METHOD FOR THE PRODUCTION OF XYLITOL

Abstract: The invention relates to a method for the production of xylitol, the method comprising (a1) providing (i) a microorganism having xylanolytic activity, and (ii) a microorganism capable of converting a pentose sugar to xylitol; or(a2) providing a microorganism having xylanolytic activity and being capable of converting a pentose sugar to xylitol, (b) culturing the microorganism of step (a1) (i) or the microorganism of step (a2) in a medium comprising polymer or oligomer materials containing pentose sugars in conditions sufficient for enabling hydrolysis of said polymers or oligomers by the microorganism; (c) producing xylitol in the microorganism of step (a1) (ii) or in the microorganism of step (a2) by bioconversion of the hydrolysis products obtained in step (b), and (d) recovering said xylitol produced. The invention also relates to a microorganism, which has xylanolytic activity and has been genetically modified (i) to enhance its xylanolytic activity, and (ii) to reduce its xylitol metabolism.

Description: Method for the production of xylitol Field of the invention The invention relates to utilisation of microorganisms in industrial processes for the production of xylitol. In particular, the invention relates to genetic modification of microorganisms in order to enhance their potential as xylitol producers, and to a process for the production of xylitol using said microorganisms. Background of the invention Xylitol is usually prepared by processes in which a xylan-containing material is first hydrolysed to produce a mixture of monosaccharides, including xylose. The xylose is then converted to xylitol, generally in a chemical process using a nickel catalyst such as Raney-nickel. A number of processes of this type have been described in the literature of the art. U.S. Patent Nos. 3,784,408 (Jaffe et al.), 4,066,711 (Melaja et al.), 4,075,406 (Melaja et al.), 4,008,285 (Melaja et al.) and 3,586,537 (Steiner et al.) may be mentioned as examples. Alternatively, xylitol can be produced from glucose. U.S. Patent No. 5,096,820, Leleu et al., describes a process in which glucose is microbiologi- cally converted to arabitol, which likewise is microbiologically converted to xy- lulose. The xylulose is then enzymatically isomerised into a mixture of xylose and xylulose, which is catalytically hydrogenated. Finally, the xylitol is recovered by chromatographic separation or crystallisation. U.S. Patent No. 5,238,826, Leleu et al., uses a similar process to obtain xylose, ultimately for the preparation of xylitol by hydrogenation. The xylose is produced by microbiological conversion of glucose via arabitol to xylulose, which is then enzymatically isomerised into a mixture of xylose and xylulose. Finally, the mixture is subjected to chromatographic separation and the xylulose fraction is recirculated into the isomerisation step. These prior methods are, however, technically complicated multi- step processes which have relatively low efficiency. The greatest problems reside in achieving an effective and complete separation of xylose from other hydrolysis by-products. Thorough purification is essential, because the catalysts used in the reduction reaction of xylose are very sensitive. As regards the purity of the final product, the separation of xylitol from the other products produced in the reduction reaction will greatly depend on the amount of other components present. The more components, the more complicated and time- consuming purification and separation processes will be needed. Altogether, the production of xylitol by these processes is very costly. Several attempts to utilise microorganisms for the biotechnological production of xylitol have also been reported. A main advantage of using microorganisms is that they are not as susceptible to varying reaction conditions as chemical catalysts. The production of xylitol by means of a biotechnological process is thus a highly attractive alternative, provided that such processes are able to provide a high quality product by a comparatively cost-effective method. It is known that many yeast strains produce reductase enzymes that catalyse the reduction of sugars to corresponding sugar alcohols. Many yeasts, in particular Pichia, Candida, Hansenula and Kluyveromyces, are also capable of reducing xylose to xylitol as an initial step in their xylose metabo- lism, and several yeast strains are able to use xylose as a sole source of carbon and energy. The reaction route or pathway of xylose utilisation for yeasts is in general the following: xylitol is synthesised in the first step by reduction of xylose to xylitol with the aid of xylose reductase. Xylitol is then metabolised by a series of successive steps. Xylitol is first oxidised to xylulose with xylitol dehy- drogenase, xylulose is phosphorylated to xylulose-5-phosphate with xylulose kinase (also called xylulokinase), and then part of the xylulose-5-phosphate is converted to pyruvate via several intermediate steps. Also ethanol and CO 2 can be formed. The relevant main products and by-products vary depending on the yeast strain and the fermentation conditions. The reactions are not tightly coupled, and consequently, some xylitol is often accumulated. It has also been reported that some bacteria, such as Corynebacte- rium sp. and Enterobacter liquefaciens, produce xylitol by the same pathway, utilising xylose as the main metabolite. However, the yields obtained have been very poor. So far, only one species of filamentous fungus, Petromyces albertensis, has been shown to produce notable amounts of xylitol from xylose [Winkelhausen E. and Kuzmanova S., J. Ferment. Bioeng. 86:1-14 (1998)]. Hence, prior to the present invention, there has been no incentive to use bacteria or filamentous fungi for xylitol production. Also as regards yeasts, the research has generally focused on attempts to identify yeast strains with enhanced ability to produce ethanol rather than xylitol. Nevertheless, xylitol production is relatively common among xylose-utilizing yeasts. For example, of 44 yeasts belonging to five different genera, Candida, Hansenula, Kluyveromyces, Pichia and Pachysolen, 42 pro J duced some xylitol into the culture media [Barbosa. M.F.S. et al., J. Indust. Mi- crobiol. 3241-3251 (1988) and Enzyme Microb. Technol. 10:66-81 (1988)]. It has been suggested to use such strains for the industrial production of xylitol. PCT publications WO 90/8193, WO 91/0740, WO 88/5467 and French published application 2 641 545 describe the use of Candida tropicalis, Candida guilliermondii and Candida parapsilosis, respectively. U.S. Patent No. 5,081 ,026, Heikkila et al., describes a process for the production of xylitol from xylose, in which an aqueous xylose solution is fermented with a yeast strain capable of converting free xylose to xylitol and free hexoses to ethanol. After fermentation, a xylitol-rich fraction is obtained by chromatographic separation, and finally, xylitol is recovered from said frac- tion. In particular Candida and Debaryomyces are mentioned as suitable yeast species. Profitable industrial production of xylitol by enzymatic bioconversion of xylose is possible only if the yield is high. No wild yeast strains have been shown to achieve this. An attempt to solve this problem has been described in WO 91/15588, Hallborn, J. et al. The inventors cloned the xylose reductase gene from Pichia stipitis into Saccharomyces cerevisiae and obtained strains capable of converting xylose into xylitol with a claimed yield of 95 % [Hallborn et al., Bio/Technology 9:1090 (1991)]. Saccharomyces cerevisiae does not normally express enzymes of the xylose pathway, but is widely accepted and commonly used in the food industry for other purposes, for example in bakery. Gong C. et al., Biotechnol. Letters 3:125-130 (1981) describe two high xylitol producing yeast mutants denominated HXP 1 and HXP 2, obtained after UV-mutagenesis of a wild strain of Candida tropicalis which originally was capable of metabolising D-xylose into xylitol. Mutants defective in xylose utilisation have also been described. Hagedorn J. et al., Curr. Genetic. 16:27-33 (1989) disclose mutants of the yeast Pichia stipitis which were unable to utilise xylose as the sole carbon source and which were deficient in either xylose reductase or xylitol dehydro- genase. Stevis, P.E. et al., Appl. Biochem. Biotechnol. 