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",isbn:"978-1-83962-292-2",printIsbn:"978-1-83962-291-5",pdfIsbn:"978-1-83962-293-9",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"5c4c0d41cf25d2e8fda944450ac46d95",bookSignature:"Prof. Constantin Volosencu",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/9976.jpg",keywords:"Fuzzy Aggregation Operators, Fuzzy Set, Hesitant Fuzzy Set, Fuzzy Decision Making, Pythagorean Fuzzy Sets, Pythagorean Fuzzy Decision, Spherical Fuzzy Set, Linguistic Fuzzy Set, Fuzzy Control System, Fuzzy Expert Systems, Fuzzy Logic, Fuzzy Neural Network",numberOfDownloads:103,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"June 4th 2020",dateEndSecondStepPublish:"September 10th 2020",dateEndThirdStepPublish:"November 9th 2020",dateEndFourthStepPublish:"January 28th 2021",dateEndFifthStepPublish:"March 29th 2021",remainingDaysToSecondStep:"4 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Dr. Volosencu is author of 10 books and has over 155 scientific papers published. He holds 27 patents and has developed electrical equipment for machine tools, spooling machines, high power ultrasound processes and other, with the homologation of 18 prototypes and 12 zero manufacturing series.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"1063",title:"Prof.",name:"Constantin",middleName:null,surname:"Volosencu",slug:"constantin-volosencu",fullName:"Constantin Volosencu",profilePictureURL:"https://mts.intechopen.com/storage/users/1063/images/system/1063.jpeg",biography:"Constantin Volosencu is a professor at the “Politehnica” University of Timisoara, Department of Automation. He is the author of 10 books and five book chapters, the editor of nine books, the author of over 150 scientific papers published in journals and\nconference proceedings, the author of 27 patents, and a manager of research grants. 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Applications",doi:"10.5772/intechopen.84531",slug:"microbial-cellulases-an-overview-and-applications",body:'\nBiomolecules derived from natural resources are playing a major role in manufacturing products needed for daily use. Enzymes are one of those molecules that are globally recognized for their multifarious applications in industries. For instance, their utility in brewing, dairy products, detergents, food and feed, pharmaceutical production, and paper and pulp industry is huge. One of those most widely used enzymes is cellulase. According to recent global cellulase market analysis reports, the demand for this enzyme is exponentially increasing.
\nCellulose, the substrate of cellulase, is the most abundant polysaccharide present on earth. It is the main substance in plant materials. Anselme Payne was the very first person to discover and isolate this amazing compound from green plants [1]. It happened more than two centuries ago. From the past, cellulosic materials have played a crucial role in daily human life. They used it to fertilize their soil for crop cultivation. It was also fodder for their cattle. It was firewood for cooking, and they were igniting cellulosic material to generate heat whenever they needed to produce energy.
\nCurrently, the role played by cellulose is not that simple. Especially, as it is recognized as a cost-effective raw material, the useful applications of cellulose in the industrial sector have become much more complex. This has laid a huge platform for scientists to do cellulose-based research in multidisciplinary approaches. One such area is hydrolysis of cellulose. In nature, this is usually accomplished by cellulases. Cellulase is catalyzing hydrolysis of cellulose.
\nHowever, cellulase is not a single enzyme. It is a group of enzymes which is mainly composed of endoglucanase and exoglucanases including cellobiohydrolases and β-glucosidase. Fungi, bacteria, and actinomycetes are recorded to be efficient cellulase enzyme producers in the natural environment. These microorganisms must secrete cellulases that are either free or cell surface bound. Their enzyme production efficiency and the enzyme complex composition are always diverse from each other. Although both aerobic and anaerobic microorganisms produce these enzymes, aerobic cellulolytic fungi, viz., Trichoderma viride and T. reesei, are excessively studied. The enzyme breaks β-1,4-linkages in cellulose polymer to release sugar subunits such as glucose. This notion is applied in industries either cellulose is utilized as a raw material or cellulose degradation is a must.
\nAccording to recent enzyme market reports, the key areas of the industry where cellulase enzyme is increasingly being applied are healthcare, textile, pulp and paper, detergent, food, and beverages. Its wide application in coffee processing, wine making, and fruit juice production is related to food and beverage segment. In other industrial applications, it is broadly used to produce laundry detergents and cleaning and washing agents. Cellulase is also being highly recognized as an effective alternative to available antibiotics for treatment of biofilms produced by Pseudomonas. Therefore, the potential of cellulases to fight against antibiotic-resistant bacteria is an amazing trend which will overcome problems in the healthcare sector [2].
\nApplication of microorganisms or microbial enzymes for pretreatment of lignocellulosic material is currently earning a huge attention of the industry. This is a result of growing interest about depletion of fossil fuel resources in the world which have inspired the production of bioethanol from lignocellulosic biomass through enzymatic hydrolysis [3]. Lignocellulosic biomass is one of the best options as a low-cost, readily available, eco-friendly raw material. However, it is not found alone. Cellulose is forming lignocellulose in combination with hemicellulose and lignin which finally becomes a compact network structure [4]. Moreover, it has a crystalline structure which is hard to break down. Therefore, cellulose is insoluble in water and causes limitations in hydrolysis. That is why it is essential to pretreat lignocellulosic material in industries like bioethanol production. During pretreatment, it will loosen up the crystalline structure and facilitate the degradability to release fermentable sugar forms. There are several methods available for pretreatment of lignocellulose, viz., physical, chemical, and biological methods. Biological pretreatment using cellulolytic microorganisms and their enzymes is found to be the best way of addressing this problem.
\nBy all means, cellulase is an enzyme which can cause a huge economic impact. However, there are some considerable bottlenecks of utilizing this enzyme in the industry. For example, the higher cost of cellulase and less catalytic efficiency are especially understood. Another important point is less understanding of the relationship between hydrolysis mechanisms and molecular structure of the enzyme. This knowledge is important to carry out further improvements in the enzyme to enhance its catalytic activity. Therefore, this chapter is discussing about the structure and function of cellulase in order to understand its mechanisms of action. The details on current applications of the enzyme have also been summarized here. Furthermore, the efforts have been taken to bring together information on novel biotechnological trends of cellulase. Moreover, it is discussing about possible low-cost, enzymatic pretreatment methods that have been practiced for lignocellulosic materials in order to use it as an efficient raw material to produce bioethanol.
\nBefore moving on to cellulase, it is essential to understand cellulose which is the substrate of cellulase enzyme. This section will provide a short description about cellulose.
\nCellulose is a linear polysaccharide. In this polymer, D-glucose subunits are attached together by formation of β-1,4-glycosidic linkages between individual glucose molecules. The molecular formula of cellulose is (C6H12O6)n. The “n” indicates the degree of polymerization (DP). It symbolizes the number of glucose subunits connected with each other. This number is varying from hundreds to thousands. Two glucose repeating units together are called cellobiose. In other words, this polymer is made by β-(1 → 4)-D-glucopyranose units in 4C1 conformation. It consists of long chains of anhydro-D-glucopyranose units (AGU) with each cellulose molecule having three hydroxyl groups per AGU with the exception of the terminal ends. Cellulose has both crystalline and amorphous regions in its structure in various proportions [5]. Those regions are intertwined to form the structure of cellulose. There are four major crystalline forms, for instance, Iα, Iβ, II, and III. This crystalline structure is a result of intramolecular and intermolecular hydrogen bonding between glucose monomers in cellulose. These hydrogen bonds construct a huge network that directly contributes to the compact crystal structure of cellulose polymer. On the other hand, this strong intramolecular and intermolecular hydrogen bond formation leads to poor solubility of cellulose.
\nIn plant cell walls, cellulose exists as different levels of structures, i.e., single cellulose chains, elementary fibrils (consisting of tens of single cellulose chains), and microfibrils (bundles of elementary fibrils). It is proposed that the macrofibril is composed by the attachment of several newly synthesized elementary fibrils. With the cellular growth, the macrofibrils divide to form individual microfibrils. Microfibril is consisting of a single elementary fibril. Although elementary fibrils and macrofibrils are composed of mere cellulose, microfibril has noncellulosic polymers like hemicelluloses along with cellulose. It is noted that others consider a microfibril as consisting of a number of elementary fibrils. Microfibril is an elementary fibril associated with noncellulosic polymers. Each microfibril might contain up to 40 cellulose chains and is about ~10 to 20 nm in diameter. Many such cellulose chains aggregate into bundles called micelles and micelles into microfibrils. Micelles are interconnected with few cellulose fibers. The plant cell wall structure is stabilized by the macrofibrils. The cross-links between hemicellulose and pectin matrices also support this stabilization process. Lignin is a complex polymer which usually fills the spaces between cellulose and pectin matrices. It forms covalent bonds with hemicellulose. This provides more mechanical strength to the plant cell wall. This structure which is present in plants is collectively called lignocellulose. Other components known as extractives including fats, phenolic, resins, and minerals are also present in lignocellulosic biomass.
