For the past 70 years, L-ascorbic acid (vitamin C) has been produced industrially from D-glucose by the well-known Reichstein method. All steps in this process are chemical except for one (the conversion of D-sorbitol to L-sorbose) which is carried out by microbial transformation. Since its initial implementation for industrial production of L-ascorbic acid, several chemical and technical modifications have been used to improve the efficiency of the Reichstein method. Recent developments of vitamin C production are summarized in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A27 (1996), pp. 547ff. Recently different steps of vitamin C production have been performed with the help of microorganisms or enzymes isolated therefrom.

Current production methods for L-ascorbic acid have some undesirable characteristics such as high-energy consumption and use of large quantities of organic and inorganic solvents. Therefore, over the past decades, other approaches to manufacture L-ascorbic acid using microbial conversions, which would be more economical as well as ecological, have been investigated. Direct L-ascorbic acid production has been reported in several microorganisms, using different cultivation methods. The disadvantage of such processes, however, is the low yield of vitamin C produced due to the instability of the product. Using, for instance, microorganisms which are known to be both capable of the production of 2-keto-L-gulonic acid (2-KGA) and vitamin C, the yield of microbiologically produced vitamin C is further limited by the relatively high production of 2-KGA which is more readily synthesized by said microorganism, leading, for instance, to ratios between the concentration of vitamin C and 2-KGA which are less than 0.1.

Thus, it is an object of the present invention to improve the microbiological production of vitamin C to get higher yields as with the processes described in the prior art.

Surprisingly, it has now been found that a process using resting cells of a microorganism capable of performing the direct conversion of a substrate to vitamin C leads to higher yields of vitamin C.

In particular, the present invention provides a process for the production of vitamin C comprising converting a substrate into vitamin C in a medium comprising resting cells of a microorganism.

L-ascorbic acid or vitamin C as used interchangeably herein may be any chemical form of L-ascorbic acid found in aqueous solutions, such as for instance undissociated, in its free acid form or dissociated as an anion. The solubilized salt form of L-ascorbic acid may be characterized as the anion in the presence of any kind of cations usually found in fermentation supernatants, such as for instance potassium, sodium, ammonium, or calcium. Also included may be isolated crystals of the free acid form of L-ascorbic acid.

On the other hand, isolated crystals of a salt form of L-ascorbic acid are called by their corresponding salt name, i. e. sodium ascorbate, potassium ascorbate, calcium ascorbate and the like.

As substrate may be used a carbon source that can be converted into L-ascorbic acid and which is easily obtainable from the D-glucose or D-sorbitol metabolisation pathway such as, for example, D-glucose, D-sorbitol, L-sorbose, L-sorbosone, 2-keto-L-gulonate, D- gluconate, 2-keto-D-gluconate or 2,5-diketo-gluconate. A further possible substrate might be galactose. Preferably, the substrate is selected from for instance D-glucose, D- sorbitol, L-sorbose or L-sorbosone, more preferably from D-glucose, D-sorbitol or L- sorbose, and most preferably from D-sorbitol or L-sorbose. The term"substrate"and "production substrate"is used interchangeably herein.

Conversion of the substrate into vitamin C means that the conversion of the substrate resulting in vitamin C is performed by the microorganism, i. e. the substrate may be directly converted into vitamin C. Said microorganism is cultured under conditions which allow such conversion from the substrate as defined above.

A medium as used herein may be any suitable medium for the production of vitamin C.

Typically, the medium is an aqueous medium comprising for instance salts, substrate (s), and a certain pH. The medium in which the substrate is converted into vitamin C is also referred to as the production medium.

Any microorganism capable of performing the conversion of the substrate to vitamin C may be used, such as for instance, yeast, algae or bacteria, either as wild type strains, mutant strains derived by classic mutagenesis and selection methods or as recombinant strains. Examples of such yeast may be, eg., Candida, Saccharomyces, Zygosaccharomyces, Scyzosaccharomyces, or Kluyveromyces. An example of such algae maybe, eg., Chlorella.

Examples of such bacteria may be, e. g., Gluconobacter, Acetobacter, Ketogulonicigenium, Pantoea, Cryptococcus, Pseudomonas, such as, e. g., Pseudomonas putida, and Escherichia,

such as, e. g., Escherichia coli. Preferred are Glticonobacter or Acetobacter aceti, such as for instance G. oxydans, G. cerinus, G. fateurii, A. aceti subsp. xylinum or A. aceti subsp. orleanus.

Microorganisms which can be used for the present invention may be publicly available from different sources, e. g., Deutsche Sammlung von Mikroorganismen und Zellkulturen (DSMZ), Mascheroder Weg 1B, D-38124 Braunschweig, Germany, American Type Culture Collection (ATCC), P. O. Box 1549, Manassas, VA 20108 USA on May 12,2003 or Culture Collection Division, NITE Biological Resource Center, 2-5-8, Kazusakamatari, Kisarazu-shi, Chiba, 292-0818, Japan (formerly : Institute for Fermentation, Osaka (IFO), 17-85, Juso-honmachi 2-chome, Yodogawa-ku, Osaka 532-8686, Japan). Examples of preferred bacteria deposited with IFO are for instance Gluconobacter oxydans (formerly known as G. melanogenus) IFO 3293, Gluconobacter oxydans (formerly known as G. melanogenus) IFO 3292, Gluconobacter oxydans (formerly known as G. rubiginosus) IFO 3244, Gluconobacterfrateurii (formerly known as G. industrius) IFO 3260, Gluconobacter cerinus IFO 3266, Gluconobacter oxydans IFO 3287, and Acetobacter aceti subsp. orleanus IFO 3259, which were all deposited on April 05,1954 ; Acetobacter aceti subsp. xylinum IFO 13693 deposited on October 22,1975, and Acetobacter aceti subsp. xylinum IFO 13773 deposited on December 08,1977. Strain Acetobacter sp. ATCC 15164, which is also an example of a preferred bacterium, was deposited with ATCC. Strain Gluconobacter oxydans (formerly known as G. melanogenus) N44-1 as another example of a preferred bacterium is a derivative of the strain IFO 3293 and is described in Sugisawa et al. , Agric.

