METHOD AND DEVICE FOR MILLING GRAIN

Abstract: A method and a device for treating grain, preferably as an initial step in a milling process, using a high-speed mill, such as a disc mill, characterised in that prior to the feed-in into the high-speed mill the grains or parts thereof are exposed to press forces ensuring that the hulls of the grains or of parts thereof crack to a small extent whereas the flour body (endosperm) cracks to a large extent. For the above feed-in the high-speed mill (1) preferably is equipped with a pair of smooth-surfaced rolls (12).

Description: METHOD AND DEVICE FOR MILLING GRAIN The present invention relates to a method and a device for treating grains, preferably as a first step of a milling process. As is well known, the purpose of milling grain most- ly is to comminute the grains in mills with a view to obtain the highest possible flour yield and high quality after completion of a large number of milling steps with intermediate sifting steps, a sifting step by definition forming the end of the preceding milling step. Various types of mills, low-speed and high-speed, are known, examples of low-speed mills being roller mills and of high-speed mills disc mills, pin mills, turbo mills, detacheurs, hammer mills. By "low-speed" and "high-speed" should be understood low (< about 20 m/s) and high (> about 20 m/s), respectively, absolute veloci¬ ty of the movable grinding components and low (normally < 5 m/s) and high, respectively, differential speed between the mill grinding components. Disc mills are faster and operate more efficiently with respect to flour yield than do roller mills (based on the results from one milling pass or step). However, such high-speed mills are disadvantageous inasmuch as their efficiency is obtained at the expense of flour quality reflected in the colour of the flour which could exhibit various shades of dark- ness, bearing witness of too high bran contents in the flour, a fact that is verified by ash analysis. The ad¬ vantage obtained with high-speed mills resides in the smaller number of milling/sifting steps required to obtain full yield. Numerous milling/sifting steps may produce a compa¬ ratively white flour - which is an advantage inherent in slow-speed, traditional roller based mills which, how¬ ever, are disadvantageous in that the number of steps is comparatively high. It has been found that high-speed mills break up a large proportion of the grain hull into small hull parts while at the same time they mill the flour body, i.e. the endosperm, with such efficiency that after one pass through a mill of this kind it is no longer possible to separate the flour from the bran, for instance by means of sifting. By breaking up is to be understood that the hull is shattered instead of being ground in the conven¬ tional sense. The energy influence of high-speed mills on the grains has not yet been clarified. One theory is, how¬ ever, that the operative members of these mills hit the hard grains with a non-resilient force, cracking the grain hull into small pieces, also when the grain hulls have been conditioned through water treatment (temper¬ ing) . In this connection it should be noted that the flour yield discussed in the above comparison is approximately the same also in the case of lighter or darker flour, due to the fact that the hull or bran fraction that can no longer be separated from the flour following for instance a disc mill pass, is but a very small, hardly measurable part by weight compared with the part by weight of the flour fraction; the presence of the hull or bran fraction manifests itself mainly as a discolouring of the flour. The purpose of the subject invention is to provide a method of treating grain, preferably in the form of an initial grain-treatment step of a milling process of a kind that radically reduces the above-outlined disadvan- tages, i.e. one that produces a flour fraction from a high-speed mill/pass or step that contains smaller amounts of hull or bran - and thus is of lighter shade - than the flour fraction from an identical high-speed mill/pass/ step than has hitherto been possible, while at the same time ensuring that the hull (bran) fraction from an identical high-speed mill/pass/step contains lesser amounts of residual flour than the hull fraction from an identical high-speed mill/pass/step than previously. The above comparison is applicable, provided that the high-speed mills have an identical setting. The purpose is achieved by means of the method de¬ fined in the claims. Surprisingly, it has been found in accordance with the invention that if, in an initial grain treatment step in a milling process, the formation of cracks that takes