\nEnzymes are known to be very useful in many industrial processes. Their broad applicability has created a significant market demand in the recent years. According to market reports on world enzyme demand (2017), they have recognized several key factors which lead to huge consumer demand for enzymes. Some of them are completely bound with economical advances. For example, increased per capita income in developing countries causes huge growth in consumer-related industrial applications [6]. A recent industry study done by Freedonia in January 2018 on “Global Industrial Enzymes” reveals that global demand for industrial enzymes is projected to grow 4.0% per year to $5.0 billion in 2021. This report also emphasizes the gains in personal incomes in developing countries as the key factor which is supporting growth in demand for enzymes. The development of scientific research on enzymes is mainly based on disciplines such as biotechnology,molecular biology and genetics. Continued advances in these areas of research, particularly related to DNA manipulation and sequencing, result in extensive increases in enzyme demand worldwide. Cellulase is one such enzyme which earns consecutively increasing demand. Therefore, collection of knowledge about this enzyme is essential for further development of fundamental and applied research on cellulase and for consequent application in human life.
\nIt is produced by fungi, bacteria, actinomycetes, protozoans, plants, and animals. According to Carbohydrate-Active Enzymes database, there is information of the glycoside hydrolase families. Glycoside hydrolases, including cellulase, have been classified into 115 families based on amino acid sequence similarities and crystal structures. A large number of cellulase genes have now been cloned and characterized. They are found in 13 different families. Furthermore, there are 3D structures of more than 50 cellulases. All of cellulases cleave β-1,4-glucosidic bonds. However, they display a variety of topologies ranging from all β-sheet proteins to β/α-barrels to all α-helical protein.
\nIn the structure of cellulase, there are catalytic modules and non-catalytic modules. The catalytic modules of cellulases have been classified into numerous families based on their amino acid sequences and crystal structures. The non-catalytic carbohydrate-binding modules (CBMs) and/or other functionally known or unknown modules may be located at the N- or C-terminus of a catalytic module. Usually, fungal and bacterial cellulase mainly has two or more structural and functional domains. Both aerobic and anaerobic microorganisms are producing this enzyme. Therefore, there are two types of cellulase systems: noncomplex and complex. A noncomplex cellulase system is produced by aerobic cellulolytic microorganisms, and it is a mixture of extracellular cooperative enzymes. In a noncomplex cellulase system, the common arrangement is joining of a catalytic domain with a cellulose-binding domain (CBD). A complex cellulase system is produced by anaerobic microorganisms and it is called “cellulosome.” Cellulosome is assembled by joining a catalytic domain with a dockerin domain. The enzyme is a multiprotein complex anchored on the surface of the bacterium by non-catalytic proteins that serves to function like the individual noncomplex cellulases but is in one unit.
\nIn addition to these two major domains in the cellulase structure, there are some other domains that are present in many cellulases, for instance, S-layer homologous (SLH) domain, fibronectin-type 111 domains, and NodB-like domain, and there are also other regions of unknown function. These domains are often connected by Pro and hydroxy amino acid (threonine and serine) enriched linker sequences. Among all these domains, catalytic and cellulose-binding domains are the most important because they are the domains which are considered participating in hydrolytic mechanisms of the enzyme.
\nCellulase catalyzes the decomposition of cellulose polysaccharide by simply breaking down β-1,4-glycosidic bonds. Three major types of enzymes are generally involved in hydrolyzing cellulose microfibrils in the plant cell wall: endoglucanase, exoglucanase, and β-glucosidase. Complete cellulose hydrolysis is mediated by the combination of these three main types of enzymes. Endoglucanase usually attacks amorphous areas of cellulose. The random attack of this enzyme on internal bonds of loosely bound, amorphous areas of cellulose creates new chain ends. These new chain ends are then easily attacked by other types of enzymes. The highest activity of this enzyme usually occurs against soluble cellulose forms or acid-treated amorphous cellulose. The function of exoglucanase is to produce glucose or cellobiose units by attacking the reducing or nonreducing end of cellulose chains. Endoglucanase is different from exoglucanase because it is usually very active against crystalline cellulose substrates such as Avicel or cellooligosaccharides. Finally, β-glucosidase can hydrolyze cellobiose to glucose from the nonreducing ends, and it is inactive against amorphous or crystalline cellulose. Although an exact mechanism is not yet finalized, fragmentation of cellulose aggregations into short fibers has been observed and reported during the beginning of cellulose hydrolysis prior to releasing any detectable amount of reducing sugars. This is known as amorphogenesis.
\nThere are two catalytic mechanisms of cellulases. They are simply introduced as retaining mechanisms and inverting mechanisms. Cellulases cleave glucosidic bonds by using acid-based catalysis. The hydrolysis is performed by two catalytic residues of the enzyme: a general acid (proton donor) and a nucleophile/base. The catalytic mechanism which occurs depends on the spatial position of the catalytic residues. The retention and inversion of the anomeric configuration of cellulose are the two mechanisms which hydrolyze cellulose. The “retaining” cellulases retain the same configuration of anomeric C bearing the target glucosidic bond even after a double-displacement hydrolysis with two key glycosylation or deglycosylation steps. “Inverting” cellulases inverts the configuration of the anomeric C configuration after a single nucleophilic displacement hydrolysis [7].
\nFor many decades, cellulases have played a crucial role as biocatalysts. They have shown their potential application in a large number of industries. Textile, paper and pulp, laundry and detergent, agriculture, medicine, and food and feed industries are some of the major industries which employ microbial cellulases. According to Coherent Market Insights, the textile industry is the dominant market for cellulases in 2017. According to most of the enzyme market research reports published in 2018, food and beverages, textile industry, animal feed, and biofuels have been reported to be the major areas of applications.
\nAccording to another Global Cellulase (CAS 9012-54-8) Market Research Report published in 2018, Asia-Pacific is the largest consumer of cellulase, with a revenue market share nearly 32.84% by 2016. Furthermore, the reported data showed 29.71% of the cellulase market demand in animal feed, 26.37% in food and beverages, and 13.77% in the textile industry in 2016. This same report forecasts that the applications of cellulases will reach 2300 million USD by the end of 2025, growing at a compound annual growth rate (CAGR) of 5.5% during the 2018–2025 period. These data suggest that the application of cellulases in industries is drastically rising annually. Novozymes and DuPont from Denmark are key cellulase enzyme producers supplying these enzymes to the global market for industrial applications. From this point forward, in this chapter, our major effort was to discuss about the current applications of cellulases in major fields that have been listed above. The novel biotechnological trends emerging in those fields while understanding the key areas of research where further studies required also surfaced to an extent.
\nThe textile industry is one of the largest industries in the world. The customer demand for fashion is increasing as they want uniqueness in styles, colors, and the clothes they wear. There was a significant growth in this industry during the last few decades as a result of this increasing customer demand. This enzyme has now become the third largest group of enzymes used in these applications [8]. This creates a very competitive market platform for manufacturers that are always looking for environmentally friendly approaches of giving their products a unique look. Cellulase is used for many purposes in the industrial sector.
\nEspecially for textile wet processing, biostoning of denim fabric, biopolishing of textile fibers, softening of garments, and removal of excess dye from the fabrics are some of the major applications of this enzyme in the industry. Fungal cellulases from Trichoderma reesei are the mostly applied enzyme in the textile industry. Apart from that, actinomycetes from the genera Streptomyces and Thermobifida and other genera of bacteria, such as Pseudomonas and Sphingomonas, are some of the sources of enzymes to be used for decolorization and degradation of textile dyes [9].
\nBiostoning and biopolishing are well known for the best applications of cellulases in the current textile industry.
\nThe conventional washing process of denim usually has three steps. The denim fabric is first treated with amylase enzyme to remove the starch coating of the fabric. This process is called desizing. During this process, starch is broken down into maltose which is a water-soluble disaccharide composed of two glucose molecules. Then, the fabric is given treatment by providing abrasion to the material in pumice stones added to the washing machine. This wash was completely achieved by adding chemicals like sodium hypochlorite or potassium permanganate. This traditional process has several disadvantages. The addition of pumice stones must be done in larger quantities. This was affecting the machine’s productivity in an adverse way causing tear effects. After the wash is completed, the manual removal of stones is needed. This is causing further reduction of the process efficiency. The excessive back-staining was another disadvantage of the traditional process. Backstaining is the reaction by which the removed dye molecules are deposited on the denim fabric again.
\nApplication of microbial cellulases was found to be an efficient alternative for pumice stone washing. It was first staged in the 1980s. The use of stones is currently replaced by cellulases in a successful way. During this process, cellulases act on the denim fabric which is made of tough cotton. The indigo dye which is used to color the fabric is trapped inside the cellulose fiber in this cotton material. Usually, the indigo dye is mostly attached to the surface of the yarn and to the most exterior short cotton fibers. When the fabric is treated with the enzyme, it hydrolyzes and breaks small fibers coming out of the fabric which loosens the dye. For this purpose, the β-1,4-linkages of cellulose chains will be broken down, and simple water-soluble sugars will be formed. This will remove the fibers which traps indigo dye. Then, the dye is easily removed from the fabric giving a faded look.
