US20090065159A1

US20090065159A1 - Chemical pulp treatment compositions and methods

Info

Publication number: US20090065159A1
Application number: US12/160,898
Authority: US
Inventors: Hui Xu; Kimberly Bloomfield; Stefanie McCloskey; Kim Borch
Original assignee: Novozymes North America Inc
Current assignee: Novozymes AS; Novozymes North America Inc
Priority date: 2006-02-14
Filing date: 2007-02-14
Publication date: 2009-03-12
Also published as: NO20083436L; AU2007214449A1; BRPI0707656A2; CA2637691A1; EP1994219A1; WO2007095575A1; JP2009526919A; CN101384769A

Abstract

The present invention relates to compositions and methods for treating pitch problems in chemical pulp by treating chemical pulp process water with a combination of a lipase and a peroxide source. The present invention also relates to compositions and methods for treating pitch problems in chemical pulp by treating chemical pulp process water with a lipase, a peroxide source and organic acid(s).

Description

FIELD OF THE INVENTION

The present invention relates to enzymatic compositions and methods for treating pitch problems in chemical pulp.

BACKGROUND OF THE INVENTION

Wood contains about 1 to 10% of pitch or extractives in addition to its main components cellulose, hemicullose and lignin. Major components of pitch are fatty acids, triglycerides, sterols, steryl esters and resin acids, such as, for example, abietic acid. Pitch causes problems in paper machines by sticking to the rollers and causing spots or holes in the paper material.
Various enzymatic processes have been used to treat pitch problems. WO 00/53843 discloses steryl esterase enzyme preparations and their use in the manufacture of paper to hydrolyze the steryl ester part of pitch.
U.S. Pat. No. 6,066,486 discloses an enzyme preparation comprising a cholesterol esterase derived from Pseudomonas fragi, and its use to hydrolyze pulp resin.
JP 2000080581 discloses the use of certain peroxidases for the decomposition of abietic acid during pulping or paper making processes.
X. Zhang; Pulp & Paper Canada. 101: 3 (2000), page 59-62, discloses studies of the ability of laccase to remove dissolved and colloidal substances.
Karlsson et al.: Reactivity of Trametes laccases with fatty and resin acids; Appl. Microbiol. Biotechnol. (2001) 55: 317-320 discloses experiments in which laccases were used to treat pitch.
U.S. Patent application 20030124710 discloses a process for manufacturing a paper material by treating a papermaking pulp process water with a fatty acid oxidizing enzyme.
U.S. Pat. No. 5,356,517 discloses the use of lipases to hydrolyze triglycerides during peroxy bleaching in the preparation of chemithermomechanical pulp.

SUMMARY OF THE INVENTION

The present invention relates to compositions and methods for treating pitch problems in chemical pulp by treating chemical pulp process water with a combination of a lipase and a peroxide source. Although not limited to any one theory of operation, it is believed that the addition of a lipase and peroxide leads to the formation of peracids which in turn oxidize unsaturated fatty acids and resin acids through an expoxidation reaction. The resulting products are hydrophilic and are readily washed from the pulp, thereby reducing pitch problems associated with chemical pulp.
In another embodiment, the present invention relates to compositions and methods for treating pitch problems in chemical pulp by treating chemical pulp process water with a lipase, a peroxide source and an organic acid.

