谷物食品技术（英文版）：08338_06

6 Dry Milling Technology Introduction The single term ‘milling’, applied in the con- text of cereals, covers a wide range of processes. In general they are methods of transforming whole grains into forms suitable for consumption or for conversion into consumable products. Milling processes do not themselves involve intentional heating, although in some cases, as in oat processing, a heating phase precedes the milling, and in maize dry milling a drying phase is included. Characteristic (but not essential) features of milling processes are: 1. Separation of the botanical tissues of the grain In milling processes involving both separation and size reduction the two operations may be carried out in two distinct phases, as in sorghum milling, or, to some extent, combined, as in bread- and soft-wheat milling, where the two processes continue throughout the multi-stage operation (although the emphasis changes as the process proceeds). In rice milling, two stages occur but neither seeks to fragment the endo- sperm. The first stage removes the husk and the second removes the bran. Even the above distinctions are not absolute as processes are in use for decorticating wheat grains before reduction of size and some rice is milled into flour. It is clear that few generalizations can be made about cereals milling, most milling technologies depend upon a series of individual Processes through which stocks Pass in sequence- (e.g. endosperm from pericarp, testa and 2. Reduction of the endosperm into flour or grits. embryo). Some milling systems include both operations (e.g. white flour milling from wheat), while others involve only one (e.g. rice milling comprises only separation, and wholemeal wheat milling seeks only to reduce particle size). 2. Separate fractions produced by (1)-type Milling schemes are conveniently classified as wet or dry, but this indicates a difference in 3. Change the temperature and/or water content degree rather than an absolute distinction as water is used in almost all separations. Damping or ‘tempering’ features even in ‘dry’ milling; it is considered in detail in Ch. 5 as it is a pre-milling treatment. This chapter is concerned with so- called dry milling; wet milling processes are dealt with in Ch. 12. Emphasis is placed on preparation for human consumption but the term ‘milling’ also applies to production of animal feeds. A brief description of feed-milling is given in Ch. 15. Mi I I i ng processes The processes are of three types, they may: 1. Change the shape and size of the feedstock. treatments. of the stocks. The processes are described below. Treatments that change shape and size Abrasion Effects depend upon severity, thus: 1. Surface abrasion is a relatively gentle process which removes all or part of the fruit coats 129 sive surface, which may be of natural stone, carborundum, sculptured or perforated metal, or other material. Where perforated screens are used, these behave as sieves also, selec- tively permitting passage of particles. In the \A/ be described as decortication. 2. Severe abrasion includes heavy or protracted DRY MILLING TECHNOLOGY 131 shallow (dull) profile disposed towards the case of water the separation usually depends on nip. The relationship between rolls may thus one or more component being denser than water be described as ‘dull-to-dull, sharp-to-sharp, and others being less dense. When air is used, dull-to-sharp or sharp-to-dull’ (Fig. 6.2). It the force of an air current supports particles of is conventional to give the fast roll disposition lesser density to a greater degree than the denser first in such descriptions. ones, allowing them to be carried upwards and 2. Flaking - flaking rolls are smooth-surfaced later deposited when the force of the current is and are generally heavier than grinding rolls reduced. The process is described as aspiration. and they are operated at zero differential. The The lighter particles frequently also have a purpose of this is to increase the surface area flat shape, which enhances their buoyancy. of the feedstock, either to facilitate subsequent Aspiration features in purifiers used in wheat separation of components (eg. germ from (particularly durum) milling to remove bran from endosperm) or to impart desired product semolina, and in rice milling, to remove pearlings characteristics, as in porridge oats. from decorticated grains. Multiple factors The paddy separator (see p. 120) is an example of a machine in which several grain characteristics are exploited in effecting their separation. Specific gravity, surface roughness and shape all combine to direct grains into appropriate streams on a surface. Changes in temperature and/or moisture content Water can be added with or without substantial change in temperature, it may be added specific- ally to achieve a required combination of the two physical conditions, as in stabilization of oats, or to change the mechanical properties of the grain components, as in wheat and rice milling, when the temperature is of less importance. Stocks produced from grains or intermediates milled at very high moisture need to be dried to permit proper processing or safe storage. Drying is performed by heating, and stocks that have been heated may subsequently require cooling. Fine grinding and air classification The contents of cells somprising the bulk of storage tissues of many legume cotyledons and cereal endosperms consist essentially of starch granules embedded in a protein matrix. In oats and some legumes an appreciable amount of oil Impacting Grains or milled stocks are thrown against a hard and possibly abrasive surface. This is usually achieved by feeding stocks into the centre of a very high-speed rotor. The process is very versatile; it may be used as a means of dehulling (as of or disinfestation, destroying all stages in the life- cycle of insect pests found in grain and flour. Fractionating processes Size This is an important criterion by which par- ticles are separated. In most milling systems which involve grinding, sieving features at some stage, to separate stocks as final products, or for further appropriate treatment. Shape In processes Such as Oat milling and rice milhgy grains are graded on a shape (and size) basis, before treatment, as machine clearances are grain-size dependent; and during the milling process, as small grains which escape treatment need to be re-fed. Shape-sensitive fractionating machines include disc separators and trieur cylinders. Specific gravity Particles differing in density may be separated on a fluid medium such as air or water. In the Oats), abrading, size-reduction, (as in Pin milhg)Y tilted vibrating table with a cunningly sculptured 132 TECHNOLOGY OF CEREALS is also present. The spaces among closely packed spherical or near spherical starch granules are wedge shaped, and where protein occupies these spaces it is compressed into the same wedge shape. It has thus been called wedge protein. Clearly the size of starch granules determines the sizes of the interstitial wedges. In the case of the Triticeae cereals the wedges among the larger population of starch granules generally have granules of the smaller population (see p. 57) When wheat endosperm is fragmented by particles, differing in size and composition (Greer et al., 1951). These may be classified into three main fractions: embedded in them. Microns grinding it is usually reduced to a mixture of 12 xx -I 0 OI 1. Whole endosperm cells (singly or in clumps), 9 03,~ segments of endosperm cells, and clusters of 2 starch granules and protein (upwards of 35 * 4 pm in diameter). This fraction has a protein content similar to that of the parent flour. 2* Large and medium sized starch granules, some with protein attached (15-35 pm in diameter). This fraction has a protein content One ha1f to two thirds that Of the parent flour* 3. Small chips (wedges) of protein, and detached small starch granules (less than 15 pm in diameter)* This fraction has a protein content approximately twice that of the parent flour (Fig. 6.3). The proportion of medium-sized and small particles (below 35 pm) in flour milled conven- tionally from soft wheat is about 50% by weight, but in hard wheat flours it is only 10%. The proportion of smaller particles can be increased at the expense of larger ones by further grinding on, for example, a pinned disc grinder, which consists of two steel discs mounted on a vertical axis, each disc being studded on the inward- facing surface with projecting steel pins arranged in concentric rings that intermesh. One disc, the stator, remains stationary while the other rotates at high speed. Feedstock enters the chamber between the discs at the centre, and it is propelled centifugally by the air current created. The particles impact against the pins and against each other, as a result of which, they are fragmented. FIG 6.3 Above: the two main types of endosperm cell - prismatic (left), polyhedral (right) - showing large and small starch granules (white) embedded in protein matrix (black). Below: exposed endosperm cell contents (left) and products of further breakdown (right): 1. detached large starch granules (about 25 pm diameter); 2. ‘clusters’ of small starch granules and protein matrix (about 20 pm diameter); 3. detached small starch granules (about 7 pm diameter); 4. fragments of free wedge protein (less than 20 pm diameter). 12xx is the representation to scale of the mesh aperture width of a typical flour bolting cloth. (Redrawn from C.R. Jones et al., J. Biochem Microbiol. Technol. Engng. 1959, 1:77 and repro- duced by courtesy of Interscience Publishers.) The reduction in particle size due to fine grinding further separates the components, as previously described, allowing increased propor- tions of starch and protein to be concentrated into different fractions. Particles below about 80 pm are considered to be in the sub-sieve range, and for making separa- tions at 15 pm and 35 pm, the flour as ground, or after fine-grinding, is fractionated by air- classification. This process involves air elutria- tion, a process in which particles are subjected to the opposing effects of centrifugal force and air drag. Smaller particles are influenced more by the air drag than by centrifugal force, while the reverse is true of the larger particles. The size at which a separation is made is controlled by DRY MILLING TECHNOLOGY 133 TABLE 6.1 Yield and Protein Content of Air-Classified Fractions of Flours With or Without Pinned-Disc Grinding* Fine Medium Coarse Parent (0-17 P.4 (17-35 pm) (> 35 pm) flour protein Protein Protein Protein content Yield content Yield content Yield content Flour (Yo) (”/.I (”/.It (”1 (“/.It (“/.) (“/.)t Hard wheat Unground 13.6 1 17.1 9 9.9 90 13.8 Ground 13.4 12 18.9 41 10.0 47 14.7 Unground 7.6 7 14.5 45 5.3 48 8.9 Ground 7.7 20 15.7 71 5.0 9 9.5 * Source: Kent (1965). t 14% m.c. basis. N x 5.7. Soft wheat varying the amount of air admitted, or by adjust- uniform particle size and granular nature are ing the pitch of baffles which divide or ‘cut’ the advantageous. airborne stream of particles. When practised commercially, air classification Classification can be continued into further is generally carried out in the mill. It is customary fractions by cuts corresponding to larger sizes. to effect separations into a protein-rich fraction As Fig. 6.4 shows, this does not lead, as might of less than 15 pm, a starch-rich fraction of be expected, to many fractions varying in size 15-35 pm, and a fraction over 35 pm consisting of only, but to fractions whose composition also cells or parts of cells that have resisted breaking varies. into discrete components. Table 6.1 shows typical The highest-protein fraction above 15 pm is yields and characteristics of fractions derived that between 44 and 55 pm, in which are concen- from fine-ground and unground flours of hard trated the cells from the outermost layer of and soft wheats. starchy endosperm. This subaleurone layer The term protein shift has been coined to define contains only few small starch granules embedded the degree of protein concentration achieved with in a solid core of protein (Kent, 1966). a given feed. Protein shift is the amount of protein shifted into the high-protein fraction plus that shifted out of the lower fractions, expressed 26 24 22 18 as a percentage of total protein in the material fractionated. 