20:327-334 (1989) dis- close the construction of yeast xylulokinase mutants by recombinant DNA techniques. EP 0 604 429, Xyrofin, describes novel yeast strains with modified xylitol metabolism, a process for the production of said strains, and the use of said strains in a process for producing xylitol. The strains are capable of reducing xylose into xylitol, but are deficient in one or more enzymes involved in the xylitol metabolism, with the effect that the xylitol produced accumulates in the culture medium and can be recovered therefrom. The yeasts described belong to the genera Candida, Hansenula, Kluyveromyces or Pichia, and the genetic modification eliminates or reduces expression of the gene that encodes xylitol dehydrogenase or xylulose kinase, or both. EP 0 672 161 , Xyrofin, describes a method for the production of xylitol from carbon sources other than xylose and xylulose by using recombinant hosts. The microorganisms either produce xylitol via an altered arabitol route involving in particular arabitol dehydrogenase, or via altered (over)expression of genes encoding the enzymes of the oxidative branch of the pentose phosphate pathway (PPP), in particular glucose-6-phosphate dehydrogenase or 6-phospho-D-gluconate dehydrogenase, thus enabling utilisation of glucose, for instance. When used, D-glucose is phosphorylated into D- glucose-6-phosphate and converted to D-ribulose-5-phosphate via 6-phospho- D-gluconate. The D-ribulose-5-phosphate is then epimerised to D-xylulose-5- phosphate, dephosphorylated to D-xylulose and reduced to xylitol. Amplification of glucose-6-phosphate dehydrogenase enzyme activity in osmotolerant yeasts is also described in FR 2 772 788, Roquette Freres. EP 0 974 646 A describes microorganisms capable of producing xylitol, or xylulose, from glucose. The microorganisms are naturally occuring and belong to the family Acetobacteracea. Novel strains of genus Asaia and Zucharibacter are mentioned as preferred. Another approach that could be taken in the bioproduction of xylitol is the enhancement of xylose production, thus providing more xylose as the primary metabolite for xylitol production. Some fungi, including Aureobasidium, Aspergillus, Trichoderma, Fusarium and Penicillium, have been reported to have xylanolytic activity and thus be able to degrade xylan-containing biopolymers and metabolise the x - Ian into xylose. In addition, several yeast species have been thoroughly stud- ied, but their hydrolytic activity has not been the main target for the studies and has not been applied in large-scale industrial processes. Kuhad R. C. et al., Process Biochemistry 33:641-647 (1998) describe a hyperxylanolytic mutant strain of Fusarium oxysporum produced by UV and N-methyl-N'-nitro-N-nitrosoguanidine (NTG) treatment, and the enhancement of its xylanase production by optimisation of several nutritional and fermentation parameters, including temperature, pH, substrate and inoculum size. Some T choderma strains have also been shown to be efficient producers of hemicellulases, in particular xylanase. The literature of the art also includes several reports on attempts to provide induction of β-xylosidase production during fermentation. Kristufek D. et al., Appl. Microbiol. Biotechnol. 42:713-717 (1995), using xylose, xylobiose and xylan as inducers for β- xylosidase induction in T choderma reesei, and Margolles-Clark E. et al., J. Biotechnol. 57:167-179 (1997), studying the expression of ten hemicellulase- encoding genes and using e.g. cellulose, xylobiose, xylan and L-arabitol as inducers, may be mentioned as examples. The enhancement of hemicellulase activity by various gene manipulation techniques have also been reported. For instance Margolles-Clark E. et al., Appl. Environ. Microbiol., October 1996, 3840-3846, describe the isolation of the genes encoding β-xylosidase and -L-arabinofuranosidase from Trichoderma reesei and cloning and expressing said genes in Saccharomyces cerevisiae. WO 97/00964, Rijksland-Bouwuniversiteit Wageningen, describes a novel β-xylosidase from Aspergillus niger, the nucleotide sequence encoding it and its use especially as a bread improver. Aspergillus, Trichoderma and Fusarium are mentioned as host cells. Hodits R. et al., ECB6: Proceedings of the 6 th European Congress on Biotechnology, 13 th to 17 th June 1993, report on recombinant Trichoderma reesei strains producing improved and tailor-made xylanases, including a method of amplifying a gene encoding β-xylosidase. U.S. Patent No. 5,837,515, Nevalainen et al., discloses a method for overproduction of hemicellulases, including endoxylanase, β- xylosidase, α-arabinosidase, α-D-glucuronidase and acetyl esterase, by cultivating Trichoderma reesei strains at least partially deficient in expressing cel- lulase enzyme(s) and transformed to include multiple copies of the gene en- coding the desired enzyme(s). Also bacteria have been studied in this respect. As an example may be mentioned Paice et al., Biotechnology and Bioengineering 32:235-239 (1988) describing in the article "Viscosity-enhancing bleaching of hardwood kraft pulp with xylanase from a cloned gene" Esche chia coli strains capable of overproduction of endoxylanase and β-xylosidase. For instance some Bacillus and Clostridium strains have also been reported to have xylanolytic activity. The background art thus describes the production of xylitol from xylan by multistep chemical processes or by multistep combinations of chemi- cal and biological processes. Further, processes utilising microorganisms, in particular yeasts, capable of producing xylitol from monosaccharide solutions or pure xylose solutions have been described. So far, no industrial processes for the production of xylitol from complex xylan-containing materials solely by means of a biotechnological method have been described, despite the fact that such a process would offer several advantages as compared to conventional processes, as far as efficient utilisation of the raw material and efficient production of xylitol in a cost-effective manner is concerned. There thus remains a constant need for further research and development in this area, with the aim of achieving efficient bioproduction of xylitol. Summary of the invention The present invention is based on a novel and inventive combination of two features: it combines utilisation of xylanolytic activity with restricted or inhibited xylitol metabolism. In one main embodiment of the invention, two microorganisms are used for the production of xylitol, one microorganism possessing xylanolytic activity and the other possessing the enzymatic activity needed for conversion of a pentose sugar, such as xylose and arabinose, preferably xylose, to xylitol. In a preferred embodiment of the invention, the xylanolytic microorganism is a mold. In another preferred embodiment, the microorganism possesses enhanced xylanolytic activity as a result of being genetically modified. The microorganism responsible for the enzymatic conversion of a pentose sugar, such as xylose and arabinose, preferably xylose to xylitol is, preferably, a yeast. In a preferred embodiment of the invention, said microor- ganism has reduced ability to metabolise xylitol. In one embodiment of the present invention, the reduced ability to metabolise xylitol is a result of genetically modifying the expression of one or more enzymes participating in the xylitol metabolism to reduce said metabolism in said microorganism. Alternatively, a microorganism can be used which in its natural form does not produce xylitol, or produces xylitol via a different pathway not utilising xylose as the main metabolite, and in which the necessary pathways have been introduced by genetic engineering. For the production of xylitol, the two microorganisms can be used simultaneously or successively or separately. Simultaneous production of both xylose and xylitol is regarded as preferred, due to less inhibitory activity, especially on the xylose production by accumulated xylose. In the other main embodiment of the invention, one microorganism, possessing both xylanolytic activity and the enzymatic activity needed for the conversion of xylose to xylitol, is used. Preferably, the microorganism is a mold or a yeast. More preferably a xylanolytic microorganism is used, which has been genetically modified to cause overproduction of the xylanolytic activity and reduced ability to metabolise xylitol. Also in this embodiment of the invention, it is possible to use a mi- croorganism which lacks xylanolytic activity in its natural form but in which genes encoding xylanolytic activity have been introduced. In a corresponding manner, it is also possible to use a microorganism which in its natural form does not produce xylitol, or produces xylitol via a different pathway, and introduce the necessary pathways by genetic engineering. The invention thus relates to a method for the production of xylitol, the method comprising (a) providing (i) a microorganism having xylanolytic activity, and (ii) a microorganism capable of converting a pentose sugar to xylitol; (b) culturing the microorganism of step (a) (i) in a medium compris- ing polymer or oligomer materials containing pentose sugars in conditions sufficient for hydrolysis of said polymers or oligomers; (c) producing xylitol in the microorganism of step (a) (ii) by biocon- version of the hydrolysis products obtained in step (b), and (d) recovering said xylitol produced. The invention further relates to a method for the production of xylitol, the method comprising (a) providing a microorganism having xylanolytic activity and being capable of converting a pentose sugar to xylitol, (b) culturing the microorganism of step (a) in a medium comprising polymer or oligomer materials containing pentose sugars in conditions suffi- cient for enabling hydrolysis of said polymers or oligomers by the microorganism; (c) producing xylitol in the microorganism of step (a) by bioconver- sion of the hydrolysis products obtained in step (b), and (d) recovering said xylitol produced. The invention also relates to genetically modified novel microorganisms which have xylanolytic activity and enhanced xylitol production as compared to the corresponding wild-type microorganisms, due to reduced ability to metabolise xylitol. In a preferred embodiment, the microorganism also has enhanced ability to produce xylose as a starting material for xylitol production. In particular, the microorganism has