WO1998031713A1 - Method for obtaining a starch-free bran extract, a refined product and material resulting from this method - Google Patents

Abstract

The invention concerns a method for obtaining a starch-free bran extract and a cellulosed refined product, consisting in the following steps: a) mixing the bran with ten times its water volume at a temperature less than 50 DEG C and filtering it so as to recuperate starch milk and starch-free bran, the starch milk being then decanted, filtered or centrifuged, then dried; this step is repeated twice to obtain a starch-free bran with a starch content less than 1 % of dry matter by weight; b) contacting the starch-free bran with an aqueous sodium solution with a concentration ranging between 2 and 12 % by volume, in a liquid/solid ratio ranging between 5 and 100, at a temperature ranging between 20 and 100 DEG C, for a duration ranging between 5 and 120 minutes; then in diluting the mixture with water, if required, until a liquid/solid ratio higher than 25 is obtained and the solid residue is separated by filtering and by drying; in concentrating the filtrate, acidifying it to a pH ranging between 4.5 and 7, and precipitating with ethanol in a ratio of 2 to 4 volumes of alcohol for one volume of solution to obtain a coagulum, which is dried for obtaining an extract rich in hemicellulose. The invention is applicable to wheat bran.

Description

 Title: Process for obtaining an extract of its starch-free product, a raffinate and a material obtained from this process.

The present invention relates to a process for obtaining an extract of de-starchy wheat bran, a raffinate and a material obtained from this process.

Wheat bran is particularly rich in insoluble fibers, in particular of the hemicellulosic type and is notably known for its actions on digestibility, intestinal transit, inhibition of colon tumors.

Hemicelluloses constitute a vast family of cell wall polysaccharides, associated with cellulose, of very variable structure, depending on the nature of the plant and the organ from which they are extracted. They are used in fields as varied as pharmacy, cosmetics or the food industry, as excipient, flavoring agent and emulsifier as described for example in the documents JP-A-07101881, JP-A-07079712, JP- A-07101882, JP-A-07099930, JP-A-07102284, JP-A-060571176 and EP-A-301440. They also have interesting effects, in particular for the regulation of cholesterol and blood pressure.

There are already known methods for fractionating wheat bran such as that described in US Patent No. 3,879,373 or in the publication "Preparation and characterization of water soluble hemicellulose (arabinoxylan) from wheat bran" in Nippon Shokukin Kogyo Gakkaish, 39, no. 12, p. 1147-1155 of 1992. These generally comprise an aqueous extraction step to remove the residual starch, followed by a step of solubilization of the hemicelluloses in an alkaline medium. Very often, wheat bran will have previously undergone one or more pretreatments such as defatting, deproteination, delignification, using different solvents as described in the publications "Constitution of a hemicellulose from wheat bran" in Canadian Journal of Chemistry, 33, p. 56-67 of 1955 and "Fractionation of wheat bran carbohydrates" in J. Sci. Food. Agric. 32, p. 243- 251 of 1981. These pretreatments associated with the multiple purification operations intended to isolate a hemicellulosic fraction as pure as possible, result in yields in hemicellulosic extracts limited and weakly active on the rheological level. Furthermore, none of these purified extracts have film-forming properties or a coating agent. One of the aims of the present invention is therefore to provide a process for obtaining an extract of de-starchy wheat bran which makes it possible to limit the number of steps and to reduce the use of solvents.

Another object of the present invention is to provide such a process which makes it possible to obtain an extract of wheat bran rich in hemicellulose with a higher yield than those of known processes.

