METHOD OF REMOVING THE FIBROUS SHELLS FROM CEREAL GRAINS

Abstract: The invention relates to a method of removing the fibrous shells from cereal grains. This method according to the invention comprises a pretreatment step, wherein the moisture content of the cereal grains is increased, e.g. in the case of corn grains from 16 to more than 20 % by weight, followed by the step of exposure of the pretreated grains to a thermal shock by means of a cold-transfer medium, such as a cryogenic medium and thereafter mechanical treatment step thereof.

Description: Method of removing the fibrous shells from cereal grains Firstly, this invention relates to a method of removing shells, which contain fibres, from cereal grains (kernels) according to the preamble of claim 1. A method of this kind is known from Belgian patent 902 584, for example. In this known method of separating the exterior shell layer or layers from the remaining portion of legumes and cereals, the agricultural products to be treated are cooled or frozen, preferably using liquid nitrogen, whereafter they are subjected to a mechanical treatment in order to separate the exterior layer or layers from the remaining portion. Bilobated legumes fall further apart into their two lobes by these treatment steps. Furthermore a similar cooling step using for example liquid nitrogen for separating the shells from cereal grains is known from DE-A-2 938 635, by which it is intended that only the decorticated grains are exposed to a milling treatment after separation and that the inert (nitrogen) gas atmosphere reduces the risk of explosion. FR-A-2 032 032 discloses a method of removing the shells from seeds, in particular oil-containing seeds, like mustard seeds, wherein this removal is carried out at a low temperature, where the fats are in a solid (solidified) state. An improvement of this prior art method is known from US-A-4090669, wherein the seeds are subjected to a thermal shock in a fluid bed, preferably using a cryogenic medium. US-A-4 436 757 discloses a method of removing the shells of sunflower seeds (decorticating) and the separation (hulling) thereof from the "meat", wherein the seeds are immersed in a bath of liquefied gas like liquid nitrogen for 1 to 60 minutes, and directly afterwards the seeds thus treated are contacted with a liquid or other aqueous heating medium having a temperature, which is at least 100 °F higher than the boiling point of the liquefied gas. In general dry methods for removing the shells from cereals, legumes, seeds and the like are preferred to wet methods, which are applied conventionally, wherein large amounts of water are required, as 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. A drawback thereof is the limited accuracy which can be achieved in such a separation. 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 and dewatering, respectively waste water, and from that point of view look very promising, these methods have not been used on an industrial scale as far as 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 constituents such as for example grits, bran and flour. Firstly, the object of the present invention is to provide an improved method for the processing of cereal grains into starch and gluten, wherein the shells of the cereal grains are removed in an efficient manner at a relatively low need for water and energy. A further object of the invention is to provide substeps, suitable in the processing of cereal grains into starch and gluten, wherein almost no water or no water at all is required. Therefor the method of removing shells, which contain fibres, from cereal grains according to the invention comprises a pretreatment step of subjecting the cereal grains to a moistening treatment . It has been found that when cereal grains, which normally contain a relatively low moisture content m the range of 10-18% after harvesting and at storage, are allowed to absorb water for a sufficient period of time, and the cereal grains thus moistened are exposed to a thermal shock by means of a cold-transfer medium, then the shell very easily splits off from the remainder of the grain by and during the subsequent mechanical operation. When the pretreatment according to the invention is applied, furthermore it has appeared that the cereal grains do not need to be cooled deeply, with the result that the exposure time to the cold-transfer medium can be retained low, which has a beneficial influence on the process and/or production rate. If the cereal grains are exposed to the cold-transfer medium for a too long period of time, so that these are thoroughly cold, there is a smaller amount of large fibres and a larger amount of broken germs, which is undesired m view of the subsequent processing steps. It is believed that moisture absorbed during the pretreatment step, e.g. by allowing the grains to steep m water for a sufficient period of time, enhances the strains and stresses, which are generated in the shell as the water freezes rapidly therein, and the cereal grains thus treated are sub ected to a mechanical operation. The term "cold-transfer medium" refers to a fluidum that is able to freeze the water (moist) in the shell. Examples of such a