\nTrichoderma reesei acidic endoglucanase II has been found to be a very efficient candidate for biostoning [10]. The neutral cellulase enzyme extracted from Humicola insolens is also reported to be commonly applied in this process [11]. The use of cellulases has several advantages over stone washing with pumice stones including high productivity; less work-intensive, safer environment; short treatment times; and less wear and tear of machines. Currently, denim with a worn-out look has a huge demand in the textile market.
\nThe major disadvantage associated with the application of microbial cellulases is again backstaining. The redisposition of dye on the fabric covers up the shaded look given by the treatment. In order to overcome this problem, several biotechnological approaches have been already experimented by researchers. Immobilization of cellulases on pumice stones is one such cost-effective way of doing this. It has also been observed that acidic endoglucanase causes a better abrasion and less backstaining compared to neutral endoglucanase. For example, the cellulase given by Trichoderma reesei is more efficient in preventing backstaining as compared to neutral endoglucanase of H. insolens.
\nThe latest trend of biostone washing is to utilize an enzyme mixture composed of amylase, cellulase, and laccase [12]. The sizing is the process by which denim material surface is covered by a compound like starch to provide rigidity and stiffness to raw denim and provide strength and friction resistance during handling. During washing, this surface layer of starch must be removed first to facilitate interaction between cellulases and cotton fibers. The amylase hydrolyzes starch from the fabric and causes desizing. Cellulase hydrolyzes small cellulose fibers, and laccase usually causes bleaching of the fabric. Laccases (EC 1.10.3.2) with intrinsic electron-donating tendency can decompose indigo in the solution as well as on the fabric creating bleaching effect on denim garments. The indigo dye is converted into isatin and anthranilic acid like chemical forms. This prevents backstaining of dye on the fabric surface. Eventually, this will give a complete faded look to the denim fabric. The purpose of using a mixture is to improve the efficiency of biostone washing process by allowing those three enzymes to work together in a sequential manner.
\nIn a recent study, it is reported that an alkali-stable cellulase in combination with xylanase from Thermomonospora sp. has a reduced tendency of backstaining [13]. However, the effluents generated during biostone washing must be pretreated to remove dye material and the intermediate chemical compounds present after the reaction. Otherwise, these dyes might pollute natural waterways and soil. Most of these dye residues are toxic and carcinogenic that would cause adverse health effects in humans and animals. Although biodegradability of enzymes is a positive advantage here, chemical by-products formed during dye removal must be neutralized.
\nThese two processes are simply similar to each other. Cellulosic fibrous materials like cotton and linen are always loosing appearance because of fuzz formation on the fabric surface. Fuzz occurred due to short fibers protruding out from the surface of the fabric. Fuzz is sometimes loosely attached to the fabric forming a ball-like appearance which gives an unattractive look to the fabric. This is called pilling. The biopolishing process basically aims on removing microfibrils of cotton. It enhances fabric look, hand feel, and color by giving a smooth and a glossy appearance. This is also leading to improvement of color brightness, hydrophilicity, and moisture absorbance by the fabric [13]. The acidic cellulases produced by T. reesei and Aspergillus niger are found to be enormously effective in this process. Biopolishing is eco-friendly because the enzymes used in this process are readily biodegradable and nontoxic.
\nThe repeated washing of a cotton garment makes it fluffy and dull. This is due to partially removed microfibrils on the fabric surface. Biofinishing by cellulases can remove these fibrils and give back the smooth surface and original color to the fabric. This will also give a soft hand feel to the material, and also this is a good way of removing stains and dirt spots that are trapped within the cotton fiber network [14, 15].
\nThis is the process that removes noncellulosic material from the surface of the cotton. This is usually done with cellulase alone or in combination with other enzymes such as pectinase. Pectinase digests the pectin substance present among cellulose fibers. This helps to remove the intact connection between the cuticle and the main body of the cellulose fiber. This helps to degrade the primary cellulosic wall of the fiber. The ultimate result is the destruction of the cuticle [16]. This reaction increases the softness of the fabric.
\nThis is a kind of a biological mode of cleaning the fabric from the cellulosic or vegetative impurities with the help of enzymes. When a pure cotton or cotton blend fabric is prepared, some traces of unwanted cellulosic material still may remain in the fabric. They may result in imperfect finishing and lower quality of the fabric. The earliest methods of carbonization involved application of sulfuric acid. It was not only expensive but also corrosive, unsafe, and hazardous. Being a nonhazardous, non-corrosive, and eco-friendly method, enzymatic carbonization was a promising alternative. This method was perfect for removal of cellulosic impurities from the material because it was least affecting the color and the hand feel of the fabric. The removal of vegetative impurities from the surface of raw wool using cellulases is called wool scouring [17]. Cellulases can be used alone or in combination with other enzymes such as pectinases to increase the efficiency of this process. These methods are doing less damage to the fabric when compared to the treatment with sulfuric acid.
\nLyocell is the generic name for a biodegradable fabric that is made out of treated wood pulp. This material is used in everything from clothing to cars. This is obtained from wood pulp using a solvent-spinning method. The solvent system which is usually applied is an organic compound called N-methylmorpholine N-oxide. Some main characteristics of lyocell fibers are that they are soft, absorbent, and very strong when wet or dry and resistant to wrinkles. One chief defect of this material is fibrillation. This is the formation of small tangled fibrils on the surface of the fabric. Cellulases can be efficiently applied to remove these fibrils and give the fabric an increased softness and an improved appearance. This is also good for preventing fuzz and pill formation.
\nAlthough the applied enzymes are nontoxic and biodegradable in the above processes, the final effluent produced will show increased biological oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), and total suspended solids (TSS) to a certain extent because the effluent may contain digested sugar and cellulosic forms. Direct release of this effluent to natural water bodies may cause water pollution. Alkalinity and pH fluctuations of the effluent will also result in polluted water which is not good for human and animal consumption. This may cause health issues like skin irritations in humans. On the other hand, the enzymatic treatments need an incubation period to facilitate the reaction between enzyme and fabric. As this is a fermentation reaction, this may release certain odors to the environment which may cause air pollution to a certain extent. Moreover, textile dyes are removed during textile processing. Most of the dyes are toxic and some are highly carcinogenic. Mixing this type of dyes with water is definitely causing adverse health effects in humans. Therefore, direct release of effluent without applying any pretreatments to neutralize these toxic compounds will breach the stability in ecosystems to which they are released. Therefore, establishment of pretreatment facilities and water quality testing procedures are essential for these enzymatic textile processing plants.
\nLyocell production has a different impact on the environment compared to the other textile polishing and finishing processes. The solvent which is used to manufacture this textile is N-methylmorpholine N-oxide. This is usually causing acute toxicity (oral, dermal, inhalation), skin irritation, serious eye damage and irritation, skin sensitization, and specific target organ toxicity. These are also hazardous to aquatic environments. These possible environmental impacts must be always addressed although application of enzymes in textile processing is eco-friendly.
\nThis is one of the largest industrial sectors in the world. According to the World Wildlife Fund (WWF), the pulp and paper industry, which includes products such as office and catalog paper, glossy paper, tissue, and paper-based packaging, uses over 40% of all industrial wood traded globally [18]. On the other hand, the latest paper industry statistics reveal China, the United States, and Japan as the three countries where the largest paper production occurs in the world. Half of the total paper manufacture of the world is done by these three countries. However, Germany and the United States are the world’s leading paper importers and exporters [19]. Moreover, the United States is reported to be the largest consumer of papers.
\nPapers and pulp are renewable resources. Therefore, recycling and reusing are two popular concepts related to this industry. Application of microbial cellulases is usually utilized for this purpose. The application of cellulases in this industry is broader. Starting from the 1980s up to now the possible applications are branching toward many areas. For instance, deinking, pulping, bioremediation of industry wastes, bleaching, and fiber enhancement can be taken.
\nThe drawbacks in mechanical pulping processes of woody raw materials such as refining and grinding resulted in pulps with higher amounts of fines, bulk, and stiffness. On the other hand, the process was high energy consuming which was not a profitable option for an industry. Meanwhile, biopulping using enzymes such as cellulases is an energy-saving way, and also it is eco-friendly [20]. The substantial energy saving is reported around 20%–40%. During the refining process, it generates small particles of the pulps. These particles reduce the drainage rate during the paper-making process. These particles can be readily degraded by cellulases in order to increase the drainage ability of the pulp. Mixtures of cellulases (endoglucanases I and II) and hemicellulases have also been used for bio-modification of coarse pulp material to improve fiber properties. It is strengthening the hand sheets. On the other hand, biological pulping has the potential to improve the quality of pulp and properties of the paper while reducing energy costs and environmental impact [21].
\nIn traditional deinking, large quantities of chemicals are used which make the method expensive and environmentally damaging and increase the release of contaminants [22]. The main advantage of bio-deinking is the ability of avoiding the alkali use during the process. This prevents yellowing of the paper. Cellulases alone, or used in combination with xylanase, are beneficial for deinking of different types of paper wastes. In most of the applications, partial hydrolysis of carbohydrate molecules releases ink from the fiber surface. This is done by a mixture of cellulases alone or in combination of cellulases and hemicellulases. The advantages associated with enzymatic deinking are clean look of paper, enhanced brightness, as well as environmental pollution reduction..
