Enhanced production of a thermostable xylanase from Streptomyces sp QG-11-3 and its application in biobleaching of eucalyptus kraft pulp

A thermostable and cellulase-free xylanase has been produced from Streptomy ces sp. QG-11-3 in solid substrate fermentation using wheat bran and eucaly ptus kraft pulp as the prime solid substrates. The maximum xylanase yield o btained using these two substrates were 2360 U/g and 1200 U/g dry solid sub strate at substrate:moisture ratios of 1:3 and 1:2.5, respectively. In immo bilized cell system using polyurethane foam (PUF) and three nonwoven fabric s, namely, polyester, silk, and cotton, the xylanase yields were enhanced b y 2.5-fold (203 U/ml), 1.91-fold (155 U/ml), 1.54-fold (125 U/ml), and 1.47 -fold (119 U/ml), respectively, compared to the xylanase yield in liquid-ba tch fermentation (81 U/ml). In the biobleaching experiments, the xylanase d ose of 3.5 U/g moisture free pulp exhibited the optimum bleach boosting of eucalyptus kraft pulp at pH 8.5 and 50 degrees C after 2 h of treatment. Wh en xylanase treated pulp was subsequently treated with 4.5% chlorine, it re sulted in reduction of kappa number by 25%, enhanced the brightness (%ISO) by 20% and improved the pulp properties such as tensile strength and burst factor by up to 63% and 8%, respectively.