Control of sticky deposits in wastepaper recycling with thermophilic esterase

Abstract

The sticky deposit problem during wastepaper recycling causes product quality decline. Enzymatic treatment is an alternative method to solve this problem. However, most enzymes are sensitive to the reaction conditions such as the temperature, pH and organic solvent, which limit their application in the pulp and paper industry. In the current study, three thermophilic esterase genes from Thermus thermophilus, Pyrobaculum calidifontis and the metagenomic library were obtained and produced in E. coli. The performance of these esterases and three commercial lipases for eliminating stickies in wastepaper recycling was evaluated. Esterase from Thermus thermophilus (TTEST) showed high stickies removal efficiency under various conditions as compared to other enzymes tested. TTEST also showed excellent thermal stability, and its combination with amylase, pectinase and xylanase had an additive effect on stickies removal. By an orthogonal experiment, the optimal ratio of TTEST, amylase, pectinase and xylanase in an enzyme mixture was determined as 0.8:0.6:0.2:0.5 (U/g, relative to bone dry pulp), resulting in enzyme degradation of 76.5% of stickies. Under the extreme conditions of 80 °C with pH 6.0 and 50 °C with pH 9.0 for a 1-h reaction, the stickies removal efficiency was kept at 59.5 and 68.9% by using the optimal enzyme mixture, respectively. Meanwhile, the physical properties of pulp after treatment by an optimal enzyme mixture have been improved. This work showed great significance in controlling sticky deposits without damage to the pulp in practical production in the papermaking industry.