xylanolytic activity and has been genetically modified to enhance its xylanolytic activity and reduce its xylitol metabolism. The invention further relates to a process for the production of said genetically modified microorganism, said process comprising (i) providing a xylanolytic microorganism; (iia) modifying the expression of one or more enzymes participating in the xylitol metabolism to reduce said metabolism in said microorganism; and optionally (iib) modifying the expression of one or more enzymes participating in the xylanolytic acitivity to enhance said xylanolytic activity in said microorganism. Drawings Figure 1 illlustrates the structure of plasmid B1073. Figure 2 illustrates the construction of plasmid pMI88. Figure 3 illustrates the construction of plasmid pMI96. Detailed description of the invention The basis for the present invention is the unique concept of using one or more microorganisms having both xylanolytic activity and ability to con- vert a pentose sugar, such as xylose or arabinose, preferably xylose, to xylitol, either in combination or separately, for xylitol production. In particular, the in- vention uses xylanolytic microorganisms which due to genetic modification show overproduction of xylanolytic activity and blocked or reduced xylitol metabolism. Thus, in accordance with the present invention, the production mi- croorganism has xylanolytic activity and can hydrolyse pentose-sugars- containing materials into metabolisable forms of pentose sugars. Furthermore, it preferably comprises at least one genetic modification leading to enhanced xylanolytic activity as compared to a corresponding unmodified organism. The present invention also utilises a microorganism having the ability to convert a pentose sugar, such as xylose or arabinose, preferably xylose, to xylitol and preferably comprising at least one genetic modification leading to reduced capability to metabolise xylitol as compared to a corresponding unmodified organism. Preferably, one and the same microorganism includes both types of genetic modification. Lignocellulosic materials are very abundant in nature and offer both renewable and cheap carbohydrate sources for microbial processing. The major components in lignocellulosic materials are cellulose, hemicellulose and lignin, the amounts and proportions of which vary with plant species. The hemicellulose portion of plant cell walls contains as a major component xylan, constituting up to 35% of the total dry weight of higher plants. Xylan consists of a homopolymeric backbone chain of beta-1 ,4-linked D-xylose units and short side chains containing e.g. arabinose, glucuronic acid or its O-methyl-ether, acetic acid, coumaric acid and ferulic acids. Lignocellulosic materials thus contain a complexity of sugars with glucose as the main hexose and xylose as the main pentose, and with arabinose, mannose and galactose as minor components. Hemicellulosic materials also comprise several monosaccharides, xylose usually being the main component and arabinose, glucose, mannose and galactose being minor components. For example, in soft wood, mannose is the main component of the hemicellulosic material. In addition to by- products from paper and pulp industry, such as spent liquor and wood hydro- lysates, also agricultural by-products, such as sugar bagasse, corn cobs, corn fiber, oat, wheat, barley and rice hulls and straw and hydrolysates thereof contain high amounts of pentosans, and can be utilised as starting materials in the present invention. As starting material, arabane or galacturonic acid con- taining polymeric materials can also preferably be utilised. One of the advantages of the present invention is the possibility to use these abundant sources of xylan-containing biomass for the production of xylose to be further metabolised to xylitol. This advantage is achieved by using a microorganism having xylanolytic activity in accordance with the present in- vention. The raw material may be pretreated before use when appropriate. Suitable pretreatment methods include hydrolysis, steam explosion and separation techniques. For instance when wood is utilised as the starting material, it is preferably ground into chips, sawdust, shavings and the like, and sub- jected to hydrolysis, or steam explosion and hydrolysis. Before fermentation it may also be appropriate to remove constituents which may be toxic or otherwise harmful to the microorganism used, or which may interfere with subsequent separation and/or purification steps. Preferably, the raw material is dissolved, or converted into a dissolved form, upon use. In connection with the present invention, "xylanolytic activity" means any enzymatic activity capable of catalysing the breakdown of hemicelluloses, pentosan-containing material and xylan-containing material, including e.g. pentosanases, xylanases, such as 1 ,4-beta-D-xylanohydrolase, hemicellulases, glycosidases, xylosidases, such as β-xylosidase, arabinosidases and arabinanases, glucuronidases, acetylxylan esterases, ferulic acid esterases, coumaric acid esterases, mannanases and mannosidases, galactanases and galactosidases, rhamnosidases, galacturonidases. The most important enzyme activities in the utilisation of xylan and/or xylo-oligosaccharide containing plant-derived materials are those performed by endoxylanases and β- xylosidases, the other activities being mainly auxiliary in nature. The xylanolytic microorganism used in the present invention can be any microorganism which comprises xylanolytic activity, either as an inherent or added property. Suitable microorganisms can be found among species belonging to fungi, i.e. yeasts and molds, and bacteria. As examples may be mentioned e.g. Trichoderma, Aspergillus, Aureobasidium, Pichia, Candida, Debaryomyces, Hansenula, Kluyveromyces, Pachysolen, Saccharomyces, Schizosac- charomyces, Streptomyces, Bacillus and Clostridium. In the context of the present invention, yeasts and filamentous fungi are regarded as preferred micro- organisms. In view of its excellent xylanolytic activity, Trichoderma sp., and in particular Trichoderma reesei and Trichoderma longibrachiatum, are regarded as most preferred. Within the framework of the present invention it is possible, as already mentioned, to use microorganisms which lack xylanolytic activity in their natural form but in which genes encoding xylanolytic activity have been introduced. In accordance with the present invention, the xylanolytic microorganism preferably contains a metabolic modification which results in overex- pression of xylanolytic activity. The xylanolytic activity can be enhanced by a number of modifications, such as overexpression of β-xylosidase and other hemicellulolytic activities. Overexpression can be achieved both by introducing multiple copies and amplification of the microorganism's homologous genes, by introducing het- erologous genes and by altering the regulation of the expression of the genes. Examples of genes encoding xylanolytic activity and appropriate for use in the present invention include, but are not limited to, β-1 ,4-xylanohydrolase, β- xylosidase, α-arabinofuranosidase and arabinanase. Overexpression of the microorganism's homologous genes encoding β-xylosidase is regarded as the most preferred embodiment. Also the xylitol metabolism can be altered in a multitude of ways. By using a xylanolytic microorganism one can assure that more xylose is available for further metabolism into xylitol, and thereby the amount of xylitol produced will be higher. Enhanced production of xylitol is very likely to lead to accumulation of xylitol into the cell, and enhanced secretion of xylitol from the cell is easily accomplished. An even more efficient measure is naturally to hamper the xylitol metabolism and thus reduce or prevent loss of xylitol produced. This can be made by inactivating, blocking, deleting or otherwise modifying the enzyme activities responsible for the further utilisation of xylitol by the cell, either on enzyme level or by acting directly on the genes encoding the enzyme activities, for instance by altering the substrate specificity cofactor requirements, or specific activity, by protein engineering. As appropriate targets for such modifications may be mentioned xylitol dehydrogenase and xy- lulokinase. As already mentioned, it is also possible to introduce a xylitol pro- ducing pathway in organisms lacking such, for instance by introducing xylose reductase and permease, or L-arabinose reductase, arabinitol dehydrogenase (L-xylulose forming) and xylitol dehydrogenase genes into such an organism. In cases where the microorganism includes an alternative pathway for xylitol production, this can be blocked, when appropriate. The xylanolytic microorganism can further be modified in a number of different ways aiming at further intensifying the xylitol production. Such auxiliary modifications include, but are in no way limited to, overexpression of the genes mentioned above. The genetic modifications can be made using conventional methods for genetic modification, such as those described in Maniatis et al. Molecular cloning. A laboratory manual, Cold Spring Harbor Laboratory, New York 1982. In a preferred embodiment, the present invention hence utilises a xylanolytic microorganism that has been genetically modified so as to produce more pentose sugars for production of xylitol or alternatively, or in addition, to inhibit further metabolism