20 s 16 fractions might be used are: Applications for which commercial classified - Parent 134% ;-14 5 12 e IO - - - - - - - - - - + Fine fraction: increasing the protein content of IL 8 bread flours, particularly those milled from low or medium protein wheats, and in the manu- facture of gluten-enriched bread and starch reduced products. Medium fraction: use in sponge cakes and pre-mix flours. Coarse fraction: biscuit manufacture where the 4 2 0 10 20 30 40 50 60 70 80 90 100 Yield, % FIG 6.4 Results of air-classification of flour into nine fractions of varying particle size. The flour, milled from CWRS wheat, was pinmilled one pass, and classified at 10, 13, 17, 22, 28, 35,44, and 55 pm nominal cut sizes. Protein content of the parent flour was 13.4%. with 7.3%. Sorghum flour fractions between Pounding grits (which are derived from the higher protein, horny parts of the grain) of 9.2-11.9% protein I Winnowing or f Coatot ion Protein shifting by air-classification has not met with success when attempted in rice. Three factors are held responsible for this: the smallness 4 Further pounding DRY MILLING TECHNOLOGY 135 decortication and cannot be readily separated from (Cecil, 1987). In a study for F.A.O. (Perten, 1977) the pericarp. The difficulties are compounded a decortication rate of 20% was recommended for when pericarps are thick (Rooney et al., 1986). good consumer acceptance. Pearling at natural Sufficient flour is produced daily for the needs moisture content was favoured as tempering of the family; three or four hours may be required reduced throughput, increased breakage of grains to produce 1.5-1.6 kg of flour from sorghum at and increased ash yield and fat content of the an extraction rate of 60-70% of the initial grain pearled grains. In India, and indeed elsewhere, weight. Bran accounts for about 12%, and the the grain is conditioned with about 2% of water same amount is lost. Even longer periods (6 h) before pearling (Desikachar, 1977). may be required to produce a family’s daily As the amount of pearlings increases so the com- requirements from millet (Varriano-Marston and position changes, reflecting the concentration of Hoseney, 1983) The problem of storage of flour fibre in the outer layers, and of protein and fibre does not arise, and this is just as well as products in the aleurone layer and the peripheral (subaleu- may have a high moisture content. Further, rone) starchy endosperm (cf. p. 37). Oil and protein the continued pounding expresses oil from the contents of the pearlings are at a maximum when embryo and incorporates it into the flour, leading about 12% of the grain has been abraded. to rancidity on oxidation. Sorghum endosperm used as a brewing adjunct leaves the mill as coarse grits: they may be produced by impaction following decortication. Embryos are removed during the process (Rooney Industrial milling of sorghum As urban drift accelerates in Tropical coun- and Serna-Saldivar, 1991). Removal of embryos tries, strain is increasingly imposed on the domes- after milling is difficult as they are the same size tic production system and industrialization of as some of the grits. They can be separated, flour production becomes more attractive. however, by virtue of their different densities; In Africa the relationship between domestic floatation on water or use of a gravity table are processing and mechanized milling is a delicate suitable methods. balance that is affected by a number of changes, Reduction to flour particle size may be achieved occurring on that continent. Processing is justi- by roller milling, impaction or pin milling and fied only if it prolongs the storage period, increases may or may not include degerming. Maintenance convenience and preserves the nutritional quality and correct setting of roller mills can be a problem of the product (Chinsman, 1984). and the use of easily maintained and adjusted Processing methods may include adapted special mills is advocated by some. The United wheat flour milling methods and specifically Milling System (now Conagra) two stage process designed abrasive methods. Most begin with a is one example. The decortication stage employs a decortication stage using mills with abrasive discs vertical rotor which hurls grains against each or carborundum stones (Reichert et al., 1982). other and against a cylindrical screen through Wholemeals are produced by use of stone, which the fragments produced by the impacts hammer, pin or roller mills. pass for collection in a cyclone. This stage was In some cases traditional methods of decortica- designed to resemble the pounding typical of tion are combined with ‘service’ milling. Such domestic processing. The disc mill that follows combinations can improve on the traditional has sawblade elements that are cheap and simple methods alone by as much as 20% extraction rate. to replace. A two tonnes installation in the Sudan An experimental milling operation using a produces flour or grits of 80% extraction rate with laboratory Biihler mill gave best results with ash yield of 0.7-1.1%. sorghum conditioned to 20% m/c. Even broom- If it is required to remove the embryo before yard sorghum (which has pales attached to the milling, this may be done using machines designed grain, and which cannot be milled by other for degerming maize. Alternatively, special methods) was successfully processed by this method machines have been produced for sorghum itself. Cotyledon -, Plumule Radicle ~ ment of this was the hominy block, fashioned from two trees, the stump of one being hollowed out as a mortar, and the springy limb of another nearby, serving as a pestle. The name ‘hominy’ is derived from a North American Indian word and it describes a coarse ground maize meal mixed with milk or water. It persists today, applied to some of the products of modern maize dry . Embryo Drying, cooling, aspiration Drying, cooling, aspiration Thros’ Thros’ Sifter Break rolls Sifter Aspiration, Sifter Oil expresser drying, cooling and filters I Flaking grits 1 I Germ oil Reduction Aspiration, drying, cooling Aspi ration, drying, cooling 138 TECHNOLOGY OF CEREALS The Beall degerm inator Possibly the most important innovation in dry maize milling was the introduction of degerming stages. The Beall degerminator is unfortunately named, as it neither reverses the process of germination, nor totally or exclusively removes the germ. It was introduced in 1906 and it is used in the majority of U.S. dry milling plants today, as an essential stage in the 'tempering-degerming' (TD) system. Its virtue lies in its potential to produce a high yield of large particle size grits with low fat content and low fibre content (about OS%), suitable for manufacture of corn flakes. the U.S.A. is No. 2 yellow dent corn. In Africa white maize is used. After cleaning and tempering to 20% moisture content, it passes to the 'Beall'. This machine consists of a cast iron cone, rotating at about 750 rev/min on a horizontal axis, within a conical, stationary housing, partly fitted with screens and partly with protrusions on the inner surface. The rotor also has protrusions on its outer surface. The maize is fed in at the small end and it works along to the large end, between the two elements. The protrusions on the rotor and the housing rub off the hull and embryo by abrasive action, also breaking the endosperm into particles of various sizes and degrees of purity. The Beall discharges two types of stock: the tail stocks which are too large to pass through the screens, consisting mainly of fragmented endosperm, and the through-stock consisting largely of bran and embryo. The proportions of different sized particles can be controlled by the finished product. Finer stocks are combined with coarser fractions of the through-stocks from the Beall, for treatment in the milling system. Milling The feed to the milling system, viz. large, medium and fine hominy, germ roll stock, and meal are mixtures of endosperm, bran and embryo. They are separated by a series of roller milling, sifting and aspiration stages before being dried and emerging as a diverse range of final products. They are fed to the mill, each entering at an appropriate point: the large and medium germ roll stock at the second roll. The milling is carried out on roller mills, using fluted rolls, a traditional flow containing up to sixteen distinct stages. The grindings with fluted rolls (15-23 cuts per cm rotating at a differential of 1.25: 1 or 1.5: 1) flatten the embryo fragments, allowing them to be removed by sieving. The products are sifted on plansifters and are aspirated. The mill is divided into a break section, a series of germ rolls and a series of reduction and quality rolls. The break system releases the rest of the embryo as intact particles, and cracks the larger grits to produce grits of medium size. The whole milling system for maize bears some resemblance to the earlier part of the wheat milling system (which is described more fully below - p. 141) as far as B2 reduction roll (2nd quality roll, in the U.S.A.) vzz. the break, coarse reduction and the scratch systems, but is extended and modified in comparison with this part of the wheat milling quantity of germ present. Modern practice is to Drying, cooling and grading use a much shortened system. Tail-stock from the Beall degerminator is dried The action of the rolls should be less severe at to 15-15.5% moisture content in rotary steam the head end of the mill than at the tail end in tubes at a temperature of 60"-70°C and cooled to order to minimize damage to the germ while 32"-38"C by aspiration with cold air. The dried simultaneously obtaining maximum yields of oil stock is sifted to produce a number of particle and oil-free grits. size fractions. The coarsest fraction, between 3.4 The finished coarse, medium and fine grits, and 5.8 mm, consists of the flaking or hominy meal and flour products are dried to 12-14% grits, originating from the vitreous parts of the moisture content on rotary steam tube driers. endosperm. They may pass through a further The germ concentrate consists largely of aspirator and drier-cooler, before emerging as a heavily damaged embryos with an oil content The favoured feed stock to the TD system in hominy at the first break, the fine hominy and setting Of the Beall (Brekke and Kwolek, 1969)' to make a mOre thorough separation of the large DRY MILLING TECHNOLOGY 139 extraction from 1645% to about 6% in the germ cake. The extracted oil is purified by filtering through cloth, using a pressure of 552-690 kN/m2 (80-100 lb/in2). The oil, which is rich in essential fatty acids, has a sp. gr. of 0.9224925, and finds use as a salad oil. Its high smoke point also makes it suitable for use as a cooking oil. Composition of products Chemical composition and nutritional value vary among the tissues of the maize grain, hence their separation leads to a concentration of components into different products (see Table 14.12). Uses for dry-milled maize products The products of maize dry milling, their par- ticle size ranges, and average yields are shown in Table 6.2. of 15%. The fat content of grits and flour is 1.5-2 .O%. Finer through-stocks pass through a drier followed by an aspirator aqd purifier, to remove bran and germ. Germ may be extracted to produce oil, the remainder being compressed into germ cake. Finer fractions of the through-stock consist of endosperm heavily contaminated with bran and germ, the mixture being known as ‘standard meal’. The various fractions are combined to give ‘hominy feed’ Impact grinding An alternative to the Beall is the European- developed system in which impact machines such as entoleters (see Fig. 5.7) or turbocrushers are used to detach the embryos. Treatment is per- formed at natural moisture content 13-15% (cf. 20% for the Beall). It is relatively economical as subsequent drying is not necessary. Separation of germ from endosperm on gravity tables is efficient, but separation of endosperm from bran by aspiration, and of vitreous endosperm from mealy endosperm, is less effective than with the TABLE 6.2 Yield and Particle Size Range of Dty-milled Maize Products Particle size range Yield Beall degerminator . The following products are (Yo by Product Mesh* mm weight) obtained from the entoleter process: Flaking grits 3.5-6 5.8-3.4 12 Maize germs, 14.5 mm, with 18-25% fat. Coarse grits 9-12 2.g1.4 Maize grits, 14.5 mm, with 1.5% fat, 0.