Delignification of wood pulp by a thermostable xylanase from Bacillus stearothermophilus strain T-6

During the bleaching of wood pulp for the paper industry, large amounts of chlorinated aromatic compounds are produced and released into the environment. These compounds are extremely toxic and are a major source of pollution. The paper and pulp industry is seeking for alternative methods for bleaching pulp. One such method involves the use of hemicellulases to release the colored lignohemicellulose. We have isolated and characterized several thermophilic bacteria which produce xylanases. One such strain, T-6, produced high levels of extracellular xylanase, free of cellulase and proteinase activities. Strain T-6 was classified as a strain of Bacillus stearothermophilus and was able to grow on defined medium containing xylose, methionine and asparagine at 65 °C. Xylanase activity was induced by either xylose or xylan; no activity was detected with other carbon sources, such as glycerol, acetate, lactose, glucose, maltose, fructose, mannose, galactose or sucrose. Xylanase constitutive mutants were obtained following mutagenesis and detection on p-nitrophenol -d-xylopyranoside containing agar plates. Xylanase T-6 was produced on large scale, and was purified and concentrated by a single adsorption-desorption step from a cation exchanger. The overall purification yield of a 1000 liter fermentation was 45%, resulting in a 98% pure enzyme. Xylanase T-6 was shown to partially remove lignin from unbleached pulp at 65 °C and pH 9.0, without loss in pulp viscosity. The enzyme-treated pulp was used to make handsheets that had higher brightness than untreated pulp.