GB2408262A - Preparation of L-arabinose from sugar beet - Google Patents

Abstract

A method of preparing L-arabinose from sugar beet pulp comprises the steps of (a) treating sugar beet pulp with aqueous alkali to provide a crude araban extract; (b) treating the extract with a precipitating agent, such as carbon dioxide, to precipitate an insoluble salt from the extract; (c) removing the insoluble salt; (d) ultra-filtration to remove impurities and concentrate the extract; (e) acid hydrolysis of the purified extract to provide a crude L-arabinose solution; (f) ultra-filtration and neutralisation (in any order) of the solution to provide a purified L-arabinose solution, and (g) concentration and crystallisation of pure L-arabinose. Preferably, the method does not comprise a chromatographic purification step. The L-arabinose solution may also undergo treatment with an anionic exchange resin to remove anionic impurities and may be contacted with a polymeric adsorption resin to decolourise the solution. The crystalline product typically comprises 97-99% L-arabinose.

Description

A METHOD OF PREPARATION OF L-ARABINOSE

The present invention relates to methods of preparation of purified Larabinose starting from sugar beet pulp.

L-(+)-arabinose is present in polysaccharide components of higher plants. For example, it is present in naturally occurring plant gums such as gum arable and mesquite gum. The polysaccharide araban, which comprises mainly L-arabinose units, is associated with pectin in the cell walls of many higher plants, and is lo abundant in sugar beet pulp.

L-arabinose is by far the most abundant naturally occurring L-pentose. It is therefore a useful starting point for the chemical synthesis of other L-pentoses and derivatives thereof. A need therefore exists for improved methods of preparing L arabinose from plant material, in particular from sugar beet pulp, with high purity and at low cost.

Sugar beet pulp is the name given to the residue remaining after conventional extraction of sucrose from sugar beet. Pressed pulp typically contains about 25% by weight of dry matter, most of which consists of plant polysaccharides. A component of the polysaccharides is araban, which consist of a chain of 1, 5-a- linked L-arabinose residues, to which other L-arabinose residues are linked (1,3)-a and/or (1,2)-a. Pure araban normally contains no more than 2-3% wt.% of non- arabinose residues.

GB-A-1182099 describes a method of preparation of L-arabinose from sugar beet by treating sugar beet slices with acid or alkali to hydrolyse the pectin and araban, filtering, and then fermenting the filtrate with yeasts to remove glucose and fructose. The remaining solution is concentrated, and the arabinose is then crystallized by addition of alcohol.

DD-C-143261 describes a process for the extraction of L-arabinose from sugar beet pulp comprising the steps of: treating sugar beet pulp with aqueous calcium hydroxide to dissolve the araban, filtering off the araban solution, treating the araban solution with sulfuric acid to hydrolyse the araban to its constituent monosaccharides, neutralising the solution and separating the L-arabinose by crystallization. The resulting L-arabinose has low purity.

US-A-4591388 describes a process for the purification of arabinose from a mixture of monosaccharides which comprises the selective adsorption of arabinose on barium-exchanged, type-X zeolite molecular sieves. This method results in some barium contamination of the arabinose, which is undesirable. Furthermore, the to method is not readily adaptable to continuous manufacture.

EP-A-0276702 describes a process for the production of crystalline Larabinose from sugar beet pulp. The process comprises treating the beet pulp with aqueous calcium hydroxide to extract crude aqueous araban, carrying out chromatography on a cationic exchanger to purify the araban extract, followed by hydrolyzing the purified araban fraction with sulfuric acid, neutralizing, and carrying out chromatography again on a cation exchanger to purify the L-arabinose. This process is similar to that of DD-C-143261, but with two additional chromatography steps. The resulting L-arabinose is pure, but the cost of carrying out two chromatography steps is high, and it is difficult to carry out chromatography steps in a continuous process unless relatively complex technology, such as a simulated moving bed, is used.

WO99/10542 describes a further method of preparing L-arabinose from sugar beet pulp. The claimed method comprises treatment of the pulp with aqueous lime to extract aqueous araban, hydrolysis of the crude araban extract with a strong acid, neutralization and filtration of the resulting solution, followed by chromatographic separation of the L- arabinose by using a cation exchanger in monovalent metal form, followed by final purification of the L-arabinose. It is alleged that the use of a cation exchange resin in monovalent metal ion form is especially effective for separation of L-arabinose, and that it is therefore unnecessary to carry out any purification on the crude araban extract before hydrolysis. However, this method still suffers from the drawbacks of the high cost and non-continuous nature of the chromatographic separation step.