&1.2% Medium grits Fine grits 1626 1.M.65 1.4-1.0 ] q60 12-16 crude fibre content: about 60% of the original Coarse meal 2648 0.65-0.3 10 maize. Fine meal (coarse cones) 48-80 0.34.17 10 size reduction of the grits. The mealy endo- Hominyfeed - sperm, of higher fat content, reduces to flour particle size more readily than does the vitreous endosperm; thus, flour has a higher fat content - about 3%, and semolina a lower fat content - 0.8-1.3%, than the grits. Maize flour >80 below 0.17 5 Semolina and flour, which may be made by Germ 6.74.5 14 11 * Tyler Standard Screen Scale sizes. Sources: Stiver, Jr (l9s5); Easter (1969), Flaking grits are used for the manufacture of the ready-to-eat breakfast cereal ‘corn flakes’ (cf. Ch. 11). Grits from yellow maize are preferred. Other uses for dry-milled maize products are discussed elsewhere: porridge (polenta) and ready-toasted cereals (Ch. 1 l), for bread and other baked foods (Ch. 8), and for industrial purposes (Ch. 15). Dy milled maize flour is not to be confused with ‘corn flour‘, the term used in the U.K. for maize starch obtained as a product of wet milling. Oil extraction from germ Solvent extraction is generally used in the dry milling industry, although mechanical pressing, e.g. with a screw press, is sometimes used. The germ from the mill is first dried to about 3% m.c. and then extracted while at a temperature of about 121°C. The oil content of the germ is reduced by Beeswing ’ on a vertical axis. Types of stone used include French burr from La Ferte-sous- Jouarre, Seine- et-Mame, millstone grit from Derbyshire, German lava, Baltic flint from Denmark, and an artificial stone containing emery obtained from the island of Paxos, Greece. The opposing surfaces of the two stones, which are in close contact, are pat- terned with series of grooves leading from the centre to the periphery. In operation, one stone is stationary while the other rotates. Either the upper stone (‘upper runner’) or the lower stone Hypodermis Thin walled cells - remnants over most of grain, but cell walls remain in crease and attachment region; includes vascular tissue in crease Cross cells Tube cells (Inner epidermis) (b) Intermediate cells (2) Seed (a) Seed coat (testa) and pigment strand (b) Nucellar epidermis (hyaline layer) (c) Endosperm Aleurone layer > Bran I DRY MILLING TECHNOLOGY 141 when steam power became available, Hungary became the centre of the milling industry. TABLE 6.5 Flour Extraction Rates in Various Countries Country Rate Country Rate Milling of white flour (Oh) (Yo) That is to say, it consists almost entirely of starchy France (1990)t 79 Germany (1990)t 79 endosperm. The ideal dispersal ‘of other grain Hungary (1988)* 74 Italy (1990)t 74 Austria (1990)t 76 Belgium (1990)t 74 Canada (1988/89)* 76 Denmark (1990/91)t 72 Malta (1988)* 75 Morocco (1988)* 79 Netherlands (1990)t 81 New Zealand 78 components is shown in Table 6.4. prepared from grain of common wheat (Triticum Spain (1990)t 72 Sweden (1990191)t 80 Switzerland (1986)* 76 Tunisia (1989)* 75 80 Zambia (1988/89)* 75 aestivum L.) or club wheat (Triticum aestivum ssp U.K. (1990)* compactum), by grinding or milling processes in and the remainder is comminuted to a suitable degree of fineness. ‘Flour fineness’ is an arbitrary particle size, but in practice, most of the material described as white flour would pass through a 140 Pm length Of side. Some flour is Produced in the milling of durum wheat (Titicum dumm) but it is not the main product. The objectives in milling white flour are: 1. T~ separate the endosperm, which is required for the flour, from the bran and embryo, which are rejected, so that the flour shall be free from bran specks, and of good colour, and so that the palatability and digestibility of the product shall be improved and its storage life lengthened. countries. The majority of flour sold in the U.K. is white. Wheat flour has been defined as the product Nomay (i99o)t 81 S. Africa (1990)* 76-79 which the bran and embryo are partly removed Sources: * International Wheat Council, t NABIM. quantity: the weight of clean wheat required to produce 100 lb (1 cwt u.s.; 45 kg) of flour. The wheat grain contains about 82% of white flour, but it is never possible to separate it exactly from the 18% of bran, aleurone and embryo, and thereby obtain a white flour of 82% extraction rate (products basis). In spite of the mechanical limita- tions of the milling process, extraction rates well in excess of 75% are now achieved. Flour extrac- tion rates prevailing in various countries are Shown in Table 6.5. They are calculated from values for wheat milled and flour Produced, Provided by official bodies in the respective flour Sieve having rectangular apertures of starchy endosperm which is required for white 2. To reduce the maximum amount of endo- Mill capacities in the U.K. were formerly sperm to flour fineness, thereby obtaining the maximum extraction of white flour from the expressed in terms of ‘sackS/h’, a Sack being 280 lb . Since metrication in 1976, use of the ‘sack’ wheat. as a unit of weight has been discontinued and mill capacity is now expressed in tonnes (of wheat) per 24 h. The accounting unit in the flour- milling industry is 100 kg. Bran and fine wheat- feed are quoted in metric tonnes. In the U.S.A. capacity is expressed as cwt of flour per 24 h. Principles Were it not for the ventral crease in the wheat grain it is likely that an abrasive removal of the pericarp, embryo and aleurone tissue would be a stage in the conventional milling of wheat, as it is in rice milling. The inaccessibility of the outer tissues in the crease region, however, has Extraction rate The number of parts of flour by weight pro- duced per 100 parts of wheat milled is known as the flour yield, or percentage extraction rate. Flour yield is generally synonymous with extrac- tion rate in the U.K.; it is calculated as a percentage of the products of milling of clean wheat, although it may also be expressed as the percentage of flour as a proportion of the dirty wheat received. It is prudent to check which system is in use, wherever possible. In the U.S.A. the term ‘yield’ is used to express a different Scratch Scalping grading dusting Semolina and middCings 1 1 1111 Purifying Reduction YI 2 e ? n 1 0 DRY MILLING TECHNOLOGY 143 3 YI L 2 2 2 f e! a 2 8 Y f 1 44 TECHNOLOGY OF CEREALS reduction system continues to purify and reduce the size of endosperm particles. A concept of the flow of the system can be gained from Fig. 6.8 and an example of a mill layout is shown in Fig. 6.9. The process includes several sieving stages, each performing a different function; names of particular sieving processes include: Scalping - sieving to separate the break stock from the remainder of the break grind; Dusting, bolting, dressing - sieving flour from the coarser particles; Grading- classifying mixtures of semolina and middlings into fractions of restricted particle size range. Purifying is the separating of mixtures of bran and endosperm particles, according to their terminal velocity in air currents. The process is particularly characteristic of durum milling (see p. 154). Stocks and materials durum wheats are not regularly milled the term ‘semolina’ is used to describe a coarse inter- mediate stock produced from the break system, in the milling of flour from T. aestivum wheats. In the U.S.A., where durum, common and club wheats are milled, the term ‘semolina’ is reserved for the durum product; and the coarse milling intermediate from the other wheats, equivalent to the U.K. ‘semolina’ is called ‘farina’. Semolina for domestic use (puddings etc.) in the U.K. is obtained from durum wheat. Middlings, break middlings - endosperm inter- mediate between semolina and flour in particle size and purity, derived from the break system. Dunst - this term is used to describe two different stocks: 1. Break dunst - starchy endosperm finer than middlings, but coarser than flour, derived from the break system. This stock is too fine for purification but needs further grinding to reduce it to flour fineness. 2. Reduction dunst - (reduction middlings in the U.S.A.) - endosperm similar in particle size to break dunst, but with less admixture of The blend of wheat types entering the milling system is known as the grist. Its composition is bran and germ, derived from semolina by usually expressed in percentages of each wheat rollermilling. type present. It is also described by reference to the product for which it is intended, e.g. a bread Flour - starchy endosperm in the form of grist. Other stocks are described as follows: particles small enough to pass through a flour Feed - material fed to, or entering, a machine. sieve (140 pm aperture). This is the definition of Grind - the whole of the ground material white flour, the term flour is also extended to delivered by a rollermill. include brown and wholemeal flours, whose particle Break stock - the portion of a break grind size ranges are less well defined. When used to overtailing the scalper cover (sieve), and forming describe the fine product of other cereals the name the feed to a subsequent grinding stage. The of the parent cereal usually appears as a prefix, corresponding fraction from the last break grind e.g. rye flour. is the bran. The modern rollermilling process for making Break release - the throughs of the scalper flour is described as a ‘gradual reduction process’ cover, consisting of semolina (farina), middlings, because the grain and its parts are broken down dunst, and flour. in a succession of relatively gentle grinding stages. Tails, overtails - particles that pass over a Between grinding stages on rollermills, stocks are sieve. sieved and the various fractions conveyed for Throughs - particles that pass through a sieve. further grinding if necessary. Although stocks are Aspirations - light particles lifted by air reground they are never returned to the machine currents. from which they came or any machine preceding Semolina, farina, sizings - coarse particles of it. Another important principle is that flour or starchy endosperm (pure or contaminated with wheatfeed made at any point in the process is bran and germ). In the