Biol. Chem. 54: 1201-1209,1990.

It is understood that the above-mentioned microorganisms also include synonyms or basonyms of such species having the same physiological properties, as defined by the International Code of Nomenclature of Prokaryotes.

The nomenclature of the microorganisms as used herein is the one officially accepted (at the filing date of the priority application) by the International Committee on Systematics of Prokaryotes and the Bacteriology and Applied Microbiology Division of the International Union of Microbiological Societies, and published by its official publication vehicle International Journal of Systematic and Evolutionary Microbiology (IJSEM). A particular reference is made to Urbance et al. , IJSEM (2001) vol 51: 1059-1070, with a corrective notification on IJSEM (2001) vol 51: 1231-1233, describing the taxonomically reclassification of G. oxydans DSM 4025 as Ketogulonicigenium vulgare.

As used herein, resting cells refer to cells of a microorganism which are for instance viable but not actively growing, or which are growing at low specific growth rates [Il], for

instance, growth rates that are lower than 0.02 h-l, preferably lower than 0.01 h-1. Cells which show the above growth rates are said to be in a"resting cell mode".

The process of the present invention may be performed in different steps or phases: preferably, the microorganism is cultured in a first step (also referred to as step (a) or growth phase) under conditions which enable growth. This phase is terminated by changing of the conditions such that the growth rate of the microorganism is reduced leading to resting cells, also referred to as step (b), followed by the production of vitamin C from the substrate using the resting cells of (b), also referred to as production phase. <BR> <BR> <P>Growth and production phase may be performed in the same vessel, i. e. , only one vessel, or in two or more different vessels, with an optional cell separation step between the two phases. The produced vitamin C can be recovered from the cells by any suitable means.

Recovering means for instance that the produced vitamin C may be separated from the production medium. Optionally, the thus produced vitamin C may be further processed.

For the purpose of the present invention, the terms"growth phase", "growing step", "growth step"and"growth period"are used interchangeably herein. The same applies for the terms"production phase", "production step", "production period".

One way of performing the process of the present invention may be a process wherein the microorganism is grown in a first vessel, the so-called growth vessel, as a source for the resting cells, and at least part of the cells are transferred to a second vessel, the so-called production vessel. The conditions in the production vessel may be such that the cells transferred from the growth vessel become resting cells as defined above. Vitamin C is produced in the second vessel and recovered therefrom.

In one aspect, the growing step can be performed in an aqueous medium, i. e. the growth medium, supplemented with appropriate nutrients for growth under aerobic conditions.

The cultivation may be conducted, for instance, in batch, fed-batch, semi-continuous or continuous mode. The cultivation period may vary depending on the kind of cells, pH, temperature and nutrient medium to be used, and may be for instance about 10 h to about 10 days, preferably about 1 to about 10 days, more preferably about 1 to about 5 days when run in batch or fed-batch mode, depending on the microorganism. If the cells are grown in continuous mode, the residence time may be for instance from about 2 to about 100 h, preferably from about 2 to about 50 h, depending on the microorganism. If the microorganism is selected from bacteria, the cultivation may be conducted for instance at a pH of about 3.0 to about 9.0, preferably about 4.0 to about 9.0, more preferably about 4.0 to about 8.0, even more preferably about 5.0 to about 8.0. If algae or yeast are used, the cultivation may be conducted, for instance, at a pH below about 7.0,

preferably below about 6.0, more preferably below about 5.5, and most preferably below about 5.0. A suitable temperature range for carrying out the cultivation using bacteria maybe for instance from about 13°C to about 40°C, preferably from about 18°C to about 37°C, more preferably from about 13°C to about 36°C, and most preferably from about 18°C to about 33°C. If algae or yeast are used, a suitable temperature range for carrying out the cultivation may be for instance from about 15°C to about 40°C, preferably from about 20°C to about 45°C, more preferably from about 25°C to about 40°C, even more preferably from about 25°C to about 38°C, and most preferably from about 30°C to about 38°C. The culture medium for growth usually may contain such nutrients as assimilable carbon sources, e. g., glycerol, D-mannitol, D-sorbitol, L-sorbose, erythritol, ribitol, xylitol, arabitol, inositol, dulcitol, D-ribose, D-fructose, D-glucose, and sucrose, preferably L-sorbose, D-glucose, D-sorbitol, D-mannitol, and glycerol; and digestible nitrogen sources such as organic substances, e. g., peptone, yeast extract and amino acids.

The media may be with or without urea and/or corn steep liquor and/or baker's yeast.

Various inorganic substances may also be used as nitrogen sources, e. g., nitrates and ammonium salts. Furthermore, the growth medium usually may contain inorganic salts, e. g., magnesium sulfate, manganese sulfate, potassium phosphate, and calcium carbonate.

In the growth phase the specific growth rates are for instance at least 0.02 h-1. For cells growing in batch, fed-batch or semi-continuous mode, the growth rate depends on for instance the composition of the growth medium, pH, temperature, and the like. In general, the growth rates may be for instance in a range from about 0.05 to about 0.2 h-1, preferably from about 0.06 to about 0.15 h-1, and most preferably from about 0.07 to about 0.13 h-1.

