Treating lignocellulosic materials

Abstract: Treatment of lignocellulosic pulp with a solution of a protease enzyme is described. Alternatively, wood chips may be treated with a solution of protease enzyme prior to forming pulp from the chips.

Description: The present invention relates to the treatment of lignocellulosic materials, i.e. pulp or wood chips. JP-A-3220389 discloses a method of treating plants with a solution containing papain and/or pancreatin as enzymes and disodium adenosine biphosphate and/or protease derived from bacteria of the genus Bacillus to remove impurities without damaging the cellulose. The resultant pulp can be used as paper stock. EP-A-0430915 relates to a procedure for the production of mechanical pulp from a fibrous product. To reduce the refining energy, the fibrous product is subjected to an enzyme treatment in which an enzyme acts on the hemicellulose and/or cellulose in the fibrous product. In addition to reducing the refining energy consumption, the enzyme treatment also improves the strength properties and the blue reflectance factor of the pulp. As can be seen from the detailed description of examples hereinbelow, the invention enables enzymatic treatment of wood chips or wood pulps which enhances fibrillation and facilitates delignification and bleaching. Previous enzymatic and biopulping processes do not take into account the presence of extensin in cell walls. Extensin is a highly crosslinked protein, rich in proline and hydroxyproline which is not affected by xylanases and peroxidases such as ligninases. It is hypothesized that lignin is bound to extensin and functions as a supporting skeleton for cells. Extensin or lignin is most likely immobilized in cell walls during formation of the secondary wall. Partial degradation of extensin would, therefore, appear to be important in making the lignin more accessible for extraction in chemical pulping and bleaching or to reduce energy consumption and damage to cellulose fibers during mechanical pulping. In accordance with a first aspect of the present invention there is provided a process which comprises treating a lignocellulosic pulp with a solution of a protease enzyme, wherein the pulp has been manufactured under alkaline conditions or has been pretreated with an alkaline solution, the treatment with an alkaline solution enhancing the infiltration of the pulp by the protease enzyme. In a second and alternative aspect thereof, the invention provides a process which comprises treating wood chips with a solution of protease enzyme prior to forming pulp from said chips, wherein the chips have been pretreated with an alkaline solution, the pretreatment with an alkaline solution enhancing the infiltration of the chips by the protease enzyme. Wood chips or wood pulps may be treated at any stage of a papermaking process but preferably prior to delignification or bleaching with the protease enzyme to enhance fibrillation and/or the efficiency of delignification or bleaching. Our tests have shown that the proteases, pepsin and papain, promote the removal of lignin from wood, possibly through cleavage of the cell wall protein matrix. Our tests have also shown that wood pulps treated with proteases exhibit a higher degree of fibrillation than pulps not treated with proteases. Our tests have shown that protease treatment produces a flatter fibre which improves paper smoothness derived from these fibres. The protease treatment may be combined sequentially or by admixture with treatment with a xylanase or ligninase. The theory which underlies the invention (as to which the applicants do not desire to be bound) is that cellulose polymer occurs in plants in an interwoven matrix of pectins, xylans (hemicellulose), lignin and protein. The structural role of the proteins has been explained using a "warp-weft" model which predicts a crosslinked protein network of extensins, hydroxyproline-rich proteins (HPRPs) and glycine-rich proteins (GRP's) which may be bound to lignin and cellulose. By hydrolyzing any of these protein systems, lignin removal is enhanced. It is also theorized that extensin may bind the microfibrils making up the fiber which may explain why enhanced fibrillation is observed with the protease treatment. Depending on the nature of the wood chips or pulp, the wood chips or pulp may be treated directly with a solution of the protease enzyme. In other cases it may be desirable to pretreat the chips or pulp with an alkaline solution