U.K. where Triticum separated out as soon as possible (this principle DRY MILLING TECHNOLOGY 145 are usually smooth (fluted rolls are used in head reductions in the U.S.A.). Break grinding The Break system consists of four or five ‘breaks’ or grinding stages, each followed by a sieving stage. Particles differing widely in size (in the grind from any one stage of rollermilling) also differ in composition: particles of starchy endo- sperm, which tend to be friable, are generally smaller than particles of bran, which tend to be tough and leathery, due to tempering. The pro- cess of sieving thus, to some extent, separates particles differing in composition one from another. A thin curtain of particles is fed into the nip between the rolls; in the first break the particles are whole grains. The flutes shear open the grain, often along the crease, and unroll the bran coats so that each consists of an irregular, relatively thick layer of endosperm closely pressed to a thin sheet of bran (see Fig. 6.10). A small amount of endosperm is detached from the bran coats, mostly in the form of chunks of up to about 1 mm3 in size (semolina); small fragments of bran are also broken off, but little flour is made. The first break grind thus consists of a mixture of particles. The largest are the bran coats (break stock), still thickly coated with endosperm; the intermediate-sized particles are either semolina, middlings and dunst, or bran snips (some are free bran, some are loaded with endosperm); the smallest are flour. is defied in some millstands, in which the pro- ducts of one roller passage pass directly to another set of rolls mounted immediately below the first; the design is said to lead to savings in energy costs (Anon, 1990)). Those fractions from other stages that can yield no useful flour are removed from the milling system to contribute to the milling by-products (offals) as bran or wheatfeed (millfeed in the U.S.A., pollard in Australia). The flour is also removed from the system as part of the finished product. Rollermilling operations The rolls used in wheat flour milling are usually 250 mm diameter and either 800 mm, 1000 mm or 1500 mm long. The feed is distributed evenly over the length of the rolls by a pair of feed rolls which also control the loading. The succession of grinding stages is grouped into three systems: the Break system removes the endosperm from the bran in fairly large pieces, producing as little bran powder as possible. The Scratch system removes any small pieces of bran and embryo sticking to the endosperm. A Sizing system may be used instead of the scratch system. The Reduction system grinds endosperm into flour, at the same time flattening the remaining bran and embryo particles, enabling them to be separated. The rolls involved in the break and scratch systems are fluted while those used in reduction 2 3 4 0 1 0- n - FIG 6.10 Stages in opening out of the wheat grain and the scraping of endosperm from bran by fluted rolls of the Break system. 1. Whole wheat grain; 2. I Bk tails; 3. I1 Bk tails; 4. I11 Bk. tails. Upper row: plan view (looking down on the inside of the bran in 24), Lower row: side view. In 2-4, endosperm particles adherent to bran are uncoloured; inner surface of bran, free of endosperm, is hatched; outer side of bran curling over is shown in solid black; Beeswing, from which bran has broken away is shown dotted. __.___ t 4’- .-. flour than sharp-to-sharp at similar releases. In any of these relationships the slow roll serves to ‘hold’ the stock while it is scraped by the fast roll surface. As the bran passes down the * DRY MILLING TECHNOLOGY 147 Wheat mn7i - K or G Four main groups of machines are shown Break and reduction rolls From H and 4th break ... Fine sieves .. ... '///I/// The flour streams are not shown but each representation of a bolting silk implies that a flour stream originates there and IS named after the rolls that feed the sifter in question FIG 6.12 Diagrammatic representation of flour milling flow. (Reproduced from C.R. Jones, Roc. Nutr. SOL, 1958, 17: 9, by courtesy of the Cambridge University Press.) 148 TECHNOLOGY OF CEREALS TABLE 6.6 Typical Break Releases and Scalping Covers an extremely important contibution to the mill Yet another variation is for the coarse semolina to pass to smooth coarse-reduction rolls followed by a flake disrupter or drum detacher before sieving. of feed to of 1st release number length The detacher operates on a similar principle to a break break feed (mm) bran finisher, with finger beaters rotating on a 1st 30 30 30 20w l.oo horizontal axis, within a drum. Flakes of endo- 2nd 52 36 66 2Ow 1 .OO sperm are broken up, enabling them to be separated 3rd 35 12 78 28w 0.71 from branny flakes. This avoids excessive cutting 4th 14 3 81 36w 0.53 flow). Break release Scalping cover Break Percentage Percentage Total Wire Aperture of bran and embryo particles. Reductjon system The reduction system consists of 8-16 grinding stages, interspersed with siftings for removal of the (reduction) flour made by each preceding grind, and a coarse (tailings) fraction from some grinds. Grinding in the reduction system is carried out on rollermills differing, in the U.K., in two important respects from the break rollermills: 1. The roll surfaces are smooth, or more often slightly matt (exceptions are those rolls used in the sizing system described above). 2. The speed differential between the rolls is lower, usually 1.25: 1, although the fast roll still runs at 500-550 rpm. The grinding effect in reduction mills is one of crushing and shearing (the the smoothness of the roll surface). The feeds to A and B rolls, despite careful purifying, inevitably contain a few particles of bran; the feeds also contain particles of embryo. By carefully controlling the roll pressure, these can be flattened without fragmentation, and sub- sequently sifted off as coarse ‘cuts’ from the grinds. These, and corresponding fractions from sub- sequent reductions, are fed to certain reduction stages, collectively known as the ‘coarse reduction system’ (U.K.) or the ‘tailing system’ (U.S.A.). A variation of the scratch system is the A simplified mill flow, illustrating the distribu- tion of stocks is shown in Fig. 6.12, and part of a roller-floor is shown in Fig. 6.13. The reduction stage in a large mill starts with A and B reductions on the coarse and fine semolina stocks and goes down as far as M scalping covers (sieves) employed in white flour milling, as quoted by Lockwood (1960) are shown in Table 6.6. Scratch system The scratch system separates particles of bran and endosperm which are still stuck together after passing through the break system. These particles are too small to have overtailed to the next break but are not ‘lean enough to g0 to the reduction system. When operated in British mills it consists of two to (rarely) four grinding stages. The feed to the scratch system contains large particles of semolina and pieces of bran with attached endo- sperm, but particles are much smaller and more cubical than break feeds. The objectives are to production of fine stock, and to reduce oversize semolina. Scratch rolls are more finely fluted than break rolls, e.g. 20 cuts per cm. The disposition of the rolls is sharp-to-sharp as in the later breaks, to ensure maximum cleaning of bran (see Fig. 6.2). Typically the speed differential is 1.8: 1. The grind from the scratch rolls is scalped, dressed, graded and purified in much the same way as that from the breaks, and fractions dis- patched respectively to subsequent scratch grind- ing stages, to offals, to ‘flour’ (as end product) or to the reduction system. extended use of fluted rolls into the head reduc- tions. This is called a sizing system. Such systems are found particularly in purifierless milling (they also feature in durum mills dedicated to producing semolina, but here purifiers also make scrape endosperm from the bran without undue balance of the two components depending on DRY MILLING TECHNOLOGY 149 reduction. In a U.K. long system, coarse and fine granules through application of high shear and semolinas from the first and second breaks are pressure (cf. p. 62). One effect of starch damage ground on the A and B rolls to provide dunst for is to increase water absorbing capacity, an impor- C rolls. This releases a certain amount of flour tant property in making doughs or pastes. For in the process. Rolls from C downwards (i.e. D, some purposes (e.g. breadmaking) a relatively E, G, H, L, M) make up the reduction system high level is desirable as this can prolong the and deal with flour stocks from the middle and storage life of bread. For biscuits (cookies) a high tail of the mill. level is undesirable as it increases the tendency Coarse stocks which overtail from A and B of the die-cut dough piece to spread, produces a flour dressers are dealt with on the coarse harder bite and requires more energy for the reduction system of B2, F and J rolls. water to be evaporated off during baking. In the short system, the A rolls deal with the largest Damage can be inflicted in all grinding stages sized stock, as in the long system, but they produce but the greater part occurs during reduction. In a feed for B rolls which in turn produce a feed for recent years the demand has been for higher levels F rolls. Apart from finishing at H or J rolls, a of starch damage as breadmaking methods with short system is similar to the long system in that short fermentation time require flours with higher B2 and F are the coarse rolls that deal with branny water absorption. This has led to heavier reduc- stocks and germ tailing over from the coarse head tion grinding; a tendency exaggerated by the reductions. adoption of shorter milling systems (Le. fewer Factors that have enabled short systems to reduction passages) and gristing for bread flours be developed include: with softer European wheats as an alternative to hard wheats from outside the E.C. As a result of 1. The introduction of the pneumatic mill. Its heavier grinding, reduction rolls become very cooler stocks allowed more work to be done hot, and water-cooled rolls are not unusual. on reduction rolls without overheating the Another consequence of the higher roll pressure stock. is the increase in production of flakes on reduc- 2. Improvements in the rollermill, including the tion rolls and the resulting need for flake dis- replacement of brass bearings with cooler rupters in the mill flow. Levels of damaged starch trouble-free roller bearings, allowing rolls to encountered in bread flours may be as high run at higher speeds and pressures. as 40 Farrand units. Damage levels in rye 3. The provision of an efficient cooling system flours tend to be lower than in wheat flours, for grinding rolls. apparently because of protection afforded by the 4. Improvements in the capacity and dressing more pentosans present in rye. efficiency of the plansifter. Flake disrupters 5. Availability of flake disrupters. Short systems have the advantage that less The tendency for greater roll speeds and pressures in the reduction grinds has led to an increase in the tendency for particles of endo- sperm to be compressed into flakes. Without treatment this would lead to the unnecessary retreatment of what is essentially flour. The problem is solved by the introduction of flake disrupters into the mill flow. The treatment is given to the grind of head reductions. Flake disrupters can be installed in the pneumatic conveying system. Each consists of a high speed rotor mounted on a vertical axis within a housing. plant is required for a given capacity. Smaller buildings are less expensive to build and