In another aspect, resting cells may be provided by cultivation of the respective microorganism on agar plates thus serving as growth vessel, using essentially the same conditions, e. g., cultivation period, pH, temperature, nutrient medium as described above, with the addition of agar agar.

If the growth and production phase are performed in two separate vessels, then the cells from the growth phase may be harvested or concentrated and transferred to a second vessel, the so-called production vessel. This vessel may contain an aqueous medium supplemented with any applicable production substrate that can be converted to L- ascorbic acid by the cells. Cells from the growth vessel can be harvested or concentrated by any suitable operation, such as for instance centrifugation, membrane crossflow ultrafiltration or microfiltration, filtration, decantation, flocculation. The cells thus obtained may also be transferred to the production vessel in the form of the original broth from the growth vessel, without being harvested, concentrated or washed, i. e. in the form of a cell suspension. In a preferred embodiment, the cells are transferred from the

growth vessel to the production vessel in the form of a cell suspension without any washing or isolating step in-between.

Thus, in a preferred embodiment step (a) and (c) of the process of the present invention as described above are not separated by any washing and/or separation step.

If the growth and production phase are performed in the same vessel, cells may be grown under appropriate conditions to the desired cell density followed by a replacement of the growth medium with the production medium containing the production substrate. Such replacement may be, for instance, the feeding of production medium to the vessel at the same time and rate as the withdrawal or harvesting of supernatant from the vessel. To keep the resting cells in the vessel, operations for cell recycling or retention may be used, such as for instance cell recycling steps. Such recycling steps, for instance, include but are not limited to methods using centrifuges, filters, membrane crossflow microfiltration of ultrafiltration steps, membrane reactors, flocculation, or cell immobilization in appropriate porous, non-porous or polymeric matrixes. After a transition phase, the vessel is brought to process conditions under which the cells are in a resting cell mode as defined above, and the production substrate is efficiently converted into vitamin C.

The aqueous medium in the production vessel as used for the production step, hereinafter called production medium, may contain only the production substrate (s) to be converted into L-ascorbic acid, or may contain for instance additional inorganic salts, e. g., sodium chloride, calcium chloride, magnesium sulfate, manganese sulfate, potassium phosphate, calcium phosphate, and calcium carbonate. The production medium may also contain digestible nitrogen sources such as for instance organic substances, e. g., peptone, yeast extract, urea, amino acids, and corn steep liquor, and inorganic substances, e. g. ammonia, ammonium sulfate, and sodium nitrate, at such concentrations that the cells are kept in a resting cell mode as defined above. The medium may be with or without urea and/or corn steep liquor and/or baker's yeast. The production step may be conducted for instance in batch, fed-batch, semi-continuous or continuous mode. In case of fed-batch, semi-continuous or continuous mode, both cells from the growth vessel and production medium can be fed continuously or intermittently to the production vessel at appropriate feed rates. Alternatively, only production medium may be fed continuously or intermittently to the production vessel, while the cells coming from the growth vessel are transferred at once to the production vessel. The cells coming from the growth vessel may be used as a cell suspension within the production vessel or may be used as for instance flocculated or immobilized cells in any solid phase such as porous or polymeric matrixes. The production period, defined as the period elapsed between the entrance of the substrate into the production vessel and the harvest of the supernatant containing vitamin C, the so-called harvest stream, can vary depending for

instance on the kind and concentration of cells, pH, temperature and nutrient medium to be used, and is preferably about 2 to about 100 h. The pH and temperature can be different from the pH and temperature of the growth step, but is essentially the same as for the growth step.

In a preferred embodiment, the production step is conducted in continuous mode, meaning that a first feed stream containing the cells from the growth vessel and a second feed stream containing the substrate is fed continuously or intermittently to the production vessel. The first stream may either contain only the cells isolated/separated from the growth medium or a cell suspension, coming directly from the growth step, i. e. cells suspended in growth medium, without any intermediate step of cell separation, washing and/or isolating. The second feed stream as herein defined may include all other feed streams necessary for the operation of the production step, e. g. the production medium comprising the substrate in the form of one or several different streams, water for dilution, and base for pH control.

When both streams are fed continuously, the ratio of the feed rate of the first stream to feed rate of the second stream may vary between about 0. 01 and about 10, preferably between about 0.01 and about 5, most preferably between about 0.02 and about 2. This ratio is dependent on the concentration of cells and substrate in the first and second stream, respectively.

Another way of performing the process of the present invention may be a process using a certain cell density of resting cells in the production vessel. The cell density is measured as absorbance units (optical density) at 600 nm by methods known to the skilled person.

In a preferred embodiment, the cell density in the production step is at least about 10, more preferably between about 10 and about 200, even more preferably between about 15 and about 200, even more preferably between about 15 to about 120, and most preferably between about 20 and about 120.

In order to keep the cells in the production vessel at the desired cell density during the production phase as performed, for instance, in continuous or semi-continuous mode, any means known in the art may be used, such as for instance cell recycling by centrifugation, filtration, membrane crossflow ultrafiltration of microfiltration, decantation, flocculation, cell retention in the vessel by membrane devices or cell immobilization. Further, in case the production step is performed in continuous or semi-continuous mode and cells are continuously or intermittently fed from the growth vessel, the cell density in the production vessel may be kept at a constant level by, for instance, harvesting an amount of cells from the production vessel corresponding to the amount of cells being fed from the growth vessel.

The produced vitamin C contained in the so-called harvest stream is recovered/harvested from the production vessel. The harvest stream may include, for instance, cell-free or cell-containing aqueous solution coming from the production vessel, which contains vitamin C as a result of the conversion of production substrate by the resting cells in the production vessel. Cells still present in the harvest stream may be separated from the vitamin C by any operations known in the art, such as for instance filtration, centrifugation, decantation, membrane crossflow ultrafiltration or microfiltration, tangential flow ultrafiltration or microfiltration or dead end filtration. After this cell separation operation, the harvest stream is essentially free of cells.

