Impact of hexenuronic acids on xylanase-aided bio-bleaching of chemical pulps Anatoly A. Shatalov * , Helena Pereira Centro de Estudos Florestais, Universidade Técnica de Lisboa, Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisbon, Portugal article info Article history: Received 27 October 2008 Received in revised form 12 January 2009 Accepted 14 January 2009 Available online 23 February 2009 Keywords: Hexenuronic acid Xylanase Bio-bleaching Chemical pulp Brightness abstract Hexenuronic acids (HexA) of hemicellulosic heteroxylan were shown to play important role in brightness development of chemical pulps during xylanase-aided bio-bleaching. Industrial wood (eucalypt) kraft pulp and a few non-wood (giant reed) organosolv pulps were pre-treated with commercial xylanase preparations (endo-1,4-b-xylanase activity; EC 3.2.1.8) and bleached by simplified bleaching sequence. The HexA performance was examined and compared with control (enzyme-free) samples. The xylan- ase-assisted direct brightening effect, noted immediately after an enzymatic stage, was proved to be caused by exclusive HexA removal with solubilized HexA-carrying xylooligosaccharide fractions. Aldo- hexa- and aldopentahexenuronic acids (Xyl 5 -HexA and Xyl 4 -HexA) were found as predominant oligosac- charides, accounting for up to 65% of total acidic oligomers in enzymatic hydrolyzates. A strong positive correlation (R 2 = 0.91) was established between further brightness improvement of xylanase-treated pulps during subsequent chemical bleaching (bleach boosting) and the content of HexA. This underlined the role of HexA as one of the key factors in definition of final brightness of bio-bleached pulps, determin- ing to a large extent the bleaching efficiency of xylanase application as a whole. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Acidic heteroxylan (arabino-) 4-O-methylglucuronoxylan is a principal non-cellulosic polysaccharide of many industrially important wood and non-wood (agro-fiber crop) species. In alka- line pulping, 4-O-methylglucuronic acid (MeGlcA) side-groups at- tached to the xylan backbone are degraded by about 75–90% (Buchert et al., 1995, 1997a). The residual MeGlcA are partially converted to unsaturated 4-deoxy-b-L-threo-hex-4-enopyranosylu- ronic acid groups (hexenuronic acid or HexA) by b-elimination of methanol directly or via the intermediate product 4-O-methylidu- ronic acid (Clayton, 1963; Johansson and Samuelson, 1977; Simkovic et al., 1986). HexA was identified as a major uronic acid (UA) substituent both of the industrial kraft pulp and of some unconventional alkali-based organosolv pulps, accounting for 83– 88% of total UA (Buchert et al., 1997a; Shatalov and Pereira, 2004). The interest to HexA has quickened in the last decade, as a re- sult of the revealed HexA capacity to affect negatively the follow- ing pulp bleaching and the properties of bleached pulps as well (Jiang et al., 2000). In fact, the bi-functionality of HexA, i.e., the presence of enol ether and unsaturated carboxylic acid group, makes it an easy target for both electrophilic and nucleophilic at- tacks during bleaching. The HexA reactions with electrophilic bleaching chemicals, such as chlorine dioxide, ozone and peracids, increase consumption of these chemicals (Vuorinen et al., 1999). The capacity of HexA to bind heavy metal ions affects indirectly consumption of other bleaching chemicals, such as hydrogen per- oxide (Devenyns and Chauveheid, 1997). Oxidative degradation of HexA (e.g., during ozonation) causes formation of calcium oxa- late deposits in the bleaching equipment (Elsander et al., 2000). HexA consume permanganate during the standard procedure of kappa number determination, commonly used (particularly in industry) for lignin quantification in pulps (Li and Gellerstedt, 1997). Finally, the HexA can modify optical properties of bleached pulps by decreasing brightness and increasing brightness reversion (Buchert et al., 1997b). The enzymatic pulp pre-treatment with highly specific hemicel- lulolytic enzymes (particularly with endo-1,4-b-xylanase activity, EC 3.2.1.8) before chemical bleaching (so-called bio-bleaching) has attracted considerable recent interest as a potential environ- mentally benign biotechnological approach. Xylanases allow to re- duce consumption of active bleaching chemicals (particularly the chlorine-based) and to increase the final brightness ceiling of bleached pulps (Tolan et al., 1996; Yang et al., 1992). The beneficial effect of xylanases on kraft pulp bleaching is attributed to the selective hydrolysis of xylan re-precipitated on the fiber surface and therefore improving fiber permeability to bleaching reagents and degradation products (Kantelinen et al., 1993), or to the in- creased extractability of lignin–carbohydrate complexes (LCC), what facilitates the pulp delignification during subsequent chemi- cal bleaching steps (Paice et al., 1992). Only limited published data are available about the effect of HexA on the bleaching performance of xylanases, whereas this 0960-8524/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2009.01.020 * Corresponding author. Tel.: +351 21 363 4662; fax: +351 21 364 5000. E-mail address: anatoly@isa.utl.pt (A.A. Shatalov). Bioresource Technology 100 (2009) 3069–3075 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech