Volume 9, Number 2
Apr 1985

Enzymes Remove Lignin

The October Tappi Journal, v.67 #10, has three articles describing the use of enzymes in commercial mills for a variety of purposes. Two of the articles (those on p. 100 and 104) describe the use of a carbohydrase type to enhance the action of biocides in controlling slime, and of cellulases to turn waste fiber into ethanol as a mill byproduct.

The most intriguing article, however, was "Biocatalysts Hold Promise of Better Pulp Quality," by Roberta Farrell, p. 31-33. Farrell is the principal investigator of Repligen Corp. (101 Binney St., Cambridge, MA 02142), a biotechnology-based company with a research program for developing biocatalysts. She describes the use during the last decade of microorganisms (bacteria and fungi) for removal or modification of lignin. White-rot fungi, especially the species Phanerochnete chrysosporium, can effect total decomposition of lignin, as measured by the decrease in kappa number; however, it is a slow procedure and does degrade the cellulose somewhat. Conditions have to be carefully controlled, and the culture may be contaminated with other organisms.

The microorganisms do their work by producing enzymes, which (like many natural products) are not pure but mixed. Repligen Corp. has been experimenting with pure lignin-modifying enzymes in order to eliminate side effects. They have found enzymes which are highly selective on the specific chemical units almost universally found in lignin, but not in cellulosic fibers. Ligninase, the first enzyme ever purified which can depolymerize lignin, was first described in the literature only last year. It is now being cloned in an organism that makes no cellulose-degrading enzymes. Another enzyme catalyzes aromatic ring cleavage, a step considered necessary in prevention of brightness reversion (darkening, like a newspaper in the sun), without dissolving the lignin and thus reducing the bulk it adds. (Almost half of newsprint may be made up of lignin.)

The purified enzymes can do in hours what fungus cultures take weeks to do. They can be used to augment or replace existing bleaching methods, and to help in the processing of pulp by weakening lignin, thus making both of these processes less destructive to wood cellulose fibers. All in all, biocatalysts can he expected to make available better paper at a lower price.

This development, important for the production of permanent/durable paper, is taking place largely not in the paper mills, which turn sheets of dried pulp into paper, but in the pulp mills, which turn trees into pulp.