In the art of removing fibrous shells from cereals, legumes, seeds and the like, two types of processes are known in general. On the one hand dry methods using a cryogenic medium have been proposed in the patent literature. On the other hand there are wet methods using large amounts of water which are applied conventionally in industry, as will be explained hereinbelow.

In the cereals processing industry, e. g. in the processing of wheat, corn, soy and tapioca into fractions containing the different constituents of the cereals, traditionally the non-usable materials like foreign matter and broken grains are separated in a first step ("cleaning") by means of screening on a vibrating table, optionally using a forced flow of air and electromagnets in order to remove metal parts. In such a separation step the cleaned cereal grains to be processed further are separated from the non-usable fraction based upon differences in size and/or weight. The cereal grains, from which the foreign matter has been removed, is used as starting material for further"wet"processing. Hereinbelow an example of the wet processing of corn into fractions of gluten and starch respectively is described in detail.

After screening of the foreign matter and broken grains from the corn, in the wet process this corn is mixed with a certain quantity of water (approximately 1,5 time the weight of corn), which if desired contains a small amount of sulphurdioxyde, and is steeped therein for a few days ("steeping") and subsequently milled into a slurry such that the germs are not damaged. The slurry thus obtained is passed over screen bendings and through hydrocyclones in order to remove the germs from the slurry. The germs separated are dewatered and dried. The slurry, from which the germs have been removed, is milled again and passed over screen bendings having smaller meshes in order to remove the fibres, which are predominantly derived from the shell of the corn kernels. The fibres are washed in countercurrent with water in order to limit the loss of starch and to recover the starch in this water. After this washing step the fibres are dewatered

and dried with the aid of conventional techniques, and stored.

The slurry, which now consists primarily of granules of starch and gluten and water, is separated into a fraction of starch and a fraction of gluten. This separation is carried out in centrifuges and hydrocyclones, into which water is fed in countercurrent. The gluten fraction thus obtained is dewatered and dried and milled to the desired dimensions. The starch fraction is subjected to a refining treatment with acid and/or enzymes in order to obtain all sorts of compositions of glucose syrups. If desired, the starch can be modified into more specific derivatives thereof.

One of the serious disadvantages of these traditional"wet" methods of processing is the large volume of water, which is consumed and which has to be removed subsequently from the separated fractions such as the germs, fibres and gluten, by means of dewatering and drying, for which operations a large need for energy exists.

Furthermore the process water, if it cannot be reused in other parts of the plant, has to be recognized as industrial waste water, which may not be discarded of as such via the sewer, so that high additional costs are involved in the disposal and processing of this kind of water.

Although the dry methods mentioned above using a cryogenic medium, wherein the shell is removed while the grains are deeply cooled, do not suffer from the disadvantages involved in the wet processing regarding drying, dewatering and waste water, and from that point of view look very promising, these methods have not been used on an industrial scale as far as presently known in the processing of cereal grains into individual fractions of starch and gluten respectively.

In this regard it has to be noted that in the cereal processing industry a distinction is made between on the one hand wet processes ("wet milling"), wherein the separation of the cereal grains into the different constituents thereof, such as starch, gluten, germs and the like is aimed for, which constituents are suitable for different end purposes, and on the other hand dry processes ("dry milling"), wherein the cereal grains are separated into components such as for example grits, bran and flour.

Thus there is still a need to provide alternatives to the conventional wet milling methods.

The object of the present invention is to provide an improved method for the fractionating of plant materials like cereal grains,

seeds and legumes into well defined constituents in order to enhance the overal efficiency of the processing of such plant materials to valuable intermediate and end products, for example the processing of corn to starch, gluten and fibrous material, e. g. useful as cattlefeed.

According to the invention the method of fractionating capillaries-containing plant material comprises applying internal mechanical pressure within the capillaries of the capillaries- containing plant material.

