**3.2. Properties and applications**

The heterogeneity and complexity of xylan have resulted in a diverse range of xylanases, which differ in their physicochemical properties, structure, mode of action and substrate specificities [10]. As the xylosidic linkages in lignocellulose are neither equivalent nor equally accessible, the production of an enzymatic system with specialized functions is a strategy to achieve superior xylan hydrolysis [28]. Together with the heterogeneous na‐ ture of xylan, the multiplicity of xylanases in microorganisms may be caused by a redun‐ dancy in gene expression. Generally, a single xylanase gene encodes multiple xylanases, and xylanase multiplicity may arise from posttranslational modifications, such as differ‐ ential glycosylation, proteolysis or both [23].

As hemicellulose is easier to depolymerize than lignin, biobleaching of pulp appears to be more effective with the use of xylanases than with lignin-degrading enzymes. This is due to the fact of the removal of even a small portion of the hemicellulose could be sufficient to open up the polymer, which facilitates removal of the residual lignin by mild oxidants [33,50].

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The use of xylanase in bleaching pulp requires the use of enzymes with special characteristics. A key requirement is to be cellulose-free, to avoid damaging the pulp fibers [3], as cellulose is the primary product in the paper industry [33]. Other desirable characteristics are stability at

Madlala *et al*. [52] used different preparations of commercial Xylanase P and crude xylanase from *Thermomyces lanuginosus* to evaluate the bleaching process of paper pulp. It was demon‐ strated that the use of enzymes could increase the pulp brightness (over 5 brightness points over the control) and reduce the amount of bleaching chemicals used (up to 30% for chlorine dioxide). Chipeta *et al*. [53] evaluated crude xylanase preparations from *Aspergillus oryzae* NRRL 3485 and *Aspergillus phoenicis* ATCC 13157 and found that at a charge of 10 U per gram of pulp it was possible to reduce the usage of chlorine dioxide up to 30% without compromising

Currently, second-generation biofuels are the primary products of the bioconversion of lignocellulosic materials. According to Taherzadech and Karimi [54], ethanol is the most important renewable fuel in terms of volume and market value, and following the fossil fuel crisis, it has been identified as an alternative fuel [48]. Despite the primarily first-generation production of ethanol, from sugar and starch, the second-generation production of ethanol has only begun to be tested in pilot plants [55]. And, unlike first-generation biofuels, secondgeneration biofuels do not compete with food production and can provide environmental,

Xylanase, together with other hydrolytic enzymes, can be used for the generation of biological fuels, such as ethanol, from lignocellulosic biomass [1,57]. However, enzymatic hydrolysis is still a major cost factor in the conversion of lignocellulosic raw materials to ethanol [56]. In bioethanol fuel production, the first step is the delignification of lignocellulose, to liberate cellulose and hemicellulose from their complex with lignin. The second step is a depolymeri‐ zation of the carbohydrate polymers to produce free sugars, followed by the fermentation of mixed pentose and hexose sugars to produce ethanol [1,58]. Simultaneous saccharification and fermentation is an alternative process, in which both hydrolytic enzymes and fermentative

Xylanase, together with pectinase, carboxymethylcellulase and amylase, can be used for the clarification of juices because the turbidity observed is due to both pectic materials and other materials suspended in a stable colloidal system [60]. Xylanase may also improve the extraction of coffee, plant oils, and starch [25]. The xylose resulting from xylan depolymerization may

high temperatures [51] and an alkaline optimal pH [48].

*3.2.2. Bioconversion of lignocellulose in biofuels*

economic, and strategic benefits for the production of fuels [56].

microorganism are present in the reaction [48,59].

*3.2.3. The pharmaceutical, food and feed industries*

the pulp brightness.

The potential applications of xylanases also include the bioconversion of lignocellulosic material and agro-wastes into fermentative products, the clarification of juices, the improve‐ ment of the consistency of beer and the digestibility of animal feedstocks [28]. One of the most important biotechnological applications of xylanase is its use in pulp bleaching [46]. Xylanases may also be applicable to the production of rayon, cellophane and several chemicals such as cellulose esters (acetates, nitrates, propionates and butyrates) and cellulose ethers (carboxy‐ methyl cellulose and methyl and ethyl cellulose), which are all produced by dissolving pulp and purifying fibers from other carbohydrates [33].

#### *3.2.1. The paper and pulp industries*

During the past several years, the use of enzymes in paper and pulp bleaching has caught the attention of researchers and industries all over the world. Xylanase enzymes have proven to be a cost-effective means for mills to take advantage of a variety of bleaching benefits [47]. Xylanases and other side-cleaving enzymes have been used in pulp bleaching primarily to reduce lignin and increase the brightness of the pulp [20,46]. The importance of xylanase in the pulp and paper industries is related to the hydrolysis of xylan, which facilitates the release of lignin from paper pulp and, consequently, re‐ duces the usage of chlorine as the bleaching agent [33].

Bleaching is the process of lignin removal from chemical pulps to produce bright or completely white finished pulp [1]. Thus, the bleaching of pulp using enzymes or ligninolytic microor‐ ganisms is called biobleaching [48]. This process is necessary due to the presence of residual lignin and its derivatives in the pulping process, which causes the resultant pulp to gain a characteristic brown color. The intensity of this pulp color is related to the amount and chemical state of the remaining lignin [33].

The bleaching of pulp involves the destruction, alteration or solubilization of the lignin, colored organic matter and other undesirable residues on the fibers [33]. Bleaching of kraft pulp usually requires large amounts of chlorine-based chemicals and sodium hydrosulfite, which cause several effluent-based problems in the pulp and paper industries. The use of these chemicals generates chlorinated organic substances, some of which are toxic, mutagenic, persistent, and highly resistant to biodegradation, in addition to causing numerous harmful disturbances in biological systems and forming one of the major sources of environmental pollution [1,33,49]. As hemicellulose is easier to depolymerize than lignin, biobleaching of pulp appears to be more effective with the use of xylanases than with lignin-degrading enzymes. This is due to the fact of the removal of even a small portion of the hemicellulose could be sufficient to open up the polymer, which facilitates removal of the residual lignin by mild oxidants [33,50].

The use of xylanase in bleaching pulp requires the use of enzymes with special characteristics. A key requirement is to be cellulose-free, to avoid damaging the pulp fibers [3], as cellulose is the primary product in the paper industry [33]. Other desirable characteristics are stability at high temperatures [51] and an alkaline optimal pH [48].

Madlala *et al*. [52] used different preparations of commercial Xylanase P and crude xylanase from *Thermomyces lanuginosus* to evaluate the bleaching process of paper pulp. It was demon‐ strated that the use of enzymes could increase the pulp brightness (over 5 brightness points over the control) and reduce the amount of bleaching chemicals used (up to 30% for chlorine dioxide). Chipeta *et al*. [53] evaluated crude xylanase preparations from *Aspergillus oryzae* NRRL 3485 and *Aspergillus phoenicis* ATCC 13157 and found that at a charge of 10 U per gram of pulp it was possible to reduce the usage of chlorine dioxide up to 30% without compromising the pulp brightness.
