Application of recombinant xylanase from Orpinomyces sp in elemental chlorine-free bleaching of kraft pulps

A xylanase coding gene (xynA16) of Orpinomyces sp. was isolated and expressed in Escherichia coli. XynA16 encoded 291 amino acids to yield a protein of molecular mass 32.6 kDa. XynA16 had a catalytic domain of glycosyl hydrolase family 11 and a dockerin domain. The enzyme expressed in E. coli showed the highest activity at pH 6.0 and 50 degrees C. The enzyme retained 78% of its initial activity after 24 h preincubation at 50 degrees C. XynA16 was used in the pretreatment of wheat straw, eucalyptus and pine kraft pulps prior to oxygen delignification, alkali extraction and hydrogen peroxide stages. It has been demonstrated that the use of XynA16 for pulp bleaching could decrease kappa number and increase brightness, however the effects differed depending on the pulp type used. In terms of kappa number reduction, increase in brightness, release of chromophores and reducing sugars, eucalyptus kraft pulp was the most susceptible to XynA16 treatment.