of the xylitol produced. The microorganism is able to perform all steps needed in the process, by using other sugars for the maintaining of growth, from the hydrolysis of the pentosan-containing oligomers or polymers, via the enhancement of xylose production to the final conversion of xylose to xylitol. When appropriate, some of the reactions can be carried out chemi- cally, such as by using chemical reduction of arabinose to arabinitol to be further converted to xylulose and finally to xylitol. Another possibility is to utilise arabanane as raw material, after reduction of which a mixture of xylulose and arabinitol is obtained, from which xylitol can be produced. This process is preferably combined with recirculation of the obtained arabinitol into the process. In addition, or alternatively, some of the reaction steps can be further intensified by externally adding suitable microorganisms and other reagents, for instance commercial enzyme products, cofactors and cosubstrates, or hexoses or pentoses in a utilisable form. The xylitol yield will naturally also depend on the fermentation con- ditions and the medium used. One of the critical parameters is the availability of necessary coenzymes, in particular pyridine nucleotide cofactors. As other key parameters the substrate concentration and oxygen supply can be mentioned. Also other cofactor, cosubstrate and nutritional requirements, availability of hexose sugars for growth, presence of inducers or inhibitors, tempera- ture and pH, etc., will have a marked influence on the xylitol yield. The optimisation of said parameters may provide better yields than those reported in the present application. The measures required are well-known to persons skilled in the art and will therefore not be described in this context. In connection with the present invention it has been proved, for instance, that by overexpressing the homologous β-xylosidase gene in Tricho- derma reesei, the β-xylosidase activity could be increased about 15-fold to 450 nkat ml "1 as compared to 32 nkat ml "1 for the parent strain. The specific β- xylosidase activity was 72.6 nkat g "1 protein and 4.9 nkat g "1 protein, respectively. By changing the substrate and using appropriate induction, the β- xylosidase activity could be further increased, up to 960 nkat ml "1 for the re- combinant microorganism of the present invention. In hydrolysis studies made, it has also been shown that the enzymes produced by the recombinant T. reesei strains are the most efficient ones to effect the hydrolysation of xylan-containing raw materials to xylan. For instance the amount of enzyme produced by the microorganisms of the pres- ent invention, which is required to hydrolyse 85% of xylo-oligosaccharides into xylose is only about 1/8 of the amount of the enzyme produced by the unmodified host strain. Altering the xylitol metabolic pathway in accordance with the present invention also led to significant improvement in the final xylitol production. By inactivating the xylitol dehydrogenase (xdh) gene, effecting the conversion of xylitol to xylulose, it was possible to increase the xylitol production up to 10- fold in the transformant as compared to the unmodified parent strain. Adding glucose to the medium used provided more cofactors for the conversion of xylose to xylitol, resulting in one particular study in a xylitol production of 5-5.5 gl " \ with the transformant, as compared to 3.1 gl "1 without glucose. By using a strain that has been genetically modified both by enhancing its xylanolytic activity and inhibiting its xylitol metabolism, the xylitol production can be improved even further. Based on the studies made, an increase of 20 times and more can be expected, as compared to the parent strain. The xylitol produced can be recovered from the fermentation broth by various techniques known per se. Such techniques include for example filtration, ultrafiltration, chromatographic methods, crystallisation etc. When using chromatography, arabinitol-rich fractions obtained can be recycled, and used as a fermentation medium component, i.e. raw material, for further xylitol production. The enzymes produced can be easily recovered from the fermen- tation broth by ultrafiltration, the retentate containing the enzymes, while the permeate contains e.g./xylitol and arabinitol. In studies made in connection with the present invention, it has also surprisingly been shown that the inactivation of xylitol dehydrogenase, as an example of an enzyme of the xylitol pathway, does not affect xylitol production in any way. Xylitol production hence occurs normally in the cell. Furthermore, the enhanced amounts of xylitol do not act as inhibitors of further synthesis, which is surprising considering that xylose on the enzyme level is known as an inhibitor of further xylose production [Kersters-Hilderson et al. (1969) Euro- pean J. Biochem. 7:434-441 ; Dekker (1983) Biotechnol. Bioeng. 30:1127- 1146; Rodionova et al. (1983) J. Appl. Biochem. 5:300-312; Poutanen and Puls. (1988) Appl. Microbiol Biotechnology 28:425-432]. As a further surprising feature it can be mentioned that, in the studies conducted so far, xylitol has not been shown to inhibit xylanolytic activity, either. This is a major benefit of the process. The methods of the present invention have several advantages as compared to known processes. Firstly, they enable the utilisation of a wide variety of raw materials. Due to its xylanolytic activity, the microorganism used as a production organ- ism is itself able to hydrolyse even complex xylan-containing raw materials, such as pentose-sugar-containing polymers and oligomers, lignocellulose- based hydrolysates and extracts, pentosans extracted from lignocellulosic materials, bagasse, sugar beet pulp, bran, straw etc. Thus, by using the methods according to the present invention, it is possible to utilise a large variety of raw materials as starting material. Secondly, the microorganism(s) is(are) capable of carrying out all the necessary reactions, from the hydrolysis of the raw material, via the metabolic conversion of the hydrolysis products, to the desired end product. Thirdly, the genetic modification of the microorganisms, irrespective of whether it involves overexpression of genes encoding for xylanolytic activity, or blocking or inactivation of genes responsible for the further metabolism of xylitol, or both, leads to enhanced xylose production. The excess of xylose produced by the increased xylanolytic activity of the cells is further metabolised into xylitol, thus leading to enhanced xylitol production. The overall xylitol production rate thus by far exceeds the rate achieved by the use of conventional microorganisms. The invention will be described in detail in the following specific examples. The examples are included herein for illustrative purposes only and are not to be construed as limiting or restricting the scope of the invention in any way. As described in the following examples, E. coli strain DH5α was used as host in E. coli clonings and Trichoderma reesei strain RutC-30 (ATCC 56765, VTT-D-86271) [Montenecourt Eveleigh Adv. Chem. Ser. (1979), 289- 301] are used in fungal transformation and expression studies. EXAMPLE 1 Construction of vectors for expression of β-xylosidase under the cbh2- and the gpdA- promoters in Trichoderma 1. Cloning of the bxl1 protein coding region The commercial plasmid pSP73 (Promega) was modified by liga- ting a linker fragment containing recognition sites for Pstl, Hindlll, Sphl, Xbal, Kpnl and Xhol between the Pstl and Xhol restriction sites in the multiple cloning region of the plasmid. The multiple cloning region in the plasmid obtained, pSP73 (MLO), contains recognition sites for restriction enzymes Bglll, EcoRV, Clal, EcoRI, Sad, Kpnl, Smal, BamHI, Xbal, Sail, Accl, Pstl, Hindlll, Sphl, Xbal, Kpnl, and Xhol. The linker oligos: 1. 5' GAAGCTTGCATGCTCTAGAGGTACC 3' (sequence Id. No. 1 ) 2. 5 * TCGAGGTACCTCTAGAGCATGCAAGCTTCTGCA3' (sequence Id. No. 2) The protein coding region of the bxh gene was subcloned in three fragments. A Sacl + Hind III fragment carrying the middle part of the gene (2047bp) was isolated from the plasmid p18SA [Margolles-Clark, E. et al., Appl. Environ. Microbiol 62:219-228, 1996)] and inserted into the plasmid pSP73(MLO) between Sacl and Hindlll. The resulting plasmid was named pMLO130. The 5'- end (252bp) and the 3'- end (192bp) were synthesised with PCR using p18SA as a template. PCR products were digested with EcoRI and Xbal and inserted into the plasmid pSP73 and the resulting plasmids now carrying the 3'- end and the 5'- end of the bxll coding region were named pMLO131 and pMLO132, respectively. The 3'-end fragment was first sequen- ced and then isolated from pMLO131 as a Hind III +Xbal fragment and inserted into the plasmid pMLO130 between Hindlll and Xbal to obtain plasmid pMLO134. The 5'-end fragment was first sequenced and then isolated from pMLO132 as a EcoRI + Sacl fragment and inserted into the plasmid pMLO134 between EcoRI and Sacl. The resulting plasmid pMLO135 now carries the whole protein coding region of the bxH gene with BamHI restriction sites in both ends. The PCR primers for the cloning of the 5'-end fragment: EcoRI BamHI + bxH 1. 5' CGGAArTCCGGGyATCCGGTGAATAACGCAGCTCTTCTCGCCGCCC- TG 3' (sequence Id. No. 3) Xbal Sacl 2. 5' GCrC AGΛGCGAGCTCCTCGAGGGTGAAGAGCGAGATGAG 3' (sequence Id. No. 4) The PCR primers for the cloning of the 3'-end fragment: EcoRI Hindlll 1. 