In one aspect, the process of the present invention leads to yields of vitamin C which are at least about 1.8 g/1, preferably at least about 2.5 g/1, more preferably at least about 4.0 g/l, and most preferably at least about 5.7 g/l. In one embodiment, the yield of vitamin C produced by the process of the present invention is in the range of from about 1.8 to about 600 g/l. The yield of vitamin C refers to the concentration of vitamin C in the harvest stream coming directly out of the production vessel, i. e. the cell-free supernatant comprising the vitamin C.

In one embodiment of the present invention, vitamin C is produced by the process as described above using resting cells of recombinant microorganisms, such as for instance recombinant bacteria. Preferably, the recombinant bacteria are selected from bacteria that can express the L-sorbosone dehydrogenase as an active form in vivo, in particular bacteria of the genera Gluconobacter, Acetobacter, Pseudomonas and Escherichia, most preferred from Gluconobacter, Acetobacter or E. coli. Even more preferred are for instance G. oxydans and E. coli and the most preferred is selected from the group consisting of G. oxydans N44-1, G. oxydans IFO 3293 and G. oxydans IFO 3244. A recombinant microorganism may be any microorganism that is genetically engineered by well known techniques to contain one or more desired gene (s) on its chromosome or on a plasmid introduced into said microorganism, leading to, e. g., an overexpression of said gene (s).

The desired gene (s) which are introduced into said microorganism may code for instance for an enzyme involved in the conversion of a substrate to vitamin C. In a preferred embodiment, the desired gene encodes an L-sorbosone dehydrogenase, catalyzing the conversion of L-sorbosone to vitamin C. A preferred L-sorbosone dehydrogenase as used in the present invention is for instance the L-sorbosone dehydrogenase (SNDHai) of G. oxydans N44-1 (Sugisawa et al. , Agric. Biol. Chem. 54: 1201-1209,1990) as represented by SEQ ID NO : 2, the nucleotide sequence encoding said SNDHai is represented by SEQ ID NO : 1. Functional derivatives of said SNDHai can also be used for the purpose of the present invention.

Methods for isolation, purification, and cloning of nucleic acid molecules, as well as methods and techniques describing the use of prokaryotic hosts and nucleic acid protein expression therein, are known to the skilled person. Determination of nucleotide sequences can be done by standard methods. It is understood that nucleotide sequences having a homology of at least 80%, preferably of at least 90%, compared with SEQ ID NO : 1 and which code for enzymes able to catalyze the conversion of L-sorbosone to vitamin C are also part of the present invention.

To express the desired gene/nucleotide sequence efficiently, various promoters may be used; e. g., the original promoter of the gene, promoters of antibiotic resistance genes such as for instance kanamycin resistant gene of Tn5, ampicillin resistant gene of pBR322, and beta-galactosidase of E. coli (lac), trp-, tac-, trc-promoter, promoters of lambda phage and any promoters which may be functional in the host cell.

For expression, other regulatory elements, such as for instance a Shine-Dalgarno (SD) sequence (e. g., AGGAGG and so on including natural and synthetic sequences operable in the host cell) and a transcriptional terminator (inverted repeat structure including any natural and synthetic sequence) which are operable in the host cell (into which the coding sequence will be introduced to provide a recombinant cell of this invention) may be used with the above described promoters.

A wide variety of host/cloning vector combinations may be employed in cloning the double stranded DNA. Preferred vectors for the expression of the SNDHai gene in E. coli may be selected from any vectors usually used in E. coli, such as for instance pQE vectors which can express His-tagged recombinant proteins (QIAGEN AG Switzerland), pBR322 or its derivatives including for instance pUC18 and pBluescript II (Stratagene Cloning Systems, Cali£, USA), pACYC177 and pACYC184 and their derivatives, and a vector derived from a broad host range plasmid such as RK2 and RSF1010. A preferred vector for the expression of the nucleotide sequence of the present invention in bacteria including Gluconobacter, Acetobacter, and Pseudomonas is selected from any vectors which can replicate in Gluconobacter, Acetobacter, or Pseudomonas as well as in a preferred cloning organism such as E. coli. The preferred vector is a broad-host-range vector such as for instance a cosmid vector like pVK100 and its derivatives and RSF1010.

Copy number and stability of the vector should be carefully considered for stable and efficient expression of the cloned gene and also for efficient cultivation of the host cell carrying the cloned gene. Nucleic acid molecules containing for instance transposable elements such as Tn5 may also be used as a vector to introduce the desired gene into the preferred host, especially on a chromosome. Nucleic acid molecules containing any DNAs isolated from the preferred host together with the SNDHai gene of the present invention may be also useful to introduce this gene into the preferred host cell, especially

on a chromosome. Such nucleic acid molecules may be transferred to the preferred host by applying any of a conventional methods, e. g., transformation, transduction, conjugal mating or electroporation, which are well known in the art, considering the nature of the host cell and the nucleic acid molecule.

The L-sorbosone dehydrogenase gene/nucleotide sequences provided in this invention may be ligated into a suitable vector containing a regulatory region such as for instance a promoter, a ribosomal binding site, and a transcriptional terminator operable in the host cell described above with a well-known method in the art to produce an expression vector.

To construct a recombinant microorganism carrying an expression vector, various gene transfer methods including for instance transformation, transduction, conjugal mating, and electroporation may be used. The method for constructing a recombinant cell may be selected from the methods well-known in the field of molecular biology. For instance, conventional transformation systems may be used for Glticonobacter, Acetobacter, Pseudomonas, or Escherichia. A transduction system may also be used for E. coli.