place in the comparatively tough hull is caused to occur to a comparatively reduced extent, preferably to the smallest possible extent, whereas the formation of cracks in the comparatively fragile flour body is caused to occur to a comparatively large extent, preferably the largest possible, a purer flour fraction and a purer hull fraction are obtained after the first milling step through a high-speed mill. Said initial treatment step thus is aimed at producing preferably maximum comminution of the flour body with minimum breaking-up of the hull. The aimed-at effect on the grain treatment is achieved in accordance with the invention by so squeezing or pressing the grains, that while being squeezed the grains will be neither polished, nor shredded or cut into fragments in the same manner as occurs when it is passed e.g. between corrugated rolls or other rolls in roller mills or stone mills, i.e. in a manner producing grain particles comprising fragments of hull as well as adher¬ ing flour body (endosperm) fragments. In other words, in accordance with the invention the shearing forces on the grains as they are being supplied to the high-speed mill are the smallest possible. The substantial elimination of the shearing forces is produced in accordance with the invention by means producing almost exclusively press forces. In accordance with the invention, these forces are exerted at such a low speed that the above aimed-for results are achieved. These results also lead to a flaked or flattened condition of the grains. Examples of means for achievement of this condition are rammers, smooth-surfaced rotating rolls that are rotated at a differential speed close to zero and through the nip cf which the grain is allowed to pass, and end- less belt conveyors the belts of which travel around grain-flaking rolls. Vibration, through mechanical or ultrasonic means, of the grains is a further alternative. In order to achieve the above-described effect in accordance with the invention (maximum crack formation in the flour body and minimum crack formation in the hull) the grain should be tempered before passing through the first treatment step of the milling process. The expert has already at his disposal knowledge of how to achieve maximum toughness in the hull and maximum fragility in the flour body in a tempering process, carried out in one or several steps. By "flattened condition" should in this context be understood that the grains are flattened or crushed but still are essentially coherent, i.e. not shredded or divided. More precisely, they are coherent because the flour body (the endosperm) is essentially coherent, part¬ ly owing to the grain hull, having crackled in the flat¬ tening operation, being more or less coherent. The surprising effect obtained by means of the invention, viz. that white flour and a pure bran fraction are obtained in one single milling step or pass, may be due to the fact that the flattened grains produced by the feed-in device, consisting for instance of a pair of smooth-surfaced rolls rotating at a differential speed of approximately zero, absorb the impact from the operative tools of the rapid mill as non-resilient bodies, a theory that the applicants put forward without however wishing to restrict themselves to. It should be underlined that in accordance with the preferred embodiment, the smooth-surfaced rolls of the high-speed mill, rotating counter one another essentially without any differential speed between the rolls, feed the flattened grains directly into the high speed mill. It has been found that the feed-in results in a predo¬ minantly coarse fraction of flattened grains wherein the hull and the flour body (endosperm) adhere to one another, i.e. there is hardly any separation of the hulls from the endosperm, which surprisingly produces respec¬ tively a higher yield of a whiter flour fraction and a higher yield of a purer hull (bran) fraction issuing from the high-speed mill than up to date. In this connection should be noted that in accor¬ dance with the conventional milling technology it is known per se to mill grain products between smooth rolls. However, this prior-art treatment comprises one or seve¬ ral milling steps/passes and is inserted as a milling step for treatment of a practically pure (i.e. dehulled) grain or grit fraction, in which the smooth-surfaced rolls are rotated at a differential speed that is sub¬ stantially different from zero (approximately 1.5:1), i.e. employing shearing forces for direct extraction of flour. The expert may set the roller nip to achieve the above-defined flattened condition without this requiring inventive activity. In the following one embodiment of a milling