DETAILED DESCRIPTION OF THE INVENTION

A “paper-making process” refers to a process wherein a chemical pulp is suspended in water, mixed with various additives and then passed to equipment for further processing, e.g., in which the paper, cardboard, tissue, towel etc. is formed, pressed and dried.
The term “paper material” refers to products which can be made out of pulp, such as paper, linerboard, corrugated paperboard, tissue, towels, corrugated containers or boxes.
The term “a papermaking pulp” or “pulp” means any chemical pulp which can be used for the production of a paper material.
A “chemical pulp” refers to chemical pulp (such as Kraft pulp or sulfite pulp) or semichemical pulp (SCP). Chemical pulp is usually manufactured by alkaline cooking whereby most of the lignin and some hemicellulose components are removed. In Kraft pulping or sulphate cooking, sodium sulphide or sodium hydroxide are generally used as principal cooking chemicals. In such pulp, as a result of the alkaline cooking, the triglyceride part of pitch will be hydrolyzed into fatty acids and glycerol.
In a particular embodiment of the use and the process of the invention, the chemical pulp is a Kraft pulp or a sulfite pulp. In particular embodiments, the Kraft pulp is bleached Kraft pulp, for example softwood bleached Kraft (SWBK, also called NBKP (Nadel Note Bleached Kraft Pulp)), hardwood bleached Kraft (HWBK, also called LBKP (Laub Hote Bleached Kraft Pulp and)) or a mixture thereof.
The Kraft pulp to be Seated may be a bleached Kraft pulp, which may consist of softwood bleached Kraft (SWBK, also called NBKP (Nadel Holz Bleached Kraft Pulp)), hardwood bleached Kraft (HWBK, also called LBKP (Laub Holz Bleached Kraft Pulp and)) or a mixture of these
The pulp to be used in the process of the invention is a suspension of chemical pulp. The pulp to be used in the process of the invention may comprise at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or 100% of chemical pulp (such as Kraft pulp or sulfite pulp). The percentage of chemical pulp lies within the range of 1-100%, in particular embodiments, the percentage of chemical pulp (such as Kraft pulp or sulfite pulp) lies within the range of 1-99%, 2-98%, 3-97%, 4-96%, 5-95%, 6-94%, 7-93%, 8-92%, 9-91%, 10-90%, 15-85%, 20-80%, 25-75%, 30-70%, 40-60%, or 45-55%.
In accordance with the present invention, chemical pulp process water is treated with a combination of a lipase and a peroxide source. Such chemical pulp process water contains pitch causing components, such as, fatty acids, triglycerides, sterols, steryl esters and/or resin acids (for example, abietic acid). The process of the invention is particularly applicable to the reduction of compounds constituting the pitch during a pulping or paper-making process, e.g. to avoid pitch troubles.
Any suitable lipase may be used. Lipases include the enzymes classified by EC 3.1.1.3. Reference is made to the Recommendations (1992) of the Nomenclature Committee of the international Union of Biochemistry and Molecular Biology, Academic Press Inc., 1992
In a preferred embodiment of the present invention, the lipase is preferably of microbial origin, in particular of bacterial, fungal or yeast origin. The lipase may be derived from any source, including, for example, a strain of Absidia, in particular Absidia blakesleena and Absidia corymbifera, a strain of Achromobactor, in particular Achromobacter iophagus, a strain of Aeromonas, a strain of Alternaria, in particular Alternaria brassiciola, a strain of Aspergillus, in particular Aspergillus niger and Aspergillus flavus, a strain of Achromobacter, in particular Achromobacter iophagus, a strain of Aureobasidium, in particular Aureobasidium pullulans, a strain of Bacillus, in particular Bacillus pumilus, Bacillus strearothermophilus and Bacillus subtilis, a strain of Beauveria, a strain of Brochothrix, in particular Brochothrix thermosohata, a strain of Candida, in particular Candida cylindracea (Candida rugosa), Candida paralipolytica, and Candida antarctica, a strain of Chromobacter, in particular Chromobacter viscosum, a strain of Coprinus, in particular Coprinus cinerius, a strain of Fusarium, in particular Fusarium oxysporurn, Fusahum solani, Fusarium solani pisi, and Fusarium roseum culmorum, a strain of Geotricum, in particular Geotricum penicillatum, a strain of Hansenula, in particular Hansenula anomala, a strain of Humicola, in particular Humicola brevispora, Humicola brevis var. thermoidea, and Humicola insolens, a strain of Hyphozyma, a strain of Lactobacillus, in particular Lactobacillus curvatus, a strain of Metarhizium, a strain of Mucor, a strain of Paecilomyces, a strain of Penicillium. In particular Penicillium cyclopium, Penicillium crustosum and Penicillium expansum, a strain of Pseudomonas in particular Pseudomonas aeruginosa, Pseudomonas alcaligenes, Pseudomonas cepacia (syn. Burkholderia cepacia), Pseudomonas fluorescens, Pseudomonas tragi, Pseudomonas maltophilia, Pseudomonas mendocina, Pseudomonas mephitica lipolytica, Pseudomonas alcaligenes, Pseudomonas plantari, Pseudomonas