maintain, clean and supervize . Damaged starch An important function of rollermilling, in addition to those described, is the production of controlled levels of starch damage. ‘Damage’ in this context has a very specific meaning: it refers to the change resulting to individual starch TECHNOLOGY OF CEREALS150 FiG 6.13 Part of the roller floor of a modem flour mill. (Reproduced by courtesy of Bl1hler Bros. Ltd, Uzwil, Switzerland). The rotors consist of two discs held apart at their outer edge by a gallery of pins. The feed enters at the centre of the plates and is thrust outward by centrifugal force. Flakes are broken by impact against the pins, the housing and each other . Drom detachers (see p. 148) may be used as an alternative or applied to the grinds of tail reductions. Sieves Two principles have been employed in the design of sifters for flour mills. Both depend on the ability of some stocks to pass through aper- tures of a chosen size while others are prevented by their size from doing so. The earlier system -centrifugals, operates on a principle similar to that described for a bran finisher .Beaters revolv- ing within a cylindrical or multi-sided 'barrel' provide turbulence to direct particles against the screen for selection. Centrifugals developed from reels in which the clothed cylinder rotated with a bed of stock within it, relying on gravity for the passage of undersize particles. All modern plants use plansifters. Plansifters consist of a series of flat rectangular framed sieves held within chests which, in turn, are suspended on flexible canes (either natural or man-made) in such a way that they can gyrate. Eccentric motion is assured by placement of weights (Fig. 6.14). Plansifters have the following advantages over centrifugals: 1. they occupy less space; 2. they use less power; DRY MILLING TECHNOLOGY 151 FIG 6.14 Plansifters. (Reproduced by courtesy of Biihler Bros. Ltd. Uzwil, Switzerland.: inch, and an indication of thread gauge. Nylon and polyester screens are described by the aper- ture sizes. Sieves are prevented from becoming blinded by sieve cleaners: small pieces of belting or similar, supported on a grid beneath each cover and able to move freely in a horizontal plane. 3. less sieving surface is required as it is used more efficiently; 4. their capital cost is less; 5. stocks are better dressed; 6. they are compatible with the pneumatic conveying system, which improves their efficiency. Grinding and sieving surface The loading of rollermills can be defined in terms of the length of roll surface devoted to grinding the stocks. It is now conventional to refer to the length applied to 100 kg of wheat ground per 24 h. Typical lengths in the break system are: 1st break, 0.7 mm; 2nd break, 0.7 mm; 3rd break, 1.07 mm; 4th break, 1.07 mm; Sth break, 0.7 mm. Total break surface in this case is 4.24 mm. Although the Break surface ranges from 4--6 mm per 100 kg of wheat ground in 24 h, the tendency is to increase roll speeds and feeds Nylon, polyester and wire covers Most sieve frames of plansifters, purifiers and small dressing machines were once clothed with woven silk. This has now been!eplaced by Plated wire, nylon (in plansifters), or polyester (in purifiers). Nylon is less expensive, less subject to change with atmospheric moisture changes and stronger than silk, allowing thinner yarn to be used and hence increasing the open area of sieves. Wire covers are light, medium or heavy , depending on the type of wire gauge used. Light plated wire is used in flour mills. Silk covers are described by a combination of a number indicating the number of threads per 152 TECHNOLOGY OF CEREALS drastically, and surface allowances as low as 2.1 mm are experienced. Much depends on the type of wheat being milled and the milling system in use. The allocated surface on reduction rolls depends upon the length of the system. A long system may require up to 40 mm per 100 kg wheat ground while 8-10 mm may be sufficient in a short system. Sifter allocation depends on the mill size and type of grist. Typical allocations of a 100 tonnes Modern flour mills are highly automated, with per 24 h mill might be: 1st Bk 1 section 22 sieves microprocessor control of many, if not all, systems 2nd Bk 1 section 22 sieves being common. Entire mills are capable of running 3rd Bk +X 1 section 22 sieves completely unattended for several days. 4th Bk 1 section 22 sieves 5th Bk f section 12 sieves Break Midds 1 section 24 sieves 24 sieves A reduction 1 section 24 sieves B reduction 1 section 24sieves C reduction 1 section B2 reduction 4 section 12 sieves D reduction f section 12 sieves 12sieves E reduction f section F reduction f section 12 sieves 12 sieves G reduction t section H reduction J reduction 4 section 12 sieves This allocation would require two 6 section into which part of the main flow in a pneumatic pipe is diverted. Protein content and moisture content are most successfully measured but claims are made for other parameters including particle size and damaged starch measurements. Measure- ments by NIR on ground materials have become routine and successful application of the technique even to whole grain is now well established. Automation Energy used in flour production It has been calculated that the primary energy requirement for milling 1 tonne of flour plus that required to transport the flour in bulk to the bakery is 1.43 GJ, or, if transported in 32 kg bags 1.88 GJ. The primary energy requirement for the fuel used to transport wheat to the U.K. flour mill is calculated as 1.46 GJ/tonne for CWRS, or 0.08 GJ/tonne for home-grown wheat. The total t section 12 sieves primary energy requirement for growing the wheat (30% CWRS, 40% French, 30% home- grown grist), transportation to the U.K. flour mill, milling the wheat to flour, and transporting this to the bakery is estimated at 6.41 GJ/tonne (Beech and Crafts-Lightly, 1980). plansifters (Anon., 1988). On-line monitoring by NIR Milling by-products In addition to the tests performed on finished flours in the mill quality control laboratory, some The principal products, in addition to flour, of monitoring of important flour characteristics is the milling process are bran and wheatfeed (both carried out during production. The main principle are included in the term millfeed in the U.S.A.). involved in such measurements is near-infrared Bran is the coarse residue from the final break reflectance (NIR) spectroscopy. The method grind. Wheatfeed (shorts in the U.S.A., pollard depends on the selective absorption of infrared in Australia) is the accumulated residues from the energy at wavelengths characteristic of the diffe- purifiers and reduction grinding. In the U.K. rent flour components. Measurements of different screenings removed during the cleaning of the components can be made simultaneously on dry wheat are generally ground and, unless contamin- flour compressed against a window. This may ated with ergot, added to the wheatfeed. The form the front of a cell presented to a laboratory major use of bran and wheatfeed is as a constituent instrument or part of a continuous sampling loop of compound animal feeds. DRY MILLING TECHNOLOGY 153 weeks by toasting or steaming. A cocoa substitute which, it is claimed, can replace 50% of the cocoa or chocolate in cakes, biscuits and other dietary products has been made from toasted, defatted wheat germ with added maize sweetners (U.K. Patent Appl. GB 2031705). The germ is defatted at 107"-155"C in the presence of a reducing sugar. The steam treatment is said to develop colour, aroma and taste characteristics resembling those of cocoa. Milling of brown flour and wholemeal Wholemeal In the U.K. wholemeal, by definition, must contain all the products of the milling of cleaned wheat, i.e. it is 100% extraction rate. The crude fibre content usually lies between 1.8 and 2.5% (average 2.2%) but this is not specified in U.K. regulations. In the U.S.A. the 100% product is known as wholewheat or GrahamfEour. In India a meal known as ana is produced. It is approximately 100% extraction flour, used for making chapattis (see Ch. 13). Types of bran Bran can be classified on the basis of particle size: Broad bran overtails a 2670 pm screen, ordinary bran overtails a 1540 pm screen, fine bran overtails a 1177 pm screen. is produced by passing ordinary bran through a bran ro11* This ro' is usudy 356 Illfn diameter; the greater diameter than pinding ro's is demanded by the heavy pressure required. For reducing the size of bran flakes, cutting rolls, grinders or bran cutters may be used. Ground bran may be added to wheatfeed, allow- ing both types to be sold as a single product. Broad bran generally has the highest va1ue' It to 1-2y0 germ oil cOntent and treated with Stearn Germ Germ is the milling by-product derived from the embryo (mainly embryonic axis) of the grain. It is not always separated and may be included in wheatfeed. The advantages of separating germ are that: it commands a higher price than wheat- feed when of good quality and its presence in reduction stocks can lead to its oils being expressed into flour during grinding, adversely affecting the keeping and baking properties of the flour. For effective separation of germ, particles pass- ing the scalping covers but overtailing covers of - Millstones - either single or multiple passes approximately 1000 pm (depending on wheat with intermediate dressing. type) are sent straight to the semolina purifiers. - Roller mills - either a shortened system Here the germ enriched stocks are directed to the or by introducing stock diversions in a con- A or scratch rolls. The germ becomes flattened ventional system. while the endosperm elements fragment, provid- - Disc grinders - either metal or stone discs. ing a means of concentrating the germ as a coarser fraction. Small pieces of embryo find their way Wholemeal may also be produced on an impact to B2, F or J rolls, where they may be retrieved mill, but this is rarely used for preparation of by a similar system to that employed on A rolls, meal for human consumption. or included in wheatfeed. Brown flours Uses of germ include incorporation in speciality breads such as Hovis and Vitbe (flours for this purpose are white, with not less than 10% According to the Bread and Flour Regula- processed germ added), and preparation of vitamin tions 1984, a brown flour must have a minimum concentrates due to its high content of (mainly) crude fibre content of 0.6% d.b. Brown flours thiamin, riboflavin, niacin and E vitamins. can be of extraction rates between 85% and Unprocessed germ becomes rancid in two weeks. 98%. The storage life can be extended to 12 weeks by For milling brown flour the white flour process defatting, to 20 weeks by drum-drying and to 26 may be modified in various ways, as follows: Wholemeal may be produced using: 154 TECHNOLOGY OF CEREALS 1. Releases are increased throughout the break system, by narrowing the roll gap, and by using scalping covers of slightly more open mesh. 2. Operation of the purifiers is altered by adjusting the air valves, altering the sieve clothing and reflowing the cut-offs so that less stock is rejected to wheatfeed, and more goes to the scratch and reduction systems. 3. The scratch system is extended by the use of additional grinding stages. 4. The extraction of flour from the reduction system is increased by more selective grinding, by changing some of the smooth grinding rolls to fluted rolls, and by employing additional grinding stages to regrind offally stock that would be rejected as unremunerative in white flour milling. the flour sieves to slightly more open mesh Milling of semolina from durum wheat sizes. 