Effect of pretreatment of mechanical pulps with yeast on depression of color reversion of mechanical pulps

Light-induced color reversion of thermomechanical pulp (TMP), groundwood (GP) and pressurized groundwood pulp (PGW) could suppress by pretreatment of unbleached TMP, GP and PGW, respectively, with yeast and followed by alkaline hydrogen peroxide bleaching. Pretreatment of unbleached TMP with yeast for 3 hours under 350 rpm and followed by bleaching was found to give highly effect on depression of the color reversion of TMP. The effect was 35% to that of the untreated pulp. Furthermore, the color reversion of TMP could also suppress 39% to that of the untreated pulp when unbleached TMP was bleached with alkaline hydrogen peroxide and followed by yeast treatment. Regardless of the difference of bleaching sequences, almost no difference in the effect was observed.The effect of pretreatment of GP and PGW with yeast on depression of the color reversion was not so high as that of TMP. However, the effects was 11 to 19% to that of the untreated pulps. In the case of GP and PGW, the effect was not affected by the bleaching sequences in the similar manner as that of TMP.In order to decrease amount of use of yeast, yeast was incorporated with sodium alginate to make immobilized yeast, and applied for depresssion of the color reversion. Pretreatment of unbleached TMP with immobilized yeast for 48 hours under 350 rpm and followed by bleaching was found to suppress 18% to that of the untreated pulp. The, results show that the effect can be increased in depression of the color reversion by changing the treatment conditions. The amount of use of yeast could be decreased in about one-half by immobilization, and it is possible to reuse in the pretreatment of mechanical pulp for depression of the color reversion.Based on the results of the reactivity between a model compound and yeast, reactions responsible for depression of the color reversion was mainly reduction reactions, but some of oxidation reactions also seems to contribute the depression.