5' CGGA4TΓCCGΛAGCTΓGAGTTTGAGTTGGTGGGAGAAGAG 3' (sequence Id. No. 5) Xbal BamHI 2. 5' GCΓC AGAGCGGΛΓCCCGCAATGGCAAAAATGGTTTGCAAACA- GAAAACTATG 3' (sequence Id. No. 6) 2. Cloning of the cbh2 promoter of T.reesei A 1000 bp fragment from the 5' non-coding region of the cbh2 gene of T. reesei strain QM9414 (ATCC 26921) was synthesised with PCR using total DNA as a template. A plasmid pAN52-Notl (Punt, P.J. et al., Gene 69:49- 57, 1988) was digested with Notl and BamHI and the released gpdA promoter of A.nidulans was replaced by the Notl and BamHI digested PCR product. The resulting plasmid was named pMLO 133. The PCR primers for the cloning of the cbh2 promoter: Sphl Notl + promoter 1. 5" ACATGCΛTGCATGCGGCCGCGGTTTGTTGCCGTAACCATTCTGTCT- CCT 3' (sequence Id. No. 7) Xbal BamHINcol-1 2. 5' GCTC AG>AGCGG/ATCCΛrGGTGCAATACACAGAGGGTGATCTTA- CAAG 3' (sequence Id. No. 8) 3. Construction of vectors for expression on β-xylosidase The BamHI fragment containing the protein coding region of the bxH gene was released from the plasmid pMLO135 and inserted inbetween the gpdA promoter and trpC terminator of the plasmid pAN52-Notl and inbetween cbh2 promoter and trpC terminator of the plasmid pMLO133. From the resulting plasmids pMLO136 and pMLO137 the β-xylosidase expression ca- sette can be released with Notl and Xbal. 4. Transformation Both pMLO136 and pMLO137 were cut with Notl and Xbal, gel purified and both fragments were together transformed into T. reesei RutC-30 using either acetamide selection or hygromycin selection. EXAMPLE 2 xdhl disruption construct with short cDNA flanking regions for integration into the xdhl locus The xdhl cDNA -containing plasmid B1073 (Fig 1.) was prepared as follows. The xdh gene of Trichoderma reesei was derived from the cDNA library constructed in the vector pAJ401 (Saloheimo A., et al., Mol. Microbiol. 13, 219-228, 1994), where the cDNA is ligated between the PGK1 promoter and terminator. Poly(A)+mRNA was isolated from T. reesei Rut-C30 cultivated on medium containing several plant polysaccharides (Stahlbrand H., et al., Appl. Environ, Microbiol. 61 , 1090-1097, 1995). The cDNA was synthesised using the ZAP-cDNA synthesis kit (Stratagene, CA, USA) and was ligated into the plasmid pAJ401 (Margolles-Clark E., et al., Appl. Environ. Microbiol. 62, 3840-3846, 1996). A 540 bp Sal\-Xho\ fragment from B1073 was ligated to Xho\ cut pARO21 (Saloheimo et al. 2000, J. Biol. Chemistry 275, §: 5817 - 5825), downstream of the pki promoter-hph-cbh2 terminator cassette conferring hygromycin resistance. A clone containing the insert in the desired orien- tation was isolated and named pMI87. The 600 bp 5' xdhl fragment was excised from B1073 as a EcoRI-Psfl fragment. This fragment was made blunt ended using the T4 polymerase and ligated to EcoRV cut pMI87 producing pMI88, which now has xdhl 5' end downstream of the cbh2 terminator and xdhl 3' end upstream of the pki promoter. The construction of the plasmid pMI88 is illustrated in Fig. 2. pMI88 cut with Asp718-Smal, the fragment containing the disruption cassette was isolated from gel and transformed to Trichoderma reesei Rut- C30 strain (Penttila et al., Gene 61 : 151 - 164, 1987) using hygromycin B se- lection. Isolation of the chromosomal copy of the xdhl gene. The xdhl cDNA was 32 P labelled and used as a probe to isolate the chromosomal copy from the genomic cosmid library of T reesei [Mantyla et al. (1992) Curr. Genet. 21 : 471-477]. The library was screened using standard methods and cosmids were isolated from colonies giving a positive hybridisation signal. One of the clones was named pMI85. The cosmid was cut with various restriction enzymes and the samples were subjected to Southern blot in order to identify fragments containing the xdhl gene. The 2.2 and 1.7 kb Pst\ fragments, both giving a positive hybridisation signal, were cloned into Pst\ cut pZErO-1 vector and named pMI94 and pMI93, respectively. xdhl disruption construct with chromosomal flanking regions for integration into the xdhl locus pMI93 was cut with EcoRV-XΛol and the 1.5 kb fragment was isolated and made blunt ended with T4 polymerase and ligated to pMI94 line- arised with Xho\ and also treated with T4 polymerase producing pMI95. Thus pMI95 contains the 5' Pst\ fragment and the 3' XΛol-Psrl fragment of xdhl separated by EcoRV and Not\ sites. pMI95 was digested with Notl-Sall, treated with T4 polymerase and ligated to the 3.2 kb blunt ended Sph\-Xba\ fragment from p3SR2 [Hynes, M.J., Corrick, CM. and King, J.A. (1983) Mol.Cell. Biol. 8: 1430-1439] containing the A. nidulans amdS gene producing pMI96. Thus the 320 bp Xho\-Sal\ fragment is removed from the xdhl protein coding region and replaced by the amdS gene in pMI96 . The construction of the plasmid pMI96 is illustrated in Fig. 3. pMI96 was cut with >4sp718-X al and the 7 kb fragment containing the disruption cassette was gel purified and transformed into T reesei Rut- C30 using acetamide selection. Screening of the transformants on xylitol and xylose plates The transformants were first purified on solid Trichoderma minimal medium (Penttila M. et al., ibid.) supplemented with either 10 mM acetamide as the sole nitrogen source instead of (NH 4 ) 2 SO 4 or with 100 μg/ml hygromycin. From the transformation made with hygromycin marker, transfor- mants were received after three rounds on selective plates and with acetami- dase marker, after two rounds on selective plates. All the transformants were single spore purified by streaking them out on selective media containing plates. Four single spore purified colonies from each transformant were tested on plates containing Trichoderma minimal medium, pH adjusted to 5,5 with KOH and supplemented either with 2% xylitol or 2% xylose as the only carbon source and without any organic nitrogen source. The possible inactivants could be found by the poor growth both on xylitol and especially on xylose containing plates. The verification of the inactivants by Southern blotting On xylitol and xylose plates tested xdhl inactivants containing the hph marker and xdhl inactivants containing the amdS marker were tested by Southern blotting technique. Fungal DNA was isolated according to Raeder, U. et al. (1985) Lett. Appl. Microbiol. 1 :17-20. In order to check correct integration of the transformed expression casette into the xdhl locus 15μg of DNA of hph transformants was digested with Pstl and 15μg DNA of amdS transformants with Xhol. The DNA fragments were size fractionated in 0,8 % agarose gels and blotted onto nylon membrane using standard methods (Maniatis et al., ibid.). Blots were probed with a 32 P-labelled fragment of the xdhl gene. The probe fragment was digested from plasmid B1073 with the plasmid EcoRI and Xhol and isolated from the gel. The probe was labeled using the Random Primed DNA Labeling Kit (Boehringer Mannheim) and [α- 32 P]dCTP (Amersham). Hybridization was done in 50% formamide-δxDenhardt's- 5xSSPE-0,1% SDS-100 μg/ml denatured herring sperm DNA-1 μg/ml polyA DNA at 42°C overnight and washed twice for 5 min in 2xSSC at room tem- perature and for 30 min in 2xSCC-0,1% SDS at 65°C. An xdhl inactivant having two copies of the transformation cassette was designated no. 63 and used in further studies. EXAMPLE 3 Construction of xdhl inactivant overexpressing β-xylosidase From the plasmid pMLO136 containing the β-xylosidase gene under the gpd promoter and trpC terminator, and from the plasmid pML0137, containing β-xylosidase under the cbh2 promoter and trpC terminator, the expression cassettes are released by cutting the plasmids with Notl and Xbal digestion. The xdhl inactivant containing the hph marker will be transformed ac- cording to Penttila et al. (ibid.) either with one or both of the isolated expression cassettes using the plasmid p3SR2 containing an acetamidase gene as a selective marker. The xdh inactivant containing the amdS marker will be transformed similarly either with one or both of the expression cassettes but using the plasmid pRLM EX 30 [Mach, R.L., Schindler, M. And Kubicek, C. (1994) Genet. 