**1. Introduction**

The enzymatic hydrolysis of xylan, which is the second most abundant natural polysaccharide, is one of the most important industrial applications of this polysaccharide [1, 2]. The primary chain of xylan is composed of β-xylopyranose residues, and its complete hydrolysis requires the action of several enzymes, including endo-1,4-β-D-xylanase (EC3.2.1.8), which is crucial for xylan depolymerization [2]. Due to the diversity in the chemical structures of xylans derived from the cell walls of wood, cereal or other plant materials, a large variety of xylanases with various hydrolytic activities, physicochemical properties and structures are known. Moreover, xylan derivatives are frequently used to induce the production of xylanases [3] by microor‐ ganisms [4], using either solid-state or submerged fermentation [5].

Xylanases and the microorganisms that produce them are currently used in the management of waste, to degrade xylan to renewable fuels and chemicals, in addition to their use in food, agro-fiber, and the paper and pulp industries, where the enzymes help to reduce their environmental impact [6]. Oligosaccharides produced by the action of xylanases are further used as functional food additives or alternative sweeteners with beneficial properties [7].

To meet the needs of industry, more attention has been focused on the enzyme stability under different processing conditions, such as pH, temperature and inhibitory irons, in addition to its ability to hydrolyze soluble or insoluble xylans. Although many wild-type xylanases contain certain desired characteristics, such as thermostability, pH stability or high activity, no individual xylanase is capable of meeting all of the requirements of the feed and food industries. Moreover, as industrial applications require cheaper enzymes, the elevation of expression levels and the efficient secretion of xylanases are crucial to ensure the viability of

© 2013 Motta et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Motta et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the process; therefore, genetic engineering and recombinant DNA technology have an important role in the large-scale expression of xylanases in homologous or heterologous protein-expression hosts.

are typically arabinoxylans [12]. Linear unsubstituted xylan has also been reported in esparto grass [15], tobacco [16] and certain marine algae [17,18], with the latter containing xylopyra‐

A Review of Xylanase Production by the Fermentation of Xylan: Classification, Characterization and Applications

Similar to other polysaccharides of plant origin, xylan has a large polydiversity and polymo‐ lecularity [20]. The degree of polymerization in xylans is also variable, with, for example, hardwood and softwood xylans generally consisting of 150-200 and 70-130 β-xylopyranose

**OH**

**O**

**O**

**O**

**-4-O-Me-GlcUA**

**HO**

**HOOC**

a

a

**O**

**OH**

**O**

**H3 CO**

**OH**

**Endoxylanase**

**O**

**O**

**OH**

**CH2 OH**

a**-Araf**

nofuranose; α-4-O-Me-GlcA: α-4-O-methylglucuronic acid. Source: Sunna and Antranikian [20].

**O**

**OH**

**Figure 1.** Structure of xylan and the xylanolytic enzymes involved in its degradation. Ac: Acetyl group; α-Araf: α-arabi‐

Based on the common substituents found on the backbone, xylans are categorized as linear homoxylan, arabinoxylan, glucuronoxylan or glucuronoarabinoxylan. Homoxylans consisting exclusively of xylosyl residues are not widespread in nature; they have been isolated from limited sources, such as esparto grass, tobacco stalks and guar seed husks [20]. However, based on the nature of its substituents, a broad distinction may therefore be made among xylans, in which the complexity increases from linear to highly substituted xylans. Four main families

**i.** Arabinoxylans, having only side chains of single terminal units of α-L-arabinofura‐

**ii.** Glucuronoxylans, in which α-D-glucuronic acid or its 4-O-methyl ether derivative

**iii.** Glucuronoarabinoxylan, in which α-D-glucuronic (and 4-O-methyl-α-D-glucuronic)

nosyl substituents. In the particular case of cereals, arabinoxylans vary in the degree of arabinosyl substitution, with either 2-O- and 3-O-mono-substituted or double (2-

**O**

**O**

**OH**

a

**O**

**O**

**-Glucuronidase**

**OH**

**O**

**O**

**HO**

**O**

**O**

**O Ac**

**Acetyl Xylan Esterase**

http://dx.doi.org/10.5772/53544

253

**-Arabinofuranosidase**

**OH**

a

**O**

**OH**

**CH2 OH**

**-Araf**

**O**

**OH**

**O**

nosyl residues linked by both 1,3-β and 1,4-β linkages [17,19].

residues, respectively [12].

**AcO**

**O**

**O**

**Ac O Ac**

**O**

of xylans can be considered [21]:

O-, 3-O-) substituted xylosyl residues.

acid and α-L-arabinose are both present.

represents the only substituent.

Considering the future prospects of xylanases in biotechnological applications, the goal of this review chapter is to present an overview of xylanase production via fermentation and to describe some of the characteristics of these enzymes and their primary substrate, xylan. Moreover, this review will discuss the fermentation processes as well as the genetic techniques applied to improve xylanase yields.