A further object of the invention is to provide a process of this type which makes it possible to obtain such an extract having a rheological and filmogenic interest, with an acceptable production cost. Another additional object of the invention is to provide a process of this type which makes it possible, after recovery of the extract, to recover the cellulosic residue in the field of heat-pressed materials, for obtaining molded, recyclable and compostable parts, instead of plastic packaging and packaging parts. These and other objects which will appear subsequently are achieved by a process for obtaining an extract of de-starchy wheat bran, and a cellulosic raffinate which is characterized, according to the present invention, by the fact that it comprises the following stages a) the wheat bran is mixed with ten times its volume of water at a temperature below 50 ° C. and filtered so as to recover a starch milk and a de-starchy bran, the starch milk then being decanted, filtered or centrifuged then dried; this step is repeated once or twice to obtain a de-starchy bran having a starch content of less than 1% of the dry matter, by weight; b) the de-starch bran is brought into contact with an aqueous sodium hydroxide solution whose concentration is between 2 and 12%, by mass, in a liquid / solid ratio between 5 and 100, at a temperature between 20 and 100 ° C. , for a period of between 5 and 120 minutes; then the mixture is diluted with water, if necessary, until a liquid / solid ratio greater than 25 is obtained and the solid residue is separated by filtration or by dewatering; the filtrate is concentrated, acidified to a pH of between 4.5 and 7, and precipitated with ethanol in a ratio of 2 to 4 volumes of alcohol per volume of solution, to obtain a coagulate, which is dried to obtain an extract rich in hemicellulose.

Advantageously, step a) is repeated once or twice to obtain a de-starchy bran having a starch content of less than 1% of the dry matter, by weight. As for the material according to the present invention, it is obtained by mixing the extract obtained above in a proportion of between 10 and 50% by mass with highly accessible activated cellulose fiber, obtained by fractionation of wheat or barley straw, and by thermomolding this mixture without adding any additive or glue.

Thus, according to the present invention, the wheat bran is de-starchized, then the de-starchy fraction is treated to obtain a hemicellulosic extract and a cellulosic raffinate. The invention is based on the demonstration of the remarkable and unknown properties of the extract of de-starchy wheat bran as a rheological and film-forming agent, usable in formulation as additives or thickening and gelling ingredients, and as a coating agent for finishing and protection of surfaces, achieved through the implementation of the process. The hemicellulosic fractions obtained under conditions different from this process, in particular when they are purer, lose their properties. The cellulosic raffinate resulting from this process provides thermally molded parts without additives or glue, based on fractionated straw fibers, better flexibility for satisfactory mechanical strength, and this unexpectedly because the incorporation of wheat bran in the panels of particles or fiber is currently known to lower their mechanical characteristics, even in the presence of glue. According to the claimed process of the present invention, wheat bran is mixed with ten times its volume of water, at a temperature below 50 ° C. Indeed, beyond 50 ° C, the appearance of gelation phenomena of starch could interfere with washing. The water-soluble fraction comprising the water-soluble compounds and the suspended starch granules is separated from the sound residue by filtration. The starch milk thus obtained is decanted and filtered, or centrifuged and then dried. Depending on the source of the bran and its initial starch content, the operation will be repeated once or twice until a de-starchy bran containing less than 1% starch is obtained. In the case of so-called “big bran” flour milling, nearly 40% of the initial dry matter is thus extracted. Wheat bran, thus de-starchy, but not dried, is brought into contact with an aqueous sodium hydroxide solution whose concentration is between 2 and 12% by mass, a liquid / solid ratio between 5 and 100, at a temperature between 20 and 100 ° C, for a period of between 5 and 120 minutes. Advantageously, the procedure is as follows: the defatted bran is brought into intimate contact with the sodium hydroxide solution at a concentration by weight set between 5 and 10%, in a liquid / solid ratio of between 6 and 10, and at a temperature between 20 and 50 ° C, for a period of less than 15 minutes, using a suitable mixer such as a pulper or a twin-screw mixer, so as to obtain a very homogeneous paste. The dough is then stored in a maturation tank for a period of between 5 and 60 minutes, without stirring, so as to allow the continuation of the hemicellulose solubilization reaction. The dough is then washed thoroughly with water, in a liquid / solid ratio of between 25 and 50, in a stirred system. The liquid / solid separation after solubilization in the alkaline phase is preferably carried out by wringing on tissue support, by filtration or by centrifugation; the latter solution requiring additional washing due to the possible formation of a gelled phase. The residue obtained is isolated, then washed or not depending on the recovery axis chosen. This one, largely enriched in fibers, finds indeed various axes of valorization such as food, animal or dietetic fields or materials.