In the method according to the invention the plant material to be treated, that contains capillaries, is separated into a number of different fractions by applying mechanical forces in the capillaries of the plant material. It is important to emphasize that unlike the conventional dry milling using externally applied forces, the mechanical pressure in the method according to the invention is applied within the capillaries themselves. It is believed that the mechanical forces applied within the capillaries induce strains and stresses in the plant material around the capillaries, resulting in a favourable fractionation in view of yield, purity and accuracy of the constituents to be separated.

The method according to the invention can be used for the removal of shells from cereal grains like corn. Other applications include the treatment of agricultural byproducts like straw, cobs, bran in order to form dietary fibres, fermentation feedstock etc Here it should be noted that an improvement of a dry method for removing fibrous shells form cereal grains using a cryogenic medium has been described in the PCT-application PCT/NL00/00270, to be published. This improved dry method comprises a premoistening step of cereal grains to raise the water content from its natural value of around 16 % to about 25%, which additional water is believed to be present predominantly in the capillaries in the shell and around the germ, followed by a step of subjecting the thus obtained cereal grains to thermal shock using a so called"cold-transfer medium", such as a cryogenic medium, thereby rapidly freezing the water in the capillaries and providing a temperature difference between the surface of each kernel and the interior thereof, and finally a step of applying mechanical force to the thus cooled cereal grains to remove the fibrous shells from the core components.

The present invention provides alternative methods for obtaining the same effect i. e. a clean separation between constituents

of capillairies-containing plant material, and which alternative methods are applicable more broadly.

According to a first embodiment the internal mechanical pressure is applied through a fluid. The mechanical forces derived from a fluid are more uniformly distributed within the capillaries than the forces obtained by generating ice crystals locally as in the case of the method according to the abovementioned PCT application.

This results in an easy fractionation.

According to a first variant thereof the method according to the invention comprises a step of allowing a liquid, present in the capillaries, to boil under sub-atmospheric pressure, e. g. in a vacuum chamber. Sub-atmospheric pressure allows to keep the temperature relatively low, for example below 60 °C in the case of water (p=150 mmHg), thereby reducing the risk of detoriation of the quality of the plant material being processed. The rapid boiling of the liquid into vapour within the capillaries generates a pressure approximately 4 times larger than atmospheric pressure, resulting in cracking the epidermis and the subsequent detachment of the materials around the capillaries. Therefor the method according to the invention is applied preferably to plant material wherein capillaries are concentrated in well defined positions, such as in the fibrous shell and around the germ in the case of most cereal grains.

A suitable way of heating the fluid, present in the capillaries, to its boiling temperature is microwave heating. This type of heating allows for a very fast increase of the temperature of the fluid, thereby generating stresses and strains in the material surrounding the capillaries, which stresses and strains are the initial points of the fractionation aimed for. Preferably microwave heating is combined with the technique previously described, e. g. by microwave heating of the plant material in a vacuum chamber.

If the natural moisture content of the capillaries-containing plant material is insufficient to provide a suitable fractionation thereof upon boiling, the moisture content can be increased in advance. In other words, the method according to the invention may comprise a step of introducing additional water into the capillaries by subjecting the plant material to a premoistening treatment, e. g. by dipping the plant material in a water bath during a sufficient time.

For example, in the case of corn a steeping time in water from about 10-180 minutes is sufficient to fill the capillaries at room temperature. Furthermore the length of the steeping time period

depends on the water temperature. In corn the moisture content is raised by such a premoistening treatment to the range from 20-30 % based on the weight of the moistened grains, whereas the initial moisture content is about 16 % by weight of corn. The percentage at equilibrium in completely filled capillaries without having moisture being penetrated into the endosperm matrix is about 25 % by weight, based on the weight of the moistened grains. Excess water which is attached to the outer surface of the plant material to be treated and which could interfere in subsequent steps is removed advantageously, for example with the aid of air knives and the like.

In the present application the expression"shell, which contains fibres"is meant to be the outer fibrous layer or layers of the kernels. In case of corn and wheat these layers is/are also indicated by the term"bran".

According to a further embodiment the method according to the invention comprises a step of introducing a solution containing a solute into the capillaries, thereby creating osmotic pressure in the capillaries, because more water is pulled in. S02 is an example of a suitable solute in water.

