THE EFFECT OF QUINONE-REDUCING AND PHENOL-METHYLATING ENZYMES ON THE YELLOWING OF MECHANICAL PULP

The effect of isolated enzymes and mycelial extracts of wood-rotting f ungi was studied in order to reveal their potential in preventing yell owing (brightness reversion) of mechanical pulp. Attempts were made to reduce quinoid structures in lignin using reducing enzymes, to block free phenolic groups using phenol methylating enzymes, and to accompli sh both reactions simultaneously with enzyme mixtures. Handsheets were irradiated and brightness was measured from reflectance curves at wav elength 457 nm (ISO brightness) before and after irradiation. With iso lated enzymes the highest prevention of yellowing was obtained when pr essurized ground wood (PGW) of spruce was treated with glucose oxidase (GLOX). When PGW was treated with GLOX under anaerobic conditions, 20 -30% less yellowing was obtained than in controls without the enzyme. Cellobiose:quinone oxidoreductase (CBQase) and catechol-O-methyltransf erase (COMT) alone or in a mixture did not prevent brightness reversio n. Yellowing was prevented by 20-30% compared with controls when PGW w as treated under anaerobic conditions using mycelial extracts from lig nin-degrading white-rot fungi Phlebia radiata or Phanerochaete chrysos porium. PGW was slightly methylated when it was treated with a mixture of CBQase and COMT. Also, when chemithermomechanical pulp (CTMP) was treated with COMT, a small increase in the amount of methoxyl groups w as obtained. However, these treatments did not prevent brightness reve rsion which indicates that other reactions than methylation may have b een responsible for the prevention of yellowing.