Washing can advantageously be carried out at a temperature below 50 ° C. in an apparatus with cylindrical walls and an elongated axis, comprising two corotative screws and axes parallel to each other and to the axis of the apparatus, divided into three upstream and downstream zones according to the direction of advance of the mixture. In the first zone, or upstream zone, wheat straw and the alkaline bran paste from the maturation tank are introduced. The set of screws located in this area has a suitable profile to impose the following effects on the mixture:

- a shearing effect which reduces the size of the particles,

- a kneading effect which allows the straws, the bran and the alkaline solution permeating the latter to be intimately contacted,

- a compression effect which promotes the diffusion of the liquid phase within the lignocellulosic matrices,

- an effect of retention of the mixture intended to increase the residence time in the apparatus, and - an effect of conveying the mixture towards the second zone.

A screw profile to obtain such a combination of effects is, for example, a sequence:

- direct pitch screw elements for conveying the mixture,

- mixer elements of suitable shapes for shearing and mixing: two-lobed, three-lobed or notched wheels, - screw elements, single or double thread with decreasing direct pitch to allow the transport of the material, with shear stress (co-penetrating screws) and axial compression (reduction of the screw pitch),

- screw elements, single or double thread, reverse pitch, the role of which is to form a plug of material which ensures the compression of the mixture; in this screw element the shearing and mixing action is also very intense. The mixture crosses the strip through the space between the screws and the wall and / or slots made in the screw.

In the second zone, or intermediate zone, the intimate straw-bran-soda solution mixture which has crossed the strip, is taken up by direct-pitch screws to be conveyed to a water injection point. The mixture is thus relaxed to promote exchanges with water in order to ensure the extraction of hemicelluloses solubilized by the action of soda, by dilution or washing effect. The liquid / solid mixture is then subjected to a mechanical mixing and compression effect thanks to the combination of a sequence of screws with decreasing direct pitch and reverse pitch. The effect of the mixture promotes the contacting of the phases, and the dilution of the solubilized hemicelluloses; the presence of straw fibers promotes the formation of a plug in the screw element with reverse pitch allowing the pressurization of the material upstream. The compression effect ensures the pressing and wringing of the mixture in order to evacuate the liquid through filter grids arranged in the cylindrical wall of the device. The collected filtrate contains the solubilized hemicelluloses.

In the downstream zone, or third zone, the wrung product which has passed through the strip, is taken up by direct-pitch screws to be sent to the outlet of the device. The mixture of bran fiber and straw may in this downstream area undergo a drying and shaping treatment, for example by extrusion through a die. For drying, the cylindrical wall will include heating means and means for removing the water vapor formed.

The matrix of cellulosic fibers, collected at the outlet of the apparatus as described above, can be molded by thermoforming to obtain molded forms sufficiently resistant to be substituted for parts obtained from plastics or composites in their applications. . The presence of bran fiber incorporated in a de-starch bran / straw weight ratio of between 0.1 and 0.5 brings better flexibility to the materials, necessary for certain applications. Such forms cannot be obtained by direct thermoforming of straw-bran mixtures, even after grinding.

The viscous alkaline solution obtained by separation of the liquid and the solid is directly concentrated, for example, by ultrafiltration on a polysulfone membrane compatible with the pH of the solution, having a cut-off threshold of 10,000 Da, an internal light greater than or equal to 1.1 mm, due to the high viscosity of the solution. The increase in this internal diameter makes it possible to reduce the duration of the stage and to increase the concentration rate of the solution. The temperature must not exceed 50 ° C so as not to alter the free polysaccharide structures in the medium.

This step can also be carried out by evaporation on a thin film evaporator, at temperatures between 35 and 50 ° C. Above 50 ° C, the solution may be altered; in addition, a solid film may form on the walls of the device. This ultrafiltration concentration step directly carried out on the basic solution has several advantages: it reduces the amount of acid necessary for subsequent acidification, therefore reducing, on the one hand, the salt content in the solution, and, d on the other hand, the production costs, and allows a recycling of the basic permeate thus obtained. This concentration step also makes it possible to considerably reduce the volumes of alcohol used subsequently during the purification by precipitation of the hemicellulosic extract.