According to a still further embodiment the method comprises a step of introducing a pressurized auxiliary fluid into the capillaries. In this way the capillaries are blown up, resulting in the desired fractionation. The fluid is preferably a gas, such as compressed air, nitrogen, SO2.

Instead of pressurizing the capillaries, according to a still further embodiment the pressure in the capillaries is reduced, e. g. by applying a vacuum, and preferably then a fluid, preferably a liquid is introduced into the capillairies having a reduced pressure, e. g. by dropping the plant material in a liquid bath for a very small time.

According to another embodiment the method comprises a step of forming hydrates within the capillaries. Such a hydrate can be formed in the capillaries by introducing suitable host molecules into the capillaries, each host molecule forming a nucleus for crystallization together with a plurality of water molecules. Propane and CO2 are examples of suitable host gasses. Such a step is preferably carried out at pressures up to 25 atm. and temperatures below room temperature. The heat of crystallization is withdrawn by the subcooled gas. Reducing the pressure liberates the host molecule, and melts down the water crystals being present.

In all the embodiments mentioned above the applied pressure

within the capillaries results in the formation of stresses and strains around the capillaries thereby causing favourable locations for the fractionation, e. g. the fractionation of cereal grains in a fraction comprising the fibrous shell and a fraction comprising the remainder of the constituents like the germ and endosperm matrix.

Compared to the conventional wet methods far less water is required.

Thereby considerable savings with respect to energy, waste treatment and equipment can be achieved. Furthermore the products obtained are comprised of a relatively dry mixture of constituents, which can be further processed in order to obtain the individual constituents, as will be explained hereinbelow using cereal grains as an example of starting material.

If necessary, the plant material treated according to the method of the invention may be subjected to an additional mechanical treatment applied externally, for example a coarse milling operation.

Advantageously sorted plant materials, i. e. material from which the foreign matter and broken or otherwise useless materials have been removed, are used as starting material in the method according to the invention. Suitable sorting techniques include electromagnetic separation, sieving and optical recognition. Optical recognition systems are commercially available, for example from Pulsarr.

A relatively dry mixture as product of the method according to the invention can be easily separated in fractions according to size and/or weight with the aid of suitable conventional techniques, such as screening, wherein a significant portion of the shells is retained as relatively large particles having a relatively low weight. A middle sized fraction contains smaller parts of the shell in addition to starch, gluten and germs. A small sized fraction contains even finer parts of the shells in addition to starch and gluten. Because of the difference in weight (density) the fibre components (fibrous shells) can be separated easily out from the middle and small sized fractions by means of a forced flow of air, such as fluidization in a fluid bed.

Alternatively, the fibre components can be separated out by means of conventional sieving. The fibres, which are entrained by the fluidization medium, are separated therefrom efficiently using for example cyclones. The fibres thus separated are stored, if necessary after a pretreatment with heat, e. g. in a heat-exchanger.

The germs, which contain oil, can be removed easily from the remaining mixture by conventional techniques, e. g. in a multistep operation. Examples thereof are inter alia ultrasonic separation,

separation on density (density difference), electronic scanning and extraction. The mixture remaining after this separation-a germ free fraction-can be further separated into starch and gluten by conventional techniques.

The mixture of starch and gluten, which remains after removal of the fibres and the germs, can be subjected to a finer milling operation, wherein the size is reduced to a maximum of about 70 microns. Then this milled mixture is advantageously separated with the aid of static electricity. As starch and gluten possess different polarities-starch is neutral, while gluten is highly positive-this difference in polarity can be utilized for the intended separation.

The movement of the gluten fraction to the respective electrode can be enhanced by incorporating the materials to be separated in a carrier gas. In order to avoid dust explosions preferably this step is carried out in an inert gas atmosphere, like nitrogen. Thereby dry starch and dry gluten are obtained as separated fractions.

The dry starch thus obtained needs only to be mixed with the precise amount of water in the preparation of a starting slurry for the refining into syrups of glucose.