Process Biochemistr_v 27 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (1992) 3 19-325 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Review Biobleaching of Kraft Pulp zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH Pratima Bajpai & Pramod K. Bajpai Chemical Engineering Division, Thapar Corporate R & D Centre, Patiala - 147 001, India (Received 20 December 1991; accepted 23 January 1992) This review reviews current research on the application of biotechnology to the bleaching of kraft pulp. Biobleaching research focuses on using white rot fungi, and ligninase and hemicellulase enzymes. Bleaching with white rot is generally very slow and the primary problems appear to be the control of cellulose degradation, the technical aspects of scale up and the avoidance of an initial lag phase. Ligninase enzymes depolymerize lignin and in combination with an extraction stage, partially bleach kraft pulp. Bleaching with hemicellulase enzymes looks promising. A reduced need.for active chlorine has been obtained by boosting the bleachability of pulp through pretreatment with hemicellulose hydrolysing enzymes zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFED INTRODUCTION Biotechnology can be put to beneficial use by the pulp and paper industry in many ways including the growing of superior trees, the processing of wood and pulp and the utilization of w astes. The integration of lignin-degrading and modifying bio- catalysts into pulping and bleaching operations as well as effluent decolorization and the use of biomass for fuels, chemicals and other products could be of great benefit to the industry in several ways. This paper is an overview of attempts being made by several research and development organ- izations around the world to use biotechnology in the bleaching of kraft pulp. The progress made and the further scope are also discussed. The kraft process is the world’s major pulping method and is likely to remain so into the Corresponding author : Dr P. Bajpai. Tel: (0175) 76157. Telex: 394 207. foreseeable future. It has evolved over a period of 100 years and has become highly refined. Currently, about 70% of the world’s annual pulp output of approximately 100 million tonnes is produced by the kraft process. Despite some shortcomings, it is the most cost effective, versatile and efficient wood delignification method available. Because of this fact and the large amount of capital already invested in kraft pulping, it is unlikely that the process will be replaced in the near future. The kraft process results in the degradation and solubilization of lignin. Wood chips are cooked in a solution of Na,S/ NaOH at about 170 “C for about 2 h to degrade and solubilize the lignin.‘-’ The lignin undergoes a variety of reactions but the most important of these results in partial depolymer- ization and formation of ionizable (mainly phenolic hydroxyl) groups ; these changes lead to the dis- solution of the lignin in the alkaline pulping liquor. The lignin reactions involved in kraft pulping have been studied extensively.3-6 About 90% of the 319 Process Biochemistry 0032.9592/92/$5,00 0 1992 Elsevier Science Publishers Ltd, England