or a cellulase enzyme solution to remove materials which may coat the fibers. This enhances the access which the protease enzyme will have to the extensin in the fibers. One manifestation of the invention is a method which comprises retaining wood chips in a solution of a protease enzyme for a period of time and under conditions sufficient to enhance delignification, bleaching or fibrillation of a pulp prepared from the wood chips. Another manifestation of the invention is a method which comprises incubating cellulosic pulp in a solution of a protease enzyme for a period of time and under conditions which enhance delignification, bleaching or fibrillation of the pulp. While the enzymes used in our methods are generally referred to as proteases, those skilled in the art will recognize that not all proteases will be equally effective against all substrates under all conditions. Any protease which is capable of hydrolyzing extensins, HPRP's and GRP's should be effective. As demonstrated in Examples 1 and 2 at the time of filing good results in terms of enhanced lignin removal have been achieved with pepsin (CAS No. 9001-75-6) and papain (CAS No. 9001-73-4). However, bromelain (CAS No. 37189-34-7) and fungal protease have not been found to be as effective in lignin removal. Particularly preferred proteases include natural or synthetic proteases which are active under less acidic or more basic conditions because acidic conditions tend to degrade pulp. For example, papain is active at a pH of 6.2 and is preferred to pepsin which requires a pH of 2.0. Preferred proteases exhibit their maximum activity at a pH greater than 4.0 and more preferably greater than 5.0. Even more preferred are proteases active under alkaline pH conditions. Representative examples of proteases useful in practical examples of our methods include subtilisins (CAS No. 9014-01-1), thermolysins (CAS No. 9073-78-3), esperase (CAS No. 9073-77-2), AlPase (CAS No. 9001-12-1) proteases exhibiting collagenase activity, proteases which hydrolyze gelatin, animal hoofs, horns, hides, ligaments, tendons or cartilage or hydrolyze other substrates which have a glycine and hydroxyproline content similar to that of plant cell wall proteins, proteases from sheep foot rot organisms, proteases from fungi that colonize wood such as blue stain fungi, white rot fungi and brown rot fungi. Suitable pulps for the practice of our methods include hardwood, softwood and other lignocellulosic pulps. By way of example, mechanical, themomechanical, chemimechanical, sulfite, kraft, soda and modified sulfite pulps may be used. The protease treatment can be carried out in any vessel of the desired size with provision for mixing and controlling the temperature of the contents. Order of addition of reactants is not critical. The basic reaction mixture comprises pulp or wood chips in water at a pH appropriate for the enzyme or enzyme mixture used. In the case of a lignocellulosic pulp the reaction mixture may range from about 1 to 16% in consistency. The protease is present in a ratio of about 1 to 1,000,000 units per gram dry pulp. One unit of protease will hydrolyze casein to produce color equivalent to 1µ mol of tyrosine per min. at 37°C (color by Folin-Ciocalteu Reagent). The reaction mixture is incubated at 20 to 80°C for about 0.1 to 6 hours. Those skilled in the art will be able to readily optimize reaction conditions for the particular enzyme system without undue experimentation. A xylanase, ligninase, pectin esterase, pectin lyase, or manganese peroxidase may also be used simultaneously or as a pretreatment or post treatment. Generally, these enzymes appear to make the extensin more accessible. For reaction of the xylanase reference can be made to International Application WO-A-91/05908. For reaction of ligninase see EP-A-0 418 201. Wood chips may be advantageously treated by soaking them in a solution of the enzyme. However, the chips are preferably destructured by passage through a screw press as described in US-A-4,869,783 to Prusas. By destructuring and compressing the chips and allowing the chips to expand in a solution of the enzyme, effective impregnation and infiltration of the chips is achieved. Treatment of pulp may be coupled with any delignification or bleaching process to enhance the efficacy of those processes. Among other processes that may be coupled with our present treatments in making paper or board are oxygen delignification, hydrogen peroxide