\nSuccessful application of cellulase and hemicellulase mixtures has been reported to modify properties of fibers. Usually in the paper industry, the making of paper is made easier by improving the beatability, runnability, and drainage of paper pulp during the process. Modifications of fiber properties are also achieved through treatment of paper by cellulase enzyme [20]. Not only that but also the enzymatic hydrolysis helps in characterization of fiber using various techniques such as scanning electron microscopy (SEM) and HPLC [23].
\nThe application of enzymes in manufacturing enzymatic washing agents or biological detergents dates back to the 1960s. Using enzymes in detergent formulae is a common practice today. In fact, according to market reports, by 2014, the detergent industry was the largest single market for enzymes at about 25–30% of total sales [22]. Another market research report published in 2017 on laundry detergent market stated that its global market size valued at 133.3 billion USD in 2016. The latest trend in the industry is to use alkaline enzymes in large amounts. For instance, protease, cellulase, α-amylase, lipase, and mannanase are broadly applied in heavy-duty laundry and automatic dishwashing detergents..
\nThe capability of enzymes to remove stains is the major focus of using them in manufacturing detergents. Cellulases are available in the market in different brands. For instance, Celluzyme® and Carezyme® are two main brands applied in detergent blends. These detergent blends are mainly applied in washing fabrics made of cotton and cotton blends. These detergents are making fiber modifications in the fabric in order to improve color brightness, softness, and particulate soil removal.
\nCellulases extracted from fungi like Trichoderma sp. (T. longibrachiatum, T. reesei, T. viride, and T. harzianum), Aspergillus niger, Humicola (H. insolens and H. griseathermoidea), and Bacillus sp. have been excessively studied so far for application in detergents. Alkaline cellulases are the most suitable additives to conventional detergents. It is because of their ability to remove soil and dirt particles from the interfibrillar spaces of the fabric. The cellulases remove the rough projections of cellulose fibers or cellulose aggregates attached to the fabric. This gives an increased gloss and smoothness to the fabric [23].
\nThe most recent innovation is to use combinations of enzymes in detergents. Cellulases are used in combination with other enzymes like proteases and lipases. The combination of enzymes is used to increase efficiency on stain cleaning and fabric care. For instance, SaniZyme® is a four-enzyme liquid detergent containing lipase, cellulase, amylase, and protease. This is a bacteriostatic enzymatic detergent for the removal of blood, protein, mucous, fats, lipids, and carbohydrates from all types of endoscopic equipment and surgical instruments. Another example is Getinge Clean MIS Detergent® which is also a formulation which includes protease, lipase, amylase, and cellulase enzymes, surfactants, sequestering agents, and corrosion inhibitors (typical pH in use dilution 8) which is specifically designed to clean complex, minimal invasive instrumentation.
\nThe application of cellulases in agriculture is usually reported in enhancement of crop growth and a control agent of plant diseases. For this purpose, combinations of cellulases, hemicellulases, and pectinases are broadly applied. Certain fungal cellulases are with the ability to degrade cell wall of plant pathogens. There are lots of details about application of bacteria such as plant growth-promoting rhizobacteria (PGPR) to improve plant performance. It is reported that these bacteria play a major role in reducing application of chemical fertilizers increasing plant development and also controlling potential plant pathogens and protecting plants from diseases. Moreover, many fungi including Trichoderma sp., Geocladium sp., Chaetomium sp., and Penicillium sp. enhance seed germination, support rapid plant growth, accelerate flowering, improve the root system and increase the crop yield. However, exact mechanisms behind these reactions are not yet clearly understood. But all these organisms have the ability to produce cellulase and related enzymes which may have a direct participation in these reactions. Some reports are about possible synergisms between bacterial cellulase production and bacterial antibiotic production against plant pathogenic fungi.
\nAccording to available information, it is evident that cellulolytic microorganisms are participating in many processes, viz., rhizosphere soil decomposition, increasing the availability of nutrient for the plant, controlling plant pathogens, facilitating root colonization, and penetration of cereal crops improving yields and nutritional contents. However, as there are no solid evidence to prove the mechanisms behind these, this area needs further research. The studies should be performed in order to characterize and improve applications of microbial cellulases in this field.
\nDuring traditional agriculture practices, especially in countries like Sri Lanka, farmers used to add straw and Gliricidia leaves like cellulosic materials into their fields. They observed that the incorporation of these types of plant material not only improved the quality of the soil but also increased the yield due to added nutrients. Therefore, it is obvious that in this type of processes cellulolytic microorganisms must have a direct contribution.
\nMedical pharmacology is currently a very active field of research that novel discoveries are coming into action. One such area is cellulases for development of medicine. By the way, humans are not cellulase producers, but the recent research on health and medicine reveals the benefits of consuming blends of enzymes including cellulase. As a result of global demand for enzyme blends, cellulase produced by the natural fermentation process of Trichoderma reesei and Bacillus licheniformis has been included in commercially available enzyme blends. This type of enzyme blends target collective digestion of cellulose-rich fibrous substances such as fruits and vegetables, cereals, legumes, bran, nuts and seeds, soy, dairy, healthy greens, sprouts, and herbs along with fats (lipids), sugars, proteins, carbohydrates, and gluten. One such example is VeganZyme®. Apart from that digestive aids (e.g., Digestin, P-A-L Plus Enzymes, Polyenzyme Plus, etc.) to treat people suffering from metabolic disorders are evolving as a promising strategy in medicine.
\nIn some records, the direct and indirect applications of cellulase in medicine have been mentioned apart from using it as consumable enzyme blends.
\nCellulase of fungal origin in combination with chitinases and lysozymes has a reported use in chitosan degradation. To obtain chitosan, a partial degradation of chitin must take place. As cellulose, chitin is a structural polysaccharide present in animals such as marine animals like shrimp and insect exoskeleton as well as participates in the formation of some parts of fungal cell walls. Chitin is a poly-ß-1,4-N-acetyl-D-glucosamine, conforming crystalline microfibrils. This polysaccharide provides structural integrity, stability, and protection to animals. Chitosan is the most important semicrystalline derivative form of chitin. This is obtained by partial deacetylation of chitin (around 50%, soluble in aqueous solution) under alkaline conditions or enzyme hydrolysis. Chitosan and its derivatives have many medical applications, viz., surgical sutures, bone rebuilding, production of artificial skin, anticoagulant, antibacterial agent, hemostatic dressings, anticancer and antidiabetic agents (in combination with metals), hypocholesterolemic effectors, elaboration of cosmetics, production of biopharmaceutics, and encapsulation of diverse materials [24, 25].
\nApart from these applications, a lot of reports have been published on several studies which discuss how cellulases hydrolyze chitosan and their potential biomedical effects. For instance, antitumor activity of cellulase-treated chitosan [26] and antimicrobial activity of low-molecular-weight chitosan obtained by Trichoderma commercial enzymes could be discussed. However, the lack of solid evidence is a common issue on this area.
\nA bezoar is a mass found trapped in the gastrointestinal system. Phytobezoars, as its name suggests, are composed of indigestible plant material (e.g., cellulose). In other words, it is a gastric concretion formed by vegetable fibers, seeds and skins of fruits, and sometimes starch granules and fat globules trapped inside the gastrointestinal tract. This is a common problem frequently reported in patients with impaired digestion and decreased gastric motility. Although sometimes surgeries are needed to remove these stagnated substances, certain minor conditions can be treated by cellulases. The common application reported is fungal cellulases. As there are no much reports about application of bacterial cellulases, research can be conducted to find out the application of potential cellulolytic bacterial cellulases to treat this condition. However, what is most important is that any of these individual enzymes or enzyme cocktails should not adversely affect the healthy body cells.
\nAnother possible direct application of cellulase in medicine is degradation of cell walls of pathogenic organisms. Acanthamoeba is a Protista which causes a very rare as well as serious corneal infection which leads to blindness. This is simply called keratitis. This amoebic keratitis is an acute sight-threatening corneal infection associated with contact lens misuse [27]. This organism has two stages in its life cycle: the cyst and trophozoite. Both these structures are available when the eye is infected by this organism. The cyst wall resembles the plant cell wall. It is plausible that amoebae secrete cellulose because its cyst wall is primarily composed of cellulose. There is evidence which has been found to prove this matter about the presence of cellulose in [28]. Therefore, it is possible to apply cellulases in controlling the pathogen which causes this disease. Cellulases can be used to break off the cyst wall and control the pathogen. However, it needs a lot of research before using cellulase as a treatment for the eye.
\nPathogenic microorganisms usually form biofilms. A biofilm is an assemblage of microbial cells that is irreversibly associated (not removed by gentle rinsing) with a surface and is enclosed in an extracellular polymeric substance (EPS) matrix. Usually, pathogenic microorganisms form this type of assemblages. They may be found on a wide range of surfaces including living tissues and indwelling medical devices. Artificial hip prosthesis, central venous catheter, prosthetic heart valve, intrauterine device, and urinary catheter are some examples for indwelling medical devices that are commonly associated with biofilms. Most of the microorganisms produce extracellular polymeric substances composed of backbone structures that contain 1,3- or 1,4-β-linked hexose residues and tend to be more rigid, less deformable, and in certain cases poorly soluble. This is the exact linkages present in cellulose polymer. Therefore, cellulases can be further studied for their efficient application in removing this type of biofilms from medical devices.
