## 1 Introduction  
Agricultural and forestry residues, municipal solid waste, industrial processing residues and energy crops are readily available, low-cost sources of large quantities of holocellulose materials that can be used to obtain chemical feedstocks, biofuels, foods and feeds [1, 2]. Dirty cotton residue is produced during cotton processing and when harvested cotton is brought to spinners where it is processed into yarn for weaving. The bales that are unsuitable for yarn processing comprise cotton powder, brittle fibers and large quantities of cotton plant waste, such as stalks, stems, leaves and seed hulls, and are referred to as dirty cotton residue. This dirty cotton residue is marketed to re-processing industries to produce new, second-quality cotton fibers and filter cotton powder [3].  
Biotechnological use of lignocellulosic residues, including dirty cotton residue, requires the action of enzymes that degrade cellulose, hemicellulose and pectin [4, 5]. The enzymes that catalyze these reactions are generally obtained from microorganisms that contain synergistic, multi-catalyst systems [6]. The utilization of abundant, low-cost agricultural by-products as substrates for xylanase production substantially reduces enzyme production costs [7].  
\(\beta\)-Xylanase (1,4-\(\beta\)-d- xylan xylanohydrolase, EC 3.2.1.8) is a representative of an important group of endo-glycosyl hydrolases. It catalyzes the breakdown of \(\beta\)-1,4 glycosidic bonds in xylan, a linear polymer of \(\beta\)-d-xylopyranosyl units [8], to yield short xylooligomers [9]. Xylooligomers are further hydrolyzed into single xylose units by side-chain enzymes [10] that are essential for the efficient breakdown of the second most abundant natural polysaccharide in hardwoods and softwoods, [11]. The xylanases of microorganisms have a number of major biotechnological applications, including clarifying fruit juices and wines, food processing in combination with cellulases, improvement of the nutritional properties of agricultural silage, grain feed and poultry feed, degumming of plant fibers, and bio-pulping [7]. The use of low-molecular-weight xylanases in prebleaching of cellulose pulp has become an alternative approach in eliminating chlorine in bleaching and reducing chlorinated organic compounds in bleach plant effluents. It is worth stressing that their small size may represent an advantage for the biobleaching application once the effects of enzymes on pulps depend largely on their penetration into the fibre wall of cellulose pulp.  
Numerous reports describe the use of filamentous fungi, primarily those of the _Aspergillus_ genus, in the production of xylanases. Among these fungi, _Aspergillus oryzae_, the so-called koji mold, has been used extensively in traditional Japanese fermentation products, such as sake (rice wine), shoyu (soy sauce), and miso (soybean paste), for more than 1000 years [12]. The long history of _A. oryzae_ in the food fermentation industries prompted the United States Food and Drug Administration (FDA) to include _A. oryzae_ in the list of Generally Recognized as Safe (GRAS) organisms [13]. In addition to its use in the manufacture of fermented foods, _A. oryzae_ is an efficient producer of xylanases.  
In light of the biotechnological importance of xylanases in plant cell wall degradation, this study aimed to use pretreated dirty cotton residue (PDCR) as a carbon source for _A. oryzae_ cultivation and the subsequent production and purification of a low-molecular-weight xylanase. Some biochemical properties, such as pH, optimum temperature, thermal stability, and hydrolysis activity, are compared with other xylanolytic enzymes described in the literature. This report is the first on the purification and characterization of a xylanase produced by _A. oryzae_ grown on textile industry residue.