Changes in the structural composition of hardwood kraft pulp during bleaching

The possibility of using xylanase preparations for hydrolyzing hemicelluloses in a non-bleached kraft pulp in order to facilitate its bleaching was studied. The effects of enzymatic preparations of fungal and bacterial origins were examined, and the optimal conditions for xylanase activity were determined. UV spectroscopy demonstrated that the treatment of kraft pulp with enzymatic preparations containing xylanase facilitated the subsequent removal of lignin and increased the brightness by 5%. The effect of enzymatic treatment was retained in the case of peroxide bleaching. The enzymatic preparations studied are promising for the development of chlorine-free pulp bleaching technologies.     

Effect of xylanases on peroxide bleachability of eucalypt (E. globulus) kraft pulp

Industrial eucalypt (E. globulus L.) kraft pulp was treated with two commercial xylanase preparations Ecopulp^® TX-200A and Pulpzyme^® HC (endo-1,4-β-xylanase activity; EC 3.2.1.8) and bleached by totally chlorine-free (TCF) three-stage hydrogen peroxide bleaching sequence, without oxygen pre-delignification. The effect of enzymatic stage on pulp properties and bleachability has been studied and compared with reference (control) pulps, processed without enzyme addition. The similar mode of enzymatic action was noted for both xylanase preparations. Final brightness of 86% ISO was achieved after complete bleaching. Direct bleaching effect caused pulp brightening (by 1.2–1.5% ISO) and delignification (by 7–10%) immediately after the enzymatic stage. The maximal bleach boosting was shown after the first peroxide stage and then diminished, despite the progressive increase in delignification over the control. The loss in efficiency of xylanase treatment by the end of peroxide bleaching was associated with specific behavior of xylan-derived chromophores, i.e., hexenuronic acids.     

Effects of surfactants on the enzymatic bleaching of kraft pulp by xylanase

A xylanase was purified from a commercial crude xylanase, Pulpzyme HC, and used for the bleaching of kraft pulp in the absence or in the presence of nonionic surfactants, Tween 20, Tween 80, and Igepal C930. The purified xylanase has a molecular weight of 23,500 as determined by a reducing SDS-PAGE. Tween 20 was most effective to enhance the efficiency of the enzymatic bleaching of kraft pulp by xylanase.     

The performance of fungal xylan-degrading enzyme preparations in elemental chlorine-free bleaching for Eucalyptus pulp

Cellulase-free xylan-degrading enzyme preparations from Acrophialophora nainiana, Humicola grisea var. thermoidea and two Trichoderma harzianum strains were used as bleaching agents for Eucalyptus kraft pulp, prior to a chlorine dioxide and alkaline bleaching sequence. In comparison to the control sequence (performed without xylanase pretreatment), the sequence incorporating enzyme treatment was more effective. Removal of residual lignin was indicated by a reduction in kappa number. Overall, enzyme preparations from T. harzianum were marginally more effective in reducing pulp viscosity and chlorine chemical consumption and improving the brightness of the kraft pulp. However, the highest reduction in pulp viscosity was mediated by the xylanase preparation from A. nainiana. Xylanase pretreatment compares very favorably with that of chemical pulping. Journal of Industrial Microbiology & Biotechnology (2002) 28, 204–206 DOI: 10.1038/sj/jim/7000227 Received 27 April 2001/ Accepted in revised form 03 November 2001     

Potential of xylanase from thermophilic Bacillus sp. XTR-10 in biobleaching of wood kraft pulp

An extracellular xylanase produced under optimal conditions by a thermophilic strain of Bacillus sp. XTR-10 was evaluated for its potential application in biobleaching of wood kraft pulp. Spectrophotometric analysis showed considerable release of lignin derived compounds and chromophoric material by the xylanase treated pulp samples. Xylanase was found to be effective in the liberation of reducing sugars in the pulp filtrates with increment in enzyme dose and reaction time. Eight hours pretreatment with 40 IU of xylanase/g of dry pulp resulted in 16.2% reduction of kappa number with 25.94% ISO increase in brightness as compared to the control. The same treatment slightly lowered the tensile strength and burst index, however. Enzyme pretreatment of the pulp saved 15% active chlorine charges in single step and 18.7% in multiple steps chemical bleaching with attainment of brightness at the level of the control. These results indicate the potential of enzymatic pretreatment of pulp for reduction in environmental discharge of hazardous waste from the pulp and paper industry.     