\nFood is essential for all living organisms to obtain nutritional support for their growth and well-being. The huge demand for food has laid a path to a very complex, interconnected global business that supplies most of the food consumed by the world’s population.
\nFood biotechnology nowadays considers cellulases as an invaluable resource due to their increased applicability in a broad range of processes. Fruit and vegetable juice clarification, reducing the viscosity of nectars, concentrating purees, alteration of fruit sensory properties, carotenoid extraction, olive oil extraction, and the quality improvement of bakery products are among the various processes in food biotechnology that cellulase is exploited worldwide.
\nThe cloudiness which is usually present in fruit and vegetable juices is a result of floating polysaccharide materials such as cellulose, hemicellulose, lignin, pectin, starch, metals, proteins, and tannins. The presence of these materials in the juice makes it low quality and draws less consumer demand. “Rapidase pomaliq” is a commercially available enzyme preparation composed of cellulase, hemicellulases, and pectinases obtained from Trichoderma reesei and Aspergillus niger. The application of this product in fruit juice clarification was beneficial to a considerable level. It is also reported that cellulase produced by bacteria such as Bacillus and Paenibacillus in combination with other enzymes such as pectinases and hemicellulases carries out the fruit and vegetable juice clarification. Apart from that, treatment of nectars and purees also found to be efficiently carried out by this enzymatic process. Rheological parameters such as viscosity of these products are brought down to a commercially acceptable level.
\nModification of sensory parameters of food is another important area where application of cellulases is highly recommended. The aroma properties, flavor, and texture of fruits are some sensory properties which play a crucial role in food biotechnology. The infusions of pectinases and cellulase enzymes have been found to be effective in altering the sensory properties of fruits and vegetables [29].
\nThese enzymes are also applied in degradation of grape fruit peels to release sugars. These sugars will be used in many industries including food production. Another important application of cellulase is extraction of phenolic compounds from grape pomace.
\nDuring extraction of olive oil, malaxing (mixing) is an indispensable step. This period allows the tiny oil droplets to attach with bigger ones and increase the oil yield which is coming from the olive paste. The use of cellulases alone or in combination with other hydrolytic enzymes like pectinases in this step has been found to have an enhancing effect on the extraction as well as the quality of olive oil. The enzymatic treatment of olive oil at the extraction stage causes significant enhancements in phenolic content and antioxidant activity of olive oil, thereby ultimately improving its quality.
\nThe enzyme cocktails of cellulases with other hydrolytic enzymes such as amylases, proteases, and xylanase result in increased loaf volume, improvement in bread quality, and production of softer crumb. Enzyme cocktail containing cellulases, hemicellulases, amylases, lipases, and phospholipases results in dough conditioning with improvement of flavor, prolonged shelf life, and increase in volume after baking [30].
\nAnother important application of cellulases is in pigment extraction from plants and plant products. Natural pigments such as carotenoids are nowadays earning a huge consumer demand as food colorants because of their natural origin, less toxicity, and availability of a wide range of colors. Brightly colored fruit peels such as orange, sweet potatoes, tomatoes, and carrot cell walls are rich in carotenoids. These are applicable as natural food colorants. The treatment of fruit peels with enzyme cocktails including cellulase leads to carotenoid extraction.
\nIn animal feed production, cellulases are applied to enhance digestibility of cereal-based food and to increase nutritive values for a higher quality of forages. There are reports about efficiently using Trichoderma cellulase as feed additives for significant improvements in feed conversion ratio as well as digestibility of the cereal-based food [31]. Forage feed of ruminants is quite complex in composition containing cellulose, hemicellulose, pectin, and lignin. There is a suggestion that it is possible to use cellulase preparations to enhance digestibility of forage [32, 33]. Usually, cellulase from bacteria such as Bacillus subtilis is used in this feed production and nutritional enhancement processes. Enzyme preparations containing cellulases and hemicellulases are utilized for numerous activities like milk yield, body weight gain, and feed. Another aspect of treating animal feed with these enzyme mixtures is to remove anti-nutritional factors present in grains and other cellulosic materials.
\nFurthermore, cellulases play an important part in increasing the rate and extent of fiber digestion. This can be used as a positive effect on natural gastrointestinal processes of the ruminants. The ultimate result will be the increased availability of absorbable nutrients by digestibility enhancement of fodder. The partial hydrolysis of lignocellulose materials also leads to better emulsification of food in the animal digestive tract resulting in ultimate improvement of nutrients availability.
\nEnergy is the life blood of the modern world. Fossil fuel, among all energy sources, holds the highest consumer demand. Since the industrial revolution, there is a drastic increase in fuel consumption. As fossil fuels are not renewable resources, the depletion of its natural deposits is inevitable. Therefore, we are in an era of limited and expensive energy. With the recent rise in fossil fuel prices, along with growing concern about its adverse effects on environment such as global warming caused by carbon dioxide emissions and subsequent climate change problems, biofuels have been gaining popularity. In our study area, we are focusing on bioethanol production using cellulolytic microorganisms as well as fermentative yeast using cellulose as the substrate.
\nBioethanol is a renewable form of energy. Especially, second-generation bioethanol production is an emerging trend because of the abundance of low-cost raw materials. The largest potential feedstock for this purpose is lignocellulosic biomass, which includes materials such as agricultural residues (corn stover, crop straws, and bagasse), herbaceous crops (alfalfa, switch grass), short-rotation woody crops, forestry residues, waste paper, and other wastes (municipal and industrial). Lignocellulose is the most abundant renewable biomass. The yield of lignocellulose can reach approximately 200 billon metric tons worldwide per year [34]. Bioethanol production from these feedstocks has certain advantages. It is an attractive method of disposing those lignocellulosic materials which is the nonedible part of plants. Less production of pollutants makes it environmentally friendly. Most importantly, bioethanol production using lignocellulosic biomass does not create any food insecurity because it does not utilize any food crops before harvesting. Finally, it is abundant all around the year as a raw material.
\nHowever, the major drawback of this production process is associated with the structure of lignocellulosic biomass. It mainly consists of lignin, cellulose, and hemicellulose which collectively form a very stable structure. In order to release fermentable sugars from this substrate, it needs to undergo a pretreatment process to break open the stable structure. One of the main focuses of this chapter is to discuss about the possible biological pretreatment of lignocellulosic biomass with special references to the current studies conducted by scientists. It also includes a description about our own attempts in this particular area of research.
\nBiomass contains about 40–50% of cellulose, a glucose polymer; 25–35% of hemicellulose, a sugar heteropolymer; 15–20% of lignin, a non-fermentable phenyl-propane unit; and lesser amounts of minerals, oils, soluble sugars, and other components. Biological pretreatment of lignocellulosic biomass uses the ligninolytic potential of certain microorganisms (fungi and bacteria and actinomycetes) to reduce the recalcitrant nature which is mainly caused by lignin component of the feedstock and enhance its digestibility by hydrolytic enzymes [35]. The breakdown of lignin barrier changes the structure of lignocellulose and enhances the access to the cellulose and hemicellulose carbohydrate components present.
\nBiological pretreatment seems to be a promising approach due to its low capital cost, low energy, and little dependence of chemicals, mild environmental conditions, eco-friendly nature, and the absence of inhibitor generation during the process which affects in bioethanol production. Moreover, this process does not release any toxic materials or any toxic effluents to the environment.
\nHowever, there are few limitations in this strategy. The main drawback against the industrial scale application is the prolonged incubation time consumed to achieve the efficient delignification [36]. This is because of low hydrolysis rate of the microorganisms. Another possible drawback that comes into mind is the possible consumption of carbohydrates as well as fermentable sugar formed by the same microorganisms used to pretreat the material. This is possible to take place because most of the lignolytic microorganisms are producing cellulolytic enzyme batteries as well. Then, the substrate left for fermentative organisms will be minimal which could consequently lead to lower bioethanol yields. Therefore, it is essential to have poor cellulolytic microorganisms for delignification process.
\nTo minimize this type of drawbacks in a biological way, it is possible to use cocultures or biofilms of efficient ligninolytic microorganisms. Introduction of fermentative yeast isolates into the same microbial coculture would also be a perfect approach. However, developing the most efficient microbial consortium is not that simple. Excessive laboratory-scale studies are required to understand the optimum physiological as well as biochemical parameter setup.
\nFungi are found to be more efficient in degrading lignocellulosic biomass. For instance, white-rot fungi, brown-rot fungi, and soft-rot fungi can be taken. The first two are basidiomycetes and soft-rot fungi are classified in ascomycete group.