extraction and bleaching, chlorine dioxide bleaching, chlorine and chlorine dioxide bleaching, etc. The particular sequence of treatment is open. Any sequence including at least one stage in which chips or pulps are incubated with a protease are useful herein. The protease treatment may be positioned to advantage at any stage of the process sequence. However, for maximum efficacy in delignification, the protease treatment must precede the delignification and bleaching stage. It is generally desirable to pretreat pulps to remove materials which may be deposited on the paper fibers and which may exhaust the protease. Many of these materials are alkaline soluble and, hence, it is desirable to wash the pulp with a sodium hydroxide solution (pH about 11 to 14) prior to the protease treatment. Where the pulp is manufactured under alkaline conditions, this may not be necessary. For example, alkaline pretreatment of kraft pulps is not necessary. Treatment with a surfactant or detergent may be used to enhance penetration of the protease into the fiber pores and to enhance washing of impurities and interfering substances. Pores may be opened by cellulase or pectinase pretreatments. Also treatment with chelators to remove metals may enhance penetration. The alkaline pretreatment is generally carried out at 10 to 80°C using about 5 to 200 parts alkali per 100 parts dry pulp. Another effective pretreatment is carried out at an alkaline pH and using about 50 to 5,000 cellulase units per 100 grams pulp or chips. The invention is illustrated in more detail by the following non-limiting examples. Example 1 Cellulase Pretreatment Refiner mechanical pulp (RMP) (750 g, 14% consistency) was mixed with 5 liters 0.1 M sodium acetate, pH 5 and 20 g cellulase from Penicillium funiculm (134,000 units, Sigma Chemical Co.) and stirred for 2 h at room temperature. The slurry was filtered, stirred with 1 liter 2 M sodium hydroxide for 30 minutes, filtered, washed with the same sodium hydroxide solution, filtered, washed with water three times and divided into 10 equal parts. Example 2 Protease Treatment Cellulase treated and alkaline washed pulp (40 g) was equilibrated in 500 ml of the buffers shown in Table 1. The pH of the solutions were adjusted to the indicated level after addition of the pulp from Example 1. Proteases (2 g) were added to each slurry according to Table 1. Protease Treatment of Wood Pulp Protease Buffer/pH (100 mM) Enzyme Units (Sigma Chemical Co.) Control (Ex.1) Phosphate, 6.2 0 Pepsin Phosphate, 2.0 250,000 Bromelain Citrate, 4.5 3,750,000 Fungal Protease Phosphate, 2.8 1,200 Papain Phosphate, 6.2 5,600 The protease treatment was conducted by stirring 1 hour at room temperature. The pulp was filtered, extracted with 100 ml 4% sodium borate in 2 M sodium hydroxide, pH 12, followed by 100 ml 2 M sodium hydroxide. Lignin removal was measured by determining the absorbance (OD) at 280 nm of each extract. The results are shown in Table 2. Protease Borate Extraction (OD 280 nm Units) Hydroxide Extraction (OD 280 nm Units) % Control Control 15.4 .34 100 Pepsin 21.4 .80 141 Bromelain 5.2 1.90 45 Fungal Protease 3.1 .64 23 Papain 5.1 1.02 39 Results from Table 2 show that significantly more lignin is removed by treatment with pepsin than was removed from the untreated pulp. Some of the differences shown may be a function of extraction time and the pH of the control (6.2) incubation. Example 3 below examines effect of extraction time and pH on the comparison of lignin removal by protease treatment. Example 3 Example 3 was conducted as Example 2 using pepsin and papain only. However, the cellulase treated pulp was reduced to 30 g wet per test. Extraction time was lengthened to 85 minutes for the borate extraction and 100 minutes for the hydroxide extraction. The results are shown in Table 3. Protease Borate Extraction (OD 280 nm Units) Hydroxide Extraction (OD 280 nm Units % Control Control, pH 2 1.36 1.87 100 Pepsin, pH 2 6.28 2.15 261 Papain, pH 6.2 2.80 2.18 154 Data shown indicated that for pepsin in particular the action of this enzyme enhances solubilization of lignin up to nearly 3-fold. These results show that proteases can promote removal of lignin from wood, possibly through cleavage of the cell wall protein matrix. Example 4 The purpose of this experiment is to demonstrate the effect of pretreatment on lignin removal. One sample of refined mechanical pulp was prepared without pretreatment. A second sample was prepared and