Impact of hexenuronic acids on xylanase-aided bio-bleaching of chemical pulps

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-β-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 xylanase-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. Aldohexa- and aldopentahexenuronic acids (Xyl₅-HexA and Xyl₄-HexA) were found as predominant oligosaccharides, accounting for up to 65% of total acidic oligomers in enzymatic hydrolyzates. A strong positive correlation (R² = 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, determining to a large extent the bleaching efficiency of xylanase application as a whole.     

The role of two Trichoderma reesei xylanases in the bleaching of pine kraft pulp

Summary The two major xylanases of Trichoderma reesei with different pI values and pH optima were compared for increasing the bleachability of pine kraft pulp. The efficiencies of the two enzymes acting on pulp substrate were very similar in hydrolysis yield, extraction kappa number or final brightness value. Only slight synergism between the two enzymes was observed in both hydrolysis and bleaching tests. The pH optimum of the pI 5.5 xylanase was similar in pulp treatment and in the hydrolysis of isolated substrates, and the bleaching result also correlated well with the hydrolysis of pulp xylan. By contrast, the pI 9.0 xylanase acted differently on pulp than on isolated xylans at different pH values and the pH optimum on pulp was increased. The bleachability of pulp by the pI 9.0 xylanase was improved more than expected at pH 7.0, although the hydrolysis of pulp xylan was substantially decreased. A similar phenomenon was also observed when the hydrolysis was performed in water instead of buffer. It thus appears that the degree of hydrolysis needed to obtain improved bleachability with pI 9.0 xylanase can be minimized by proper adjustment of the hydrolysis conditions. Correspondence to: J. Buchert     

Prebleaching of kraft pulp with full-length and truncated forms of a thermostable modular xylanase from Rhodothermus marinus

Full-length and truncated forms of a modular thermostable xylanase (EC 3.2.1.8., glycoside hydrolase family 10) were used in bleaching sequences of hardwood and softwood kraft pulps. Enzymatic treatment led to brightness gains of all pulps but the result depended on the pulp source. The presence of the additional domains in the full-length enzyme (including carbohydrate-binding modules) did not improve the bleaching process. No significant change in viscosity was seen after enzyme treatments indicating an unaffected pulp fibre length.     

Modification of high-lignin kraft pulps with laccase. Part 2. Xylanase-enhanced strength benefits

The effects of xylanase pretreatment of high lignin content softwood (SW) kraft pulp on subsequent pulp treatment with laccase in combination with gallic acid were investigated. Although xylanase pretreatment was ineffective in enhancing the laccase-facilitated biografting of gallic acid to kraft fibers, it was beneficial for subsequent treatment with laccase exclusively. Treating pulp fibers with xylanase followed by laccase provided a collective 25% and 46% increase in dry and wet tensile strength properties, respectively.     

Laccase for biobleaching of eucalypt kraft pulp by means of a modified industrial bleaching sequence

Biobleaching of kraft pulp is a possible application of laccase, but it has not been described in detail for complete industrial bleaching sequences yet. Therefore, in this work, the biobleaching of Eucalyptus globulus kraft pulp was performed using a modified industrial totally chlorine‐free sequence. The modification consisted in the substitution of an enzymatic delignification stage, based on the application of laccase from Trametes villosa, for the first alkaline extraction one. The enzymatic stage was performed with several synthetic and natural mediators, namely 1‐hydroxybenzotriazole (HBT), violuric acid (VA), methyl syringate, and syringaldehyde. Several pulp properties were analyzed after each stage of the bleaching process—kappa number, ISO brightness, viscosity, and optical properties of CIEL*a*b* system. The new biobleaching sequence improved the pulp properties, in comparison to the conventional bleaching sequence, if HBT or VA was used as mediators. VA was selected as the best mediator of those tested and the effect of its concentration in the enzymatic stage was subsequently studied. Reducing the initial concentration by 30%, the same pulp quality was obtained, but if the reduction attained 60%, an important decrease in pulp integrity was detected. The modified bleaching sequence could improve the bleached pulp properties (kappa number 10%, ISO brightness 1%, and viscosity 5%) in comparison to the mill sequence. © 2012 American Institute of Chemical Engineers Biotechnol. Prog., 2012     

