between hemicelluloses, cellulose, and lignin; break down the fiber bundles into small and loose particles
Acid:
Sulfuric acid, Hydrochloric acid, Phosphoric acid/acetone
Hemicellulose degradation, Remove most lignin and hemicellulose, destroy the cellulose crystallinity
Oxidizing agents:
Hydrogen peroxide, Wet oxidation, Ozone
Removal of lignin; dissolves hemicellulose and causes cellulose decrystallization
Ionic Liquids
Hydrolyze lignin and hemicellulose
Biological pretreatments
Fungi and actinomycetes
Delignification; reduction in degree of polymerization of cellulose; partial hydrolysis of hemicellulose
1.3.1 Physical Pretreatment Techniques
Physical methods of pretreatment like milling and steam treatment will reduce particle sizes thereby increasing the available surface area for enzymatic attack. Steam explosion loosens the crystalline complex and also removes the pentose while increasing the surface area. However, the drawback of the process is that steam treatment may generate certain cellulase inhibitors that can interfere with the enzymatic hydrolysis of the cellulosic substrate.
1.3.1.1 Milling
Milling can be employed to alter the inherent ultrastructure of lignocelluloses and degree of crystallinity, and consequently make it more accessible to enzymatic degradation. Milling and particle size reduction have been applied prior to enzymatic hydrolysis, or even other pretreatment processes with dilute acid, steam or ammonia, on several lignocellulosic waste materials [31, 32]. Among the milling processes, colloid mill, fibrillator and dissolver are suitable only for wet materials, while the extruder, roller mill, cryogenic mill and hammer mill are usually used for dry materials. The ball mill can be used for both dry and wet materials. Grinding with hammer milling of waste paper is a favorable method [33]. Milling can improve susceptibility to enzymatic hydrolysis by reducing the particle size and degree of crystallinity of lignocelluloses, which improves enzymatic degradation of these materials.
1.3.1.2 Irradiation
Irradiation by gamma rays, electron beam and microwaves can improve enzymatic hydrolysis of lignocelluloses. The combination of the preradiation and other methods, such as acid treatment, can further accelerate degradation of cellulose into glucose. The cellulose component of the lignocellulose materials can be degraded by irradiation to fragile fibers and low molecular weight oligosaccharides and even cellobiose, that could be due to preferential dissociation of the glucoside bonds of the cellulose chains by irradiation in the presence of lignin. But a very high irradiation can lead to the decomposition of oligosaccharides and the glucose ring structure [34].
1.3.2 Chemical Pretreatment
In general, chemical pretreatment processes selectivity degrades the biomass component, but they involve relatively harsh reaction conditions, which may not be ideal in a biosaccharification scheme due to adverse effects on downstream biological processing. Different chemical pretreatments that are generally practiced include acid, alkaline, ozonolysis, oxidative H2O2 delignification, organosolv, etc. [31, 35]. Besides these, their combinational effects have also been found suitable. However, utilization of various chemicals in the pretreatment procedures is a major drawback and affects the total economy of the bioconversion of the lignocellulosic biomass.
1.3.2.1 Acid Hydrolysis Pretreatment
Both concentrated and diluted acids such as H2SO4, HCl and perchloric acids have been used to treat lignocellulosic materials. Pretreatment with acid hydrolysis can result in improvement of enzymatic hydrolysis of lignocellulosic biomasses to release fermentable sugars. Although they are powerful agents for cellulose hydrolysis, concentrated acids are toxic, corrosive, hazardous, and thus require corrosion resistant reactors, which makes the pretreatment process very expensive. In addition, the concentrated acid must be recovered after hydrolysis to make the process economically feasible [36].
1.3.2.2 Alkaline Hydrolysis
The effect of alkaline pretreatment depends on the lignin content of the lignocellulosic materials. Alkali pretreatment processes can be effective at lower temperatures and pressures than many other pretreatment technologies, but it requires longer times on the order of hours or days. Compared with acid, alkaline pretreatments cause less sugar degradation, and many of the caustic salts can be recovered and/or regenerated [37]. Sodium, calcium, potassium, and ammonium hydroxides are widely used alkaline pretreatment agents. Out of these, sodium hydroxide has been studied the most. However, calcium hydroxide (slake lime) also has been shown to be an effective pretreatment agent and is the least expensive.