cold-transfer medium include cold air, e.g. having a temperature of about -30°C and cryogenic media, such as liquid nitrogen, liquid carbon dioxyde etc.. Preferably the pretreatment with moisture is carried out m such a manner that the moisture penetrates only into the capillaries, which are present m the shell (between aleurone and cross cells and tube cells) and around the germ (between a so called "cementing layer" and endosperm matrix) . Experiments with corn have shown that a steeping time from about 10-180 minutes, preferably from about 15-120 minutes and more preferably from about 15 minutes to about 1 hour is sufficient to fill the capillaries with water at room temperature. With respect thereto it is believeα that the capillary between germ and endosperm is filled three times as fast as the capillary m the shell itself. Furthermore the length of the steeping time period depends on the water temperature. In corn the moisture content is raised to the range from 20-30 %, preferably 23-26 % by weight, based on the weight of the moistened grains, whereas the initial moisture content is about 16 % by weight of corn. The percentage at equilibrium m 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. Water which is attached to the periphery of the kernel and which would deteriorate the operation of the thermal shock, is removed advantageously, for example with the aid of air knives and the like, in advance of the thermal shock. Therefore the inventive method differs from the conventional steeping step of wet methods according to the prior art, wherein the grains are wetted throughout . 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 the invention preferably sorted cereal grains, i.e. cereals from which the foreign matter and broken cereal grains have been removed, and afterwards the moisture content of which has been elevated sufficiently, are subjected to a thermal shock, so that because of the differences in thermal expansion coefficients and heat transfer coefficients between the fibrous shell and the remaining portion of the grain, comprising the germ and the endosperm matrix, the shell is splitted off, which is enhanced during the mechanical operation. Preferably the method is carried out by exposing the cereal grains to an environment of a cryogenic medium, such as liquid nitrogen or carbon dioxyde, for example by immersion m such a cryogenic medium or by spraying of the cryogenic medium onto the cereal grains or m a reactor having a fluidized bed of cereal grains . Another suitable method involves mixing the cereal grains and the cold-transfer medium. Thereby tne moisture sucked into the capillaries becomes supercooled and freezes while ice is formed, which generates the stresses and strains within the shell and around the germ. The liquid nitrogen and/or carbon dioxyde evaporate after establishing the thermal shock and these gases can be disposed off m an unhindered manner. Furthermore it has been found unexpectedly that m exposing corn grains to a thermal shock, which grains have been pretreated according to the invention, not only the shell is removed, but also the germ is detached from the shell as well as from the endosperm matrix without damage. Advantageously the cereal grains are subjected to a coarse milling operation immediately following the exposure to the thermal shock. In other words when the water m the capillaries is still frozen. Preferably this coarse milling operation is carried out m a milling device m such a manner by adjusting the rate and fineness that the germs will remain intact. This coarse milling operation contributes to the detachment of the shell and the germ. A mill of the centrifuge type is a preferred device for carrying out such a milling operation, m particular one mounted with an impact blade. The fibrous shells are crimped from the cereal grains by the combination of pretreatment, thermal shock and mechanical operation, thereby a relatively dry mixture of the different constituents being obtained. This relatively dry mixture can be easily separated m 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 m addition to starch, gluten and germs. A small sized fraction contains even finer parts of the shells m addition to starch and gluten. Because of the difference m 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 m 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. m a heat-exchanger. The germs, which contain oil, can be removed easily from the remaining mixture by conventional techniques, e.g. m 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. As indicated heremabove briefly, preferably the method according to the invention comprises a sorting step preceding the pretreatment step, wherein the cereal grains are separated into a fraction of whole cereal grains and a fraction, which comprises foreign matter and/or damaged cereal grains. This sorting step may be carried out m a conventional manner using wmdsievmg (and if necessary electromagnets) . A preferred sorting technique is based on optical recognition, e.g. using socalled vision systems, whereby an improved separation can be achieved. Optical recognition systems are commercially available, for example from Pulsarr, and these systems are already used for sorting peas and beans. This improved method of optical sorting cereal grains and foreign matter can also be applied advantageously m the existing processing of cereals, both dry and wet processing. In order to acquire the information about the starting material which is needed for the pretreatment step and cryogenic crimping step, the fraction of whole cereal grains is analysed and examined, e.g. on moisture content, size, color, (number of) cracks and the like during or after the optical sorting operation. 