The present inventors have found that L-arabinose can be prepared from sugar beet pulp with high purity, in a continuous process, without any need for a chromatographic separation step.

The present invention provides a method of preparing L-arabinose from sugar beet pulp comprising the steps of: (a) treatment of a sugar beet pulp with aqueous alkali to provide a crude aqueous araban extract; (b) treatment of the crude araban extract with a precipitating agent to precipitate an insoluble salt from the araban extract; (c) removal of the insoluble salt sludge; (d) ultra-filtration to remove low molecular weight impurities and concentrate the araban extract; (e) acid hydrolysis of the purified araban extract after ultra-filtration to provide a crude aqueous L-arabinose solution; (0 ultra-filtration of the crude L-arabinose solution to remove high molecular weight impurities and provide a purified L-arabinose solution; (g) neutralization of the L-arabinose solution; and (h) concentration and crystallization of the purified L-arabinose solution to provide a purified L-arabinose.

The steps are generally carried out in the order specified above, optionally with further steps inserted as discussed further below. In some embodiments, the order of steps (f) and (g) may be reversed.

Use of ultra-filtration stages on both the araban extract and the Larabinose solution after hydrolysis enables the preparation of high purity L-arabinose, without chromatography.

The starting material for the process according to the present invention may be any sugar beet pulp from which sucrose has already been extracted by diffusion.

The process can start from stored pulp, frozen pulp or dried shreds. The pulp is preferably washed with excess water at about 50 C to about 70 C, suitably for a period of from about 1 to about 2 hours. This step removes residual sucrose and soluble impurities. The pulp is then dewatered. The dewatered pulp typically has a solids content of from about 10% to about 30%, for example about 20% to about 25% by weight. This dewatered pulp is then fed to step (a) of the process.

Step (a) of the process is extraction of araban from the beet pulp into alkali solution. The alkali is usually hot aqueous lime. The lime may be added as calcium oxide or calcium hydroxide. The alkali is preferably added to hot aqueous beet pulp, preferably already heated to at least about 60 C, more preferably to at least about 80 C, since adding the lime to cold pulp can result in the precipitation of calcium pectinates and hydrolysis of pectins to uranic acids. Suitable conditions are: temperature about 80 C to about 100 C, preferably about 90 C to about 95 C; pH about 10 to about 12, preferably pH about 11; time about 45 minutes to about 3 hours, preferably about 1 hour; solids content about 2 wt.% to about 10 wt.%, suitably about 4 wt.% to about 6 wt.%.

Following step (a), the alkali solution of araban is separated from the pulp sludge, for example by decanting. The araban solution is then treated to neutralise the solution and precipitate an insoluble salt of the alkali. The treatment may for example be carried out by treatment with a phosphoric acid. Preferably, the neutralization and precipitation is carried out with carbon dioxide (carbonatation).

The CO2 reacts with calcium hydroxide to precipitate calcium carbonate. The calcium carbonate precipitate also contains a number of impurities. The carbonatation is suitably carried out in similar fashion to the wellknown carbonatation step used for the purification of sugar beet juice in the manufacture of sucrose. Typical conditions are: temperature about 70 C to about 90 C, preferably about 80 C; time about 10 minutes to about 60 minutes, suitably about minutes to about 40 minutes; pH (final) about 6 to about 9, suitably about 8 to about 9. s

The precipitation step (b) may be followed by conventional filtration and/or centrifugation steps to remove the insoluble salt precipitate sludge and other insoluble matter and entrained impurities. Preferably, the process comprises S centrifugation, for example with a disk-stack centrifuge, followed by filtration, for example a plate-filtration or cartridge filtration stage, for example through 8 micrometer plate, to remove small particles of insoluble matter from the araban solution.

to The crude aqueous araban extract is then further purified by ultrafiltration. The ultra-filtration is preferably carried out using a membrane having a nominal molecular weight cut off of from about 5,000 to about 50,000, suitably from about 5,000 to about 30,000, depending on the membrane type. The ultrafiltration is generally carried out with a cross-flow of the solution in order to minimise filter blocking. The ultra-filtration step has two important functions. Firstly, it concentrates the araban in the retentate from an initial concentration of from about 2% to about 4% by weight to a final concentration of from about 10% to about 20% araban by weight. The ultra-filtration also removes low molecular weight impurities into the permeate. In particular, it removes saccharides and soluble salts such as calcium acetate.

Preferably, the ultra-filtration step is followed by a diafiltration step. That is to say, the concentrated araban extract from the ultrafiltration step is diluted with water and then ultra-filtered again to concentrate the araban as above, and to achieve a further reduction in the concentration of low molecular weight impurities. The diafiltration step may be carried out using a diafiltration membrane having similar nominal molecular weight cut-off values as the ultrafiltration membrane.

