REPEATED TREATMENTS WITH AUREOBASIDIUM-PULLULANS HEMICELLULASES AND ALKALI ENHANCE BIOBLEACHING OF SULFITE PULPS

The yeast Aureobasidium pullulans when grown on xylose produces hemice llulolytic enzymes with predominant xylanase and beta-xylosidase activ ity and no cellulase activity. At pH 4.5 and a temperature of 50 degre es C, the enzyme preparation produced by the means of ultrafiltration and dialysis retained 96 and 62% of its xylanase activity when incubat ed for 1 and 24 h, respectively. Aureobasidium pullulans enzymes were able to hydrolyze selectively the hemicellulose portion of unbleached sulphite pulps directed toward the manufacture of dissolving pulp. Hyd rolysis conditions for the efficient removal of zylan from pulp by the hemicellulase preparation were optimized in terms of pulp consistency (9%), temperature (55 degrees C), time (1 h), enzyme loading (15 IU x ylanase g(-1) oven-dry pulp), and number of repetitions (three). The r epeated consecutive enzymatic and alkali-oxygen treatments were studie d in a totally chlorine-free bleaching sequence for their effects on p ulp characteristics related to hemicellulose and lignin content of pul p. The best results were obtained when on enzyme-oxygen treatment repe ated three times was applied: Pentosan content of pulp was reduced two -fold, the kappa number decreased by 60%, brightness was enhanced by 1 8 points, and alpha-cellulose content was enriched by 3 points. Monoca rbohydrate analysis indicated that treated pulp contained less xylan ( 48%) and glucomannan (15%) than the untreated reference of dissolving pulp.