After concentration, the solution is acidified to a pH between 4.5 and 7, with a preferred pH of 5.5. In fact, a slightly acidic pH favors the precipitation of proteins and hemicelluloses A in the medium, and also favors the precipitation of the extract in alcohol by forming a firmer precipitate. Below 5.5 a large amount of acid is required to reach lower pH values, which increases costs and salt content.

The solution thus acidified, characteristic by its aspect of blank, is precipitated in ethanol in a ratio of one volume of solution for 2 to 4 volumes of ethanol, 2.5 volumes of ethanol being the volume preferably retained. It is left for the time necessary for precipitation at room temperature, or placed at 4 ° C in order to promote the formation of the coagulate.

To dry the hemicellulosic fraction thus obtained, a filter press is used in order to transform the coagulate formed into a powder having, in addition to a very light color, a strong thickening and film-forming power and an excellent surface coating capacity. This stage is, moreover, completely extrapolated on a large scale at very competitive costs. The powder obtained after grinding has a dry matter of between 25 and 30% at the outlet; it can thus be left in the air for subsequent drying or be dried in a column in air or other gas in order to avoid the formation of micro-aggregates. The remaining ethanolic phase is distilled and then reused.

According to the process of the present invention, yields of isolated hemicellulosic extract are obtained comprised between 21 and 30% of the dry matter of bran, and between 35 and 50% of the dry matter of de-starchy bran, characterized by: - a majority composition in non-cellulosic wall polysaccharides, greater than 75% of the dry matter by weight of the arabinoxylan and glucan type with arabinose / xylose weight ratios of between 0.3 and 0.7, a glucose content of between 8 and 25% of dry matter by weight,

- a minority composition in protein materials of between 1 and 25% of the dry matter by weight,

- a mineral ash content of less than 15% of the dry matter by weight.

As a consequence of these characteristics, modulated as a function of the operating conditions for obtaining them, hemicellulosic extracts have the following properties: - rheological behavior of Newtonian and thixotropic type in dilute aqueous solution of less than 1%, and rheofluidifier beyond. From a concentration of 2% in water, the hemicellulosic extract develops a strong increase in the viscosity of the medium which can reach values greater than 1,500 centipoises at 3% in water, at low speed gradients, this which makes it comparable to commercial synthetic thickeners such as cellulose derivatives. This viscosity is, remarkably, stable in a large gram of pH (4 to 10) and not very sensitive to the salt content. A very strong synergy develops with carob extracts leading to viscosities higher than 4,000 centipoises at low speed gradient and 300 centipoises at high speed gradient for concentrations of 1% of each of the polysaccharides in water. At higher concentrations (4 to 6%), the hemicellulosic extract leads to the production of gels comparable to those obtained from carrageenans.

- a film-forming power allowing a good quality surface coating, with breaking strengths of the films obtained without the addition of superior plasticizers at 40 M Pa, which places them in the range of films obtained based on cellulose derivatives, starch or polyethylenes.

Examples of carrying out the method according to the present invention are given below in order to better allow the skilled person to implement it and to illustrate its advantages.

The examples which follow are intended to illustrate the flexibility of the process which makes it possible to obtain extracts of its starch-free bran with yields and a modular rheological quality according to the operating conditions. All percentages are expressed by weight unless otherwise indicated.

Example 1

In this example, a large wheat bran is used without prior grinding which contains approximately 90% dry matter. Its composition is as follows in% of dry matter: Hemicelluloses 38%

Lignins 7% Celluloses 9% Starch 13% Proteins 15% Mineral ash 5%

1 ° Deamylation

The device used is a stirred reactor fitted with an integrated filter. The operating conditions in this example are as follows: The wheat bran is mixed with ten times its volume of water at a temperature of 40 ° C for 15 minutes, then the liquid phase, containing the suspended matter, is drained by through the filter. The starch milk obtained contains 4.3% dry matter. The suspension is allowed to settle. The residual sound is again mixed at ten times its volume at 40 ° C for 15 minutes; the starch milk obtained in this second phase contains 1.3% of dry matter. This second suspension is allowed to settle. Finally, the residual sound is mixed again with ten times the volume of water at 40 ° C for 15 minutes (the duration of the last stages can be reduced to 5 minutes). The starch milk obtained contains 0.6% of dry matter. This third suspension is allowed to settle. After complete decantation, the three supernatant phases are combined and can be reused as a wash phase on a new sound. The three fractions of starch jobs are also gathered, concentrated and then dried: they will find different areas of recovery (textiles, paper, glue, etc.).