25: 567-570] containing the hph gene as a selective marker. The construct can be made also by transforming the xdhl cassette released from the plasmid pMI88 which contains the hygromycin gene (described above) to the strain overexpressing β-xylosidase (VTT-D-98694) using hygromycin as a selective marker. EXAMPLE 4 Production of xylanase and β-xylosidase by genetically modified Trichoderma reesei Trichoderma reesei VTT-D-98694, derived from T. reesei Rut C-30 as described in Example 1 , by inserting a homologous β-xylosidase gene into the genome under the control of the cbh2 promoter, was used to study the production of xylanase and β-xylosidase. The microorganism produces β- xylosidase when induced to produce cellulases, e.g. CBHII. Both the production of xylanase and the production of β-xylosidase are achieved under induction. The parent strain T reesei Rut C-30 was used as a control. Inoculations of the microorganisms were prepared by growing them in the following medium: Sloka floe cellulose 20,0 gl "1 Wheat based distiller's grain 10,0 gl "1 K 2 HPO 4 15,0 gl "1 (NH 4 ) 2 S0 4 5,0 gl "1 I 1 x 200 ml was grown on agar substrate 2 d, 200 rpm, at 29 °C. II 5 x 200 ml, 1 d, 200 rpm, 29 °C. The fermentation medium was the following: Sloka floe cellulose 40,0 gl "1 → 600 g Wheat based distiller's grain 20,0 gl "1 → 300 g K 2 HPO 4 5,0 gr 1 → 75 g (NH 4 ) 2 SO 4 5,0 gl "1 → 75 g The fermentations were carried out by using a 20 I fermentor, Che- mab LF 20, containing 14 I fermentation medium and inoculation, made up to 15 I, for four days at a temperature of 29 °C, a pH of 6.0 - 6.5 (regulated by NH 4 OH and H 3 PO 4 ), while mixing at about 600 rpm and aerating to keep the level of dissolved oxygen over 30 %. After fermentation, the broths were cooled to under 20 °C and cen- trifuged in a Cryofuge, 5000 rpm, 20 min, <10 °C. The centrifuged broths were concentrated by ultrafiltration to about 1.0 - 1.5 1. At various stages of the process samples were taken and the following enzyme activities measured: endoglucanase [HEC, Bailey and Nevalainen (1981) Enzyme Microb Technol 3:153-157., IUPAC (International Union of Pure and Applied Chemistry) (1987) Pure and Appl Chem 59:257- 268], xylanase, β-xylosidase, FPU [Filter Paper Unit, Mandels et al., Biotechnol. Bioeng. Symp. 6 (1976) 21-33. John Wiley and Sons, NY., IUPAC (International Union of Pure and Applied Chemistry) Pure and Appl. Chem. 59 (1987) 257-268], acetyl xylan esterase (AXE), α-glucuronidase (αGLU) and α- arabinosidase. In addition, the protein content was determined using the Lowry TCA method. The results obtained at the end of fermentation are presented in Table 1. . The results show that the production of cellulases (HEC, FPU) was rather weak on the cellulose medium used, due to the high pH value. However, the FPU value, 3.1 units ml "1 , and the protein content, 6.2 gl 1 , both indi- cate that there were moderate amounts of cellulases in the broth. The xylanase production was on the expected level [Bailey et al.(1993), Appl. Micro- biol. Biotechnol. 40: 224 - 229]. The β-xylosidase activity shown by the genetically modified strain was about 450 nkat ml "1 , which result was significantly higher than the result obtained by the parent strain, 32 nkat ml "1 . The specific activities were 72.6 nkat/g protein and 4.9 nkat/g protein, respectively. Table 1 Protein content and enzyme activities produced after fermentation (92h) In conclusion, addition of the β-xylosidase gene resulted in a ma- nyfold increase in β-xylosidase production while the production of other enzymes remained essentially on the original level. EXAMPLE 5 Production of xylanase and β-xylosidase by genetically modi- fied Trichoderma reesei The previous example was repeated by using a different substrate based on lactose and xylan. The two microorganisms were separately grown on the following medium: Lactose 20.0 g I "1 Peptone 4.0 g I "1 Yeast extract 1.0 g I "1 KH 2 PO 4 15.0 g l "1 (NH 4 ) 2 SO 4 2.8 g I "1 MgSO 4 x 7H 2 O 0.5 g I "1 CaCI 2 x 7H 2 O 0.4 g I "1 Mineral solution 1.0 ml Mineral solution: FeSO 4 • 7H 2 O 5.0 mg/l MnSO 4 • H 2 O 1.6 mg/l ZnSO 4 • 7H 2 O 1.4 mg/l CoCI 2 • 6H 2 O 3.7 mg/l and the inoculations were prepared as described in the previous example. The fermentation medium was the following: Lactose 40,0 g I "1 -» 600 g Peptone 4,0 g I "1 -» 60 g Yeast extract 1 ,0 g l "1 - 15 g KH 2 PO 4 4,0 g I "1 -» 60 g (NH 4 ) 2 S0 4 2.8 g I "1 - 42 g MgSO 4 x 7H 2 0 0.6 g I "1 - 9 g CaCI 2 x 2H 2 O 0.8 g I "1 -> 12 g Mineral solution 2,0 ml I "1 -» 30 ml In addition, 2.0 I of Stake-xylan solution, Cultor (see example 6), as a 1 :2 dilution was fed with a MSC-WW5 pump, Chemap, hose 4/7 mm. The feed was started after a growth period of about 8 h. During the first 24 h, the feed rate was about 30 mlh "1 , thereafter about 40 mlh "1 . The fermentations were carried out using a 20 I fermentor, Che- mabCF 2000 containing 14 I fermentation medium and inoculation, up to 15 I, for four days at a temperature of 29 °C, a pH of 4.0 - 5.0 (regulated by NH 4 OH and H 3 PO 4 ), while mixing (500 - 800 rpm) and aerating to keep the level of dis- solved oxygen over 30 %. After fermentation, the broths were processed and samples taken as described in the previous example. The results obtained at the end of fermentation are shown in Table 2. Table 2 Protein content and enzyme activites produced after fermentation (92h) The production of both xylanase and, in particular, β-xylosidase was significantly higher as compared to the previous example. In this connection, it is worth pointing out that these examples describe preliminary tests and thus, by systematic optimisation of different parameters, such as fermentation me- dium composition and xylan feed, much better yields can be obtained. The example clearly shows that a soluble lactose-xylan medium provides better results than the cellulose-based medium used in the previous example. The xylo-oligosaccharide solution has been an efficient inducer both for the production of cellulases and, in particular, of native β-xylosidase. Again, the par- ent strain produces significantly less, about 22 %, β-xylosidase than the genetically modified organism. In this induced fermentation process condition the specific activity of β-xylosidase was 106 nkat/g protein for the genetically modified microorganism and 27 nkat/g protein for the parent strain. A more than five-fold increase in β-xylosidase production gives significantly better yields in the ultimate hydrolysis processes. EXAMPLE 6 Hydrolysis efficacy of the enzymes produced by genetically modified Trichoderma reesei The aim of this study was to test the efficacy of the enzymes produced by the genetically modified Trichoderma reesei for the hydrolysis of xylo-oligomer raw materials like steam exploded birch wood extract (Stake- liquor). A steam exploded birch wood extract was produced by Cultor and stored at a concentration of 32 %. The composition of the Stake-liquor was the following: After acid hydrolysis (1 h, 120 °C, pH1) Dry solids 33.8 % (Rl*), 28 % (in oven, 105°C, 24 h) Oligosaccharides 62 % DS (Rl) Xylose 10 % " 60.6 % Galactose+rhamnose 1.1 % " 3.75 % Arabinose+mannose 1.9 % 5.6 % Glucose 3.1 % pH 3.6 Color (lcumsa) 86 500 Conductivity 3600 uS/cm Rl = Refractometry Index Four different enzymes were produced in pilot fermentation as described above. Codes and analysis of the enzymes produced by the geneti- cally manipulated Trichoderma reesei and by the unmodified parent strain with and without induction with Stake-liquor are those disclosed in Tables 1 and 2. Multifect GC-140, Genencor Intern. Europe, Jamsankoski, Finland, was used as a commercial reference. Analysis of Multifect XK 803 showed a protein content of 82 g/l (Lowry), a β-xylosidase activity of 600 nkat/ml, and a xylanase activity of 780000 nkat/ml. Enzymatic hydrolysis of xylo-oligosaccharides or xylan was carried out at a concentration of 4 %, which has been proved to be the level where end product inhibition of xylose is at a tolerable level. The hydrolysis was carried out at 45 °C, agitating 100 rpm, initial pH 4.5 and hydrolysis time 24 - 48 h. The dosage of enzymes, as β-xylosidase, varied between 12 - 150 nkat/g of d.s. of Stake-liquor. The carbohydrates of the hydrolysate were analysed with an HPLC apparatus equipped with a Rl-detector on a column in Pb + + -form at 65 °C, flow rate 0.4 ml/min, injection volume 20 μl, with water as the mobile phase. The hydrolysis yields were calculated from the detected monomeric carbohydrates. The enzymatic conversion of xylo-oligosaccharides in Stake-liquor with the tested enzymes is shown in table 3. All the enzymes produced rather equal amounts of xylose from Stake-liquor when comparing the efficiency as β-xylosidase activity used per g dry solids. The commercial enzyme Multifect XS 803 was much more efficient at the same β-xylosidase level than the test enzymes because Multifect XS 803 contained a higher amount of xylanase, which has also a strong contribution to the degradation of xylooligo- saccharides. The efficacy of the enzyme preparates was compared also by making calculations based on the amount of enzyme (with an equal protein content) which was required to achieve the same hydrolysis level. With this type of calculation the raw materials used for fermentation of the enzyme preparation are at an almost equal level. The amount of an individual enzyme solution of each test enzyme preparation used for hydrolysis was converted to the amount of enzyme (liter/ton dry solids), which corresponded to enzyme at a protein level of 45 g/l. The reference level of 45 g protein/I was chosen to be the same as the level of protein in Genencor's commercial hemicellulase Multifect GC-140. The results calculated on the protein level 45 g/l are shown in table 3. According to Table 3 the enzymes produced by the recombinant 7 " . reesei strains are clearly beneficial when comparing the hydrolysis of Stake- liquor to xylose on the bases of required amount of enzyme protein. Table 3 Xylose yields obtained from Stake-liquor (Dosage of enzymes was converted to the protein level 45 g/l) The enzymes produced by the recombinant 7 " . reesei strains are the most efficient ones to hydrolyse Stake-liquor to xylan. The combination of recombinant strain and inducer did not improve the hydrolysis efficiency of protein significantly (Table 4) as compared to the enzyme protein produced by the recombinant strain without inducer. Both enzymes have very similar specific β- xylanase activity but rather high differences in specific acetylesterase activity. Probably the high β-xylosidase content compensates for the importance of acetyl esterase. However, the improved efficiency of the enzyme product produced with the inducer is probably partly due to the higher acetylesterase content when comparing the enzymes produced by the unmodified strains. A summary of the results is shown in Table 4 wherefrom it can be seen that the amount of enzyme produced by the recombinant strain, which is required to hydrolyse 85 % of xylo-oligosaccharides to xylose, is only 1/6 - 1/8 of the amount of the enzyme produced by the unmodified parent strain. Table 4 Enzyme dosage (per 45 g protein/I) required to obtain a xylose yield of 85 % (= 51.5 % of D.S.) from xylo-oligosaccharides of Stake-liquor. EXAMPLE 7 Production of xylitol in shake flask cultivations on Lenzing xylan with xdhl inactivant and the host strain RutC-30 Xdhl inactivant 63 and host strain RutC-30 were cultivated at 200 rpm at 28°C in 2 I shake flasks (200 ml of culture medium) on buffered Trichoderma minimal medium (Penttila et al., ibid.) at pH 5.5 supplemented with 3% Lenzing xylan and 0.2 % proteose peptone. 