washed in 2M NaOH. A third was treated with cellulase and washed with 2M NaOH. Each sample was treated with papain (phosphate buffer, pH = 6.2) in the amounts shown in Table 3 followed sequentially by a water wash to remove protein and a borate extraction at pH = 12 to remove lignin which separates the 280nm contribution of the protein from that of the lignin and any other carbohydrates. The results are shown in Table 4 as percent of control (i.e., no protease enzyme). Effect of Pretreatments on OD 280 (Lignin) Extractibles from Refiner Mechanical Pulp. Percent Over Control (No Enzyme) - Borate Extraction OD 280 Papain (grams) No Pretreatment Base Wash 2 M NaOH Pretreatments Cellulase Pretreatment + 2 M NaOH Wash 0.5 0.7 39 61 1.0 3.1 74 160 1.5 22 77 165 2.0 12 98 216 Example 5 The effect of incubation time was determined by treating a mechanical pulp which had been treated with 2M NaOH pretreatment and incubated with 1g papain under the time conditions shown in Table 5. Effect of Time of Incubation with Papain on Base Extractible from Refiner Mechanical Pulp. Time of Papain Incubation Minutes Percent Increase in 280nm Extractibles Over Zero Time Control 15 111 30 134 45 160 60 130 The results in Table 5 show that the occurrence of increased extractibles is not an artifact of the treatment, but rather a time-dependent result of papain catalysis. Example 6 A kraft brownstock was treated with papain or bromelain without pretreatment under the condition shown in Table 6. Amount of Protease Grams Increase in Extractibles over Control (%) Papain: 0.5 50 1.0 83 1.5 94 2.0 202 Bromelain: 0.5 22 1.0 17 1.5 33 2.0 56 The results in table 6 show an increased effect of protease concentration on release of lignin from Kraft Brownstock pulp without further pretreatment. Both bromelain (protease from pineapple stems) and papain are effective, but the superior selectivity of papain is illustrated here.

Claims: A process which comprises treating a lignocellulosic pulp with a solution of a protease enzyme, wherein said pulp has been manufactured under alkaline conditions or has been pretreated with an alkaline solution, said treatment with an alkaline solution enhancing the infiltration of said pulp by said protease enzyme. A process according to Claim 1, wherein said treatment is carried out at a temperature of about 20 to 80°C. a process according to Claim 1 or Claim 2, wherein said pretreatment includes treating said pulp with sodium hydroxide solution. A process according to any one of the preceding claims, wherein said pretreatment includes treating said pulp with a cellulase enzyme solution. A process according to any one of the preceding claims, wherein said process includes the additional step of treating said pulp with a xylanase enzyme. A process according to any one of the preceding claims, wherein said protease is capable of hydrolyzing extensin, hydroxyproline-rich protein or glycine-rich proteins. A process according to Claim 6, wherein said protease is active at pH greater than 4.0. A process according to Claim 7, wherein said protease is papain. A process according to any one of Claims 1 to 5, wherein said protease is selected from subtililisins, thermolysins, esperase, Alpase, collagenases, proteins from sheep foot rot organism, blue stain fungi, white rot fungi and brown rot fungi. A process according to any one of the preceding claims, wherein said pulp is a mechnical pulp. A process according to any one of the preceding claims, wherein said process further comprises delignification and/or bleaching of said pulp after said treatment with said protease enzyme. A process according to Claim 11, wherein said delignification or bleaching process is selected from oxygen delignification or bleaching, hydrogen peroxide extraction or bleaching, and chlorine or chlorine dioxide bleaching. A process which comprises treating wood chips with a solution of protease enzyme prior to forming pulp from said chips, wherein said chips have been pretreated with an alkaline solution, said pretreatment with an alkaline solution enhancing the infiltration of said chips by said protease enzyme. A process according to Claim 13, wherein said chips are destructured prior to contacting said chips with said protease enzyme.

Inventors: JOHNSON MARK A, POKORA ALEXANDER R, JOHNSON, MARK A., POKORA, ALEXANDER R.
Applicants: MEAD CORP, THE MEAD CORPORATION
Application year: 1992
Main classification: D21C5
IPC codes: C12P21, A61P25, A61K9, D21C5