DIFFERENTIAL AND SYNERGISTIC ACTION OF STREPTOMYCES ENDOXYLANASES IN PREBLEACHING OF KRAFT PULPS

Microbial endoxylanases reduce chemical demand when they are applied t o pulps prior to bleaching sequences, However, their mechanism of acti on is not fully understood, and the criteria for determining which enz ymes are most effective are not well known. We examined the effects of two types of well-characterized endoxylanases from Streptomyces sp. T UB B-12-2 on the bleachability of kraft softwood and hardwood pulps, x yl 1a is a low molecular mass, high pI xylanase that forms mainly xylo triose but no xylose from oat spelts xylan. xyl 3 is a high molecular mass, low pI xylanase that forms mainly xylose and xylobiose as end pr oducts. xyl 3 released more reducing sugars than did xyl 1a, but both enzymes released about the same amount of total sugars. High performan ce liquid chromatography analysis of product sugars from pulp showed t hat at pH 7.0, xyl 1a formed oligosaccharides with a high degree of po lymerization, but at pH 9.0 the action patterns of the two enzymes wer e similar. xyl 3 released 12% more chromophores from hardwood and 30% more from softwood pulp than did xyl 1a. Both enzymes enhanced brightn ess while reducing chemical demand, but xyl 3 was more effective. Syne rgism was noted in the prebleaching of softwood but not hardwood pulp. Both enzymes were active at pH 9, and the presence of pulp increased thermal stability.