Optimization of medium composition for alkali-stable xylanase production by Aspergillus fischeri Fxn 1 in solid-state fermentation using central composite rotary design S.R. Senthilkumar a , B. Ashokkumar a , K. Chandra Raj b , P. Gunasekaran a,  a Department of Microbial Technology, School of Biological Sciences, Centre for Excellence in Genomic Sciences, Madurai Kamaraj University, Madurai 625 021, India b Department of Biotechnology, Indian Institute of Technology Madras, Chennai 600 036, India Received 29 June 2004; received in revised form 2 November 2004; accepted 13 November 2004 Available online 20 January 2005 Abstract Response surface methodology and central composite rotary design (CCRD) was employed to optimize a fermentation medium for the production of alkali-stable cellulase-free xylanase by Aspergillus fischeri in solid-state fermentation at pH 9.0 with wheat bran as substrate. The four variables involved in this study were sodium nitrite, potassium dihydrogen phosphate, magnesium sulphate and yeast extract. The statistical analysis of the results showed that, in the range studied, only sodium nitrite had a significant effect on xylanase production. The optimized medium containing (in g/l) NaNO 2 —7.0, K 2 HPO 4 —1.0, MgSO 4 —0.5 and yeast extract—5.0 resulted in 1.9-fold increased level of alkali-stable xylanase (1024 U/g wheat bran) production compared to initial level (540 U/g) after 72 h of fermentation, whereas its value predicted by the quadratic model was 931 U/g. The level of protease activity was con- siderably decreased in optimized medium, thus helping to preserve the xylanase activity and demonstrating another advantage of applying statistical experimental design. Ó 2005 Published by Elsevier Ltd. Keywords: Xylanase; Alkali-stable; Aspergillus fischeri; Optimization; RSM; CCRD 1. Introduction Xylan is the major hemicellulosic polysaccharide of wood and agricultural wastes, where it comprises up to 20–35% dry weight. Xylan is a hetero-polysaccharide and is composed of a xylose backbone, linked by b-1,4- xylosidic bonds, substituted with arabinosyl, glucorono- syl and acetyl residues. Among xylanolytic enzymes, endoxylanase and b-xylosidase are the key enzymes that cleave the xylan backbone into lower xylooligomers and xylose units. Tolerance to high pH and temperature are desirable properties of xylanases for effective use in pulp pre-treatment, which improves the efficiency of conven- tional chemical bleaching and therefore assists with pol- lution control. The alkali-stable xylanase can reduce the pollution by chloro-organo compounds in the paper- pulp industry due to usage of chlorine for bleaching, which is also a significant health hazard (Buchert et al., 1994). Fungal enzymes are commonly used in industries due to various technical reasons, including the feasibility of obtaining enzymes in high concentra- tion by solid-state fermentation (Mitchell and Lonsane, 1992). A few fungal strains produce alkali-tolerant, cellulase-free xylanase when grown under alkaline conditions (pH 8–10) (Bansod et al., 1993). Aspergillus 0960-8524/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.biortech.2004.11.005 * Corresponding author. Tel.: +91 452 2458478; fax: +91 452 2459181. E-mail address: pguna@eth.net (P. Gunasekaran). Bioresource Technology 96 (2005) 1380–1386