pseudoalcaligenes. Pseudomonas putida, Pseudomonas stutzeri, and Pseudomonas wisconsinensis, a strain of Rhizoctonia, in particular Rhizoctonia solani, a strain of Rhizomucor, in particular Rhizomucor miehei, a strain of Rhizopus, in particular Rhizopus japonicus, Rhizopus microspore and Rhizopus nodosus, a strain of Rhodosporidium, in particular Rhodosporidium toruloides, a strain of Rhodotorula, in particular Rhodotorula glutinis, a strain of Sporobolomyces, in particular Sporobolomyces shibatanus, a strain of Thermomyces, in particular Thermomycas lanuginosus (formerly Humicola lanuginosa), a strain of Thiarosporella, in particular Thiarosporella phaseolina, a strain of Trichodema, in particular Trichoderma harzianum, and Trichoderma reesei, and/or a strain of Verticillium.
In a preferred embodiment, the lipase is derived from a strain of Aspergillus, a strain of Achromobacter, a strain of Bacillus, a strain of Candida, a strain of Chromobacter, a strain of Fusarium, a strain of Humicola, a strain of Hyphozyma, a strain of Pseudomonas a strain of Rhizomucor, a strain of Rhizopus, or a strain of Thermomyces.
Preferred lipases include the lipases described in U.S. Pat. No. 6,074,863 and WO 02/055679, Preferred commercial lipases include Resinase A2X and Resinase NT (Novozymes A/S). In another preferred embodiment, the lipase is the Candida antarctica lipase A (CALA) or the Candida antarctica lipase 8 (GALS) (available from Novozymes A/S)
As used herein, a “peroxide source” or “hydrogen peroxide source” refers to hydrogen peroxide itself or components which can generate peroxide. The hydrogen peroxide source may be added at the beginning or during the lipase treatment process, e.g., at a concentration of about 0.001-100 mM, particularly 0.01-50 mM. One source of hydrogen peroxide includes precursors of hydrogen peroxide, such as, e.g., a perborate or a percarbonate. Another source of hydrogen peroxide includes enzymes which are able to convert molecular oxygen and an organic or inorganic substrate into hydrogen peroxide and the oxidized substrate, respectively. These enzymes produce only low levels of hydrogen peroxide. Examples of enzymes which are capable of producing hydrogen peroxide include, but are not limited to, glucose oxidase, urate oxidase, galactose oxidase, alcohol oxidase, amine oxidase, amino acid oxidase and cholesterol oxidase.
Although not limited to any one theory of operation, it is believed that the addition of a lipase and peroxide leads to the formation of peracids which in turn oxidize unsaturated fatty acids, resin acids and other pitch components through an epoxidation reaction. The resulting products are hydrophilic and are readily washed from the pulp, thereby reducing or eliminating pitch problems.
Organic acid(s) may also be added to the pulp, e.g., to enhance the pitch treatment. Organic acids refer to any organic substance which contains at least one acidic group. Examples of organic acids are acetic acid, butyric acid, and linoleic acid. The concentration of organic acid is preferably between 0.001-500 mM.
In the case of paper and pulp processing, the process according to the invention can be carried out at any pulp production stage. The enzyme can be added to any holding tank, e.g. to a pulp storing container (storage chest), storage tower, mixing chest or metering chest. The enzyme treatment is preferably applied before or after pulp bleach process or in between the pulp bleaching stages. The enzyme can be added to the circulated process water (white water) originating from bleaching. In a particular embodiment of a Kraft pulping process, the enzyme is added during the brown-stock washing.
In the present context, the term “process water” can include water added as a raw material to the paper manufacturing process; intermediate water products resulting from any step of the process for manufacturing the paper material; as well as waste water as an output or by-product of the process. In a particular embodiment, the process water is, has been, is being, or is intended for being circulated or re-circulated, i.e., re-used in another step of the process. The term “water” in turn means any aqueous medium, solution, suspension, e.g., ordinary tap water, and tap water in admixture with various additives and adjuvants commonly used in paper manufacturing processes, in a particular embodiment the process water has a low content of solid (dry) matter, e.g., below 20%, 18%, 16%, 14%, 12%, 10%, 8%, 7%, 6%, 5%, 4%, 3%, 20% or below 1% dry matter.
The process of the invention may be carried out at conventional conditions in the paper and pulp processing. The process conditions will be a function of the enzyme(s) applied, the reaction time and the conditions given.
The enzyme and peroxide should be added in an effective amount. By the term “effective amount” is meant the amount sufficient to achieve the desired effect of reducing or inhibiting pitch components, such as, by degrading or converting such components into a form which can be more readily removed from the pulp or pulp process water.