Conjugal mating system may be widely used in Gram-positive and Gram-negative bacteria including for instance E. coli, P. putida, and Gluconobacter. An example of conjugal mating is disclosed in WO 89/06,688. The conjugation may occur in for instance liquid medium or on a solid surface. Examples for a recipient for SNDHai production include for instance microorganisms of Gluconobacter, Acetobacter, Pseudomonas, or Escherichia. To the recipient for conjugal mating, a selective marker may be added; e. g., resistance against nalidixic acid or rifampicin is usually selected.

Natural resistance may also be used, e. g., resistance against polymyxin B is useful for many Gluconobacters.

The recombinant microorganism, such as for example G. oxydans N44-1, may comprise several copies of SNDHai cloned on a suitable plasmid or integrated on its chromosome.

Plasmid copies which may be suitable for the present invention are for instance in the range of about 2 to about 15, preferably in the range of about 5 to about 10 per transformed microorganism. The number of plasmid copies may be determined by, for instance, comparison of the intensity of a respective band visible on SDS-PAGE.

When using recombinant microorganisms for the process of the present invention, the growth and production step can be essentially the same as described above. If a recombinant microorganism comprising SNDHai is used, such as for example recombinant G. oxydans N44-1 with increased SNDHai dosage, the growth medium may contain for instance D-sorbitol, L-sorbose, glycerol or D-glucose either alone or mixtures thereof, one or more suitable nitrogen sources and salts. The production medium may

contain for instance D-sorbitol and/or L-sorbose and salts. Harvesting of vitamin C can be performed as essentially described herein.

In a further aspect, the process of the present invention may be combined with further steps of separation and/or purification of the produced vitamin C from other components contained in the harvest stream, i. e., so-called downstream processing steps.

These steps may include any means known to a skilled person, such as, for instance, concentration, crystallization, precipitation, adsorption, ion exchange, electrodialysis, bipolar membrane electrodialysis and/or reverse osmosis. Vitamin C may be further purified as the free acid form or any of its known salt forms by means of operations such as for instance treatment with activated carbon, ion exchange, adsorption and elution, concentration, crystallization, filtration and drying. Specifically, a first separation of vitamin C from other components in the harvest stream might be performed by any suitable combination or repetition of, for instance, the following methods: two-or three- compartment electrodialysis, bipolar membrane electrodialysis, reverse osmosis or adsorption on, for instance, ion exchange resins or non-ionic resins. If the resulting form of vitamin C is a salt of L-ascorbic acid, conversion of the salt form into the free acid form may be performed by for instance bipolar membrane electrodialysis, ion exchange, simulated moving bed chromatographic techniques, and the like. Combination of the mentioned steps, e. g., electrodialysis and bipolar membrane electrodialysis into one step might be also used as well as combination of the mentioned steps e. g. several steps of ion exchange by using simulated moving bed chromatographic methods. Any of these procedures alone or in combination constitute a convenient means for isolating and purifying the product, i. e. vitamin C. The product thus obtained may further be isolated in a manner such as, e. g. by concentration, crystallization, precipitation, washing and drying of the crystals and/or further purified by, for instance, treatment with activated carbon, ion exchange and/or re-crystallization.

In a preferred embodiment, vitamin C is purified from the harvest stream by a series of downstream processing steps as described above without having to be transferred to a non-aqueous solution at any time of this processing, i. e. all steps are performed in an aqueous environment. Such preferred downstream processing procedure may include for instance the concentration of the harvest stream coming from the production vessel by means of two-or three-compartment electrodialysis, conversion of vitamin C in its salt form present in the concentrated solution into its acid form by means of bipolar membrane electrodialysis and/or ion exchange, purification by methods such as for instance treatment with activated carbon, ion exchange or non-ionic resins, followed by a further concentration step and crystallization. These crystals can be separated, washed and dried. If necessary, the crystals may be again re-solubilized in water, treated with

activated carbon and/or ion exchange resins and recrystallized. These crystals can then be separated, washed and dried.

Example 1 : Production of L-ascorbic acid from L-sorbosone using cells grown on mannitol broth agar medium IFO strains 3293,3292, 3244,3260, 3266,3287, 3259,13693, and 13773 as well as Acetobacter sp. ATCC 15164 and Gluconobacter oxydans N44-1, a derivative of the strain IFO 3293, were used for the production of L-ascorbic acid from L-sorbosone.

Strains IFO 13693 and IFO 13773 were grown at 27°C for 3 days on No. 350 medium containing 5 g/1 Bactopeptone (Difco), 5 g/1 yeast extract (Difco), 5 g/l glucose, 5 g/l mannitol, 1 g/1 MgS04-7H20, 5 ml/I ethanol, and 15 g/1 agar. All other Acetobacter strains and all Gluconobacter strains were grown at 27°C for 3 days on mannitol broth (MB) agar medium containing 25 g/1 mannitol, 5 g/1 yeast extract (Difco Laboratories, Detroit, Mich. , USA), 3 g/l Bactopeptone (Difco), and 18 g/l of agar (Difco).