Enzymatic bleaching of kraft pulp by xylanase from Aspergillus sydowii SBS 45

Crude xylanase from Aspergillus sydowii SBS 45 was tested for enzymatic bleaching of kraft (Decker) pulp. After optimization of three parameters, consistency of pulp, retention time and enzyme dose, considerable increase in the release of UV and visible absorbance spectra of materials and reducing sugars was observed, which clearly indicated the action of xylanase on pulp. Final brightness of pulp was increased from 29.42 to 70.42% and kappa number was reduced from 15.93 to 1.61, when 25 U of xylanase was given with a retention time of 5 h and at a consistency of 10%. When 10 U g⁻¹ xylanase was given, 14.3% elemental chlorine and 14.3% H₂O₂ could be reduced and when 25 U g⁻¹ xylanase was given 14.3% elemental chlorine and 28.6% H ₂O₂ could be reduced thereby retaining the brightness at control level.     

Production of thermostable pectinase and xylanase for their potential application in bleaching of kraft pulp

A very high level of alkalophilic and thermostable pectinase and xylanase has been produced from newly isolated strains of Bacillus subtilis and Bacillus pumilus respectively. Enzyme production for pectinase was carried out under SSF using combinations of cheap agricultural residues while xylanase was produced under submerged fermentation using wheat bran as substrate to minimize the cost of production of these enzymes Among the various substrates tested, the highest yield of pectinase production was observed by using combination of WB + CW (6592 U/g of dry substrate) supplemented with 4% yeast extract when incubated at 37 °C for 72 h using deionized water of pH 7.0 as moistening agent. The biobleaching effect of these cellulase free enzymes on kraft pulp was determined. Both xylanase and pectinase showed stability over a broad range of pH from 6 to 10 and temperature from 55 to 70 °C. The bleaching efficiency of the pectinase and xylanase on kraft pulp was maximum after 150 min at 60 °C using enzyme dosage of 5 IU/ml of each enzyme at 10% pulp consistency with about 16% reduction in kappa number and 84% reduction in permanganate number. Enzyme treated pulp when subjected to CDED₁D₂ steps, 25% reduction in chlorine consumption and upto 19% reduction in consumption of chlorine dioxide was observed for obtaining the same %ISO brightness. Also an increase of 22 and 84% in whiteness and fluorescence respectively and a decrease of approximately 19% in the yellowness of the biotreated pulp were observed by pretreatment of the pulp with our enzymatic mixture.     

Xylanase and Mannanase enzymes from Streptomyces galbus NR and their use in biobleaching of softwood kraft pulp

Enzymatic pretreatment of softwood kraft pulp was investigated using xylanase and mannanase, singly or in combination, either sequentially or simultaneously. Enzymes were obtained from Streptomyces galbus NR that had been cultivated in a medium, containing either xylan of sugar cane bagasse or galactomannan of palm-seeds, when they were used as sole carbon sources from local wastes in fermentation media. No cellulase activity was detected. Incubation period, temperature, initial pH values and nature of nutritive constituents were investigated. Optimum production of both enzymes was achieved after 5 days incubation on a rotary shaker (200 rpm) at 35 degrees C and initial pH 7.0. Partial purification of xylanase and mannanase in the cultures supernatant were achieved by salting out at 40-60 and 60-80% ammonium sulphate saturation with a purification of 9.63- and 8.71-fold and 68.80 and 62.79% recovery, respectively. The xylanase and mannanase from S. galbus NR have optimal activity at 50 and 40 degrees C, respectively. Both enzymes were stable at a temperature up to 50 degrees C. Xylanase and mannanase showed highest activity at pH 6.5 and were stable from 5.0 to 8.0 and from 5.5 to 7.5, respectively. The partial purified enzymes preparations of xylanase and mannanase enzymes showed high bleaching activity, which is an important consideration for industry. Xylanase was found to be more effective for paper-bleaching than mannanase. When xylanase and mannanase were dosed together (simultaneously), both enzymes were able to enhance the liberation of reducing sugars and improve pulp bleachability, possibly as a result of nearly additive interactions. The simultaneous addition of both enzymes was more effective in pulp treatment than their sequential addition.     