6. Still additional flour extraction can be obtained, at the sacrifice of good colour, by bringing the Durum wheats are milled to produce coarsely wheat on to the 1st. break rolls at 1-1.5% ground endosperm particles known as semolina, lower moisture content than the optimum for somewhat similar to grits produced by dry milling white flour-milling. maize and sorghum. Semolina is used in the manufacture of pasta, it is also cooked unprocessed as the North African staple ‘COUSCOUS’ (See Ch. 10). The desirable characteristics in a durum semo- lina include a clear, bright golden appearance, free- dom from dark specks originating from impurities, and a high enough content of protein to provide an elastic gluten, thus ensuring the best eating quality of the pasta produced. The maximum ash limit for semolina (U.S.A.) is 0.92%, and for farina 0.6%. The milling process Because of the extreme hardness of durum wheat the usual practice is to temper to the high moisture content of 1616.5% before grinding, further details of tempering are given in Ch. 5. Grinding Milling itself consists of three systems, all dependent upon rollermills. The first is the break steam jets. The wheat fills the tube, and the rate of throughput is adjusted to allow the grain an adequate time of treatment. To achieve the same objective, flour may be similarly treated, but with indirect heat and limited amounts of steam, to avoid gelatinization and lumping. Structure of the U.K. milling industry Most millers in the U.K. are members of the trade organization NABIM (National Association of British and Irish Millers), which represents 41 companies operating 84 mills. Of these, the three largest members contribute approximately 75% of production. The proportions of flour types produced have remained fairly consistent, at least over the last five years, figures for which are reproduced in 5. Additional flour may be obtained by changing Table 6.7. Grists Changes in the composition of the bread grist in the U.K. since 1973, are discussed in Ch. 8. (Fig. 8.1). The proportion of home-grown wheat has increased from about 25% to nearly 80%. As the home-grown element of the grist has increased, the importance of its quality has also increased. As a result the selective purchasing of favoured varieties has become normal practice. Steam treatment of wheat When wheat is treated with live steam, alpha- amylase and other enzymes are inactivated, and the protein is denatured so that gluten cannot be recovered (See Ch. 3). Inactivation of amylase is rapid, but not instantaneous; to ensure complete inactivation, the wheat is held at or very near 100°C for 24 min in a steamer, which may take the form of a vertical tube well supplied with DRY MILLING TECHNOLOGY TABLE 6.7 6.7 U.K. Flour Production by Type 155 Type of flour 1987/8 1988/9 1989/90 1990/1 1991/2* Breadmaking White 53.4 53.5 54.2 54.6 53.9 Brown 3.4 3.5 3.5 3.4 4.1 Wholemeal 6.8 6.3 6.1 6.1 6.1 Biscuit 13.7 14.6 15.2 14.3 13.8 Cake 1.9 1.9 1.9 1.8 1.6 Self-raising 2.7 2.3 2.3 2.2 2.8 For starch manufacture 3.2 3.1 2.8 2.8 3.7 Others 10.8 11.4 10.9 11.3 10.8 Pre-packed Household 4.1 3.4 3.1 3.5 3.3 *Estimates. Source: National Association of British and Irish Millers, 1991 and 1992. FIG 6.15 Purifiers in a modern Durum semolina mill (reproduced by courtesy of Buhler Bros. Ltd. Uzwil, Switzerland). system and its function is to open the grains and progressively scrape endosperm from the bran, as in the milling of common wheat. Durum milling is characterized by a long break system, with five or six relatively gentle passages. The gentle grinds of the breaks, with rolls set sharp- to-sharp, are designed to release large chunks of endosperm with a minimum of fines. The same number of detaching passages as break passages are included. The detacher system is similar to the scratch system used in bread wheat milling, its purpose being not so much to grind but lightly to scrape semolina particles free of any small flakes of bran remaining on them. Detacher rolls 156 TECHNOLOGY OF CEREALS have a finer fluting than the corresponding break rolls. durum milling is concerned with sizing and reduc- tion. The sizing rolls reduce large endosperm particles to a uniformly smaller size. The rolls are finely fluted. Few smooth reduction rolls are needed in the traditional durum mill as little flour is required. Particles below the size specified for semolina have to be ground to flour size however by reduction rollermills which are characterized by smooth rolls. Successful purifying depends upon maintaining a continuous layer of pure endosperm along the The third rollermilling system featured in entire length of the sieve. Thus, only the finest pure endosperm must be allowed to pass through at the beginning, and progressively larger endo- sperm particles are allowed through as the stocks progress. The lightest impurities are removed by aspiration while overtails are directed to appro- priate further treatment. The capacity of a purifier (half the machine) is 0.5-1.4 t/h. It depends on the type of stock being treated, the highest capacities being associated with coarser stocks (Bizzarri and Morelli, 1988). Within the durum mill, stocks are conveyed through pneumatic lines. The air-flow also serves to cool the mill, particularly the roller mills, which produce heat as a result of grinding. The performance of the mill is continuously monitored by strategically sited weighers, indicating any changes in the output of a particular stage of the process, and providing a warning of faulty tempering or machine condition. Purifying Stocks leaving the roller mills pass to plansifters to be graded and streamed to machines appropriate to their further treatments. Before further grinding sieved stocks are purified. Abundant purifiers are characteristic of a mill dedicated to durum wheat milling. A modern purifier is usually a double assembly, in which twin machines work side by side in the same frame (Fig. 6.15). Each half can be adjusted Semolina particle size separately and may treat different stocks. A purifier consists of a long oscillating sieve, at a The particle-size of semolina and farina in the slight downward slope from head to tail, which U.S.A. is such that all the product passes through is divided into four or more sections. The cover a No. 20 sieve (840 pm aperture size) but not becomes progressively coarser from the head more than 30% passes a No. 100 sieve (149 pm to tail. Individually controlled air currents rise aperture size). through the cover, aspirating stocks as they move The degree of grinding and the consequent over the oscillating sieve cover. When particles particle size range considered desirable has of similar density but different particle-size are changed. With increasing emphasis on con- shaken together, the heavier ones tend to sink. tinuous processing, a uniform rate of hydration Grading is effected on the basis of size, density is required, so a uniform particle size is particularly and air resistance. Generally, the more bran and important. less endosperm there is in a particle of a given In some mills in the U.S.A. the growing size, the lower its density and the higher its air demand for durum flour, which can be processed resistance will be. more quickly than semolina, has been met by order: The advantages of rapid hydration must be balanced against the greater starch damage associ- Light bran and beeswing; ated with smaller particles, as losses during Heavy bran; cooking increase as a function of damaged starch Large composites; level. A desirable range of semolina particle size Large pure endosperm particles; is 150-350 pm, but 100-500 pm is a more realistic Small pure endosperm particles. expectation. Stratification thus results in the following installation of additional reduction capacity. DRY MILLING TECHNOLOGY 157 North American practice is to grind higher protein, smaller sized grains into fine flour with no particular quality requirements. Western European practice is to grind lower Milling extraction rate Extraction rate for durum milling may be expressed as semolina extraction, or total extraction give commercial averages of 6045% for the granular product with about 3.0% flour, for production in the U.S.A. Figures from the U.S. Department of Commerce suggest higher total extraction rates of 7678% (mean 75.3%) between may result from different modes of expression, i.e. on the basis of feed weight or product weight. (semo1ina Plus flour)* Dick and Youngs (1988) protein, larger sized grains into fine flour with strict quality requirements. Eastern European rye mills grind all types of rye into a coarse flour according to local quality specifications. Rye is milled at a moisture content similar to (tempering) periods suffice as water penetrates into the grain more quickly than into wheat; between 6 and 15 h are generally used in North America. Rye milling An important difference between wheat and The milling methods for rye processing have rye mills lies in the surface of the rolls: break developed in parallel with those of wheat, but rolls for rye have shallower and duller grooves to since the end of the last century there have release more break flour. Instead of being smooth been significant differences between them. The or frosted, reduction rolls in a rye mill are finely departure began when wheat millers changed fluted. This is to cope with the sticky nature of from stone to roller grinding. The reasons for the rye endosperm due to the high pentosan content. departure are dependent on both the nature of On smooth rolls excessive flaking would occur. the grain and the requirements of the consumer. A rye mill in the U.S.A. described by Shaw (1970) Compared with wheat, rye grains have a softer has five break passages, a bran duster, and ten endosperm which breaks down to flour particle reduction passages (Sizings, Tailings and eight size more readily, and greater difficulty is experi- Middlings reductions). Purifiers do not feature enced in separating endosperm from bran. Possibly in rye mills. The specific surface in a modern rye as a consequence of the latter factor, rye consumers mill would be 18-24 mm per 100 kg of rye milled have developed a taste for products made from a per 24 h. Rye flour produced in North America coarser meal with a higher bran content than in may be treated with chlorine (0.19-0.31 g/kg) to the case of wheat flour. The yield of reasonably improve the colour of white flour. pure flour from rye is 6445%. At increasing In Europe, particularly in Germany, the use of extraction rates the flour becomes progressively combination mills, grinding both wheat and rye, darker and the characteristic rye flavour more has proved successful (Bushuk, 1976). pronounced. In the U.S.A. a good average yield Rye milling in the former U.S.S.R. was said of rye flour would represent a 83% extraction rate to be highly developed (Kupric, 1954). The charac- (Shaw, 1970). teristics of production are: coarser flour granula- Rye meal may be of any extraction rate, but tion, darker flour colour, and high throughput of rye wholemeal is 100% extraction. the plants. To achieve these features coarser flour Rye milling varies considerably according screens and coarser and deeper roll corrugations to the extraction rate required (wholemeal is were used. produced by two break passages applied to dry grain) and to the geographical region where it is processed; three major regions have been identified as characteristic: North America, Western Europe, and Eastern Europe, including the states of the former U.S.S.R. (Bushuk, 1976). 