Cells were scraped from the agar plates, suspended in distilled water and used for resting cell reactions conducted at 30°C for 20 h in 5 ml tubes with shaking at 230 rpm. The reaction mixtures (0.5 ml) contained 1% L-sorbosone, 0.3% NaCI, 1% CaCO3 and cells at a final concentration of 10 absorbance units at 600 nanometers (OD600). At the conclusion of the incubation period, the reaction mixtures were analyzed by high performance liquid chromatography (HPLC) using an Agilent 1100 HPLC system (Agilent Technologies, Wilmington, USA) with a LiChrospher-100-RP18 (125 x 4.6 mm) column (Merck, Darmstadt, Germany) attached to an Aminex-HPX-78H (300 x 7.8 mm) column (Biorad, Reinach, Switzerland). The mobile phase was 0.004 M sulfuric acid, and the flow rate was 0.6 ml/min. Two signals were recorded using an UV detector (wavelength 254 nm) in combination with a refractive index detector. In addition, the identification of the L-ascorbic acid was done using an amino-column (YMC-Pack Polyamine-II, YMC, Inc., Kyoto, Japan) with UV detection at 254 nm. The mobile phase was 50 mM NH4H2PO4 and acetonitrile (40: 60).

An Agilent Series 1100 HPLC-mass spectrometry (MS) system was used to identify L- ascorbic acid. The MS was operated in positive ion mode using the electrospray interface. The separation was carried out using a LUNA-C8 (2) column (100 x 4.6 mm) (Phenomenex, Torrance, USA). The mobile phase was a mixture of 0.1% formic acid and methanol (96: 4). L-Ascorbic acid eluted with a retention time of 3.1 minutes. The identity of the L-ascorbic acid was confirmed by retention time and the molecular mass of the compound.

To exclude the presence of D-isoascorbic acid, the identification of L-ascorbic acid was additionally done by retention time using an amino-column (YMC-Pack Polyamine-II, <BR> <BR> YMC, Inc. , Kyoto, Japan) with UV detection at 254 nm. The mobile phase was 50 mM NH4H2PO4 and acetonitrile (40: 60).

The Gluconobacter and Acetobacter strains produced L-ascorbic acid from L-sorbosone as shown in Table 1.

Table 1. Production of L-ascorbic acid from L-sorbosone Strain L-ascorbic acid (mg/L) G. oxydans IFO 3293 1740 G. oxydans N44-1 570 G. oxydans IFO 3292 410 G. oxydans IFO 3244 1280 G. frateurii IFO 3260 50 G. cerinus IFO 3266 140 G. oxydans IFO 3287 60 A. aceti subsp. Orleanus IFO 3259 30 A. aceti subsp. Xylinum IFO 13693 40 A. aceti subsp. Xylinum IFO 13693 120 Acetobacter sp. ATCC 15164 310 Blank Not detected

Blank; reaction was done in the reaction mixture without cells.

Example 2: Production of L-ascorbic acid from D-sorbitol, L-sorbose or L-sorbosone using cells grown on 3BD agar medium Cells of G. oxydans N44-1 were grown at 27°C for 3 days on No. 3BD agar medium containing 70 g/1 L-sorbose, 0.5 g/l glycerol, 7.5 g/l yeast extract (Difco), 2.5 g/l MgSO4 7H2O, 10 g/l CaC03 and 18 g/l agar (Difco). The resting cell reactions (1 ml

reaction mixture in 10 ml tube) were carried out with 2% D-sorbitol, 2% L-sorbose, or 1% L-sorbosone at 30°C for 24 h as described in Example 1. Strain N44-1 produced 280, 400 and 1780 mg/1 of L-ascorbic acid from D-sorbitol, L-sorbose, and L-sorbosone, respectively.

Other reactions (0.5 ml reaction mixture in 10 ml tube) were carried out with N44-1 cells grown on No. 3BD agar medium in reaction mixtures containing 2% D-sorbitol, 2% L- sorbose or 2% L-sorbosone for 2 days as described in Example 1. Strain N44-1 produced 1.8, 2.0 and 5.1 g/1 of L-ascorbic acid from D-sorbitol, L-sorbose, and L-sorbosone, respectively.

A reaction using cells of G. oxydans IFO 3293 was carried out with 2% L-sorbosone as described above. Strain IFO 3293 produced 5.7 g/l of L-ascorbic acid in 2 days.

Example 3: Production of L-ascorbic acid from D-sorbitol using cells grown in liquid medium Cells of G. oxydans N44-1 were grown in 200 ml of No. 5 medium containing 100 g/1 D- sorbitol, 0.5 g/l glycerol, 15 g/l yeast extract (Fluka BioChemika, Buchs, Switzerland), 2.5 g/1 MgS04-7H20 and 15 g/l CaCO3 in a 2-1 baffled shake flask at 30°C with shaking at 180 rpm. After 24 h, the culture was centrifuged at 3220 g (Eppendorf 5810R, Hamburg, Germany), and the cells were resuspended in 0.9% NaCI solution, centrifuged again at 3220 g and the cell pellet was used to inoculate one baffled 500 ml shake flask containing 50 ml of full growth medium (100 g/l D-sorbitol, 0.5 g/l glycerol, 15 g/l yeast extract, 2.5 g/1 MgSO¢. 7H20,15 g/l CaC03) and another baffled 500 ml shake flask containing 50 ml production medium (100 g/l D-sorbitol, 3 g/l NaCI, 10 g/l CaC03). The initial cell density, measured as optical density at 600 nm (OD600), in both flasks was 10. Both flasks were incubated at 30°C with shaking at 180 rpm. After 48 h, the cell suspension in growth medium and production medium had accumulated 1.06 and 1.18 g/1 L-ascorbic acid, respectively. No additional growth was observed in full medium during the incubation period time.

Example 4: Production of L-ascorbic acid from L-sorbosone or D-sorbitol by recombinant microorganisms with increased SNDHai gene dosage The SNDHai gene of G. oxydans N44-1 (SEQ ID NO : 1) with upstream and downstream flanking regions was amplified by PCR with chromosomal DNA of strain N44-1 as template and the primer set N1 (SEQ ID NO : 3) and N2 (SEQ ID NO : 4).