\nAmong these efficient ligninolytic microorganisms that have been studied so far, white-rot basidiomycete fungi are found to be more versatile in the process. Most research has been concentrated on species such as Phanerochaete chrysosporium (Sporotrichum pulverulentum) which is considered as the model organisms for lignin degradation as it completely mineralizes lignin to CO2 and water. Ceriporiopsis subvermispora, Phlebia subseralis
Recently, some bacterial laccases have also been characterized from Azospirillum lipoferum, Bacillus subtilis, etc. Unlike fungi, the bacteria are considered as low potential for lignin degradation. However, the three groups of bacteria, namely, actinomycetes, α-proteobacteria, and γ-proteobacteria, are known to have ligninolytic systems. Some actinomycetes were studied for their role in lignin biodegradation [37]. These degraded lignin into low-molecular-weight fragments. Some studies have shown potential of Penicillium camemberti for lignin degradation.
\nThe biological pretreatment can be performed by growing the microorganism directly on the feedstock or using the enzyme extracts. Solid-state fermentation is the method of choice for biological delignification. Thus, from the reports available, it is evident that white-rot fungi and actinomycetes can be used to remove lignin from lignocellulosic substrates. However, further studies are required to shorten the incubation time and to optimize the delignification process.
\nThe importance of enhancing enzymatic hydrolysis has been increased because of the urgent need for efficient biological pretreatment processes. For this purpose it is essential to search for high enzyme-producing organisms from the natural environment. Selecting the most effective strain and its culture conditions can make the process more efficient. Another important aspect is to find unique microbial communities for biological pretreatment. These communities can be called consortia. The efficient biodegradation of lignocellulosic biomass could be achieved by the synergistic action of various bacteria and fungi in a microbial consortium. There are a number of advantages in using a microbial consortium for biological pretreatment. The increase of adaptability, improved productivity, improved efficiency of enzymatic saccharification, control of pH during sugar utilization, and increase in substrate utilization are some of them. With the development of biotechnology and molecular biology, the production of hyperlignolytic mutants by genetic modification of wild-type species is one approach that could be studied further. Furthermore, complete understanding of the theoretical basis behind the mechanisms of actions of these hydrolytic enzyme systems is very useful in the process of enhancing hydrolytic efficiency.
\nVarious process parameters affecting biological pretreatment like incubation temperature, incubation time, inoculums concentration, moisture, aeration, and conditions of pH have to be optimized. This must be done with well-planned laboratory-scale experiments. It is essential to pay attention to the microorganism used as well as the type of lignocellulosic material utilized because these parameters obviously change based on these two factors. Accessory enzymes are those enzymes which act on less abundant linkages found in plant cell walls. These include arabinases, lyases, pectinases, galactanases, and several types of esterases. Some studies have reported that addition of these accessory enzymes will improve hydrolysis efficiency.
\nRecently, several studies have been conducted in Sri Lanka on efficient lignocellulose-degrading microorganisms isolated from the natural environment. The effect of coculturing these fungal isolates for degradation of lignocellulosic material has also been reported. Coculturing of Trichoderma spp. with other cellulolytic fungi has found to improve the activity of lignocellulose-degrading enzymes compared to its monocultures [36]. In a different study, 18 basidiomycete isolates from the natural environment of Sri Lanka has been evaluated for their lignocellulose-degrading enzyme production. An Earliella scabrosa species with higher laccase activity (79,600 U/l) when cultured in 50 g/l rice bran has been reported [37]. Thus, it can potentially be used for industrial production of laccase using rice bran as a cheap carbon source for high laccase production.
\nThe current progress in applications of cellulases is truly remarkable and attracting worldwide attention. It has already conquered the global market in an unbeatable way. Microbes are an attractive topic of interest for the production of cellulases due to their immense potential for cellulase production. However, it is apparent that more efficient species are still out there in the environment unnoticed by researchers. Further exploration and understanding of hidden mechanisms behind the activity of these enzymes are much more important. Microbial cellulases are preferred for their potential applications in a broad range of industries. Their ventures are expanding day by day. More and more researches are required to produce scientific knowledge to meet the growing demands for microbial cellulase. The advances in the emerging fields such as biotechnology, microbiology, and molecular biology will open up novel strategies to magnify the still-unlocked potentials of these enzymes. Eventually, it will be able to fine-tune the areas which still are dragging on the way to their utmost success.
\nSpecial thanks go to Research Assistant, Mr. K. Mohanan and Technical Officer Mrs. Kumuduni Karunarathna at Bioenergy and Soil Ecosystems Research Project, National Institute of Fundamental Studies, Kandy Sri Lanka and the National Research Council (Grant No: 12-021) for financial support.
\nNo conflicts of interest.
\nDuring the last decades, an impressive technological development has been achieved permitting the manipulation of single photons with a high degree of statistical accuracy. However, despite the significant experimental advances, we still do not have a clear physical picture of a single photon state universally accepted by the scientific community, especially involved in quantum electrodynamics. In this chapter, based on the present state of knowledge, we make a synthesis of the physical characteristics of a single photon put in evidence by the experiments, and we advance theoretical developments for its representation. Accordingly, the concept of the wave-particle nature of a single photon becomes physically comprehensive and in agreement with the experimental evidence.
\nHowever, before advancing in the theoretical developments, we consider that it is important starting with a brief historical review on the efforts carried out previously for understanding the nature of light while simultaneously making a synthesis of the main experimental results which are of crucial importance for the comprehension of the birth of the photon concept.
\nThe very first scientific publications on the nature of light are due to ancient Greeks who believed light is composed of corpuscles [1, 2]. Around 300 BC Euclid published the book Optica in which he developed the laws of reflection based on the rectilinear propagation of light. Two centuries later, Ptolemy of Alexandria published the book Optics, in which he included extensively all the previous knowledge on light. In this book, colours as well as refraction of the moonlight and sunlight by the earth’s atmosphere were analysed. After Ptolemy of Alexandria, almost no progress has been reported until the seventeenth century.
\nIn the year of 1670, Newton revived the ideas of ancient Greeks and advanced the theory following that light is composed of corpuscles that travel rectilinearly [3]. Ten years later, Huygens developed the principles of the wave theory of light [1, 4, 5]. Huygens’ wave theory was a hard opponent to Newton’s corpuscle concept. In the beginning of the nineteenth century, Young obtained experimentally interference patterns using different sources of light and explained some polarisation observations by assuming that light oscillations are perpendicular to the propagation axis [1, 6]. Euler and Fresnel explained the diffraction patterns observed experimentally by applying the wave theory [6]. In 1865, Maxwell published his theory on the electromagnetic waves establishing the relations between the electric and magnetic fields and showing that light is composed of electromagnetic waves [7]. A few years later, Hertz confirmed Maxwell’s theory by discovering the long-wavelength electromagnetic radiation [1, 7]. Thus, at the end of the nineteenth century, the scientific community started to accept officially the wave nature of light replacing Newton’s theory.
\nNevertheless, new events supporting the particle nature of light occurred in the beginning of the twentieth century. Stefan and Wien discovered the direct relationship between the thermal radiation energy and the temperature of a black body [8, 9]. However, the emitted radiation energy density as a function of the temperature calculated by Rayleigh failed to describe the experimental results at short wavelengths. Scientists had given the name “UV catastrophe” to this problem revealing the necessity of a new theoretical approach. Planck managed to establish the correct energy density expression for the radiation emitted by a black body with respect to temperature, in excellent agreement with the experiment [8]. For that purpose, he assumed that the bodies are composed of “oscillators” which have the particularity of emitting the electromagnetic energy in “packets” of hν, where ν is the frequency and h is a constant that was later called Planck’s constant. During the same period, the experiments carried out by Michelson et al. [10] demonstrated that the speed of light in vacuum is a universal physical constant corresponding to the product of the frequency ν times the wavelength λ, that is, c = λ ν.
\nIn 1902, Lenard pointed out that the photoelectric effect, discovered by Hertz 15 years earlier [11], occurs beyond a threshold frequency of light and the kinetic energy of the emitted electrons does not depend on the incident light intensity. Based on Planck’s works, Einstein proposed a simple interpretation of the photoelectric effect assuming that the electromagnetic radiation is composed of quanta with energy hν [12]. He advanced that the energy of a light ray when spreading from a point consists of a finite number of energy quanta localised in points in space, which move without dividing and are only absorbed and emitted as a whole. Although that was a decisive step towards the particle theory of light, the concept of the light quanta was still not generally accepted, and Bohr, who was strongly opposed to the particle concept of light [13], announced in his Nobel lecture (1922) that the light quantum hypothesis is not compatible with the interference phenomena and consequently it cannot throw light in the nature of radiation. Bohr’s statement was rather surprising because Taylor’s experiments, consisting of repeating Young’s double slit diffraction at extremely low light intensities, had already demonstrated since 1909 that light rays are composed of discrete parts whose spots compose the diffraction patterns by gradual accumulation on the detection screen [14]. Compton published his studies on X-rays scattered by free electrons in 1923 advancing that the experimental results could only be interpreted based on the light quanta model [15].
\nThus, the photoelectric effect and Compton scattering have been initially considered as the undoubtable demonstrations of the particle nature of light and historically were the strongest arguments in favour of the light quanta concept, which started to be universally accepted, and Lewis introduced the word “photon”, from the Greek word phos (Φωs, which means light) [1, 4].