1.3.2.3 Oxidative Delignification by Peroxide
Lignin biodegradation has been reported to be catalyzed in the presence of H2O2. The pretreatment of cane bagasse with hydrogen peroxide greatly enhanced its susceptibility to enzymatic hydrolysis [38]. About 50% of the lignin and most of the hemicellulose were solubilized by 2% H2O2 at 30 °C within 8 h, and 95% efficiency of glucose production from cellulose was achieved in the subsequent saccharification by cellulase at 45 °C for 24 h. Wet oxidation combined with base addition readily oxidizes lignin from wheat straw, thus making the polysaccharides more susceptible to enzymatic hydrolysis. Furfural and hydroxymethylfurfural, known inhibitors of microbial growth when other pretreatment systems are applied, were not observed following the wet oxidation treatment.
1.3.2.4 Organosolv Process
The organosolvation method is a promising pretreatment strategy, and it has attracted much attention and has proven potential for utilization in lignocellulosic pretreatment. In this process, an organic or aqueous organic solvent mixture with inorganic acid catalysts (HCl or H2SO4) is used to break the internal lignin and hemicellulose bonds [39]. The commonly used solvents in the process are methanol, ethanol, acetone, ethylene glycol, triethylene glycol, and tetrahydrofurfuryl alcohol. Other organic acids like oxalic, acetylsalicylic, and salicylic acids can also be used as catalysts in the organosolvation process. Treatment of lignocellulosic materials with these organosolvs at temperatures ranging from 140 to 220 °C causes lignin breakdown into fragments which are quite soluble in the solvent system [40]. This technique yields three separate fractions: dry lignin, an aqueous hemicellulose stream, and a relatively pure cellulose fraction.
1.3.2.5 Ozonolysis Pretreatment
Ozone pretreatment is one way of reducing the lignin content of lignocellulosic wastes which results in an increase of the in-vitro enzymatic digestibility of the treated material, and unlike other chemical treatments, it does not yield toxic products [41]. Although ozone can be used to degrade lignin and hemicellulose in many lignocellulosic materials, such as wheat straw, rice straw, bagasse, peanut, pine, cotton straw, sawdust, etc., the degradation is mainly limited to lignin. In this process hemicellulose portions are slightly affected, but cellulose is not. Ozonolysis pretreatment has an advantage in that the reactions are carried out at room temperature and under normal pressure. Furthermore, after pretreatment ozone can be easily decomposed by using a catalytic bed or increasing the temperature to minimize environmental pollution. A drawback of ozone pretreatment is that a large amount of ozone is required, which can make the process expensive [42].
1.3.2.6 Ionic Liquids Pretreatment
Another technology for lignocellulose fractionation is using ionic liquids. Ionic liquids (ILs) are organic salts which exist as liquids at low temperatures; often well below 100 °C. They have negligible (or very low) vapor pressures, generally good thermal stability and there is a variety of combinations of anions and cations that can be used to synthesize ILs [43]. Recent studies have showed that cellulose and lignin both can be dissolved in a variety of ILs and can be easily regenerated from these solutions by means of addition of a non-solvent [44]. Dadi et al. [45] used 1-n-butyl-3-methylimidazolium chloride to dissolve cellulose. The regenerated cellulose had an amorphous structure allowing a greater number of sites for enzyme adsorption and improving the enzymatic hydrolysis rate by 50-fold.
1.3.3 Physico-Chemical Pretreatment
Physico-chemical pretreatment is a combination of different processes for chemical and physical treatments. In these procedures, milder chemical conditions are often used, but under more extreme operational conditions like biorefinery, relatively harsh techniques are used. Different physicochemical pretreatment techniques include mainly liquid hot water (hydrothermolysis, aqueous or steam/aqueous, uncatalyzed solvolysis aquasolv), steam explosion (autohydrolysis with and without chemical addition), ammonia fiber explosion (AFEX), and CO2 explosion.