The PCR was done with the GC-rich PCR system (Roche Diagnostics GmbH) according to the instructions of the supplier. The amplified DNA fragment was inserted into vector

pCR2. 1-TOPO (Invitrogen, Carlsbad, CA, USA). The resulting plasmid was then digested with HindIII and XhoI. The HindIII-XhoI fragment including the SNDHai gene was ligated to vector pVK100 (available from the American Type Culture Collection, catalog no. ATCC 37156) previously treated with HindIII and XhoI. The ligation mixture was used to transform E. coli TG1. The desired plasmid, designated pVK-P-SNDHai-T, was isolated from E. coli, and introduced into G. oxydans strain N44-1 by electroporation using standard methods (Electrocell manipulator ECM600, BTX Inc. , San Diego, CA, USA).

Three independent transformants, designated N44-1 (pVK-P-SNDHai-T) done number 1,2, and 3, together with the parental strain G. oxydans N44-1, were each grown on No.

3BD agar and MB agar media. The cells were scraped from the plates and used for resting cell reactions (1% L-sorbosone as the substrate) as described in Example 1. After growth on No. 3BD agar, in the resting cell assay strain N44-1 produced 2.5 g/l L- ascorbic acid, while strains N44-1 (pVK-P-SNDHai-T) clones 1,2 and 3 produced 4.2, 4.1 and 4.2 g/l L-ascorbic acid, respectively. After growth on MB agar, in the resting cell assay strain N44-1 produced 0.12 g/1 L-ascorbic acid, while strains N44-1 (pVK-P- SNDHai-T) clones 1, 2 and 3 produced 1.8, 2.5 and 0.94 g/1 L-ascorbic acid, respectively.

Another reaction was carried out using cells of G. oxydans N44-1 and clone 2 (see above) cultivated in 50 ml of No. 5 medium (100 g/l D-sorbitol, 0.5 g/l glycerol, 15 g/l yeast extract, 2. 5 g/l MgSO4-7H2O, 15 g/l CaCO3) in duplicate 500 ml baffled shake flasks at 30°C with shaking at 220 rpm for 3 days. From one flask for each strain, the resulting broth was centrifuged at 500 rpm to remove CaCO3. The supernatant from this step was then centrifuged at 5,000 rpm to pellet the cells. The collected cells were re-suspended in 10 ml of 0.9% NaCl solution, and again centrifuged at 5,000 rpm to pellet the cells. The collected cells were re-suspended in water and used to inoculate 1 ml of production medium (20 g/l D-sorbitol, 3 g/l NaCl, 10 g/l CaCO3) in 10 ml reaction tube at a final resting cell density corresponding to 5 OD units at 600 nm. After 20 h reaction time at 30°C and 220 rpm, the supernatant harvested from the production flask contained 360 and 760 mg/1 L-ascorbic acid, respectively for strains N44-1 and N44-1 overexpressing SNDHai. In contrast, after 72 h the supernatant harvested from the remaining growth medium contained 0 and 440 mg/1 L-ascorbic acid, respectively.

Example 5: Production of L-ascorbic acid from L-sorbosone in E. coli The SNDHai gene without stop codon named SNDHai-1, corresponding to nucleotides 1-2364 of SEQ ID NO : 1, was amplified from strain N44-1 chromosomal DNA by PCR (Roche High Fidelity kit) using the primer pair SNDHai-Nde (SEQ ID NO : 5) and SNDHaiHis-X (SEQ ID NO : 6).

The amplified DNA was cloned into pCR2. 1-TOPO (Invitrogen, Carlsbad, CA, USA) to obtain pCR2. 1-TOPO-SNDHai-1, whose SNDHai sequence was confirmed to be correct by nucleotide sequencing. Then the SNDHai-1 gene was cut out with NdeI and XhoI and ligated between NdeI and XhoI sites of pET-21b (+) (Novagen, Madison, WI, USA) to produce pET21b-SNDHaiHis ; 6xHis was added at the C-terminus of SNDHai. The pET21b-SNDHaiHis was introduced into E. coli BL21 (DE3).

Five ml of one overnight culture of E. coli BL21 (DE3)/pET2lb-SNDHaiHis in LB with carbenicillin 501lg/ml was inoculated into 200 ml of the same medium. The cells were cultivated at 230 rpm at 37°C for 2 h, then induced with 1 mM IPTG and continued to be cultivated at 230 rpm at 25°C for 3 h. The resulting culture was centrifuged and washed twice with saline and the cell pellet was resuspended in 2 ml of water. The cells were used for resting cell reaction with the reaction mixture (500 ul in 5 ml tube) containing cells at OD600=10,1% sorbosone monohydrate, 5 uM PQQ, 5mM MgCl2, 0.3% NaCI, and 1% CaCO3 conducted at 30°C for 15 h. 0.14 g/L of L-ascorbic acid was produced after incubation for 15 h. When the resting cell reaction was done with 1 uM PQQ (the other conditions were same as those described above), 0.05 g/L of L-ascorbic acid was produced after incubation for 3 h.