A total of 40.6% of the initial dry matter was thus extracted and the composition of the defatted wheat bran is as follows, as a percentage of the dry matter:

Hemicelluloses 60%

Lignins 8%

Cellulose 15.5%

0.5% starch

Protein 15%

1% mineral ash

2 ° Obtaining the hemicellulosic extract

The alkaline treatment of the wheat bran thus de-starchy is carried out in a stirred reactor under the following conditions:

Sodium concentration 4% Reaction temperature 50 ° C Liquid / solid ratio 50 Reaction time 1 hour De-starchy bran 20 g dry

The mixture is then filtered through a propylene fabric with a porosity between 1 and 5 μm. The filtrate is concentrated 2 times under reduced pressure, adjusted to a pH = 5.5 by addition of acetic acid, then mixed with 3 volumes of ethanol and placed 12 hours at 4 ° C to promote the formation of a coagulate . This is then taken up in ethanol then filtered on paper support and dried at room temperature. The yield of hemicellulosic extract is 35.5% relative to the dry starchy bran, or 21.3% compared to the initial dry bran.

The hemicellulosic powder thus isolated has the following characteristics:

Arabinose / xylose ratio 0.41 Glucose content 12.1% of the dry matter Protein content 7.75% of dry matter Mineral ash content 10.4% of dry matter

The viscosity of a 2% organic matter solution of the hemicellulosic extract in water, measured using a constrained rheometer

Carrimed 50, at a temperature of 20 ° C and a pH of 7, is 210 cp at 0.5 N / m ² and 158 cp at 2 N / m ² .

Example 2 The wheat bran used in this example belongs to the lot of de-starchy bran obtained in Example 1, and therefore has the same characteristics. The alkaline treatment conditions are as follows:

Sodium concentration 6% Reaction temperature 25 ° C Liquid / solid ratio 75 Reaction time 1 hour 30 De-starchy bran 20 g dry

The protocol concerning the following steps is in all respects similar to that described in Example 1. The hemicellulosic powder thus isolated has the following characteristics:

Ratio arabinose / xylose 0.36 Glucose content ^'13.5% of dry matter Protein content: 9% of the dry matter content mineral ash: 11, 5% of dry matter

The extraction yield obtained under these conditions is 34.5% relative to the de-starchy bran, or 20.7% compared to the initial dry bran. The viscosity measured under the same conditions as in Example 1 is 411 cp at 0.5 N / m ² and 241 cp at 2 N / m ² . Example 3

The wheat bran used in this example belongs to the same batch of its starch-free bran obtained in Example 1, and therefore has the same characteristics. The alkaline solubilization conditions are as follows:

Soda concentration 6% Reaction temperature 75 ° C Liquid / solid ratio 75 Reaction time 1 hour 30 De-starchy bran 20 g dry

The protocol concerning the following steps is similar in all respects to that described in Example 1. The extraction yield obtained under these solubilization conditions is 45.5% relative to the defatted bran, or 27% relative to the bran initial dry. The hemicellulosic powder thus isolated has the following characteristics:

Arabinose / xylose ratio 0.48

Glucose content 11.3% of dry matter

Protein content: 3% of the dry matter Mineral ash content: 11.5% of the dry matter

The viscosity measured under the same conditions as in Example 1 is 61 cp at 0.5 N / m ² and 54 cp at 2 N / m ² .

These three examples illustrate that the best rheological qualities of the extract of de-starchy wheat bran are obtained for a rather low arabinose / xylose ratio (0.36) corresponding to a less substituted hemicellulosic arabinoxylan fraction, for a rather high protein content. (9%), and a rather high glucose content (13.5%) corresponding to the presence of glucans.

The examples which follow are intended to compare different conditions of implementation of the process and their repercussions on the qualities of the extract of its starch-free extract.