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 m 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 m a carrier gas. In order to avoid dust explosions preferably this step is carried out m an inert gas atmosphere, like nitrogen. Thereby dry starch and dry gluten are obtained as separated fractions. 5 The substeps discussed above can be beneficially used as such m the existing wet processes according to the prior art. It will be appreciated by the skilled person that the maximum advantage regarding water and energy consumption will be obtained, when subsequent to the pretreatment step all substeps are carried out 0 without the addition of water and/or chemicals as far as possible. Furthermore the use of a number of expensive and energy consuming devices, long steeping times as well as long storage periods are superfluous m the method steps according to the invention. The dry starch thus obtained needs only to be mixed with the 5 precise amount of water m the preparation of a starting slurry for the refining into syrups of glucose. Hereinbelow the invention is illustrated m more detail referring to the drawing and further examples. In the drawings Fig. 1 s a block diagram of the processing of corn according 0 to the invention into starch and gluten; and Fig. 2 is a further block diagram, showing the initial steps of another embodiment of the method according to the invention. In an embodiment as shown m fig. 1, corn is fed via a feeding conduit 1 into a pretreatment unit 2. The corn is sorted optically 5 m the pretreatment unit 2 -damaged grains and foreign matter being discharged via discharge conduit 3-, and after measurement of the initial moisture content the sorted corn is moistened with a predetermined amount of water, which is supplied via concuit 4. After the moisture content has been raised to about 25 % by weight, 0 based on the wet grains, the corn is passed to a thermal insulated chamber 6 via connecting conduit 5, m which chamber the corn is immersed m a bath of liquid nitrogen, which liquid nitrogen is supplied via a conduit 7 and directly afterwards the corn is subjected to a coarse milling operation. As a result of these Ξ treatment steps a relatively dry mixture is produced, wherein all constituents of the corn grains initially charged are present. After the coarse milling operation, a coarse fraction of light parts of the fibrous shell is separated, which is passed to a fibre separation unit 16 via conduit 28. Tne remaining mixture is passeα to separation units arranged m series via conduits 8, 9 and 10, which separation units comprise a fluidization apparatus 11 for separating the lighter parts of the shell, a classifying unit 12 and a vibrating table 13, which is mclmedly arranged, for removal of germs and separated m the respective constituents. Nitrogen gas is used m the fluidization device 11 as fluidization medium, which gas is supplied via conduit 14. The fraction of flbre-containmg shells is discharged from the fluidization device 11 through discharge conduit 15 into an additional separation unit 16 and subsequently via conduit 17 and optional heat-exchanger 18 to fibre storage 19. The remaining particles of starch and gluten and the germs pass into degermmg device 13 via classifying unit 12, m the latter occurring a further separation m size and/or weight. In the degermmg device 13 the germs are seperated by vibration and discharged to storage 21 via conduit 20. The remaining mixture is separated into a gluten fraction and starch fraction using an electrostatic separator 22, which is operated under a nitrogen atmosphere. Optionally a finer milling operation (not shown) is applied preceding the electrostatic separation. The gluten fraction is discharged to storage 25 via conduit 23 and an additional separator 24. The starch fraction is removed via conduit 26 and discharged to storage 27, optionally after being predried and subjected to a heat exchange m the pretreatment device 2 with fresh supplied corn. Insufficiently milled material is returned to the inlet of the chamber 6 via return conduit 29. The electrostatic separator is maintained under an atmoshere of nitrogen gas m order to exclude the risk of a dust explosion. In the embodiment, shown fig. 2, corn is fed via a feeding conduit 51 onto a 6 mm sieve 52. Damaged grains of corn and foreign material are rejected and removed from the 6 mm sieve 52 oy conduit 53. The thus sorted corn is conveyed via conduit 54 to a pretreatment unit 55 where the corn is wetted by allowing the corn to soak water for 45 minutes. After soaking the corn thus pretreated is passed to unit 57 via conduit 56 where excess water is removed from the