FR2762616A1 - High dextrose content starch hydrolysate preparation - Google Patents

Abstract

Preparation of a starch hydrolysate of high dextrose content comprises:(a) liquefaction of starch milk using alpha -amylase; (b) inhibiting the alpha -amylase; (c) saccharification using a glycogenic enzyme; and (d) molecular sieving and recovering the hydrolysate. Also claimed is the preparation of sorbitol by hydrogenation of the above hydrolysate.

Description

PROCESS FOR PRODUCING A HIGH STARCH HYDROLYSATE
DEXTROSE CONTENT
The invention relates to a method for manufacturing a starch hydrolyzate with a high dextrose content.

 The invention also relates to a process for the manufacture of sorbitol from a starch hydrolyzate with a high dextrose content obtained by the implementation of the process according to the invention.

It is known to manufacture starch hydrolysates for which the value of the dextrose equivalent (reducing power expressed in glucose on dry matter, below
DE) is from 2 to 98 and which, depending on this value, can contain up to 96% of true dextrose. These different qualities of starch hydrolysates are obtained by the choice of the conditions for the hydrolysis of the starting starch. The nature of the hydrolysis, that is to say the fact that it is acidic or enzymatic, also plays a role.

 Starch hydrolysates rich in dextrose, although having many fields of application, are mainly used as a raw material for the manufacture of crystallized dextrose or as a substrate for the manufacture of fructose by isomerization. For these two applications, the highest possible conversion is sought, that is to say the highest possible dextrose content, with a minimum of impurities.

 Starch conversion processes using an acid lead to starch hydrolysates whose dextrose content does not exceed 85 to 90%. These processes favor the competing reactions of reversion and internal dehydration of dextrose.

 Starch conversion processes using both an acid and an enzyme (generally a glucoamylase) lead to starch hydrolysates whose richness in dextrose is never greater than 93 t.

Indeed, in such processes, the acid hydrolysis of starch produces highly branched saccharides which resist the action of glucoamylase.

 The starch hydrolysates obtained by double enzymatic conversion to a-amylase and to amyloglucosidase (or glucoamylase), generally titrate from 93 to 95% of true dextrose, and contain from 5 to 7% of oligosaccharides and residual polysaccharides, the major part of which consists of disaccharides (maltose and isomaltose).

 These hydrolysates are obtained conventionally by a liquefaction of starch to a DE of 12 to 20, followed by a saccharification with amyloglucosidase, but under these conditions, the content of true dextrose cannot exceed 94 to 95 %.

 In order to achieve higher true dextrose contents, several methods have been proposed for either improving the conversion of starch by limiting the formation of co-products, or improving the efficiency of the dextrose / co-product separation (oligosaccharides and polysaccharides).

 Thus, a first process consists in operating the liquefaction and saccharification steps with very low dry matter (of the order of 5 to 10 t). But even at such a low dry matter content, the richness in true dextrose does not exceed 95 to 97%. Furthermore, such a process is not at all economically profitable because of the energy required for the evaporation of water.

 Another method consists in operating the saccharification in the presence of an enzyme hydrolyzing the 1-6 bonds of the starch, but even in this case, the dextrose content is at most only 96 to 97%.

 Yet another method consists in separating, in a manner known per se, the dextrose and the oligosaccharides and polysaccharides by passing the hydrolyzate over a column of a molecular sieve such as a cationic resin. In such a process, the aqueous starch hydrolyzate having previously undergone a pretreatment such as a concentration, filtration and / or discoloration, is adsorbed on the column, the co-products (the polysaccharides and part of the oligosaccharides) ending up in the raffinate excluded from the sieve. The dextrose is then desorbed by elution with water, the latter then being partially or completely eliminated to form a concentrated solution of dextrose or of crystallized dextrose.

Another process, based on the same principle as that mentioned above, consists in separating the dextrose and the oligosaccharides and polysaccharides by passing the starch hydrolyzate over membranes with tangential filtration. Such a process is described in the documents
EP-A-452,238, US-A-4,429,122 and US-A-4,511,654.

 These last two processes effectively make it possible to obtain a starch hydrolyzate having a high content of true dextrose, greater than 98-99%, but the yields obtained are unfortunately too low (of the order of 60 to 65%). to justify from an industrial and economic point of view such processes.