In a particular embodiment, the dosage of the lipase enzyme is from about 0.1 mg enzyme protein to about 100.000 mg enzyme protein (of each enzyme) per ton of paper pulp.
The enzymatic treatment can be done at consistency, e.g., 0.5-10% dry substance, in particular embodiments, the consistency is within the range of 0.5-45; 0.5-40; 0.5-35; 0.5-30; 0.5-25; 0.5-20; 0.5-15; 0.5-10; 0.5-3; 0.5-6; or 0.5-5% dry substance.
The enzymatic treatment may be carried out at a temperature of from about 10 to about 100° C. Further examples of temperature ranges (all “from about” and “to about”) are the following; 20-100, 30-100, 35-100, 37-100, 40-100, 50-100, 60-100, 70-100, 10-90, 10-80, 10-70, 10-60, and 30-60° C., as well as any combination of the upper and lower values here indicated. The temperature may be from about 20 to 90° C., or 20 to 95° C., preferably from about 40 to 70° C., or 40 to 75° C.
The enzymatic treatment may be carried out at a pH of from about 2 to about 12. Further examples of pH ranges (all “from about” and “to about”) are the following: 3-12, 4-12, 5-12, 6-12, 7-12, 8-12, 9-12, 2-11, 2-10, 2-9, 2-8, 4-10, 5-8 as well as any combination of the upper and lower values here indicated. The pH range may be from about 2 to 11, preferably within the range from about 3-9.
A suitable duration of the enzymatic treatment may be in the range from a few seconds to several hours, e.g. from about 30 seconds to about 48 hours, or from about 1 minute to about 24 hours, or from about 1 minute to about 18 hours, or from about 1 minute to about 12 hours, or from about 1 minute to 5 hours, or from about 1 minute to about 2 hours, or from about 1 minute to about 1 hour, or from about 1 minute to about 30 minutes. The reaction time may be from about 10 minutes to 3 hours, 10 minutes to 10 hours, preferably 15 minutes to 1 hour, or 15 minutes to 2 hours.
Various additives over and above the enzyme and peroxide treatment can be used in the process or use of the invention. Surfactants and/or dispersants are often present in, and/or added to a papermaking pulp. Thus the process and use of the present invention may be carried out in the presence of an anionic, non-ionic, cationic and/or zwitterionic surfactant and/or dispersant conventionally used in a papermaking pulp. Examples of anionic surfactants are carboxylates, sulphates, sulphonates or phosphates of alkyl, substituted alkyl or aryl. Fatty acids are examples of alkyl-carboxylates. Examples of non-ionic surfactants are polyoxyethylene compounds, such as alcohol ethoxylates, propoxylates or mixed ethoxy-/propoxyiates, poly-glycerols and other polyols, as well as certain block-copolymers. Examples of cationic surfactants are water-soluble cationic polymers, such as quartenary ammonium sulphates and certain amines, e.g. epichlorohydrin/dimethylamine polymers (EPI-DMA) and cross-linked solutions thereof, polydiallyl dimethyl ammonium chloride (DADMAO), DADMAC/Acrylamide co-polymers, and ionene polymers, such as those disclosed in U.S. Pat. Nos. 5,681,862; and 5,575,993. Examples of zwitterionic or amphoteric surfactants are betains, glycinates, amino propionates, imino propionates and various imidazolin-derivatives. Also the polymers disclosed in U.S. Pat. No. 5,256,252 may be used.
Also according to the invention, surfactants such as the above, including any combination thereof may be used in a paper making process. The amount of each surfactant in such composition may amount to from about 8 to about 40% (w/w) of the composition. In particular embodiments the amount of each surfactant is from about 10 to about 38, or from about 12 to about 36, or from about 14 to about 34, or from about 16 to about 34, or from about 18 to about 34, or from about 20 to about 34, or from about 22 to about 34, or from about 24 to about 34, or from about 26 to about 34, or from about 28 to about 32% (w/w).
It is to be understood that the term enzyme, as well as the various enzymes and enzyme classes mentioned herein, encompass wild-type enzymes, as well as any variant thereof that retains the activity in question. Such variants may be produced by recombinant techniques. The wild-type enzymes may also be produced by recombinant techniques, or by isolation and purification from the natural source.
In a particular embodiment the enzyme in question is well-defined, meaning that only one major enzyme component is present. This can be inferred e.g. by fractionation on an appropriate Size-exclusion column. Such well-defined, or purified, or highly purified, enzyme can be obtained as is known in the art and/or described in publications relating to the specific enzyme in question.
The term “applied together with” (or “used together with”) means that the additional enzyme may be applied in the same, or in another step of the process of the invention. The other process step may be upstream or downstream in the paper manufacturing process, as compared to the step in which the papermaking, pulp or process water is treated with lipase and peroxide source.