200 ml of growth medium in 2 I shake flask was inoculated with 1.2-10 8 spores. Samples were taken from the shake flasks after 3, 4, 5, 6 and 10 days. Xdhl inactivant and RutC-30 were cultivated similarly on buffered Trichoderma minimal medium at pH 5.5 supplemented with 3% Lenzing xylan, 0.2% proteose peptone and 2% lactose as a cosubstrate. Samples were taken as described above. Supernatant samples were separated by filtering through a GF/B glass fibre filter (Whatman ) and stored at -20°C. The mycelia were washed with sterile water and stored at -70°C. The amount of xylitol, xylose, lactose and glucose were analysed from the supernatant samples by HPLC with HPX-87H column (Pharmacia) flow rate 0.3 ml/min. The amount of xylitol was confirmed with Cobas MIRA Plus (Roche Diagnostic Systems) using the D-Sorb itol/Xy lito I kit (Boehringer Mannheim, Cat. no 670 057), lactose with Lactose/D-Galactose kit (Boehringer Mannheim, Cat.no 176 303) and glucose with Glucose/GOD-Perid Method (Boehringer Mannheim, Cat.no MPR 3 124 036). In order to simulate the production of xylitol with a strain overexpressing β-xylosidase and having its xylitoldehydrogenase activity inactivated, purified and concentrated β-xylosidase enzyme (290 nkat/ml) was added to the medium described above. The cultivations were carried out as described above in shake flasks with the xdh disruptant and the parent strain RutC-30 for 10 days on minimal medium containing Lenzing xylan as the sole carbon source and peptone as the organic nitrogen source with and without addition of purified β-xylosidase, lactose or both. The xdh inactivant grew and consumed xylose slower than the parent strain on the media used. After consuming both xylose and lactose (after 4 - 5 days) both strains began to use the xylitol in the culture medium. On pure xylan without any additions the amount of xylitol produced by the inactivant was 1.3 g/l after 4 days. If lactose was added, the amount increased to 2.0 g/l. With the addition of β-xylosidase the amount increased to 1.4 g/l. However, if both lactose and β-xylosidase were added at the beginning of the cultivation, the amount of xylitol could increase up to 4.9 g/l which confirms that by over- expressing β-xylosidase the production of xylitol can be improved. The maximum amount of xylitol produced by the parent strain RutC-30 was 0.3 g/l. As a summary, on the xylan based media the best production of xylitol (4.9 g/l) could be achieved when both β-xylosidase and lactose were added to the medium. The inactivant produced around 20-fold more xylitol than the parent strain RutC-30. The yield was 0.17 g xylitol/g xylan and 0.273 g xylitol/g dry weight. The addition of β-xylosidase increased the amount of xylitol produced 3.3-fold on that particular medium. The results are summarised in Table 5A. EXAMPLE 8 Production of xylitol in shake flask cultivations on xylose, arabinose and a mixture of xylose and arabinose with xdhl inactivant and the host strain, with and without daily addition of glucose Xdhl inactivant 63 and host strain RutC-30 were cultivated at 200 rpm at 28°C in 2 I shake flasks (300 ml of culture medium) on buffered Trichoderma minimal medium (Penttila et al., ibid.) at pH 5.5 supplemented with ei- ther 3% xylose, 3% arabinose or a mixture of 1.5% xylose and 1.5% arabinose, and 0.2% proteose peptone and 0.2% glucose. 300 ml of growth medium in a 2 I shake flask was inoculated with 1.8-10 8 spores. Samples were taken from the shake flasks after 2, 3, 4, 5, 6, 7, 10 and 14 days. Xdhl inactivant and RutC-30 were cultivated similarly on buffered Trichoderma minimal medium at pH 5.5 supplemented with either 3% xylose, 3% arabinose or a mixture of 1.5% xylose and 1.5% arabinose, and 0.2% proteose peptone and 0.2% glucose as a cosubstrate. In addition glucose was added daily to the final concentration 0.2%. Samples were taken as described above. Supernatant samples were separated by filtering through GF/B glass fibre filter (Whatman) and stored at -20°C. The mycelia were washed with sterile water and stored at -70°C. The amount of xylitol, xylose, arabinose and glucose were analysed from the supernatant samples by HPLC with HPX-87H column (Pharmacia) flow rate 0.3 ml/min. The xylitol amount was confirmed with Cobas MIRA Plus (Roche Diagnostic Systems) using the D-Sorbitol/Xylitol kit (Boehring Mannheim, Cat. No 670 057), arabinose with Lactose/D-Galactose kit (Boehringer Mannheim, Cat. No 176 303) and glucose with Glucose/GOD-Perid Method (Boehringer Mannheim, Cat. no MPR 3 124 036). Simulation of using an xdh inactivant overexpressing β-xylosidase is carried out as described in Example 7. The inactivant grew much slower on xylose than the host strain but it grew even more slower on arabinose. Daily addition of glucose improved the growth but nevertheless the inactivant was slower in growth as well as in the consumption of xylose and arabinose. The host strain had consumed both the xylose and arabinose after 5 days, but it took 6 - 7 days for the inactivant to consume the xylose and even 14 days to consume the arabinose if glucose was not added. The addition of glucose daily increased the amount of the my- celia and enhanced especially the consumption of arabinose. Xylitol was produced in an amount of 4.4 g/l on pure xylose medium, 5.3 g/l on pure arabinose and about 4.1 g/l on the mixture of xylose and arabinose. This indicates that Trichoderma is capable of producing xylitol from arabinose as well. Because of the slow consumption rate of arabinose it seems that the conversion efficiency from arabinose to xylitol is better than from xylose to xylitol. The xylitol produced is consumed when all the other carbon sources have been depleted. If glucose was fed daily, the production of xylitol could be increased slightly to 5.0 g/l on xylose, to 5.1 g/l on arabinose and to 4.2 g/l on the mixture of xylose and arabinose. Since glucose was present in the medium because of daily addition, the xylitol produced was not consumed during the cultivation. The results are summarised in Table 5B. Table 5 Production of xylitol with xdh inactivated T. reesei When cultivating either on pure xylose or arabinose the best production was received when cultivating on xylose with glucose feed (5.0 g/l). EXAMPLE 9 Production of xylitol in shake flask cultivations on higher con- centrations of xylose, arabinose with xdhl inactivant, with and without daily addition of glucose Xdhl inactivant 63 was cultivated at 200 rpm at 28°C in 2 I shake flasks on a buffered Trichoderma minimal medium (Penttila et al., ibid.) at pH 5.5 supplemented with 4 % lactose and 0.2 % proteose peptone. 300 ml of growth medium in a 2 I shake flask was inoculated with 1.8-10 8 spores. The mycelia of the inactivant were collected after 6 days of cultivation by centri- fuging for 5 minutes at 3000 rpm and transferred to fresh 300 ml buffered Trichoderma minimal medium pH 5.5 in a 2 I shake flask supplemented with either 10 % xylose or 10 % arabinose, and 0.2 % proteose peptone and 0.2 % glucose. After addition of mycelia the concentration of xylose or arabinose was around 8 %. The inactivant was cultivated at 200 rpm at 28 °C. Samples were taken from the shake flasks after 1 , 2, 3, 4, 6, 7, 8, 9, 10 and 15 days. The Xdhl inactivant was cultivated similary on buffered Trichoderma minimal medium at pH 5.5 supplemented with either 8% xylose, 8% arabinose and 0.2 % glucose as a cosubstrate. In addition glucose was added daily to the final concentration 0.2%. Samples were taken as described above. Supernatant samples were separated by filtering through a GF/B glass fibre filter (Whatman) and stored at -20°C. The mycelia were washed with sterile water and stored at -70°C. The amount of xylitol, xylose, arabinose and glucose were analysed from the supernatant samples by HPLC with HPX-87H column (Pharmacia) flow rate 0.3 ml/min. The xylitol amount was confirmed with Cobas MIRA Plus (Roche Diagnostic Systems) using the D-Sorbitol/Xylitol kit (Boehring Mannheim, Cat. No 670 057), arabinose with Lactose/D-Galactose kit (Boehringer Mannheim, Cat. No 176 303) and glucose with Glucose/GOD-Perid Method (Boehringer Mannheim, Cat. no MPR 3 124 036). The results are summarised in Table 5C. EXAMPLE 10 Production of xylitol in shake flask cultivations on L-arabitol with the xdhl inactivant and the host strain, with and without daily addition of glucose L-arabitol is the next compound from arabinose in the metabolic pathway from L-arabinose to xylitol. In order to see whether L-arabitol is converted to xylitol as efficiently in the inactivant 63 and Rut-C30 were grown on 3 % L-arabitol with and without 0.2 % glucose feed. Xdhl inactivant 63 and host strain Rut-C30 were first cultivated at 200 rpm at 28 °C in 2 I shake flasks on buffered Trichoderma minimal medium at pH 5.5 supplemented with 4 % lactose and 0.2 % proteose peptone. 