Process Biochemistry 35 (1999) 149 – 157 Biobleaching of cellulose pulp from wheat straw with enzymes and hydrogen peroxide L. Jime ´nez a, *, E. Navarro a , J.L. Ferrer a , F. Lope ´z b , J. Ariza b a Departamento de Ingenierı ´a Quı ´mica, Facultad de Ciencias, Uniersidad de Co ´rdoba, 14004 Cordoba, Spain b Departamento de Ingenierı ´a Quı ´mica, Quı ´mica Fı ´sica y Quı ´mica Orga ´nica, Escuela Te ´cnica Superior, Uniersidad de Huela, Huela, Spain Received 16 November 1998; received in revised form 26 February 1999; accepted 19 March 1999 Abstract The influence of the operating conditions (temperature, time, enzyme concentration, pH and pulp consistency) used in the enzymic step of an XP (Cartazyme – hydrogen peroxide) sequence for bleaching soda pulp from wheat straw on various properties of the resulting pulp (yield, brightness, viscosity and kappa number) and of the paper sheets obtained from it (brightness, breaking length, stretch, burst index and tear index) was studied. A 2 m-1 central composite rotatable design was used for this purpose. The results of the 27 experiments performed were processed using the BMDP software suite, which provided equations that reproduced the values of the dependent variables with errors less than 10%. Application of the steepest ascent method to the equations thus derived allowed the identification of the most suitable conditions for optimizing the values of the dependent variables. Based on these results, using an enzyme concentration of 0.04% by dry pulp weight, a temperature of 35°C, a pH of 4.7 and a consistency of 12% for 2 h in the enzymic step provided paper sheets of acceptable quality. The XP bleaching sequence decreases pulp yield by 28.3% (19.2% in step X and 9.1% in step P), and the kappa number and viscosity of the pulp by 37.2 and 6.4%, respectively; on the other hand, it increases the brightness, breaking length, stretch, burst index and tear index of the paper sheets obtained from the pulp by 42.7, 32.4, 21.9, 49.7 and 7.7%, respectively. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Biobleaching; Wheat straw; Pulp; Enzymes; Hydrogen peroxide www.elsevier.com/locate/procbio 1. Introduction Agricultural cropping residues such as wheat straw are arousing increasing interest as raw materials for manufacturing cellulose pulp in countries that produce large amounts of this type of waste and have scanty wood resources. Such is the case with Spain, which produces more than 7 ×10 6 tonnes of cereal straw each year; at a typical yield of 25–30%, such a vast source of paper pulp would provide an amount of paper and board in excess of the current Spanish annual produc- tion [1], which comes largely from wood fibre and recycled paper, mostly imported. On the other hand, the need to decrease pollution from cellulose pulp mills has promoted the search for alternative bleaching and, especially, biobleaching pro- cesses [2–4]. The new bleaching procedures replace the classical steps based on chlorine and/or chlorinated compounds with others that use alternative reagents such as hydro- gen peroxide, oxygen, ozone, etc. Under mild conditions, hydrogen peroxide has no delignifying effect; it bleaches pulp with little yield loss, so it is widely used to bleach high yield (mechanical and semi-chemical) pulps. Under more drastic conditions, however, hydrogen peroxide acts as a delignifier and can be applied to chemical pulps in order to obtain high brightness and stability with little degradation, as well as to avoid the polluting effects of industrial effluents [2,3]. In addition, hydrogen peroxide is easy to manipulate and apply, flexible in use as a reagent, convenient to transport and store, and scarcely volatile. Moreover, its reaction products are of relatively low toxicity. How- ever, it decomposes easily, so it must be used under carefully controlled alkaline conditions [2]. * Corresponding author. Tel.: +34-957-218624; fax: +34-957- 218606. 0032-9592/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII:S0032-9592(99)00046-1