In particular embodiments the additional enzyme is an enzyme which has protease, xylanase, cutinase, oxidoreductase, cellulase, endoglucanase, amylase, mannanase, steryl esterase, and/or cholesterol esterase activity. Examples of oxidoreductase enzymes are enzymes with laccase, and/or peroxidase activity.
The term “a step” of a process means at least one step, and it could be one, two, three, four, five or even more process steps. Thus, the lipase and peroxide source may be applied in at least one process step, and the additional enzyme(s) may also be applied in at least one process step, which may be the same or a different process step as compared to the step where the lipase and peroxide source is used.
The term “enzyme preparation” means a product containing at least one lipase enzyme. In addition to the enzymatic activity such a preparation preferably contains at least one adjuvant. Examples of adjuvants, which are used in enzyme preparations for the paper and pulp industry, are buffers, polymers, surfactants and stabilizing agents.
Any enzyme having protease, xylanase, cutinase, oxidoreductase, cellulase endoglucanase, amylase, mannanase, steryl esterase, and/or cholesterol esterase activity can be used as additional enzymes in the use and process of the invention. Below some non-limiting examples are listed of such additional enzymes. The enzymes written in capitals are commercial enzymes available from Novozymes A/S, Krogshoejvej 36, DK-2880 Bagsvaerd, Denmark. The activity of any of those additional enzymes can be analyzed using any method known in the art for the enzyme in question, including the methods mentioned in the references cited.
Examples of cutinases are those derived from Humicola insolens (U.S. Pat. No. 5,827,719); from a strain of Fusarium, e.g. F. roseum culmorum, or particularly F. solani pisi (WO 90/09446; WO 94/14964, WO 94/03578), The cutinase may also be derived from a strain of Rhizoctonia, e.g. R. solani, or a strain of Alternaria, e.g. A. brassicicola (WO 94/03578), or variants thereof such as those described in WO 00/34450, or WO 01/92502.
Examples of proteases are the ALCALASE, ESPERASE, SAVINASE, NEUTRASE and DURAZYM proteases. Other proteases are derived from Nocardiopsis, Aspergillus, Rhizopus, Bacillus alcalophilus, B. cereus, B. natto, B. vulgatus, B. mycoide, and subtilisins from Bacillus, especially proteases from the species Nocardiopsis sp. and Nocardiopsis dassonvillei such as those disclosed in WO 88/03947, and mutants thereof, e.g. those disclosed in WO 91/00345 and EP 415296.
Examples of amylases are the BAN, AQUAZYM, TERMAMYL, and AQUAZYM Ultra amylases. An example of a xylanase is the PULPZYME HC hemicellulase. Examples of endoglucanases are the NOVOZYM 613, 342, and 476 enzyme products.
Examples of mannanases are the Trichoderma reesei endo-beta-mannanases described in Ståhlbrand et ai. J. Biotechnol. 29 (1993), 229-242.
Examples of steryl esterases, peroxidases, laccases, and cholesterol esterases are disposed in the references mentioned in the background art section hereof. Further examples of oxidoreductases are the peroxidases and laccases disclosed in EP 730641; WO 01/98469; EP 719337; EP 765394; EP 767836; EP 763115; and EP 788547. In the present, context, whenever an oxidoreductase enzyme is mentioned that requires or benefits from the presence of acceptors, enhancers, mediators and/or activators, such compounds should be considered to be included if not already present Examples of enhancers and mediators are disclosed in EP 705327; WO 98/56899; EP 677102; EP 781328; and EP 707637. If desired a distinction could be made by defining an oxidoreductase enzyme system (e.g. a laccase, or a peroxidase enzyme system) as the combination of the enzyme in question and its acceptor, and optionally also an enhancer and/or mediator for the enzyme in question.
These are particular embodiments of the present invention: Use of a lipase and peroxide source for reducing the deposition of pitch in the paper making process. A process for reducing deposition of pitch in the paper making process, wherein the process comprises treating the pulp and/or process wafer with a lipase and peroxide source. Use of a lipase, peroxide source and organic acid for reducing the deposition of pitch in the paper making process. A process for reducing deposition of pitch in the paper making process, wherein the process comprises treating the pulp and/or process water with a lipase, a peroxide source and organic acid.
The invention described and claimed herein is not to be limited in scope by the specific embodiments herein disclosed, since these embodiments are intended as illustrations of several aspects of the invention. Any equivalent embodiments are intended to be within the scope of this invention, indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description. Such modifications are also intended to fail within the scope of the appended claims. In the case of conflict, the present disclosure including definitions will control.
Various references are cited herein, the disclosures of which are incorporated by reference in their entireties.

