Pulp Bleaching with Xylanases

The kraft process accounts for 85% of the total pulp production in the United States. Bleached kraft pulp is a relatively high-value component of the total production of kraft paper. Kraft pulping removes most of the lignin and dissolves and degrades hemicelluloses without severely damaging cellulose. The kraft process results in excellent pulp from a wide variety of wood species. Unfortunately, kraft pulping also generates large quantities of chromophores. Chromophores are composed of residual lignin and carbohydrate degradation products. They are hard to extract because they are physically entrapped in and covalently bound to the carbohydrate moieties in the pulp matrix. Manufacturers use elemental chlorine and chlorine dioxide to bleach the chromophores and then they extract them, along with the residual lignin to make white pulp. Because of consumer resistance and environmental regulation of chlorine in bleaching, pulp makers are turning to oxygen, ozone, and peroxide bleaching, even though these may be more expensive and less effective than chlorine. One new technology has evolved to decrease the use of chlorine for bleaching, and that is the treatment of the pulp with xylanase enzyme. The use of xylanase enzymes to enhance the bleaching of the pulp was first reported in 1986.1 The Finnish forest companies were the first in the world to start millscale trials in 1988.