Example 4

The wheat bran is de-starchy under the same conditions as those described in Example 1. The liquid / solid mixture is separated in an not

bowl reuse with a capacity of 50 liters on propylene fabric with a porosity between 1 and 5 μm. The cellulosic residue obtained is washed until pH 7.

The basic filtrate obtained is concentrated 1.7 times and partially demineralized by ultrafiltration, at a temperature below 40 ° C., on a hollow polysulfone fiber membrane having a cutoff threshold of 10,000 Daltons, a light of 1.1 mm. and a filtration surface of 0.45 m ² . The permeate which contains part of the sodium hydroxide is stored for later recycling. The pH of the retentate is adjusted to 5.5 by adding acetic acid, mixed with 2.5 volumes of ethanol and stored at 4 ° C for 12 hours. The coagulate formed is removed and introduced into a filter press tank, for example of the LAROX PF 0.1 H2 type equipped with a calendered multifilament polyester filter cloth, with an amount of ethanol such that the suspension has a content of 3 % of dry matter. The capacity is 1, 22 kg / m ² and a dryness of the hemicellulosic powder cake of 25 to 30% allows an automatic stripping. The powder thus obtained is dried under a stream of air. The yield of hemicellulosic extract is 27% compared to the dry starchy bran, or 16% compared to the initial dry bran. The hemicellulosic powder thus isolated has the following characteristics:

Arabinose / xylose ratio 0.36

Glucose content 10% of dry matter Protein content: 7.4% of dry matter

Mineral ash content: 4% of the dry matter

The viscosity measured under the same conditions as in Example 1 is 270 cp at 0.5 N / m ² and 103 cp at 2 N / m ² . The applicability as a rheological additive is illustrated by the qualities of a matt white acrylic emulsion paint, formulated with hemicellulosic extract in substitution for a commercial rheological agent of the cellulose derivative type, everything being identical elsewhere:

Paint formula: Aqueous gel of the rheological additive: 308 g

Wetting agent Coatex P 90 1.78 g Nopco NXZ defoamer 1.04 g Titanium dioxide pigment Titration RL 68 62.34 g Mineral filler Talc 20 MO 51.97 g Calcium carbonate omyacarb extra CL 129.56 g Durcal 5 331, 94 g

Rhodopas DS 910 styrene-acrylic emulsion: 108.92 g

Texanol coalescing agent 3.12 g

Preservative Parmetol K 40 1, 4 g

Quality compared to painting:. Manufacturing: no difficulty

. Storage stability 30 days at room temperature: identical to the control. Rheological behavior: neighbor of the control but with a more marked rheofluidifying and thixotropic character

. Applicability of the paint compared to the witness:

- easier brush spreading

- less marked brushing marks

- better appearance of stretch after drying - better homogeneity and satin effect

- better drying.

. Mattness: specular reflection almost identical to the witness. Water resistance or leachability: identical to the control.

The film-forming power of the hemicellulosic extract is measured by the breaking stresses by stretching of the films obtained by drying a controlled quantity of aqueous solution spread out in a flat layer of thin thickness:

Exemple 5

Example 5

In this example, the sound processing protocol is identical to that of Example 4, up to and including the concentration step by ultrafiltration. The retentate obtained is then demineralized by diafiltration on the same membrane as that of ultrafiltration. The demineralized aqueous solution obtained is separated into four equivalent fractions which will be dried according to four different drying modes: Lyophilization, Lyophilization with additive, Atomization, Atomization with additive.

The viscosity measurements of aqueous solutions containing 2% organic matter of the powders isolated after these four drying treatments are grouped in the following table:

This example illustrates the sensitivity of the qualities of the hemicellulosic powder to the conditions of its isolation.

