Marguerite Sykes

Differential and synergistic action of Streptomyces endoxylanases in prebleaching of kraft pulps

Abstract Microbial endoxylanases reduce chemical demand when they are applied to pulps prior to bleaching sequences. However, their mechanism of action is not fully understood, and the criteria for determining which enzymes are most effective are not well known. We examined the effects of two types of well-characterized endoxylanases from Streptomyces sp. TUB B-12-2 on the bleachability of kraft softwood and hardwood pulps. xyl 1a is a low molecular mass, high pl xylanase that forms mainly xylotriose but no xylose from oat spelts xylan. xyl 3 is a high molecular mass, low pl xylanase that forms mainly xylose and xylobiose as end products. xyl 3 released more reducing sugars than did xyl 1a, but both enzymes released about the same amount of total sugars. High performance liquid chromatography analysis of product sugars from pulp showed that at pH 7.0, xyl 1a formed oligosaccharides with a high degree of polymerization, but at pH 9.0 the action patterns of the two enzymes were similar. xyl 3 released 12% more chromophores from hardwood and 30% more from softwood pulp than did xyl 1a. Both enzymes enhanced brightness while reducing chemical demand, but xyl 3 was more effective. Synergism 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.

Enzymatic Solutions to Enhance Bonding, Bleaching and Contaminant Removal,

Microbial enzymes can enhance the properties of virgin pulps and secondary fibers. Enzymes added to pulp slurries at key steps during fiber processing alter structure and composition of native substrates in specific ways. Lipases hydrolyze fatty acid esters thereby facilitating depitching and deinking. Cellulases remove fines and change surface properties, thereby enhancing drainage, deinking, and contaminant removal of secondary fibers. The binding domains of cellulases exfoliate or roughen the surface of cellulose fibers. This feature may be used to enhance inter-fiber bonding. Xylanases and ligninases enhance lignin and chromophore removal thereby facilitating bleaching of kraft pulps. Microbial xylanases reduce chemical demand for bleaching by 35 to 50% in softwood and hardwood pulps, respectively. Many extracellular microbial enzymes are stable at 50 to 60°C and between pH 4 and 7, and some function well at greater extremes. In some instances crude enzymes can be used, but by cloning and expressing pure enzymes at high titers more precise and economical applications can be achieved.