corn by an air flow. Thereafter the corn is conveyed to unit 59 via conduit 58, wherein the corn is mixed with liquid nitrogen twice by means of transporting the soaked corn tnrough unit 59 comprising an insulated chamber and spraying liquid nitrogen, supplied from a liquid nitrogen source 65 via conduits 60, on the corn on two occasions while the corn is mixed. The outlets of the conduits 60 into the chamber are spaced apart m the conveying direction of the corn. Via conduit 61 the corn having a frozen outer layer is transported to unit 62, which is a centrifuge type mill mounted with an impact blade, wherein the corn is milled coarsely. A relatively dry mixture of corn components is transported to a storage 64 via conduit 63. The invention is further illustrated by the following non- limitmg examples. A quantity of corn grains (1000 g) was steeped for 1 hour a large volume of water (1,5 1), whereby the moisture content initially being 16.0 % by weight was raised to 25.05 % by weight. The corn thus preconditioned was completely immersed m a bath of liquid nitrogen (at about - 190 °C) for 1 sec, thereby cooling the shell strongly and rapidly, while the interior was cooled to a much lesser extent. Immediately following this thermal shock the corn was milled m a mill of the centrifuge type, available at MicroTec. This mill having a housing with a conical shape, which functions as a stator, can be provided with 3 blades, an upper blade, which is called an impact blade, and two adjacent blades, disposed below the impact blade. The distance between the blades and the housing was adjusted at 5 mm, so that any case the germs would not be damaged. In example 1 the impact blade was not used. The finest product did fill the stator which was provided with protrusions, with the result that m fact the quantity of the fraction having dimensions of < 1.4 mm was higher. In Example 2 the mill had all 3 blades, while m Example 3 only the impact blade and that blade which is situated directly below the impact blade were used. The number of revolutions was set at the same value m all Examples. TABLE 1 , Please note that the value represented in the columns "WEIGHT" includes the weight of the cup (67.9 g) , wherein the fractions were weighed. The value in the columns "WT.%" is calculated, for a given fraction, as follows: [WEIGHT] fraction - [WEIGHT]^ [SOM NET WEIGHT] all fx __ 0 _ The fibres are contained mainly in the fraction X > 6.3 mm, together with some starch and gluten. The finer fibres and the remaining gluten and starch are divided over all other fractions. The germs are contained in the fractions of 1.4 mm through 4.0 mm. When the obtained fractions of Example 3 are separated according to the invention into the respective components, the following results are achieved. TABLE 2 * : the value presented ( ) is the percentage by weight of gluten **: based on total dry weight of fibres, germs, coarse and fine milling ***: based on total dry weight of the corn Representative results of a conventional separating method according to the prior art are 21 % of fibres, 6 % of germs, 5% of gluten and 67 % of starch, based on total dry weight. It is apparent that m the method according to the invention it is possible to work substantially without water, but that also the efficiency of the separation is improved, thereby achieving a higher yield of starch and gluten. Although the above example is directed to a method according to the invention starting from corn grains it is believed that other cereals, which have a similar shell structure having a capillary therein, as well as a capillary surrounding the germ, can be processed into fractions of starch and gluten respectively m a similar manner, wherein the time of the moistening treatment will vary from kind to kind which time s necessary to allow the capillaries being filled completely. Example 4 In a batch mode, a quantity of corn was sieved using a 6 mm sieve and subsequently steeped m water having a temperature of about 20°C for 45 minutes. Thereafter excess water attached to the exterior of the corn was removed by transporting the corn on a so- called "dewatering belt" and by subjecting the steeped corn to a flow of air. Then the corn was subjected to a thermal cold shock by spraying liquid nitrogen onto the steeped corn, while the corn was conveyed by means of " a transporting screw. Mixing of the corn while spraying the corn with liquid nitrogen caused a thermal cold shock to occur to the outer layers of all corn kernels, while the interior of the corn kernels remained relatively warm. The corn kernels having low temperature outer layers were quickly transported to a mill (Micro Tec UTM 2000) mounted with an impact cross blad and two adjacent blades, disposed below the impact blade. The rotor-stator gap of the mill was set at 6 mm and the mill was operated at a speed of 1500 rpm. The mixture of tissue components after milling can be separated into different fractions by methods known in the art. The tissue rich fractions obtained after the separation of the mixture obtained after milling comprise fractions such as the so-called endosperm fractions, the germ fraction, the fibers fraction and rest fractions.