 A first object of the present invention is therefore to propose a method for manufacturing a starch hydrolyzate with a high dextrose content which overcomes the limits and / or drawbacks of the methods known from the prior art.

 Another object of the present invention is to provide a process for manufacturing a starch hydrolyzate with a high dextrose content greater than 97%, preferably greater than 98% and more preferably still greater than 99%.

 Another object of the present invention is to provide such a process, simple and economically efficient, making it possible to obtain, with very satisfactory yields, hydrolysates with such a high content of true dextrose.

To this end, the invention provides a method of manufacturing a starch hydrolyzate with a high dextrose content comprising the steps consisting in:
(a) carrying out a hydrolysis of a starch milk using an Ot-amylase so as to obtain a liquefied starch milk
(b) saccharifying the liquefied starch milk, using a glucogenic enzyme, to obtain a saccharified hydrolyzate
(c) performing molecular sieving of said saccharified hydrolyzate to collect a hydrolyzate with a high dextrose content;
this process being characterized in that, between step (a) and step (b), a step of inhibition of the α-amylase is carried out.

 The process for manufacturing a starch hydrolyzate with a high dextrose content which is the subject of the present invention is in fact based on the observation, hitherto neglected, that a good separation technique, whether chromatographic or otherwise, is effective only if the characteristics of the product to be separated, as such, effectively allow its separation by the technique or techniques considered.

 In the present case, an effective separation can only be obtained from a starch hydrolyzate having a particular composition, having in particular a bimodal carbohydrate spectrum, that is to say having besides a high content of true dextrose, a significantly reduced content of disaccharides and trisaccharides, and on the contrary a high content of polysaccharides easily separable from dextrose.

 It is therefore sought, in the present invention, to ensure that the impurities are in the form of long-chain polysaccharides rather than in the form of disaccharides or oligosaccharides of molecular mass close to dextrose.

 The particular bimodal carbohydrate spectrum of the starch hydrolyzate is obtained, in accordance with the method of the invention, by inhibiting the α-amylase at the end of the liquefaction step.

 This inhibition of α-amylase can preferably be done thermally, by proceeding from the liquefaction to a thermal shock of a few seconds at a temperature greater than or equal to 1300C.

 This inhibition of the liquefaction enzyme avoids the continuation of its action during the saccharification stage, thus disadvantaging the formation of disaccharides (maltose, isomaltose) which are difficult to separate from dextrose, in favor of the branched polysaccharides (degree of polymerization DP = 7) resistant to amyloglucosidase and easily separable from dextrose.

 To reinforce the bimodal distribution of the starch hydrolyzate, it is preferred, in the process according to the invention, to carry out a controlled hydrolysis of the starch milk so as to obtain a liquefied starch milk with low transformation rate.

Thus, in the process according to the invention, stage (a) of liquefaction is preferably carried out up to a
DE between 2 and 10, and more particularly up to DE between 4 and 8.

 Liquefying starch milk at an extremely low ED of 2 to 10 (and preferably 4 to 8), in combination with inhibiting the liquefying enzyme at the end of liquefaction, promotes l 'Obtaining a final hydrolyzate having the desired characteristics, that is to say having a high dextrose content and a particular composition in non-dextrose resulting essentially from the presence of polysaccharides and not of di- or oligosaccharides.

Preferably, the liquefaction step is carried out in two sub-steps, the first consisting in heating, for a few minutes and at a temperature between 105 and 108 "C, the starch milk in the presence of the enzyme (type
THERMAMYL 120L sold by the company NOVO) and a calcium-based activator, the second consisting in heating the starch milk thus treated, at a temperature between 95 and 100 "C for one to two hours.

 Once the liquefaction step is complete, under the conditions of dry matter content, pH, level of enzyme and calcium well known to those skilled in the art, and after inhibition of the liquefying enzyme, the procedure is carried out. step (b) of saccharification of the liquefied starch milk.

 During this step, the liquefied starch milk is subjected to the action of a glucogenic enzyme, in particular chosen from the group consisting of amyloglucosidase, glucoamylase or any other glucogenic enzyme.