1.3.3.1 Liquid Hot-Water Pretreatment
Beginning several decades ago, treatment in liquid hot water has been one of the pretreatment methods applied for lignocellulosic materials. Pressurized water can penetrate into the biomass, hydrate cellulose, and remove hemicellulose and part of the lignin. The major advantages of this process are that no addition of chemicals and no corrosion-resistant materials are required for hydrolysis. In addition, the process has a much lower need for chemicals for neutralization of the produced hydrolyzate, and produces lower amounts of inhibitory products compared to acid or alkali pretreatment [46]. Hemicelluloses are dissolved as soluble oligosaccharides and can be separated from insoluble cellulose and lignin fractions. Due to enlargement of the accessible surface area of the cellulose, hydrolytic enzymes become more accessible for saccharification [47].
1.3.3.2 Steam Explosion
Steaming with or without explosion has received substantial attention in the pretreatment of lignocellulosic materials. The pretreatment removes most of the hemicellulose, thus improving the enzymatic digestion. In this method, lignocellulosic biomass is treated with high-pressure saturated steam, and then the pressure is suddenly reduced, which makes the materials undergo an explosive decompression. High pressure and temperature (between 160 and 260 °C) for a few seconds (e.g., 30 s) to several minutes (e.g., 20 min), are used in the steam explosion process [48], which cause hemicellulose degradation and lignin transformation due to high temperature and increase the potential of cellulose hydrolysis. Its energy cost is relatively moderate, and it satisfies all the requirements of the pretreatment process. For this reason, this process is being utilized in pilot processes by several research groups and companies [49].
1.3.3.3 Ammonia Fiber Explosion (AFEX)
In the AFEX process, the lignocellulosic biomass is treated with liquid ammonia at moderate temperatures (e.g., 90–100 °C) and high pressure for a period of time (e.g., 30 min), followed by the rapid release of pressure. As a result, cellulose crystallinity is decreased, the fiber structure is expanded, and the accessible surface area to enzymes is increased. It also depolymerizes or alters lignin structure via ammonia reactions with lignin macromolecules. This pretreatment yields optimal hydrolysis rates at low enzyme loadings and is particularly suited for herbaceous and agricultural residues [31]. The major advantage of AFEX pretreatment is minimization the formation of sugar degradation inhibitory by-products, yet, part of phenolic fragments of lignin degraded products may remain on the cellulosic surface. Therefore, washing with water is necessary to remove these inhibitory components. However, AFEX is more effective on the biomass that contains less lignin, and it does not significantly solubilize hemicellulose. Besides, ammonia must be recycled after the pretreatment to reduce the cost and protect the environment [50].
1.3.4 Bological Pretreatment
Biological pretreatment includes microorganisms for the treatment of lignocellulosic biomass and to enhance enzymatic hydrolysis. Microorganisms usually secrete extracellular enzymes to degrade lignin and hemicellulose. Cellulose is degraded to a lesser extent since it is more recalcitrant to the biological attack. Several brown-, white-, and soft-rot fungi are used for this purpose. White-rot fungi are among the most effective microorganisms for biological pretreatment of lignocelluloses. The biological delignifcation of lignocellulosic materials has been attemped by different strains like Aspergillus, Streptomyces, Phelebia, and Pleurotus [51, 52]. Lignin degradation by fungi occurs through the action of lignin-degrading enzymes such as peroxidases and laccase [53]. Although the biological pretreatment have several advantages such as low-capital cost, energy input and high yields without generating inhibitory by-products, the hydrolysis rate of most biological pretreatment processes is very low [20]. Long treatment time and degradation of the residual carbohydrates are also some of the drawbacks of such processes.
1.4 Hydrolysis of Lingocellulose
The hydrolysis of cellulose is usually performed by acids, alkali or by enzymes. Acid hydrolysis of hemicelluloses and cellulose is performed by concentrated or diluted acids. Acid catalyzes the breakdown of long carbohydrate chains to shorter chain oligomers and then to monomeric sugars. Due to their amorphous nature, hemicelluloses require less severe conditions for their hydrolysis in comparison with crystalline cellulose. When concentrated acids, such as H2SO4 or HCl (10–30%), are used during the pretreatment, cellulose is degraded concomitantly. In this case, pretreatment and hydrolysis are carried out in one step. The advantages of acid hydrolysis are that the acid can penetrate lignin without pretreatment and the rate of acid hydrolysis is faster than enzyme hydrolysis; but it causes corrosion problems in the equipment, which is one of the major disadvantages of this process. When a dilute acid hydrolysis is chosen (2–5%), high temperatures are needed to achieve good rates of cellulose conversion. In this case, the high temperature increases the rates of lignocellulose biomass-derived sugars decomposition, thus causing the formation of toxic compounds which further decreases the yields of fermentable sugars. Acetic acid is released from the acetyl groups of hemicellulose, while furfural and 5-hydroxymethylfurfural (HMF) are formed from the degradation of sugars (xylose and glucose, respectively). On the other hand, subsequent degradation of these aldehydes leads to the formation of formic acid and levulinic acid [54]. The degradation of lignin produces phenolic compounds, such as vanillic acid, vanillin, syringic acid, or syringaldehyde, depending on the type of the lignin present in the biomass [50]. These are very toxic compounds to the microorganisms which will be using the hydrolysates in a subsequent step. Several detoxifying steps are required to remove these inhibitors [55]. Examples of detoxification methods are evaporation, overliming, activated charcoal treatment, membrane filtration, ion exchange resins, and biological treatments [55–57].