step shown in the enclosed drawing figure will be explained in closer detail. The drawing illustrates a disc mill 1 in accordance with applicant's Swedish Published Specification No. 8403125-1 (442,826), having an outlet 3, grinding discs 4, 5 one of which may be stationary whereas the other one is rotated by means of a drive shaft 16. Further, the disc mill has a feed-in conduit 7 for supply of grain, a milling chamber 8, corrugated sections 10 formed on the grinding discs 4, 5 and a housing 11. In accordance with the invention a pair of smooth- surfaced flaking rolls 12 are provided in the feed-in conduit 7, which rolls are rotated at the same speed by means of a motor, not shown. In other words, the pair of rolls 12 serves as a feed-in device supplying material into the disc mill, the product issuing from the feed-in device being of the nature defined in the aforegoing. Example. By means of the above-defined disc mill white flour fractions (of wheat) have been produced, having ash contents of 0.55-0.70 and hull fractions that were almost entirely free of flour and grits. On the other hand, with an identical starting product which was supplied directly to a disc mill of the kind describ¬ ed above but devoid of the feed-in device equipped with smooth-surfaced flaking rolls, a flour fraction having ash contents of between 0.70 and 0.85 was obtained. These observations and results are accounted for in the Table below, wherein likewise are given the results from tests carried out with a "Fine Grinder" - a type of turbo mill - produced by UNITED MILLING SYSTEMS A/S, Denmark. This grinder too is equipped with a feed-in device equipped with a pair of smooth rolls. Milling equipment Ash contents % Disc mill 0.82 0.78 0.73 Disc mill with feed-in 0.69 0.61 0.58 according to the invention Fine Grinder 0.85 0.92 Fine Grinder with feed-in 0.76 0.81 according to the invention The invention is not limited to use for grinding purposes in a first step of a milling process but could likewise be applied to grinding in later stages of a milling process. In other words, the starting material need not be whole grains or grains still in possession of their original hull, even when the invention is applied in a first step of a milling process. For instance, the starting material could be partly dehulled whole grains or cut-up grains. It should also be underlined that the invention is not intended to be applied for the sole purpose of simul¬ taneously obtaining pure flour and pure bran but also for the purpose of obtaining either pure bran or pure flour. For instance, a pin mill equipped with the feed-in device specified in the aforegoing, does not produce pure bran but pure flour, "pure" meaning higher purity than the purity results obtained without the feed-in device in accordance with the invention.

Claims: CLAIMS 1. A method of treating grain, preferably as a first step of a milling process, using a high-speed mill, such as a disc mill, c h a r a c t e r i s e d in that prior to being fed into the high-speed mill the grains or parts thereof, are exposed to press forces, whereby the hulls of the grains or of parts thereof crack to a limited extent whereas the flour body (endosperm) cracks to a large extent. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the press forces are exerted in conjunction with said feed-in. 3. A method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the press forces are exerted by means of a pair of smooth-surfaced rolls that are being rotated at a peripheral differential speed of essentially zero. 4. A method as claimed in any one of claims 1-3, applied to obtain a pure bran fraction. 5. A method as claimed any one of claims 1-3, applied to produce a pure flour fraction. 6. A device for treating grain, preferably as a first step in a milling process, using a high-speed mill, such as a disc mill, c h a r a c t e r i s e d in that the high-speed mill (1) is equipped with a feed-in device ( 12) essentially exerting only press forces on the grains or parts thereof, whereby the hull of the grains or of parts thereof crack to a limited extent whereas the flour body (endosperm) cracks to a large extent. 7. A device as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the feed-in device consists of a pair of smooth-surfaced rolls and of a drive system for rotation of the rolls at a peripheral differential speed of essentially zero.

Inventors: HOEHNDORF THOMAS, LOEFQVIST BO, HOEHNDORF, THOMAS, LOEFQVIST, BO
Applicants: HOEHNDORF THOMAS, LOEFQVIST BO, UNITED MILLING SYSTEMS A S, HOEHNDORF, THOMAS, LOEFQVIST, BO, UNITED MILLING SYSTEMS A/S
Application year: 1996
Main classification: B02B3
IPC codes: B02B3, B02C7, B02C11