TCF mill trial on softwood pulp with Korsäs thermostable and alkaline stable xylanase T6

Abstract: Use of hemicellulases, including xylanases, for delignification in the paper industry has been slowed down by the lack of large-scale availability of enzymes which are active at a high pH (above 8) and a high temperature (above 60°C), conditions prevailing in many bleaching processes. During the past years, acidic or neutral hemicellulases, working at temperatures below 60°C, were used in most mill experiments. The Korsäs T6 xylanase from Bacillus stearothermophilus , which is active at a pH above 9.0 and at a temperature above 65°C, was produced on a large scale in collaboration with Gist-brocades and was employed on a full scale mill trial to produce a Total Chlorine chemical-Free (TCF) pulp from softwood. The bleaching sequence used was (OO)BQQPP. where O stands for oxygen delignification. B for the enzymatic treatment, Q for the chelating agent step and P for the hydrogen peroxide step. The enzyme bleaching step was performed during a period of 4 h at 63 ± 1°C and pH 8.7 ± 0.1. The results of the mill trial show that the TCF pulp produced had a brightness of 78% ISO and, at the same time, it preserved the same strength properties as chlorine dioxide-bleached pulp. The saving of hydrogen peroxide was 20%. The results on brightness, strength and chemical saving of this first full scale trial with T6 xylanase indicate that, after optimization, a TCF bleaching sequence including an enzymatic step with a xylanase working at a high pH and a high temperature, such as T6 xylanase, can be used to produce a high-strength bleached pulp. The advantages of a high pH and a high temperature enzymatic bleaching step are discussed.     

Microscopy studies reveal delignification and sterol removal from eucalypt kraft pulps by laccase-HBT

Fungal laccases in the presence of mediators are powerful biocatalysts to degrade lignin. Pycnoporus cinnabarinus laccase and 1-hydroxybenzotriazole (HBT) have been successfully used to delignify eucalypt kraft pulp once integrated in a totally chlorine-free bleaching sequence. Real time delignification of kraft pulp by laccase-HBT was verified in situ by monitoring the loss of lignin autofluorescence during the enzymatic treatment using confocal laser scanning microscopy. The highest delignification of pulp fibers occurred over a very short time-span (5 min). Moreover, we demonstrate the removal of sterols, responsible for pitch deposits in hardwood kraft pulps, as an additional effect of laccase-HBT. Spherical structures between pulp fibers localized by low temperature scanning electron microscopy were removed by laccase-HBT. The use of filipin, a specific stain, revealed the sterol nature of many of these structures. At the end of the enzyme-aided bleaching sequence, the fluorescent sterols-filipin signals were almost completely absent.     

Bleach boosting and direct brightening by multiple xylanase treatments during peroxide bleaching of kraft pulps

The effects of multiple xylanase treatments were assessed during the peroxide bleaching of three pulps: Douglas-fir (kraft); Western hemlock (oxygen delignified kraft); and trembling Aspen (kraft). The addition of a xylanase treatment stage, either before or after the peroxide bleaching stage(s), resulted in the enhanced brightening of all pulps. A higher brightness was achieved using two enzyme treatments, one before and one after the peroxide stage(s). Both bleach boosting and direct brightening seemed to contribute to the enhancement of the peroxide bleaching. Compared to xylanase prebleaching, xylanase posttreatment of peroxide bleached pulps solubilized less lignin and chromophores and made smaller amounts of these materials alkaline soluble. Nevertheless, the final brightness achieved by xylanase posttreatment was similar or superior to that achieved with xylanase prebleaching of the corresponding unbleached pulps. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 54: 312-318, 1997.     

