Bleaching of kraft pulp by a commercial lipase: Accessory enzymes degrade hexenuronic acids

The potential of a commercial lipase for bleaching kraft pulp was investigated. Enzymatic treatment of hardwood and softwood kraft pulps before and after oxygen delignification resulted in a significant reduction of kappa number and hexenuronic acid content. Pretreatment of unbleached hardwood kraft pulp followed by a DE bleaching sequence gave the same kappa number as a control sequence but with considerably less chlorine dioxide. Two enzyme fractions containing high accessory enzyme activities were purified by size exclusion chromatography. These fractions demonstrated a high bleaching efficiency as well as a high selectivity, with a significant reduction in sugar released in the filtrate compared to commercial xylanase bleaching. This work provides the first evidence that the accessory enzymes from a commercial enzyme preparation can degrade hexenuronic acids and bleach kraft pulp.