Repeated treatments with Aureobasidium pullulans hemicellulases and alkali enhance biobleaching of sulphite pulps

The yeast Aureobasidium pullulans when grown on xylose produces hemicellulolytic enzymes with predominant xylanase and beta-xylosidase activity and no cellulase activity. At pH 4.5 and a temperature of 50 degrees C, the enzyme preparation produced by the means of ultrafiltration and dialysis retained 96 and 62% of its xylanase activity when incubated 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. Hydrolysis 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 xylanase g(-1) oven-dry pulp), and number of repetitions (three). The repeated consecutive enzymatic and alkali-oxygen treatments were studied in a totally chlorine-free bleaching sequence for their effects on pulp characteristics related to hemicellulose and lignin content of pulp. The best results were obtained when on enzyme-oxygen treatment repeated three times was applied: Pentosan content of pulp was reduced two-fold, the kappa number decreased by 60%, brightness was enhanced by 18 points, and alpha-cellulose content was enriched by 3 points. Monocarbohydrate analysis indicated that treated pulp contained less xylan (48%) and glucomannan (15%) than the untreated reference of dissolving pulp.