Production of a recombinant xylanase in plants and its potential for pulp biobleaching applications

The purpose of this study was to produce recombinant xylanase in transgenic plants and to test its potential application for pulp bleaching. The xynII xylanase gene from Trichoderma reesei was inserted into the Arabidopsis genome. Many transgenic plants produced biologically active XYNII and accumulated in leaves at level of 1.4-3.2% of total soluble proteins. The bleaching ability of XYNII on Kraft pulp was demonstrated by a reduction in the kappa number and the residual lignin contents. The bleaching efficiency of transgenic plant produced XYNII was similar to commercial xylanase on unbleached Kraft pulp. The effect of xylanase treatment on Kraft pulp was also investigated by SEM. Clear physical change on the pulp fiber surface was observed and was related to the amount xylan removed and microfibrils were visible on the fiber surface. This report demonstrates the potential application of plant produced recombinant xylanase for pulp and paper bleaching.     

Effect of Organic Acids on Reactions Catalyzed by Manganese Peroxidase from Phanerochaete chrysosporium

The effect of several organic acids on the oxidation of Mn(II) catalyzed by manganese peroxidase was studied. Reactivities of manganese peroxidase and chemically prepared Mn(III) organic acid complexes towards phenolic compounds were compared. If lactate appears to be the best complexant for manganese peroxidase activity, chemically prepared Mn(III)—lactate complex is a less effective oxidant towards phenolic compounds than other Mn(III)—complexes. Our results agree with the hypothesis that certain organic acids are involved in the catalytic cycle of manganese peroxidase. Malonate and lactate seem to be the most attractive complexants for practical applications of manganese peroxidase and were used in enzymatic treatment of hardwood kraft pulp. Bleaching of kraft pulp was studied and after alkaline extraction, a significant decrease of kappa number was measured. The bleaching was enhanced in lactate buffer.     

Biological bleaching of chemical pulps

Use of biotechnology in pulp bleaching has attracted considerable attention and achieved interesting results in recent years. Enzymes of the hemicellulolytic type, particularly xylan-attacking enzymes, xylanases are now used commercially in the mills for pulp treatment and subsequent incorporation into bleach sequences. The aims of the enzymatic treatment depend on the actual mill conditions and may be related to environmental demands, reduction of chemical costs or maintenance or even improvement of product quality. The use of oxidative enzymes from white-rot fungi, that can directly attack lignin, is a second-generation approach, which could produce larger chemical savings than xylanase but has not yet been developed to the full scale. It is being studied in several laboratories in Canada, Japan, the U.S.A. and Europe. Certain white-rot fungi can delignify kraft pulps increasing their brightness and their responsiveness to brightening with chemicals. The fungal treatments are too slow but the enzyme manganese peroxidase and laccase can also delignify pulps and enzymatic processes are likely to be easier to optimize and apply than the fungal treatments. Development work on laccase and manganese peroxidase continues. This article presents an overview of developments in the application of hemicellulase enzymes, lignin-oxidizing enzymes and white-rot fungi in bleaching of chemical pulps. The basic enzymology involved and the present knowledge of the mechanisms of the action of enzymes as well as the practical results and advantages obtained on the laboratory and industrial scale are discussed.     

Family-10 and Family-11 xylanases differ in their capacity to enhance the bleachability of hardwood and softwood paper pulps

Enzyme-aided bleaching of softwood and hardwood kraft pulps by glycosyl hydrolase family-10 and -11 xylanases and a family-26 mannanase was investigated. The ability to release reducing sugar from pulp xylan and to enhance bleachability is not a characteristic shared by all xylanases. Of the six enzymes tested, two xylanases belonging to family 11 were most effective at increasing bleachability and improving final paper brightness. None of the enzymes had a deleterious effect on pulp fibre integrity. The efficiency of individual xylanases as bleach enhancers was not dependent on the source microorganism, and could not be predicted solely on the basis of the quantity or nature of products released from pulp xylan. Cooperative interactions between xylanase/xylanase and xylanase/mannanase combinations, during the pretreatment of softwood and hardwood pulps, were investigated. Synergistic effects on reducing-sugar release and kappa number reduction were elicited by a combination of two family-10 xylanases. Pretreatment of kraft pulp with mannanase A from Pseudomonas fluorescens subsp. cellulosa and any one of a number of xylanases resulted in increased release of reducing sugar and a larger reduction in kappa number than obtained with the xylanases alone, confirming the beneficial effects of family-26 mannanases on enzyme-aided bleaching of paper pulp. Received: 6 January 1997 / Received revision: 10 April 1997 / Accepted: 19 April 1997