 To avoid reversion reactions and the formation in particular of dissaccharides (maltose, isomaltose) by repolymerization of dextrose, the saccharification step is carried out, under conditions and in a manner known per se, so as to obtain a DE of l 'final hydrolyzate not exceeding 95, in particular not exceeding 90 and more particularly between 80 and 90.

 In fact, the preferred substrate for glucogenic enzymes is of high molecular weight, and the o (-1.4 bonds of starch are hydrolyzed much faster than the a1, 6 bonds. Consequently, at the start of saccharification, the large Since molecules and α-1,4 bonds are predominant, the production of dextrose is extremely rapid whereas the production of reversion products is very slow due to the low concentration of dextrose in the reaction medium.

 As saccharification continues, small molecules and a-1,6 bonds becoming predominant, the rate of production of dextrose gradually decreases while the production of reversion products (highly branched oligosaccharides) accelerates.

 To remedy this phenomenon, it may be advantageous to combine with the glucogenic enzyme an enzyme which specifically hydrolyzes the a-1,6 bonds of starch.

This addition of a debranching enzyme makes it possible on the one hand to accelerate the hydrolysis reactions without simultaneously accelerating the reversion reactions, and on the other hand, to reduce the quantity of highly branched oligosaccharides normally resistant to the action of the glucogenic enzyme.

Preferably, the debranching enzyme is isoamylase or pullulanase.

The quantities and the conditions of action of the different enzymes used in the process according to the invention are chosen from the following
- M-amylase: 20 to 2,000 KNU (Kilo Novo Units) per kilogram of dry substrate, temperature from 80 to 1500C, duration of action from 2 to 15 minutes.

 - amyloglucosidase: 4,000 to 400,000 international units per kilogram of dry substrate, temperature from 50 "C to 60" C, duration of action from 30 to 72 hours, pH from 5.0 to 6.0.

 - pullulanase: 150 to 15,000 ABM units.

 The enzymes used can be of bacterial or fungal origin.

 The hydrolyzate thus saccharified is then filtered on a precoat filter or preferably by microfiltration on membranes, then demineralized.

 Molecular sieving is then carried out on this saccharified and purified hydrolyzate to separate the dextrose and the component oligosaccharides and polysaccharides and thus collect a starch hydrolyzate with a high dextrose content.

 This molecular sieving step can consist, for example, of a chromatographic separation step or a separation step on membranes.

 The chromatographic fractionation step is carried out in a manner known per se, discontinuously or continuously (simulated moving bed), on adsorbents of the cationic resin type, or on strongly acidic zeolites, preferentially charged using alkaline ions. or alkaline earth metals such as calcium or magnesium but more preferably using sodium ions.

Examples of such methods are described in particular in documents US-A-3,044,904, US-A-3,416,961, US-A-3,692,582, FR-A-2,391,754, FR-A-2,099,336 , US-A-2 985 589, US-A-4 024 331, US-A-4 226 977, US-A-4 293 346, US-A-4 157 267, US
A-4 182 633, US-A-4 332 623, US-A-4 405 455, US-A-4 412 866,
US-A-4,422,881 and WO-A-92/12179.

According to a preferred embodiment, the chromatographic fractionation is carried out using the method and the apparatus described in the American patent.
US-A-4,422,881 of which the applicant company is the holder.

Whatever the chromatographic process chosen, recourse is preferably had, as regards the adsorbent, to a strong cationic resin used in sodium or potassium form and crosslinked with approximately 4 to 10% of divinylbenzene. The resins are advantageously of homogeneous particle size and between 100 and 800 micrometers.

 The choice of chromatographic fractionation parameters, among which there is more particularly the elution rate, the feed rate of starting hydrolyzate, the rate of extraction of the fraction containing the starch hydrolyzate with a high dextrose content. , the flow rate of the fraction containing the high molecular weight impurities and the composition of the desorption, adsorption and enrichment zones is carried out in such a way that the first enriched fraction is recovered with the best possible yield dextrose with a dextrose content greater than 99%.