Beside acids, alkaline hydrolysis is used for pretreatment of lignocellulosic materials. The mechanism of alkaline hydrolysis is believed to be saponification of intermolecular ester bonds crosslinking xylan hemicelluloses and other components. With the removal of the crosslinks, the porosity of the lignocellulosic materials increases. Generally, alkaline hydrolysis enhances digestibility of the lignocellulose and reduces inhibitors formation [58, 59].
Enzymatic hydrolysis of cellulose is superior to the inorganic catalysts, because enzymes are highly specific towards its substrates and can work at mild process conditions. The cellulase enzyme system is a mixture of endo-β-1,4-glucanglucanhydrolases, exo-β-1,4-glucancellobiohydrolases, and β-glucosidase, which effectively breaks down cellulose to cellobiose and subsequently to glucose. Besides cellulases, a number of other enzymes, such as glucuronide, acetylesterase, xylanase, β-xylosidase, galactomannase, and glucomannase, are present, which can effectively degrade hemicellulose. These enzymes work synergistically to break down both cellulose and hemicellulose in the lignocellulosic materials [60]. In spite of several advantages, the use of enzymes in industrial processes is still limited due to their relative instability at high temperatures and high costs of their purified forms. Currently, extensive research is being carried out with improved thermostability, since high temperatures could speed up the hydrolysis reaction time [59].
1.5 Lignocellulose Biomass as Substrate for PHA Production
Among various types of lignocelluloses, forest biomass represents an enormous reservoir of renewable carbon-rich material. Globally, approximately 80 billion tons of woody biomass is generated per annum, with the production of total plant matter estimated at roughly 180 billion tons annually. There is abundant availability of such agricultural wastes and they are rich sources of carbohydrates. A process diagram of PHA production from lignocellulosic biomass is shown in Figure 1.2. These wastes are mostly used as cattle feed since they have little economic value. Several bacterial species have the innate ability to utilize such diverse and cheap carbon wastes as they possess hydrolytic enzymes capable of metabolizing these complex residues. An overview of different microbial strains utilizing lignocellulosic materials is presented in Table 1.3. Wood hydrolysate is a potentially inexpensive and renewable feedstock that can be produced through enzymatic or dilute acid hydrolysis of cellulose or hemicellulose to fermentable sugars, such as glucose, galactose, xylose, and mannose. Considering this, Pan et al. [55] utilized Sugar maple hemicellulosic hydrolysate containing 71.9 g/l of xylose as an inexpensive feedstock to produce polyhydroxyalkanoates (PHAs) by Burkholderia cepacia ATCC 17759. The inhibitory effects of selected inhibitors from wood hydrolysate were evaluated for effects on cell growth, PHA production, and physical-chemical properties of PHAs. Subsequently, membrane-purified wood hydrolysate, detoxifed to remove phenolics, was used to produce PHAs by fermentation. Wood biomass was again utilized by Bowers et al. [61] for PHAs production. In their study, Pinus radiata wood chips were subjected to high-temperature mechanical pretreatment or steam explosion in the presence of sulphur dioxide before being enzymatically treated to produce corresponding hydrolysates. Two potent bacteria, Novosphingobium nitrogenifigens and Sphingobium scionense, were grown on these hydrolysates and the highest PHB yields of 0.4 g L–1 were observed in Sphingobium scionense.