Typically, the process according to the present invention does not comprise any concentration step on the araban solution other than the ultra-filtration step and optional diafiltration step. The process steps described above result in a solution comprising typically about 10% to about 20% araban by weight, together with some large anionic impurities, some large organic impurities and some polysaccharide impurities, in particular galactan. Overall, the purified araban extract typically comprises (mainly as polysaccharides) about 65-75 wt.% arabinose, about 815 wt.% D-galactose, about 3-6% ash, about 4-7wt.% proteins, and the balance comprising other saccharides such as uranic acids, S rhamnose Dxylose, D-glucose and D-fructose.

Preferably, without further concentration, and preferably without any chromatographic purification step, the araban extract then undergoes hydrolysis in step (e) of the process according to the present invention.

The purified araban extract is treated with sulfuric acid in conventional fashion to hydrolyse the polysaccharides. The hydrolysis is carried out with H2SO4 in an amount of about 5 to about 10wt.%, preferably about 7 to about 8wt.%, calculated as pure H2SO4 on total dry substance in the araban extract. The hydrolysis is carried out preferably at a temperature of from about 90 C to about 100 C, typically at from about 93 C to about 97 C, for a time of from about 10 minutes to about 2 hours, suitably 30 minutes to about 90 minutes, for example about 1 hour.

The conditions preferably result in hydrolysis of the araban, but not of the galactan.

The hydrolysed solution having dry solids content typically about 7 to 9 wt.%, of which about 60% is L-arabinose, is then subjected to ultrafiltration through a membrane, usually by cross-flow filtration. The membrane suitably has a nominal molecular weight cut-off in the range of 1000-30,000, for example about 5000 to 30,000. The galactan and other non-hydrolysed polysaccharides are retained, and the L-arabinose is removed in the permeate stream at a total solids content of about 6 to 8 wt.%, or which about 75% to about 90% is L-arabinose. The permeate is substantially free of saccharides other than L-arabinose.

Preferably, the retentate is subjected to little or no diafiltration, since the water added for diafiltration results in further dilution of the L-arabinose stream.

Neutralisation of the acidic L-arabinose solution is achieved by anion exchange, for example on a suitable, commercially available resin. In fact, this may take the pH above 9 and additional acid is required to bring the pH back to about 7.

Currently this step is carried out after the ultrafiltration but it may be preferable to carry out the neutralization before the ultrafiltration in order to minimise acid damage to the membranes. The anionic exchange resin also has the effect of removing anionic impurities such as acetate from the solution.

The purified L-arabinose solution from the ultra-filtration and optional diafiltration steps still contains some impurities, such as salts and colour impurities.

Therefore, the method of the invention preferably further comprises treating the L- arabinose solution with a polymeric adsorption resin to decolorise the solution.

Any of the decolorising resins used in the decolorising of sucrose juice in the manufacture of sucrose may be used for this purpose.

Finally, the L-arabinose solution is crystallized to yield pure, crystalline L- arabinose. The crystallization step (h) may comprise for example the stages of neutralizing the solution, concentrating the solution, cooling crystallization, centrifugation and drying. Multiple crystallization stages may be carried out.

The resulting crystalline product typically comprises L-arabinose 97-99%, D- galactose < 1%, Ash< 0.2 %, total dry solids> 99.5 %.

An embodiment of the invention will now be described further, by way of example.

Example 1

Sugar beet pulp from which the sucrose had been extracted by conventional diffusion methods was washed by soaking in a 5-fold volume excess of water at 62 C for 2 hours. The dirty water containing sucrose and other soluble impurities was removed by screw-pressing, and the pressed pulp having a dry matter content of about 25% by weight was passed to the alkali treatment step (a).

Water was added to the pressed pulp to form a mobile slurry. The amount of water added was 5 times the weight of the pressed pulp. The slurry was then heated to 95 C, and calcium hydroxide was added with stirring to raise the pH to pH 11. Suitably, the amount of calcium hydroxide added is approximately 50g/kg of pressed pulp.

Stirring was continued for 45 minutes, and the alkali solution containing the araban was then decanted from the pulp sludge. The crude araban extract was then passed to the carbonatation stage (b) as follows.

Carbonatation was carried out by passing CO2 through the crude araban extract maintained at 80 C with stirring for 30 minutes, to a final pH of 8.

The neutralised crude araban extract when then centrifuged (disk stack) to remove l 5 the lime sludge and other insoluble matter. The araban extract was then subjected to plate filtration ( 8 micrometer plate) to remove small particles of lime sludge and other insoluble matter.