To achieve this result, these parameters are chosen as follows when the chromatographic fractionation is carried out using the methods and apparatus described in US Pat. No. 4,422,881 and when the adsorbent used is a cationic resin of small particle size crosslinked to 8% of divinylbenzene and that it is used in sodium form
- elution rate of 70 to 700 l / h / m3 of adsorbent,
- flow rate of supply of starting hydrolyzate from 10 to 100 l / h / m3 of adsorbent,
- extraction rate of the fraction enriched with dextrose from 80 to 800 l / h / m3,
- extraction rate of the fraction enriched with dextrose from 80 to 800 l / h / m 'of adsorbent,
- flow rate of the fraction enriched in oligo- and polysaccharides from 20 to 200 1 / h / m3 of adsorbent.

 Instead of the chromatographic separation step, it is possible, in the process according to the invention, to implement a separation step by nanofiltration on membranes.

 Membranes of different pore diameters are made from numerous polymers and copolymers of the polysulfone, polyamide, polyacrylonitrile, polycarbonate, polyfuran, etc. type.

 Currently, four classes of membranes are sold: reverse osmosis, nanofiltration, ultrafiltration and microfiltration.

 Examples of the use of such membranes are described in particular in documents EP-A-452,238, US-A4 429,122 and US-A-4,511,654.

 According to a preferred embodiment, the separation on membranes is carried out using a membrane with a porosity close to 10 angstroms. Thus, the membrane advantageously used in the process according to the invention is of the FILMTEC NF 40 type sold by the company DOW.

 According to an advantageous embodiment of the invention, the part excluded from the membranes or from the chromatography, comprising the impurities of high molecular weight, is recycled upstream of the saccharification step.

 Thanks to the process in accordance with the invention which takes advantage of the benefits obtained from both the hydrolysis stages used and the molecular sieving stage, it is possible to obtain, with yields greater than 80%, a starch hydrolyzate with a dextrose content greater than 99%. Until now, such purity could only be obtained with a crystallized dextrose redissolved.

 The starch hydrolyzate with a high dextrose content obtained in accordance with the process of the invention can then be easily catalytically hydrogenated.

 The hydrogenation of such a hydrolyzate is carried out in accordance with the rules of the art which lead, for example, to the production of sorbitol from glucose.

 Both ruthenium catalysts and Raney nickel catalysts can be used for this step. However, it is preferred to use Raney nickel catalysts which are less expensive.

 In practice, from 1 to 10% by weight of catalyst is used relative to the dry matter of the hydrolyzate subjected to hydrogenation. The hydrogenation is preferably carried out on a hydrolyzate whose dry matter is between 15 and 50%, in practice close to 30 to 45%, under a hydrogen pressure between 20 and 200 bars. It can be carried out continuously or discontinuously.

 When operating discontinuously, the hydrogen pressure used is generally between 30 and 60 bars and the temperature at which the hydrogenation takes place is between 100 and 150 "C. Care is also taken to maintain the pH of the hydrogenation medium by the addition of sodium hydroxide or sodium carbonate for example, but without exceeding a pH of 9.0 This procedure makes it possible to avoid the appearance of cracking or isomerization products.

 The reaction is stopped when the content of reducing sugars in the reaction medium has become less than 1%, more preferably less than 0.5% and more particularly less than 0.1%.

 After the reaction medium has cooled, the catalyst is removed by filtration and the sorbitol thus obtained is demineralized on cationic and anionic resins. At this stage, the syrups contain at least 98% of sorbitol and it is easy to purify the latter by crystallization after concentration and cooling of the solutions.

 Other characteristics and advantages of the invention will appear clearly on reading the examples which follow. They are however given here only by way of illustration and not limitation.

EXAMPLE 1
Starch milk is liquefied in a conventional manner using 2%. of THERMAMYL 120L (a-amylase sold by the company NOVO) up to a DE of 6.5.

 The reaction medium is then heated for a few seconds at 140 ° C. so as to inhibit the amylase.

 Is then carried out, in a manner known per se, the saccharification of the hydrolyzate to 35% of dry matter in the presence of 0.8%. of amyloglucosidase G990 sold by the company ABM.

After 42 hours of saccharification, a hydrolyzate is obtained having the following carbohydrate spectrum
glucose: 95.6%
DP2: 1.9 t
DP3: 0.3%
Higher DP: 2.2% on the understanding that the abbreviation "DP" means degree of polymerization.