1982 and 1987 (Mattern, 1991). The discrepancy that ofsoft wheat (14.5-15.5%). Short conditioning Triticale milling Milling procedures for triticale resemble those utilized for wheat and rye. Even with the best hexaploid triticales, however, the yield of flour 158 TECHNOLOGY OF CEREALS is considerably lower than that of wheat flour. Macri et al. (1986) compared milling performance with Canadian Western Red Spring wheat on a Buhler experimental mill. Triticales produced between 58 and 68% flour of 0.44-0.56% ash, while the CWRS gave 71.75% at 0.45% ash. Triticales were tempered to 14.5% mc. Little if any triticale is milled commercially as a separate mill feed. whitening process and the measure of discoloration of the endosperm. Village processing The Engleberg huller is the best known and most widely used small-scale rice mill. It combines the two stages, dehulling and milling, in a single machine. The work is done by a ribbed cylinder rotating on a horizontal axis within a cylindrical chamber, the lower half of which is formed of a screen through which fine material may pass. Rough rice is introduced at one end and passes The main product of rice milling is the dehulled, down the gap between the rotating cylinder and decorticated endosperm known as milled rice, or, the chamber wall. An adjustable steel blade after further processing, polished rice. Unlike the determines the size of the narrowest gap through milled products of most cereals the endosperm which the grain passes and thus determines the remains intact as far as this is possible. Rice flour level of friction experienced. The rate of flow also is produced, but this is of minor importance. serves as a means of controlling the severity of As with the processing of most cereals through- the treatment. The directions of the ribs on the out the world the scale of industrial plants is central cylinder vary along its length; the major increasing, but, in the case of rice the rate of part of the length has ribs parallel to the cylinder change is slow and industrial processing accounts axis but in the early stages dedicated to dehulling, for only about half the world crop, the remainder they run in a diagonal curve, helping to feed the being stored and processed at a village level. At stock in as well as to abrade it. The hulls removed whatever level, the objective of the milling is to in the initial phase help to abrade the grain surface remove hull, bran, and embryo as completely as in the later phase. They are ultimately discharged possible without breaking the remaining cores of through the slotted screens with fragments of endosperm. There are two stages, the first being bran, embryo and broken endosperm while the dehulling and the second ‘whitening’ or milling. largest endosperm pieces are discharged as over- The moisture at which untreated rice is processed tails of the screen. The proportion of whole is relatively low, around 11-12% (Sharp 1991) endosperm is as variable as the skills of the but parboiled rice (see Ch. 5) undergoes heat operators and the condition of the machine. The treatment at 32-38% mc. before drying back to fine materials are used for feeding domestic stock 14%. The method of drying is very important as and, as such, form a valuable by-product. it influences the degree of breakage occurring Small-scale alternatives to the Engelberg huller during milling. Drying in the sun, or with heated include two-stage processors in which dehulling air can lead to cracking and much breakage is performed by rubber rollers, and whitening is during milling (Bhattacharya, 1985). achieved by friction methods. Both processes may Parboiling is applied to about one fifth of the be enclosed within a single housing. world’s rough rice (cf. Ch. 10). Its effects on milling performance result from changes brought Commercial processing about in both endosperm and hull. In endosperm, starch gelatinization occurs, leading to swelling On an industrial scale, processing follows the of the grain which disrupts the ‘locking’ of the same principles as those employed at the village lemma and palea (cf. Ch. 2). Disadvantages of level, but it is usual for the processes to be carried parboiling lie in the additional costs involved, the out in separate machines. Capacities vary from increased difficulty in removing bran during the 1-3 t/h up to 100 t/h but the scale of machinery Rice milling Dehuller 1 Whole rice Broken I rice 160 TECHNOLOGY OF CEREALS by a small adjustable gap, through which the friction whitener comprises a rotating cylinder rough rice passes, having been fed in from a within a chamber of hexagonal section, and central hopper. walls of screening with slotted perforations. The Optimal hulling results from a clearance of horizontal cylinder has air passages through its slightly more than half the grain length. Approx- centre that pass to the surface as a series of holes imately 25% of the grains escape hulling, owing in parallel alignment. Rotation of the cylinder to their small size, and have to be ‘returned’ for causes friction within the grain mass surrounding retreatment, not in the same machine but in one it, and air from the cylinder cools the stock and with a smaller gap. Such dehullers lead to more supports the discharge of bran. Bran and germ breakage of grains than the rubber roller type and pass out through the perforated screens. A recent they are unsuitable for the automation associated innovation introduced in Japanese rice processing with current practice. is ‘humidijied friction whitening’. This concept Products of dehullers need to be separated one includes the injection of a small amount of from another and the usual method for this is atomized water in the pressurized air stream aspiration. As with other rice machinery, designs through the cylinder airways. The process serves abound but the Japanese designed closed circuit a similar purpose to tempering of maize or wheat; hull aspirator probably has the most advantages. it toughens the bran, facilitating its removal. The The air produced by the fan does not leave the ultimate result is the reduction in endosperm machine but is recirculated. A further advantage breakage, and the increased recovery of head is that the abrasive hulls do not pass through the rice. Humidification may also be combined with fan, thus reducing wear. A more traditional cooling before whitening. The cooling hardens machine performing the function is the paddy the grain leading to less breakage. In the more separator, described in Ch. 5. modern, larger mills a high degree of automation exists in the whitening process, including micro- computer control. Whitening Having been dehulled, the brown rice is treated Po,ishing to remove bran and embryo. Two principles are involved in design of whitening machines, the The rice emerging from the whitening process most common is the abrasive cone. It is used in has had the outer layers of bran removed but the single or multipass systems, sometimes with the inner layers remain. The remaining fragments are other type of machine, the friction type. The cone removed by the process known as polishing or rotates on a vertical axis, its surface is covered refining (‘polishing’ is used by the Japanese to with abrasive material and it is surrounded by a describe the milling process - terminology can perforated screen. Rubber brakes are installed at be very confusing!). In conventional rice mills intervals around the cone to provide a smaller polishers are like whitening cones, but instead of clearance than that between the rotor and the abrasive coverings, the cone is covered with many screen. Raising the cone increases the gap between leather strips. No rubber brakes are applied. it and the screen and the brakes can be adjusted Polishing extends the storage life of the product independently, to determine the severity of the as the aleurone layer is removed, thus reducing pearling. The size of cone is inversely related to the tendency for oxidative rancidity to occur in the number of passes involved in the system. that high-oil tissue. The innovative vertical whitener (Fig 6.17) For a description of coated rice see Ch. 14. employs abrasive rollers and, it is claimed, produces smoother rice with fewer grains breaking during processing. Friction machines always occur in multipass systems in which abrasive cones also feature. A Grading of milled rice products Milled rice produced by the milling section of commercial rice mills is a mixture of entire and 161 FiG 6.17 The Vertical Rice Whitener. (Reproduced by courtesy of Satake, UK, Ltd.) broken endosperms of various sizes. The number of grades separated depend on the sophistication of the system and of the market. In developing countries only 'brewers rice' may be separated from the main product. Brewers rice passes a sieve with 1.4 mm round perforations (F.A.O. definition). Other markets may justify grading into 'head rice' (grains entire except for the tip at the embryo end), 'large brokens' and 'small brokens' as well. Separation is by trieurs or disc separators (see Ch. 5). Rice flour Rice flour, bakers' cones and ground rice are the products of grinding milled rice; the head rice 162 TECHNOLOGY OF CEREALS is rarely used as a feedstock because of its high value, but the 'second head' product, comprising endosperm particles of about half grain size, is used. Polishing. Approximate proportions of products (as per- centages of paddy) are husk, 20-30%; milled rice, 63-72%; bran, 7-8%. Pearling of blocked barley or large barley groats (to produce pearl barley). Aspiration, grading, sifting. For making barley flakes, barley groats or pearl Damping. Steam cooking. Flaking - on flaking rolls. Drying - on a hot air drier. barley is milled on roller mills. barley are subjected to: Barley milling Barley is milled to make blocked barley, pearl barley flakes and barley flour for human consump- tion. Removal of the hull or husk, which is largely indigestible, is an important part of the milling process. Blocking and pearling The hardness of barley grains is a characteristic dependent upon type and variety; types with blue coloured aleurone layers tend to be the harder types. For milling purposes, the harder types are preferred, as the objective is generally not to produce flour but to remove the hull and bran retain the shape of the whole grain. With this type of processing, softer grains would tend to first quality products' Bar1ey for mi11ing shou1d have as low a hull content as possible. The presence of damaged grains lowers the quality of milling barley. Such grains frequently reveal areas of exposed endosperm where fungal attack may occur, leading to discoloration. Such grains would contribute dark particles to the finished product. Thin grains also lower the milling quality because of the increased proportion of hull. Processes in barley milling Following cleaning and conditioning and pos- sibly bleaching e.g. in Germany (not practised in the U . K .) barley is treated in the following sequence to produce the various products: burley (GrauPen in German)Y burley floats (Greutz)Y For making barley flour, blocked or pearl Both blocking (shelling) and pearling (round- ing) of barley are abrasive scouring processes, differing from each other merely in degree of removal of the superficial layers of the grain. Blocking removes part of the husk. This process must be accomplished with the minimum of stages, removes the remainder of the husk and part of the endosperm. The products of these barley, respectively. The three processes remove about 5%, 15%) and 11% respectively, to yield a final product representing about 67% of the grain. Three types of blocking and pearling machine are in general use. The first two are used in Britain, all three are in use in continental Europe: 1. Batch machine, consisting of a large circular stone, faced with emery-cement composition, rotating on a horizontal axis within a perforated metal cage. 2. Continuous-working machine which is of Swedish make, consisting of a rotor faced with abrasive material, rotating on a horizontal axis within a semicircular stator, lined with the same material. The distance between rotor and stator is adjustable. 