\nTherein, it is extremely important to mention that Wentzel in 1926 [16] and Beck in 1927 [17], as well as much later Lamb and Scully in the 1960s [18], demonstrated that the photoelectric effect can be interpreted remarkably well by only considering the wave nature of light, without referring to photons at all [19]. Furthermore, the Compton scattering has been fully interpreted by Klein and Nishina in 1929 [20] also by considering the electromagnetic wave nature of light without invoking the photon concept. On the other hand, Young’s experiment, initially presented as the most convincing argument for the wave nature of light, was applied by Taylor at very low intensities to demonstrate the particle concept of light [14]. Indeed, much later Jin et al. [21] published an excellent theoretical interpretation of Young’s diffraction experiments based only on the particle representation of light.
\nThus, the picture on the nature of light in the 1930s was rather confusing since both opposite sides defending the wave or the particle nature advanced equally strong arguments. Hence, Bohr, inspired by de Broglie’s thesis on the simultaneous wave character of particles, announced the complementarity principle according to which light has both wave and particle natures appearing mutually exclusively in each specific experimental condition [1, 2, 19].
\nThe development of lasers [22] in the 1960s and the revolutionary parametric down-convertion techniques [23, 24] in the 1970s, have made it possible to realise conditions in which, with a convenient statistical confidence, only a single photon may be present in the experimental apparatus. In this way, the double-prism experiment [25] realised in the 1990s contradicted for the first time Bohr’s mutual exclusiveness demonstrating that a single photon exhibits both the wave and particle natures in the same experimental conditions.
\nAccording to the experimental investigations, it has been always stated that a photon has circular, left or right, polarisation with spin \n
The lateral expansion of a single photon, considered locally as an indivisible entity, was always an intriguing part of physics. With the purpose of studying the lateral expansion of the electromagnetic rays, Robinson in 1953 [32] and Hadlock in 1958 [33] carried out experiments using microwaves crossing small apertures and deduced that no energy is transmitted through apertures whose dimensions are smaller than roughly ∼λ/4. In 1986, for the same purpose, Hunter and Wadlinger [34, 35] used X-band microwaves with λ = 28.5 mm and measured the transmitted power through rectangular or circular apertures of different dimensions. They concluded that no energy is transmitted when the apertures are smaller than ∼λ/π confirming that the lateral expansion of the photons is a fraction of the wavelength.
\nThus, the experiments have shown that the single photon is not a point and cannot be localised at a coordinate, as stated by Einstein, while it can exhibit both the wave and particle natures in the same experimental conditions contradicting Bohr’s mutual exclusiveness. However, quantum electrodynamics (QED) has been developed during the 1930s to 1960s based upon the point particle model for the photon [36, 37, 38, 39]. In fact, the point photon concept has permitted to establish an efficient mathematical approach for describing states before and after an interaction processes [19, 39, 40, 41], but it is naturally inappropriate for the description of the real nature of a single photon.
\nFinally, what we can essentially draw out by summing up the experimental evidence is that a single photon is a minimum, local, indivisible part of the electromagnetic field with precise energy hν and momentum hν/c, having circular left or right polarisation with spin \n
In what follows, we present first the standard theoretical representation of the electromagnetic field quantization resulting in photons, and next we proceed to recent advances based on the vector potential quantization enhanced to a single photon state.
\nSince the formulation of Maxwell’s equations, the vector potential \n
where \n
In 1949, Ehrenberg and Siday were the first to put in evidence the influence of the vector potential on charged particles [42] deducing that it is a real physical field. Ten years later, Aharonov and Bohm re-infirmed the influence of the vector potential on electrons in complete absence of electric and magnetic fields [43]. That was confirmed experimentally by Chambers [44], Tonomura et al. [45], and Osakabe et al. [46] demonstrating without any doubt the reality of the vector potential field end its direct influence on charges.
\nFrom a theoretical point of view [43], the behaviour of a particle with charge q and mass m in the vicinity of a solenoid where the vector potential is present is described by the Hamiltonian:
\nwith \n
If the solenoid is extremely long along the z axis, then the magnetic field is uniform in the region inside and zero outside. The scalar potential Φ can be put to zero by assuming that the solenoid is not charged. In this case, in the outside region, the electric and magnetic fields are zero, but the vector potential is not zero and depends on the magnetic field flux in the solenoid:
\nwhere r is the radial distance from the z axis of the solenoid, S is the surface of the circle with radius r perpendicular to z, and \n
The Schrödinger equation for a charged particle outside the solenoid, where the vector potential is not zero, writes in complete absence of any other external potential:
\nwith \n
where \n
The exponential part of the wave function of Eq. (6) entails that two particles have equal charge and mass moving both outside the solenoid at the same distance from the axis, but the first in the same direction with the vector potential \n
Interference patterns for electrons in analogue conditions have been observed experimentally [44, 45, 46] demonstrating that the vector potential is a real physical field and interacts directly with charged particles in complete absence of magnetic and electric fields and of any other potential.
\nThe vector potential, being a real field, is considered as the fundamental link between the electromagnetic wave theory issued from Maxwell’s equations and the particle concept in quantum electrodynamics (QED) [19, 36, 39]. We will show analytically how this link is established.
\nIn the classical theory [5, 7], the energy density of a mode k of the electromagnetic wave writes:
\nwhere ε0\n and μ0\n are the electric permittivity and magnetic permeability of the vacuum, respectively, related to the speed of light in vacuum c by \n
In the case of a monochromatic plane wave with angular frequency \n
where \n
Introducing Eqs. (10) and (11) in Eq. (9), the energy density now depends on the square of the vector potential amplitude:
\nThe mean value over a period, thus over a wavelength, is time independent:
\nNote that the last equation expressing the mean energy density over a period of the mode k of the electromagnetic wave is independent on any external volume yielding that in the classical description, a free of cavity electromagnetic radiation mode expands naturally within a minimum volume. In a given cavity, this volume corresponds roughly to that imposed by the boundary conditions and the cut-off wave vectors [4, 5, 7].
\nOn the other hand, in the quantum description, the energy density for a number \n
In order to link the classical to the quantum description [4, 9, 19], the classical mean energy density over a period, expressed by Eq. (13), is imposed to be equivalent to the quantum mechanics expression of Eq. (14) for \n
The last relation is the fundamental link between the classical and quantum theory of light which is used to define in QED the vector potential amplitude operators for a single photon [19, 26, 29, 36, 37, 38, 39, 40, 41]:
\nwhere \n
Therein, it is worth noting that an external arbitrary volume parameter V appears in the vector potential amplitude of the single photon, expressed by Eq. (15), which is supposed to be an intrinsic physical property. This could entail the unphysical interpretation that a single photon in an infinite cavity has zero vector potential, thus zero electric and magnetic fields and consequently zero energy. This ambiguity, which is scarcely quoted in the literature, is lifted by considering that, in the case of a single photon, the volume V in Eq. (15) is equivalent to that defined by the boundary conditions in a cavity for the single radiation mode k.
\nThe energy of the electromagnetic field in a volume V considered as a superposition of different k-modes and λ-polarisations is obtained directly from Eq. (13):
\nwhere the summation over the λ-polarisations takes only two values corresponding to circular left and right [19, 36, 37, 38, 39, 40, 41].
\nReplacing in Eq. (17) the vector potential amplitude and its conjugate by the relations of the vector potential amplitude operators defined in Eq. (16), we get the “normal ordering” radiation Hamiltonian corresponding to the order \n
and the “anti-normal ordering” Hamiltonian corresponding to the order \n
where we have used the fundamental commutation relation in quantum electrodynamics:
\nIn Dirac’s representation the eigenfunctions take the simple expression \n
The successive action of both operators in the normal order corresponds to the photon number Hermitian operator \n
In this representation the normal and anti-normal ordering radiation Hamiltonians write, respectively:
\nWe obtain a harmonic oscillator Hamiltonian for the electromagnetic field by considering the mean value of the normal ordering and anti-normal ordering Hamiltonians:
\nThus, in QED the electromagnetic field is considered to be an ensemble of harmonic oscillators each represented by a point particle, the photon, whose eigenfunction is denoted simply by \n
Although we have no experimental facts showing the harmonic oscillator nature of a single photon, this representation has been adopted since the 1930s [37].
\nIn a different way, a harmonic oscillator representation for the electromagnetic field can be obtained by the intermediate of the canonical variables of position \n
Introducing the last expressions in Eq. (17), we get the electromagnetic field energy:
\nwhere the (+) sign is obtained when Eq. (17) is considered initially to be in the “normal order”, \n
With the purpose of establishing a harmonic oscillator representation for the electromagnetic field, it is generally considered that \n
Replacing in the last equation the classical canonical variables of position and momentum with the corresponding Hermitian operators [19, 29, 41]:
\nand putting \n
At that level it is important to note that, for a harmonic oscillator of a particle with mass \n
to the quantum mechanics Hamiltonian:
\nwhere \n
Consequently, the harmonic oscillator Hamiltonian for a particle of mass m expressed by Eq. (31) is a quite physical result (e.g., phonons in solid-state physics) obtained with a perfect correspondence between the classical canonical variables of momentum and position \n
Conversely, this is not the case for the electromagnetic field [19, 29, 39] because commutations between the canonical variables \n
Obviously, as frequently quoted [2, 19, 39], the fundamental mathematical ambiguity consisting of cancelling the commuting classical variable term \n
In fact, since no experiment has yet demonstrated that a single photon is a harmonic oscillator, the main reason for considering the electromagnetic field as an ensemble of harmonic oscillators lies in the importance of the zero-point energy (ZPE) issued in absence of photons from the eigenvalue \n
The summation of the last expression over all modes and polarisations is infinite and represents the principal singularity in the QED formalism [19, 26, 29, 36, 39].
