Abstract:
The invention relates to a method of removing the fibrous shells from cereal grains. This method according to the invention includes 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 cryogenic medium and thereafter mechanical treatment step thereof.

Description:
This is a continuation of prior application Ser. No. 09/432,621, filed Nov. 3, 1999, which is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     Firstly, this invention relates to 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 cryogenic medium and a step of mechanical treatment thereof. 
     DESCRIPTION OF THE PRIOR ART 
     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 U.S. Pat. No. 4,090,669, wherein the seeds are subjected to a thermal shock in a fluid bed, preferably using a cryogenic medium. 
     U.S. Pat. No. 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. 
     OBJECTS OF THE INVENTION 
     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 such a separation into the different constituents is not intended, but instead thereof a flour is obtained, which is composed of all constituents of the cereal grains. 
     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. 
     SUMMARY OF THE INVENTION 
     The method of removing fibrous shells from cereal grains according to the invention comprises a step of exposure of the cereal grains to a thermal shock by a cryogenic medium and a step of mechanical treatment thereof, wherein the method also comprises a pretreatment step, wherein the cereal grains are subjected to a moistening treatment. 
     It has been found that when cereal grains, which normally contain a relatively low moisture content in 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 cryogenic 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 cryogenic 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 cryogenic 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 in view of the subsequent processing steps. 
     It is believed that moisture absorbed during the pretreatment step, e.g. by allowing the grains to steep in 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 subjected to a mechanical operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawing a block diagram of the processing of corn according to the invention into starch and gluten is schematically shown. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferably the pretreatment with moisture is carried out in such a manner that the moisture penetrates only into the capillaries, which are present in 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 of about 15 minutes to about 1 hour is sufficient to fill the capillaries with water at room temperature. With respect thereto it is noted that the capillary between germ and endosperm is filled three times as fast as the capillary in 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 23-26% by weight, based on the weight of the moistened grains, whereas the initial content is about 16% by weight. 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. 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, which layer or 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 liquid nitrogen or carbon dioxyde, for example by immersion in such a cryogenic medium or by spraying of the cryogenic medium onto the cereal grains or in a reactor having a fluidized bed of cereal grains. Thereby the 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 in an unhindered manner. 
     Furthermore it has been found unexpectedly that in 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 in the capillaries is still frozen. Preferably this coarse milling operation is carried out in a milling device in 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. 
     The fibrous shells are crimped from the cereal grains by the combination of pretreatment, thermal shock and mechanical operation, thereby a dry mixture of the different constituents being obtained. This dry mixture can be easily separated in 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 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. 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. Examples thereof are inter alia ultrasonic separation, separation on density (density difference), electronic scanning and extraction. The mixture remaining after this separation can be further separated into starch and gluten by conventional techniques. 
     As indicated hereinabove 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 in a conventional manner using windsieving (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 in 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 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 substeps discussed above can be beneficially used as such in the existing wet processes according to the prior art. These substeps as such are defined in claims 19 and 20. 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 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 in the method steps according to the invention. 
     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. 
     Corn is fed via a feeding conduit  1  into a pretreatment unit  2 . The corn is sorted optically in 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, based on the wet grains, the corn is passed to a thermal insulated chamber  6  via connecting conduit  5 , in which chamber the corn is immersed in 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 treatment steps a 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 . The remaining mixture is passed to separation units arranged in 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 inclinedly arranged, for removal of germs and separated in the respective constituents. Nitrogen gas is used in the fluidization device  11  as fluidization medium, which gas is supplied via conduit  14 . The fraction of fibre-containing 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 degerming device  13  via classifying unit  12 , in the latter occurring a further separation in size and/or weight. In the degerming 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 in 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 in order to minimalize the risk of a dust explosion. 
     EXAMPLES 
     The invention is further illustrated by the following non-limiting examples. 
     A quantity of corn grains (1000 g) was steeped for 1 hour in a large volume of water (1,5 l), whereby the moisture content initially being 16.0% by weight was raised to 25.05% by weight. The corn thus preconditioned was completely immersed in 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 in 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 in 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 in fact the quantity of the fraction having dimensions of &lt;1.4 mm was higher. In Example 2 the mill had all 3 blades, while in 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 in all Examples. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
               
             
          
           
               
                 DIMENSION X 
                   
                   
                   
                   
                   
                   
               
               
                 (MM) 
                 WEIGHT (g) 
                 WT. % 
                 WEIGHT (g) 
                 WT. % 
                 WEIGHT (g) 
                 WT. % 
               
               
                   
               
             
          
           
               
                 X &lt; 1.4 
                 111.9 
                 10.9 
                 187.3 
                 19.3 
                 183.7 
                 18.3 
               
               
                 1.4 &lt; X &lt; 2.4 
                 127.3 
                 14.7 
                 155.9 
                 14.2 
                 133.4 
                 10.4 
               
               
                 2.4 &lt; X &lt; 4.0 
                 262.0 
                 48.1 
                 362.3 
                 47.7 
                 297.6 
                 36.3 
               
               
                 4.0 &lt; X &lt; 6.3 
                 146.7 
                 19.5 
                 166.2 
                 15.9 
                 218.6 
                 23.8 
               
               
                 X &gt; 6.3 
                 95.3 
                 6.8 
                 85.5 
                 2.8 
                 138.5 
                 11.2 
               
               
                   
               
             
          
         
       
     
     The fibres are contained mainly in the fraction X&gt;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 
               
             
             
               
                   
                   
               
               
                   
                 EXAMPLE 3 
                   
               
             
          
           
               
                   
                 WEIGHT (g) 
                 WT. % 
               
               
                   
                   
               
             
          
           
               
                   
                 FIBERS 
                  48.99 
                 10.4 
               
               
                   
                 GERMS 
                  37.48 
                  8.0 
               
               
                   
                 COARSE MILLING 
                 210.77 
                 44.8 
               
               
                   
                 PINE MILLING 
                 172.85 
                 36.8 
               
               
                   
                 STARCH &amp; GLUTEN 
                 383.62 
                 81.6 (9)* 
               
               
                   
                   
               
               
                   
                 *the value presented in () is the percentage by weight of gluten  
               
             
          
         
       
     
     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. It is apparent that in 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 in a similar manner wherein the time of the moistening treatment will vary from kind to kind which time is necessary to allow the capillaries being filled completely.