300 ml of growth medium in a 2 I shake flask was inoculated with 1.8 • 10 8 spores. The mycelia of the inactivant were collected after 6 days cultivation and of Rut- C30 after 5 days cultivation by centrifuging 5 minutes at 3000 rpm and trans- ferred to fresh 300 ml buffered Trichoderma minimal medium pH 5.5 in a 2 I shake flask. The medium was supplemented with 3 % L-arabitol, 0.2 % proteose peptone and 0.2 % glucose as a cosubstrate. The inactivant grew on both media even though in the beginning of the cultivation the growth was slower compared to the host strain Rut-C30. Rut-C30 also consumes L-arabitol much quicker than the inactivant. Xylitol is converted from L-arabitol even though not as efficiently as from L-arabinose. The yields and productivities are also slightly lower. The results are presented in Table 6. The yields (g xylitol/g arabitol) and productivities (mg xylitol/g cell dry weight per hour) are calculated from parallel flasks from the exponential growth phase. The amount of arabitol corresponds to the amount left in the medium after the exponential growth phase. Table 6 Production of xylitol with xdh indicated 7- reesei EXAMPLE 11 Production of xylitol in fed batchfermentations The inactivant 63 was grown in a fermentor as a fed-batch cultiva- tion. The fermentor used was BioFlo IV A, working volume 4 litres, and with automatic control of dissolved oxygen concentration (DO>20 %), temperature (29 °C) and pH (between 4.0 and 5.0, controlled by NaOH and H 3 PO 4 ). Aeration was 5 l/min and agitation between 400 - 800 rpm (dependent of the level of DO). The inoculum was cultivated in shake flasks (2 x 200 ml) for 3 days at 29 °C at 200 rpm on the medium containing glucose 20 g/l, peptone 4 g/l, yeast extract 1 g/l, KH 2 PO 4 15 g/l, (NH 4 ) 2 SO 4 2.8 g/l, MgSO 4 x 7 H 2 O 0.6 g/l, CaCI 2 x 2 H 2 O 0.8 g/l, FeSO 4 x 7 H 2 O 0.005 g/l, MnSO 4 x H 2 O 0.0016 g/l, ZnSO 4 x 7 H 2 O 0.0014 g/l, CoCI 2 x 6 H 2 O 0.0037 g/l. The strain was cultivated in a fermentor first on the similar medium (2 % glucose) as in the shake flasks except that only 4 g/l of KH 2 PO 4 was used. After 22 hours, when the glucose has been consumed, a first batch of xylose was added (6 % xylose in the broth) and the glucose feed was started (0.1 g/l /h, which was about 10 % of the glucose consumption rate in the be- ginning of the cultivation). After 71.5 hours, a second batch of xylose was added and the glucose feed was increased up to 0.25 g/l/h. A third batch of xylose was added after 99 hours as well as other medium components (nutrients, nitrogen). The rate of glucose feed was not changed. After 142 h a final batch of xylose was added , but no other components , in order to study how efficient the conversion from xylose to xylitol is when the fungus is not able to grow any more. The cultivation was finished after 286 hours. Supernatant and mycelium samples were treated as in the shake flask cultivations. In the end of the cultivation, the amount of xylitol was 29.9 g/l. The results are presented in Table 7. Table 7 Xylitol production, xylose consumption, yields (g xylitol / g xylose) and productivities (mg xylitol/ g cell dry weight per hour) in different time periods during the cultivation. EXAMPLE 12 Testing of possible inhibiting effect of xylitol on β-xylosidase 300 nkat concentrated β-xylosidase was incubated with 0, 1 , 5, 10, 50, 100, 200 and 300 g/l of xylitol at 28°C. Samples were taken after 0, 0.5, 1 , 2, 4, 6, 8, 24, 48 and 72 hours incubation and stored at -20°C. The activity of β-xylosidase was measured as described in Poutanen and Puls (1988) Appl. Microbiol. Biotechnol. 28:425-432. The activity of β-xylosidase remained stable in all the samples taken and thus it is not inhibited by xylitol.

Claims: Claims 1. A method for the production of xylitol, the method comprising (a) providing (i) a microorganism having xylanolytic activity, and (ii) a microorganism capable of converting a pentose sugar to xylitol; (b) culturing the microorganism of step (a) (i) in a medium comprising polymer or oligomer materials containing pentose sugars in conditions sufficient for enabling hydrolysis of said polymers or oligomers by the microorganism; (c) producing xylitol in the microorganism of step (a) (ii) by biocon- version of the hydrolysis products obtained in step (b), and (d) recovering said xylitol produced. 2. The method of claim 1 , wherein said microorganisms are used simultaneously. 3. The method of claim 1 , wherein said microorganism (a) (i) is a mold and said microorganism (a) (ii) is a yeast. 4. A method for the production of xylitol, the method comprising (a) providing a microorganism having xylanolytic activity and being capable of converting a pentose sugar to xylitol, (b) culturing the microorganism of step (a) in a medium comprising polymer or oligomer materials containing pentose sugars in conditions sufficient for enabling hydrolysis of said polymers or oligomers by the microorganism; (c) producing xylitol in the microorganism of step (a) by bioconver- sion of the hydrolysis products obtained in step (b), and (d) recovering said xylitol produced. 5. The method of claim 4, wherein said microorganism is a mold or a yeast. 6. The method of claim 1 or 4, wherein the microorganism used possesses cellulolytic activity. 7. The method of claim 1 or 4, wherein anyone of the microorganisms used has been genetically modified. 8. The method of claim 1 or 4, wherein the genetic modification enhances the xylanoytic activity of the microorganism. 9. The method of claim 8, wherein the genetic modification is made in the gene(s) encoding endoxylanase activity. 10. The method of claim 9, wherein the genetic modification is made in the gene encoding β-xylosidase activity. 11. The method of claim 1 or 4, wherein the genetic modification reduces the xylitol metabolism in said microorganism. 12. The method of claim 11 , wherein the genetic modification is made in a gene belonging to the group of genes encoding xylitol dehydrogenase or xylulokinase activity. 13. The method of claim 12, wherein the genetic modification is made in the gene encoding xylitol dehydrogenase activity. 14. The method of claim 4, wherein the xylanolytic microorganism has been genetically modified both to enhance its xylanolytic activity and to reduce its xylitol metabolism. 15. The method of claim 1 or 4, wherein the xylanolytic microorganism is xylanolytic as an inherent property. 16. The method of claim 1 or 4, wherein the xylanolytic microorganism is xylanolytic as an added property. 17. The method of claim 16, wherein the xylanolytic microorganism has been made xylanolytic by genetically introducing the gene encoding β- xylosidase. 18. The method of claim 1 or 4, wherein the microorganism reduces a pentose sugar to xylitol as an inherent property. 19. The method of claim 1 or 4, wherein the microorganism reduces a pentose sugar to xylitol as an added property. 20. The method of claim 19, wherein the microorganism has been genetically modified by introducing a gene encoding xylose reductase. 21. The method of claim 1 or 4, wherein the microorganism has been further modified by altering the activity of auxiliary enzymes. 22. The method of claim 1 or 4, wherein the polymer or oligomer material containing pentose sugars belong to a group consisting of lignocellu- losic prehydrolysates, pentosan extracted from lignocellulosic material, bagasse, sugar beet pulp, corn cobs, corn fibers, oat, wheat, barley and rice hulls and straw, and hydrolysates thereof. 23. The method of claim 1 or 4, wherein the pentose sugar is xylose or arabinose. 24. The method of claim 1 or 4, wherein arabinose is reduced and converted via arabitol to xylulose, and finally to xylitol and the arabinitol produced is recirculated into the process. 25. The method of claim 1 or 4, wherein the microorganism is a fungus or bacteria. 26. The method of claim 25, wherein the microorganism is a yeast, mold or filamentous fungus. 27. The method of claim 26, wherein the microorganism belongs to the genera Trichoderma, Aspergillus, Aureobasidium, Pichia, Candida, De- baryomyces, Hansenula, Klyveromyces, Pachysolen or Saccharomyces. 28. The method of claim 27, wherein the microorganism is a Trichoderma sp. 29. The method of claim 21 , wherein the microorganism is Trichoderma reesei. 30. A microorganism, which has xylanolytic activity and has been genetically modified (i) to enhance its xylanolytic activity, and (ii) to reduce its xylitol metabolism.

Inventors: HEIKKILAE HEIKKI, ILMEN MARJA, OJAMO HEIKKI, PENTTILAE MERJA, SARKKI MARJA-LEENA, UUSITALO JAANA, VEHKOMAEKI MAIJA-LEENA, HEIKKILAE, HEIKKI, ILMEN, MARJA, OJAMO, HEIKKI, PENTTILAE, MERJA, SARKKI, MARJA-LEENA, UUSITALO, JAANA, VEHKOMAEKI, MAIJA-LEENA
Applicants: HEIKKILAE HEIKKI, ILMEN MARJA, OJAMO HEIKKI, PENTTILAE MERJA, SARKKI MARJA LEENA, UUSITALO JAANA, VEHKOMAEKI MAIJA LEENA, XYROFIN OY, HEIKKILAE, HEIKKI, ILMEN, MARJA, OJAMO, HEIKKI, PENTTILAE, MERJA, SARKKI, MARJA-LEENA, UUSITALO, JAANA, VEHKOMAEKI, MAIJA-LEENA
Application year: 2001
Main classification: C12P7
IPC codes: C12P7