EXAMPLES

Example 1

Deresination of Kraft Pulp Pitch by Candida antarctica Lipase B (CALB)

Dammar resin was obtained from Fluka and used as a model pitch to simulate Kraft pulp pitch. TAED was also obtained from Fluka. Put 75 mg of Dammar resin in a flask and add 75 mL of Dl water. Add chemicals or enzymes according to the conditions shown in Table 1. Stir at ambient temperature overnight. Turbidity of the solutions was determined by UV-vis at 600 nm.


Sample ID	Chemical/Enzyme	Dose per beaker	600 nm

1	Control		0.043
2	CALB	50 mg	0.222
3	Peroxide	25 mg	0.084
4	CALB and peroxide	50 mg/25 mg	0.847
5	Peroxide and TAED	25 mg/360 mg	0.501

After mixing the samples overnight, it was observed that the lipase (CALB) and peroxide treated sample turned into milky emulsion whereas the control sample was still as clear as water with the resins deposited either at the bottom on the wail of the of the flask. The turbidity results clearly showed that a combination of peroxide with lipase could lead to better emulsification of dammar resin. Peracetic acid generated in situ by peroxide and a bleach activator (TAED) was also fairly effective, but not as effective as peroxide and lipase combination.

Example 2

Deresination of Kraft Pulp Pitch by Resinase A2X and Resinase HT

The experiment was carried out in the same manner as illustrated in Example 1 except that the flasks were stirred overnight at 40° C. The lipases used in this study were Resinase® A2X and Resinase® HT (available from Novozymes A/S).


Sample ID	Chemical/Enzyme	Dose per beaker	600 nm

6	Control		0.104
7	Resinase ® A2X	50 mg/25 mg	0.178
8	Resinase ® HT	50 mg/25 mg	0.219
9	Resinase ® A2X and	50 mg/25 mg	0.535
	H₂O₂
10	Resinase ® HT and	50 mg/25 mg	0.614
	H₂O₂

It is evident that both Resinase® A2X and Resinase® HT worked to emulsify the Kraft model pitch.

Claims

1. A method for reducing pitch problems in a chemical pulp, comprising treating a chemical pulp with a lipase and a peroxide source in amount effective to reduce pitch problems.

2. The method of claim 1, wherein said treating is performed before, after or in between bleaching stages.

3. The method of claim 1, wherein said chemical pulp is a Kraft pulp.

4. The method of claim 1, wherein said chemical pulp is a sulfite pulp.

5. The method of claim 1, wherein said chemical pulp is bleached Kraft pulp.

6. The method of claim 1, further comprising the step of preparing paper, linerboard, corrugated paperboard, tissue, towels, corrugated containers or boxes from said pulp.

7. The method of claim 1, wherein said lipase is derived from a strain of Aspergillus, a strain of Achromobacter, a strain of Bacillus, a strain of Candida, a strain of Chromobacter, a strain of Fusarium, a strain of Humicola, a strain of Hyphozyma, a strain of Pseudomonas, a strain of Rhizomucor, a strain of Rhizopus, or a strain of Thermomyces.

8. The method of claim 1, wherein said lipase is derived from a strain of Candida.

9. The method of claim 1, wherein said lipase is derived from a strain of Candida antarctica lipase A or the Candida antarctica lipase B.

10. The method of claim 1, comprising treating a chemical pulp with a lipase, a peroxide source and an organic acid.

11. The method of claim 1, wherein peroxide source is hydrogen peroxide.

12. The method of claim 1, wherein peroxide source is a precursor of hydrogen peroxide.

13. The method of claim 12, wherein the precursor of hydrogen peroxide is perborate or a percarbonate.

14. The method of claim 1 wherein the peroxide source is an enzyme that converts molecular oxygen and an organic or inorganic substrate into hydrogen peroxide.

15-16. (canceled)