\nNevertheless, the zero-point energy is very important because it is considered to be the basis for the explanation of the vacuum effects such as the spontaneous emission, the Lamb shift and the Casimir effect. However, as pointed out by many authors [19, 26, 39, 41], it is important to underline that the explanation of the spontaneous emission and the Lamb shift in QED is not due to Eq. (33) but precisely to the commutation properties of the photon creation and annihilation operators, \n
Conversely, the zero-point energy expressed by Eq. (33) is useful for the explanation of the spontaneous emission and the Lamb shift in the classical description of radiation [2, 39, 47].
\nRegarding the Casimir effect, it is often commented that caution has to be taken concerning the interpretation of its physical origin because it has been demonstrated by different methods [48, 49, 50] that it can be easily explained using classical electrodynamics without invoking at all the zero-point energy.
\nHence, in view of the above, the normal ordering Hamiltonian is the one mainly used in QED, casting aside the vacuum singularity issued from the harmonic oscillator formalism, while the zero-point energy issued from the harmonic oscillator Hamiltonian is principally useful in the classical formalism for the interpretation of the vacuum effects [2, 19, 39, 47].
\nWe have analysed in Section 3.1 the electromagnetic field energy quantization according to the harmonic oscillator representation. Now, we will analyse the vector potential field quantization following the second quantisation process.
\nConsidering the natural units \n
with
\nwhere \n
Using Eq. (36) in Eq. (34), the vector potential becomes:
\nwith \n
For \n
Suppressing the natural units (i.e., introducing c and \n
On the basis of the density of state theory, the quantization of a field in a cavity of volume V permits to transform the continuous summation over the modes to a discrete one [19, 51]:
\nThe last transformation is only valid for an ensemble of modes k whose wavelengths \n
Switching now to Heisenberg’s representation:
\nGeneralizing the coordinate system, adapting the phase and using Eq. (40), the vector potential of the electromagnetic field writes in QED [19, 29, 39, 41, 51]:
\nConsidering the scalar potential to be constant, the electric field is:
\nThe last expressions represent in a given volume V the quantized vector potential and the electric field of the electromagnetic radiation composed of a large number of modes k each with angular frequency \n
The amplitudes in Eqs. (42) and (43) have been obtained using the density of state theory and are valid only on the condition of Eq. (44). Furthermore, the boundary conditions of the electromagnetic waves considered in cavities and waveguides impose the wave vectors k of the modes to be higher than a characteristic cut-off value \n
The last equations represent the vector potential and the electric field of a large number of modes k of the quantized electromagnetic field in a finite volume V with \n
We have seen in Section 3.1 that according to the energy quantization procedure, a k-mode and λ-polarisation photon is considered to be a point harmonic oscillator represented by the simplified eigenfunction \n
As mentioned in Section 2.2, the classical expression of the mean energy density over a period for a single electromagnetic mode k, represented by Eq. (13), can be considered equivalent to that for a single photon in the quantum representation, given by Eq. (14) for \n
From a theoretical point of view, this is also compatible with the density of state theory according to which the spatial volume corresponding to a single state of the quantized field is proportional to \n
On the other hand, the dimension analysis of the vector potential issued from the general solution of Maxwell’s equations yields that it is proportional to an angular frequency [5, 7, 9]:
\nwhere J is the current density (C m−2 s\n−1) and μ the magnetic permeability.
\nIndeed, it is well established experimentally that the energy density radiated by a dipole is proportional to \n
This result is gauge independent since it concerns the natural units of the vector potential.
\nAccording to the previous considerations, for a free single k-mode photon with λ-polarisation (left or right circular), the vector potential can be written in quantum and classical formalism:
\nwhere, following to the above analysis, the amplitude writes:
\nwith \n
We can evaluate \n
where \n
Thus, the characteristic volume of a free single photon writes in agreement with Eq. (47):\n
\nReplacing \n
\nEquations (50) and (53) express the quantized vector potential amplitude and the spatial extension of a single photon with the constant \n
For a free k-mode photon, the volume Vk\n corresponds to the space in which the quantized vector potential oscillates at the angular frequency \n
which are independent on any external arbitrary volume parameter and are directly proportional to the square of the angular frequency [2, 54, 55].
\nWe can now express the quantum properties of the photon, energy, momentum, and spin by integrating the classical electromagnetic expressions over the volume \n
With the same token considering circular polarisation [4, 5, 7, 9] for the amplitudes of the electric and magnetic fields in Eq. (55), the momentum is:
\nAccording to the classical electromagnetic theory, the spin can be written through the electric and magnetic field components; hence, using again the circular polarisation, we get:
\nwhere we have taken the mean value \n
The fact that the quantum properties, energy, momentum, and spin, of the photon can be expressed through the classical electromagnetic fields integrated over the volume Vk\n signifies that the photon has naturally a spatial extension, and consequently when employing the term “wave-particle”, one must have in mind that a single photon is a “three-dimensional particle”.
\nWe can now obtain Heisenberg’s uncertainty relation for position and momentum using Vk\n. Indeed, replacing V in Eq. (16) by Vk\n, we get the photon vector potential amplitude operators:
\nThe corresponding position \n
Thus, introducing Eq. (59) in Eq. (60) and using Eq. (20) with Eq. (53), Heisenberg’s commutation relation, a fundamental concept in quantum theory, results directly [2]:
\nThe fundamental properties of the photon, energy \n
Considering Heisenberg’s energy-time uncertainty principle:
\nwe directly deduce from Eq. (62) the vector potential-time uncertainty:
\nThe energy and vector potential uncertainties with respect to time are intrinsic physical properties of the wave-particle nature of the photon.
\nObviously, the photon vector potential function \n
as well as the vector potential energy (wave-particle) equation for the photon [2, 54]:
\nwhere the vector potential operator \n
It is worth remarking the symmetry between the pairs \n
Now, when considering the propagation of a k-mode photon with wavelength \n
In fact, from a theoretical point of view, for a photon propagating in the z direction, Heisenberg’s uncertainty for the position z and momentum \n
Notice that the momentum uncertainty along the propagation axis is expressed through the uncertainty over the inverse of the wavelength.
\nConsidering now the vector potential function with the quantized amplitude \n
Obviously, the shorter the wavelength of the photon, the higher the localization probability in agreement with Heisenberg’s uncertainty and the experimental evidence.
\nThe photon vector potential is composed of a fundamental function \n
In this way, the general equation for the vector potential of the electromagnetic wave considered as a superposition of plane wave modes writes:
\nand that of a large number of cavity-free photons in quantum electrodynamics is:
\nAccording to Eqs. (55) and (62), for \n
The field \n
Combination of the expression \n
Using again Eq. (51) and recalling that the electron mass may be written as \n
entailing that the mass derives also from the EFGS and is proportional to the charge square.
\n\nEquations (50), (74), and (75) show the strong physical relationship between photons and electrons-positrons which are all related directly to the EFGS through the amplitude ξ. Obviously, photons and electrons-positrons, also probably leptons-antileptons, are issued from the same quantum vacuum field. This may be at the origin of the physical mechanism governing the photon generation during the electron-positron (and probably lepton-antilepton) annihilation and that of the electron-positron (lepton-antilepton) pair creation during the annihilation of high-energy gamma photons in the vicinity of very heavy nucleus.
\nIn this chapter we have presented recent theoretical developments complementing the standard formalism with the purpose of describing a single photon state in conformity with the experiments. We resume below the principal features.
\nThe quantization of the vector potential amplitude \n
A single photon, as a local three-dimensional entity of the electromagnetic field, is absorbed and emitted as a whole and propagates guided by the non-local vector potential function (Eq. (49)), which appears to be a natural wave function for the photon satisfying both the propagation equation (Eq. (65)) and the vector potential - energy equation (Eq. (66)). The probability for detecting a photon around a given point on the propagation axis is obtained by the square modulus of the vector potential and is proportional to the square of the angular frequency \n
Finally, the electromagnetic field ground state (EFGS) at zero frequency, a real quantum vacuum component, issues naturally from the vector potential wave function putting in evidence that photons are oscillations of the vacuum field. Furthermore, the electron-positron charge and mass are directly proportional to the vector potential amplitude quantization constant showing the strong physical relationship with the photons. Obviously, the origin of the mechanisms governing the transformations of photons to electrons-positrons and inversely lies in the nature of the electromagnetic field ground state.
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