The araban extract was then subjected to ultra-filtration using a membrane having a nominal molecular weight cut off of 30,000. This concentrated the araban retentate to approximately 12wt.% solids, and removed sugars and other low molecular weight soluble impurities in the permeate.

The purified retentate was then subjected to diafiltration as follows. 500 liters of the retentate was diluted with 300 liters of water, and then ultrafiltered through a membrane of the same type as for the first ultrafiltration until a retentate volume of 500 liters was again reached. The steps of dilution and ultrafiltration were then repeated three more times Finally, the purified araban solution was treated with a strong cation resin to remove calcium ions and lower the pH of the solution to approximately pH 4. At this point the solution contains approximately 12 wt.% araban, together with some l galactan and other large organic soluble species. This solution was then passed to hydrolysis step (e) as follows.

Hydrolysis was carried out by adding sulfuric acid in an amount of 7wt.% acid (as HANSON), based on the dry solids weight of the araban solution, and stirring for one hour at 97 C. These conditions result in hydrolysis of the araban to L-arabinose, without substantial hydrolysis of the galactan.

The crude L-arabinose solution was then purified by ultrafiltration through a MW l () 30,000 cut-off membrane. The resulting purified Larabinose solution was treated with an anionic exchange resin [XE583 Rohm and Hass (OH form)] to neutralise the solution and remove anionic impurities. The neutralized solution was then treated with an adsorption resin [XAD16, Rohm and Haas] to decolorize the solution. The pH of the solution was then adjusted to about 7 with sulfuric acid.

The resulting purified L-arabinose solution having a dry solids content of about 5 to 7wt.% was concentrated to 50-55bx at 35-40 C, followed by cooling crystallization at 20-35 C for 30 minutes to 2 hours, basket centrifuging and drying of the crystalline arabinose under vacuum at 60 C. The purity of the solution would be good enough to get a second and third crystallization, if desired.

The product arabinose comprised L-arabinose 97-99%, D-Galactose <1%, ash < 0.2%, dry solids > 99.5%. )

Claims (12)

1. A method of preparing L-arabinose from sugar beet pulp comprising the steps of: (a) treatment of a sugar beet pulp with aqueous alkali to provide a crude aqueous araban extract; (b) treatment of the crude araban extract with a precipitating agent to precipitate an insoluble calcium salt from the araban extract; (c) removal of the insoluble calcium salt sludge; JO (d) ultra-filtration to remove low molecular weight impurities and concentrate the araban extract; (e) acid hydrolysis of the purified araban extract after ultra-filtration to provide a crude aqueous L-arabinose solution; (0 ultra-filtration of the crude L-arabinose solution to remove high molecular weight impurities and provide a purified L-arabinose solution; (g) neutralization of the L-arabinose solution; and (h) concentration and crystallization of the purified L-arabinose solution to provide a purified L-arabinose.

2. A method according to claim 1, wherein the precipitating agent is carbon dioxide.

3. A method according to claim 1 or 2, wherein the method does not comprise a chromatographic purification step.

4. A method according to any preceding claim, wherein the ultrafiltration is carried out using a membrane having a nominal molecular weight cut off of from about 5000 to about 50,000.

5. A method according to any preceding claim, wherein the ultrafiltration step (d) is followed by a diafiltration step prior to the step (e) of hydrolyzing the araban.

I

6. A method according to any preceding claim, wherein the concentration of the araban in the retentate from the ultrafiltration step (d) and the optional diafiltration step is from about 10% to about 20% by weight.

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7. A method according to any preceding claim, wherein the method does not comprise any substantial concentration step carried out on the araban solution before the hydrolysis step (e), other than the ultra-filtration step and optional diafiltration step.

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8. A method according to any preceding claim, wherein the ultrafiltration step (0 is carried out using a membrane having a nominal molecular weight cut off of from about 1000 to about 30,000.

9. A method according to any preceding claim, wherein the neutralization step (9) is carried out before the ultrafiltration step (f).

10. A method according to any preceding claim, wherein the L-arabinose solution undergoes treatment with an anionic exchange resin to remove anionic impurities and optionally also to neutralise the L-arabinose solution.

11. A method according to any preceding claim, wherein the L-arabinose solution further undergoes treatment with a polymeric adsorption resin to decolorise the solution.

12. A method according to any preceding claim for the preparation of crystalline L-arabinose comprising L-arabinose 97-99%, D-galactose < 1%, Ash< 0.2 %, total dry solids> 99.5 %.