Example 6: Production of L-ascorbic acid from D-sorbitol by recombinant microorganisms with increased SNDHai gene dosage Cells of G. oxydans N44-1 overexpressing SNDHai are grown in 50 ml of No. 5 medium containing 100 g/1 D-sorbitol, 0.5 g/l glycerol, 15 g/l yeast extract (Fluka BioChemika, Buchs, Switzerland), 2.5 g/1 MgS04-7H20 and 15 g/1 CaC03 in a 500-ml baffled shake flask at 30°C with shaking at 180 rpm for 48 h. The resulting cell suspension is used to inoculate a 2-L bioreactor, called growth vessel (Biostat-MD, B. Braun Melsungen, Melsungen, Germany) containing 1. 251 of medium composed of 100 g/1 D-sorbitol, 15 g/l yeast extract (Fluka BioChemika, Buchs, Switzerland), 2.5 g/1 MgS04-7H20, 0.3 g/l KH2PO4 and 0.12 g/1 CaS04. Cells are cultivated at 30°C, 11/min aeration rate, the pH is controlled to 5.7 with a 25% solution of Na2CO3, dissolved oxygen is controlled to 10% saturation by varying the stirring speed. After 24 h, the cell density measured as absorption units at 600 nm is 20. At this time point, a feed solution containing 100 g/l D-sorbitol, 15 g/1 yeast extract (Fluka BioChemika, Buchs, Switzerland), 2.5 g/l MgS04-7H20, 0.3 g/l KH2PO4 and 0.12 g/1 CaS04 is fed into the growth vessel at a feed rate of 125 ml/h, and broth is continuously harvested at a harvest rate of 125 ml/h. By this means, the volume in the growth vessel is kept constant at 1. 25 1. Other process parameters continue to be controlled as mentioned above.

This broth is continuously fed at a rate of 125 ml/h into a second reactor, called production vessel, filled with 51 production medium containing 100 g/l D-sorbitol, 0.3 g/1 NaCl and 0.12 g/1 CaS04, and the temperature is kept at 30°C, pH at 7.0 by controlling with a 20% solution of NaOH. The aeration rate is kept constant at 101/min, and dissolved oxygen is controlled at 20% by varying the stirrer speed. Production medium with the same composition is also continuously fed to the production vessel at a feed rate of 375 ml/h. The vessel volume is kept constant at 51 by continuously harvesting supernatant at 500 ml/h rate, resulting as a filtrate stream from a crossflow ultrafiltration module with 500 kDa pore size (UFP-500-E-9A, Amersham Biosciences), through which the cell suspension harvested from the production vessel is pumped at 501/h using a Masterflex pump. The retentate flow is pumped back into the vessel. Once the cell density in the production vessel reaches 100 absorption units at 600 nm, cells start to be harvested from the concentrated cell stream flowing back into the production vessel at a rate of 25 ml/h, in order to keep the cell density in the production vessel constant.

The harvest stream of cell-free supernatant contains 4 g/1 L-ascorbic acid and is continuously fed at a rate of 500 ml/h into a collecting vessel with a double jacket at 30°C (Ecoline Rel 12, Lauda, Lauda-Koenigshofen, Germany). This vessel feeds continuously supernatant to the diluate compartment of a two-compartment electrodialysis unit (stack containing 10 cell pairs with cation exchange membranes CMX-S and anion exchange membranes ASM, total membrane area 0.2 m, from Eurodia Industries, Wissous, France) at a rate of 1801/h, and a constant stream is pumped out of the vessel to keep its volume constant at 2 1. Another vessel with double jacket containing initially deionized water at 30°C is continuously fed with fresh deionized water at a rate of 62.5 ml/h, pumps constantly aqueous solution into the concentrate compartment of the electrodialysis unit at a rate of 2001/h, and a constant harvest stream is pumped out of the vessel. Feed solutions are pumped to the electrodialysis stack using peristaltic pumps (7518-00, Masterflex, USA), and recirculation of solutions through each electrodialysis compartment is done with help of rotary pumps (MD-20, IWAK, Tokyo, Japan). During the whole process, 14 V are applied to the electrodialysis stack (power source FuMATech TS001/5, St. Ingbert, Germany). The concentration of L-ascorbic acid in the harvest stream is 16 g/1.

Example 7: Purification of L-ascorbic acid produced by a resting cell reaction via downstream processing steps The harvest stream of Example 6 containing 16 g/1 L-ascorbic acid is fed to a chelating resin (Amberlite IRC 748, Rohm and Haas, Philadelphia, PA, USA) to eliminate divalent cations from the stream. It is then collected in a cooled vessel (feed vessel), and when 10 1 have been collected, they are processed in batch mode through a bipolar membrane

electrodialysis unit (stack containing 7 Neosepta BP1/CMB membranes, total membrane area 0.14 m, from Eurodia Industries, Wissous, France). This solution is pumped at 2001/h through the feed compartment of the electrodialysis unit, and recycled into the feed vessel. Another cooled vessel (concentrate vessel) containing initially 51 of a 2 g/l NaOH solution is pumped at 1001/h through the concentrate compartment of the bipolar membrane electrodialysis unit. By applying a maximal voltage of 25 V and maximal electric current of 20 A, sodium cations from the feed compartment are transferred to the concentrate compartment, and thus the sodium form of L-ascorbic acid present in the feed stream is converted into the corresponding free acid form. After reaching 90% conversion yield, the process is stopped. In the concentrate vessel, 61 of solution containing 7.5 g/1 NaOH are collected in the diluate vessel, 91 solution containing about 16 g/l L-ascorbic acid in its free acid form and 1.6 g/1 L-ascorbic acid in its sodium salt form are further processed through a cation exchange resin (Amberlite FPC 21 H, Rohm and Haas, Philadelphia, PA, USA), in order to increase conversion yield of the sodium salt into the free acid form to about 99%. Alternatively, the 101 solution containing 16 g/1 L-ascorbic acid in its sodium salt form coming from the electrodialysis step is directly treated by cation exchange resin, being converted to the free acid form at 99% yield. The stream of L-ascorbic acid in the form of the free acid, obtained by either of the methods described above, is then further processed by a sequence of the following steps: anion exchange, activated carbon treatment, concentration, crystallization, filtration of the crystals, and drying. The final purity of the obtained crystals is 98%, and the yield obtained with the combined downstream processing steps is 80%.