Figure 1.2 A process diagram of PHA production from lignocellulosic biomass.
Table 1.3 Microbial utilization of different lignocellulosic materials for polyhydroxyalkanoates production.
Microorganisms
Carbon source
PHA production g L
–1
Azotobacter beijerinickii
Coir pitch
Cupriavidus necator
Bagasse hydrolysate
Bacillus firmus
NII 0830
Rice straw hydrolysate
Burkholderia sacchari
Wheat straw hydrolysate
Burkholderia cepacia
ATCC 17759
Sugar maple hemicellulosic hydrolysate
Ralstonia eutropha
Bagasse hydrolysate
Brevundimona svesicularis
Acid hydrolyzed sawdust
Cupriavidus necator
MTCC-1472
Water hyacinth hydrolysates
Bacillus megaterium
Oil palm empty fruit bunch
Sphingopyxis macrogoltabida
LMG 17324
Hydrolyzed pine saw dust
Apart from wood biomass, agriculture and food industry waste streams are another promising source of lignocellulose-based materials which can be used as a substrate for PHA production. These materials are generated in enormous amounts during processing of agricultural plants. Hence, there are numerous papers dealing with the conversion of these waste materials into PHAs. To confirm the feasibility of using agrowastes to replace glucose in the production of PHA, Gowda and Shivakumar [15] successfully utilized different carbon sources (4%, w/v), like bagasse, jowar, ragi husk, straw, rice husk, wheat bran, mango peel, jack fruit seed powder and potato peel residue, for PHA accumulation by B. thuringiensis IAM 12077 in shake flask cultures. The strain showed PHB production on all the substrates tested and the maximum PHA yield was observed with mango peel (4.03 g/L; 51.3%), followed in decreasing order by bagasse (1.26 g/L; 46.15%); rice husk (1.56 g/L; 32.7%); jackfruit seed powder (3.93 g/L; 29.32%); ragi husk (0.96 g/L; 23.2 %), etc. Bacillus species were further explored for their potential to produce poly hydroxy butyrate (PHB) using different low-cost agro-industrial materials. Ghate et al. [62] utilized different agro-industrial materials like Jawar stem, Neera, Cashew apple pulp, Sugar cane bagasse, Coconut pulp and Grapes pulp. Highest cellular PHB content was obtained from Bacillus subtilis with Neera as source of carbon, which was found to be 0.284 g/L.
Chaleomrum et al. [63] attempted to investigate the potential of cassava starch wastewater for producing polyhydroxyalkanoate (PHA) from sequencing batch reactor (SBR) treatment system seeded with Bacillus tequilensis MSU 112, a PHA-producing bacterial strain. Under the optimized condition, 3,346 mg/L of PHA was produced. Another very promising waste substrate for PHA production is spent coffee grounds (SCG). This can be considered as a very promising substrate for PHA production. SCG contain approximately 15% of oil, which can be simply extracted and converted into PHB by Cupriavidus necator [64, 65]. The residual solids after oil extraction contain a significant portion of hemicelluloses and cellulose. Therefore, they were hydrolyzed and converted into PHAs employing Burkholderia cepacia. Hexoses (predominantly mannose and galactose) were substantially dominating sugars of the hydrolysate, which may be an important factor positively influencing the production of PHAs. Moreover, hydrolysate contained levulinic acid, which served as a precursor of 3-hydroxyvalerate, resulting in accumulation of P (3HB-co-3HV) copolymer.
1.6 Conclusion
Biocomposites usefulness is no longer in question and more and more reports are focused on applicative aspects in the environment, packaging, agriculture devices, biomedical fields, etc. Lignocellulose materials seem to be very promising substrates for various industrial and biotechnological processes, because utilization of these resources might decrease our dependence on petroleum and reduce the impact of its gradual depletion and increasing price. The economic feasibility of using lignocellulosic hydrolysates as carbon sources to biologically produce biopolyesters strongly depends on the capacity of microorganisms to consume both the hexoses and the pentoses released from the lignocellulosic biomass and to convert these sugars into products at high conversion yields. Hence there is a necessity of utilizing potent microorganisms capable of fermenting different types of sugar for better production of PHA. This might help these valuable environmentally friendly polymers to compete with petrochemical-based plastics and, therefore, partially replace them in appropriate applications.
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