Example 6

3 kg of dry matter of its starch-free bran under the conditions described in Example 1 are brought into intimate contact with an 8% sodium hydroxide solution, in a liquid / solid ratio of 7.2 at 50 ° C., in a pulper equipped at its base a helical stirring turbine allowing efficient mixing of the dough by a simultaneously circular and upward movement. After dilution with water until a liquid / solid ratio of 50 is obtained, the hemicellulosic powder extracted is isolated according to the same protocol as described in Example 4. The results obtained are collated in the following table:

Example 7 Wheat bran, de-starchy under the conditions identical to those cited in Example 1, is brought into intimate contact with an 8% sodium hydroxide solution, at 50 ° C., in a twin-screw reactor-mixer of the type CLEXTRAL BC 45 with a screw profile fitted with two-lobe mixers and striplings to obtain effective mixing. The flow rate of its de-starchy food is 5.3 kg of dry matter per hour, for a flow rate of 8% sodium hydroxide solution of 43 kg / h, defining a liquid / solid ratio of 8.1. For a screw rotation speed of 50 rpm, the residence time is 2 minutes on average. The pasty mixture obtained at the outlet of the twin-screw reactor-mixer is stored in a buffer tank for a variable duration, diluted with water until a liquid / solid ratio of 50 is obtained before being treated according to the same protocol. as in example 4.

The isolated powder has the following characteristics:

These examples illustrate the best rheological qualities of the hemicellulosic extract obtained by very effective contact between the sound and the alkaline solution with limited contact times and a low liquid / solid ratio (7 to 8).

EXAMPLE 8 In this example, the bran of de-starchy wheat under the conditions identical to those cited in Example 1, is brought into intimate contact with a 6.8% soda solution, with a liquid / solid ratio of 7, to 50 ° C, in a reactor-mixer identical to that described in Example 7. The pasty mixture is stored for 15 minutes in a buffer tank, diluted with water to reach a liquid / solid ratio of 12 and introduced into the first zone of the cylindrical wall apparatus described above, with a flow rate of 35.9 kg / h, simultaneously with wheat straw with a flow rate of 5.47 kg of dry matter per hour. This device or reactor-extractor is for example of the twin-screw type such as that sold by the company CLEXTRAL. The speed of rotation of the screws is 170 revolutions / min and the temperature of the sleeve maintained at 50 ° C. In the second zone, water is injected with a flow rate of 85 kg / h, the liquid / solid ratio then being 48.5 relative to the introduced bran and 13.2 relative to the total bran + straw. The filtrate is collected through the grids of the wringing module and treated according to a protocol identical to that described in Example 1. The yield of isolated hemicellulosic powder is 22.5% relative to the dry dry starch introduced. The yield of mixture of straw fiber and bran at the outlet of the reactor-extractor is 89% relative to the dry matter of bran and straw introduced. The qualities of this mixture of straw fiber and bran are illustrated by the fact that they lead to heat-pressed materials of very similar quality to those obtained for materials based on straw fiber treated under the same conditions. The qualities of the hemicellulosic powder are illustrated by the protective and finishing film obtained by coating the surface of a thermo-molded material based on straw and or bran, with a 4% aqueous solution. This film gives the room an excellent finish and a good surface finish.

Claims

1. Method for obtaining an extract of its starch-free bran and a cellulosic raffinate, characterized in that it comprises the following stages: a) the bran is mixed with ten times its volume of water at a temperature below 50 ° C. and it is filtered so as to recover a starch milk and a de-starchy bran, the starch milk then being decanted, filtered or centrifuged and then dried; this step is repeated once or twice to obtain a de-starchy bran having a starch content of less than 1% of the dry matter by weight; b) the de-starch bran is brought into contact with an aqueous sodium hydroxide solution whose concentration is between 2 and 12%, by mass, in a liquid / solid ratio between 5 and 100, at a temperature between 20 and 100 ° C. , for a period of between 5 and 120 minutes; then the mixture is diluted with water, if necessary, until a liquid / solid ratio greater than 25 is obtained and the solid residue is separated by filtration or by dewatering; the filtrate is concentrated, acidified to a pH of between 4.5 and 7, and precipitated with ethanol in a ratio of 2 to 4 volumes of alcohol per volume of solution, to obtain a coagulate, which is dried to obtain an extract rich in hemicellulose. 2. Method according to claim 1, characterized in that step a) is repeated once or twice to obtain a de-starchy bran having a starch content of less than 1% of the dry matter by weight. 3. Material characterized in that it is obtained by mixing the solid residue obtained according to claims 1 or 2, or in proportion of between 10 and 50% by mass with highly accessible activated cellulose fiber, obtained by fractionation of straw wheat or barley, and said mixture by thermomolding without the addition of additive or glue.