Claims: WHAT IS CLAIMED IS: 1. A method of removing fibrous shells from cereal grains, the method comprising a step of exposure of the cereal grains to a thermal shock by a "cold-transfer medium and a step of mechanical treatment thereof, characterized in that the method comprises a pretreatment step of subjecting the cereal grains to a moistening treatment . 2. A method according to claimd 1, characterized in that the cold-transfer medium is a cryogenic medium. 3. A method according to claim 1 or claim 2, characterized in that the moistening treatment comprises the filling of empty capillaries, which empty capillaries are present in the shell, as well as between the germ and the endosperm matrix. 4. A method according to one of the preceding claims, characterized in that the cereal grains are allowed to steep in water during a short period of time. 5. A method according to claim 4, characterized in that the cereal grains are corn grains and that the moisture content of the cereal grains is increased to the range of 20-30 % by weight, based on the weight of the moistened cereal grains. 6. A method according to claim 5, characterized in that the moisture content of the corn grains is increased to the range of 23-26% by weight, based on the weight of the moistened corn grains. 7. A method according to claim 5 or 6, characterized in that the corn grains are allowed to steep in water during a period of time from 10-180 minutes. 8. A method according to claim 7, characterized in that the corn grains are allowed to steep in water during a period of time from 15-120 minutes. 9. A method according to claim 7 or claim 8, characterized in that the corn grains are allowed to steep in water for a period of time from 15-60 minutes. 10. A method according to one of the preceding claims, characterized in that attached water is removed after being steeped. 11. A method according to one of the preceding claims, characterized m that the mechanical treatment comprises the milling of the cereal grains. 12. A method according to claim 11, characterized that the milling is carried out m a mill of the centrifuge type. 13. A method according to claim 12, characterizeα m that the mill of the centrifuge type is mounted with an impact blade. 14. A method according to one of the preceding claims, characterized m that the method comprises a sorting step of sorting the cereal grains, preceding the pretreatment step, wherein the cereal gra s are separated into a fraction of whole cereal grains and a fraction, which comprises foreign matter and/or damaged cereal grains . 15. A method according to claim 14, characterized that the sorting step is carried out using optical recognition techniques. 16. A method according to one of the preceding claims 14 or 15, characterized m that the initial moisture content of the fraction containing whole cereal grains is determined. 17. A method according to one of the preceding claims, characterized that the method comprises a further separating step, wherein the cereal grains mechanically treated are separated into a fraction of decorticated cereal grams and a fraction of the fibrous shells. 18. A method according to claim 17, characterized that the method comprises a further step, wherein the germs are removed from the decorticated cereal grains. 19. A method according to claim 18, characterized m that the method comprises a second separation step, wherein the germs thus removed are separated from a germ free fraction. 20. A method according to claim 19, characterized m that the second separation step is carried out on an inclined and vibrating conveyor. 21. A method according to one of the preceding claims, characterized that the method comprises a third separation step, wherein the germ free fraction is separated into a starch fraction and a gluten fraction. 22. A method according to claim 21, characterized m that the third seperation step is carried out using static electricity. 23. A method according to one of the preceding claims, characterized that the method is conducted an inert gas atmosphere. 24. A method of preparing a slurry of starch for refining thereof into glucose syrup, " characterized m that the starch is mixed with water, which starch has been obtained by the method according to one of the claims 21-23. 25. A method of separating decorticated cereal grains, which are reduced size and which do not contain germs, into a starch fraction and a gluten fraction, characterized m that the starch fraction and gluten fraction are separated from one another by static electricity. 26. A method according to claim 25, characterized m that the separation is conducted m an inert gas atmosphere.

Inventors: VAN BOMMEL BERNHARD GEORGE, VAN BOMMEL, BERNHARD, GEORGE
Applicants: BOMMEL BERNHARD GEORGE VAN, CARGILL BV, CARGILL B.V., VAN BOMMEL, BERNHARD, GEORGE
Application year: 2000
Main classification: B02B3
IPC codes: B02B3, B02B5