 The hydrolyzate thus saccharified is then filtered by microfiltration on membranes and then demineralized.

The saccharified and purified hydrolyzate is then fractionated in the continuous chromatographic separation installation, the details of construction and operation of which are described in the American patent.
US 4,422,881.

We thus obtain
a first fraction enriched with dextrose, with a yield greater than 87% by weight and having a composition which is as follows
dextrose: 99.4%
DP2: 0.6%
- a second fraction enriched in oligo- and polysaccharides having the following composition
dextrose: 70%
DP2: 10%
DP3: 2.3 z
Superior PD: 17.7%
EXAMPLE 2
The same liquefaction and saccharification steps as those described in example 1 above are carried out on a starch milk, except that the aamylase is not inhibited.

After 42 hours of saccharification, a hydrolyzate is obtained having the following carbohydrate spectrum
glucose: 95.1%
DP2: 2.3%
DP3: 1.3%
Higher DP: 1.1%
The hydrolyzate thus saccharified is then filtered by microfiltration on membranes and then demineralized.

After chromatographic fractionation, we recover
- a first fraction enriched with dextrose, with a yield of 78% by weight, and having the following composition
dextrose: 99.4%
DP2: 0.5%
DP3: 0.1%
- a second fraction enriched in oligo- and polysaccharides having the following composition
dextrose: 80%
DP2: 8.6 z
DP3: 5.5%
Higher DP: 5.9%
The chromatographic yield of 78% by weight obtained in this example is to be compared to that of 87% by weight obtained in Example 1 in accordance with the invention.

EXAMPLE 3
The fraction enriched with dextrose of Example 1, concentrated to a dry matter of 45%, is subjected to a catalytic hydrogenation in the presence of 5% by weight relative to the dry matter of nickel from Raney.

The operating conditions are as follows
temperature: 1400C
pressure: 60 bars
duration: 2 hours
The hydrogenation is stopped when the content of reducing sugars in the reaction medium is less than 600 ppm.

 After the reaction medium has cooled, the catalyst is removed by filtration, then the syrup obtained is demineralized and finally concentrated to 70% dry matter.

The dry composition of the syrup thus obtained is as follows
sorbitol: 98.8%
mannitol: 0.4%
iditol and cracking products: 0.2 t

Claims (10)

 1. Process for the manufacture of a starch hydrolyzate with a high dextrose content comprising the steps consisting in:  (a) carrying out a liquefaction of a starch milk using an -amylase so as to obtain a liquefied starch milk;  (b) saccharifying the liquefied starch milk, using a glucogenic enzyme, to obtain a saccharified hydrolyzate  (c) performing a molecular sieving of said saccharified hydrolyzate to collect a hydrolyzate with a high dextrose content  characterized in that, between step (a) and step (b), a step of inhibiting the am-amylase is carried out.  2. Method according to claim 1, characterized in that a controlled liquefaction of the starch milk is carried out so as to obtain a liquefied starch milk with low transformation rate.  3. Method according to claim 1 or 2, characterized in that the step (a) of liquefaction is carried out up to a DE of between 2 and 10, preferably between 4 and 8.  4. Method according to any one of claims 1 to 3, characterized in that the step (b) of saccharification is carried out up to a DE not exceeding 95.  5. Method according to claim 4, characterized in that the step (b) of saccharification is carried out up to a DE not exceeding 90.  6. Method according to claim 5, characterized in that the step (b) of saccharification is carried out up to a DE between 80 and 90.  7. Method according to any one of claims 1 to 6, characterized in that one associates with the glucogenic enzyme an enzyme hydrolyzing the 1-6 bonds of starch.  8. Method according to any one of claims 1 to 7, characterized in that step (c) is a step of chromatographic separation.  9. Method according to any one of claims 1 to 7, characterized in that step (c) is a step of separation by nanofiltration on membranes.  10. Process for the manufacture of sorbitol by hydrogenation of a starch hydrolyzate with a high dextrose content, characterized in that the said hydrolyzate is obtained by implementing the process according to any one of claims 1 to 9.