3. Continuous-working machine comprising a pile of small circular stones rotating on a vertical axis within a metal sleeve, the annular space between the stones and the sleeve, occupied by the barley, being strongly aspirated. by supeficia1 abrasion, yielding particles which injury to the grains. Pearling, carried OUt in two fragmentY leading to a reduction in the yield Of processes are blocked barley, seconds, and pearled Blocking. Aspiration - to remove husk. Sifting. Cutting - on groat cutter (for barley groats). DRY MILLING TECHNOLOGY 163 TABLE 6.8 Chemical Composition of Milled Barley Products* (Dry Matter Basis) Protein Fat Ash Crude fibre Carbohydrate Energy Material (“h) (%) (Yo) (Oh/.) (%) kJ/lOOg Pearl barley 9.5 1.1 1.3 0.9 85.9 1676 Barley flour 11.3 1.9 1.3 0.8 85.4 1693 Barley husk 1.6 0.3 6.2 37.9 53.9 5 60 Barley bran 16.6 4.0 5.6 9.6 64.3 - Barley dust 13.6 2.5 3.7 5.3 74.9 - * Values calculated from Kent, 1983. The barley falls between the rotor and the stationary part of each machine; in bouncing from one surface to the other, the husk is split or rubbed off. The degree of treatment, resulting in either blocking or pearling, is governed by the abrasiveness of the stone facing, the distance between rotor and stator, and by the residence period in the machine. Aspiration of the blocked or pearled grain to remove the abraded portionsy and cutting Of the blocked barley into grits, are similar to the cor- responding processes in oatmeal milling. Cutting of blocked barley is not commonly carried out in the U.K. where it is the practice to pass the whole blocked barley grains into the pearling machine. In Germany, however, the blocked barley is first cut into grits, the grits graded by size, and then rounded in the pearling machine. Pearl barley is polished on machines similar to those used for pearling, but equipped with stones made of hard white sandstone instead of emery composition. The practice of adding talc (mag- nesium silicate) in polishing, once customary in Germany, has been discontinued. It was never used in Britain. Barley flakes Pearl barley is converted to flakes by steaming and flaking on large diameter rolls, as described for oats. The flakes have been used as a flavouring ingredient in speciality breads in the U.S.A. In the U.K. grades of pearl barley are limited to Pot barley, First Pearl and Second Pearl. In Germany an elaborate scheme of grading and sizing exists which divides the grain into as many as twelve different sizes. Much of the pearl barley used in the U.K. is imported (unbleached) from Germany. Barley flour Barley flour is milled from pearled, blocked or hull-less barley. Optimum tempering conditions are 13% mc. for 48 h for pearl barley, 14% mc. for 48 h for unpearled, hull-less grain. The milling system uses roller mills with fluted and smooth rolls, and plantsifters, in much the same way as in wheat flour milling. Barley flour is also a by-product of the pearling and polishing processes. Average extraction rate of 82% of barley flour is obtained from pearl barley representing 67% of the grain, i.e. an overall extraction rate of 55% based on the original whole grain (including the hull). By using blocked grain an overall extraction rate of 59% of the whole grain could be obtained, but the product would be considerably less pure than that produced from pearl barley. It is possible to mill a mixture of wheat and hull-less barley in ratios between 90: 10 and 80:20. Bleaching The bleaching of barley is not permitted by law in Britain, but is generally practised in Germany. Imported barley is preferred to domestic grain for milling in Germany because of its greater hardness and yielding capacity, and it is the foreign barley, the aleurone which has a bluish colour, which is said to require bleaching. 164 TECHNOLOGY OF CEREALS LIPASE IN OATS Lead to bitter bite in oatmeal and oatflakes for 3 minutes inactivates FIG 6.18 Main points in the relationship between fat and the enzyme lipase in oatmeal milling and oatcake baking. This reduces the tendency for by-products of barley milling to overload the system. Other milled barle y products Malted barley flour is made by grinding or milling barley malt. The chemical composition of milled barley products is shown in Table 6.8. Oat milling 2 and 5 times as high as that of wheat, and the groat also contains an active lipase (lipid-splitting enzyme). The lipase is located almost entirely in the pericarp of the groat (Hutchinson et al., 1951), whereas the lipid occurs in the starchy endosperm, aleurone and embryo, the last two being particularly rich sources. There is little, if any, lipid in the pericarp. Thus, in the intact groat of raw oats the lipase and the lipid do not come into contact with each other and, hence, little or no hydrolysis of the lipid occurs. However, when the grain is milled, the enzyme and substrate come together, and hydrolysis ensues. The substrate consists essen- tially of acyl glycerols, and hydrolysis leads to the production of glycerol and free fatty acids, mainly oleic, linoleic and palmitic acids. The glycerol is neutral and stable, but the free fatty acids progres- sively give the product a bitter flavour as they are slowly released. Thus, the inactivation of the enzyme, by a process called stabilization, is an essential step in the milling of oat, a process The processing of oats in the mill has two complications, one of which arises from the structure of the oat grain, the other from its chemical composition. The oat grain consists of the groat, or caryopsis, and a surrounding hull or husk. Only the groat is required for the milled products; the hull is indigestible and must therefore be separated and removed in a dehulling (or shelling) stage. In this respect, oats resemble barley and rice. The groat has a lipid content which is between DRY MILLING TECHNOLOGY 165 that is not required in the milling of any other cereal. 1. width grading, 1. width grading, If the lipase is not inactivated, then its effects become apparent in oatcakes baked from oat- meal, fat and water. For making oatcakes, the raising agent frequently used is sodium hydrogen carbonate NaHC03. If free fatty acids are present in the oatmeal, they react with the raising agent to form the sodium salts of fatty acids, which are soaps. The resulting oatcakes thus have a soapy Grading flavour. Furthermore, the fat added to the oat- meal in oatcake baking may be animal fat, e.g. beef dripping, or vegetable fat. All types of fat The cleaning operations directed to the removal consist of glycerol plus fatty acids, the particular of foreign matter from oats are similar to those fatty acids concerned varying from fat to fat. The employed for other cereals; they are described in use of certain vegetable fats, e.g. palm kernel and Ch. 5. Before milling, however, oats are subjected coconut oils, in oatcake baking is particularly to a further treatment because of a problem undesirable, if the oatmeal contains an active peculiar to them. The treatment removes oats of lipase, because the fatty acids in these fats are narrow width, known as ‘light grains’, which chiefly lauric and myristic; the former, when consist of a husk enclosing only a rudimentary released by lipase, has an unpleasant soapy groat. The length of these grains is not necessarily taste. less than that of normal grains, so they may defy A summary of the main points on the relation- the selective processes of the cleaning treatments. ship between lipase and fat in oatmeal milling The grading apparatus comprises a Perforated and oatcake baking is shown in Fig. 6.18. cylinder slowly rotating on a horizontal axis. The screen may be slotted or a specially designed mesh. If not removed thin grains cause many problems in the later stages of milling. Stabi/ization For the inactivation of enzymes the grains pass through a continuous cooker into which live steam is injected. The best results are obtained by treating the raw or ‘green’ oats before kiln- 14% inactivation is incomplete). The grain is quickly raised to a temperature of 96°C by injection of steam at atmospheric pressure, and thereafter maintained at that temperature for 2- 3 min by controlling the rate of throughput of the steamer. When the green-shelling process was first intra- duced, stabilization was carried Out on the whole oats, as in the dry-shelling process. But in the modern application of the green-shelling process, Stabilization is carried out on the groats after shel- ling. Adoption of the current practices is justified Modern (green shelling) 2. shelling (by impact), 3. stabilization, 4. kiln-drying, 5. length grading, Traditional (dry-shelling) 2. stabilization, 3. kiln-drying, 4. length grading, 5. shelling (on stones), 6. cutting, 7. grinding (for oatmeal, oatflour and oat bran), 8. steaming and flaking (for rolled oats). Processes in oat milling Two systems of oat milling have been estab- lished. They differ principally as regards the moisture content of the oats at the shelling stage, and in the method of shelling employed, viz. by abrasion on stones, or by impact. system formerly used in most oat mills in the U.K., and is still currently in use in a few small mills in Scotland. The modern green-she11ing system has now replaced the traditional dry-shelling system in most U.M. mills. In the U.S.A. one process in use involves whole oats being pan roasted before shelling; this imparts a caramelized or toasted flavour to the product. The sequence of operations in both systems is as follows: The traditiona1Y Or dry-shellingJ system was the drying, at a moisture cOntent of 1420% (below 166 TECHNOLOGY OF CEREALS mainly on economic grounds, as energy is not expended on heating the husk, from which lipase is absent and which is removed from the high- value product anyway. It might also be argued that inactivation of lipase is more effective on the unprotected groat, because of easier access of the inactivation agent to the site of the enzyme. A check on the completeness of lipase inactiva- tion may be made by applying the tetrazolium test (cf. p. 113). If, after stabilization, a red colour develops during the test at the embryo end of the groat, it follows that the heat treatment was insufficient to inactivate dehydrogenase enzymes, and that other enzymes, notably lipase, have probably survived also. Tests for other enzymes, e.g. tyrosinase, may also be employed. Excessive steaming, beyond that required for enzyme in- activation, is to be avoided, because oxidation of the fat, resulting in oxidative rancidity, may be encouraged by heat treatment. 2. To facilitate the subsequent shelling of the oats by increasing the brittleness of the husk. 3. To develop in the oats a characteristic flavour, often described as ‘nutty’. In the modern green-shelling system, the func- tion of kilning is limited to the reduction of the moisture content, shelling already having taken place. Herein is a complication of the green- shelling process, because the high temperatures in the later part of the process that have been found necessary for the development of flavour during the kilning of oats are quite unsuitable for the kiln-drying of groats; the groats no longer have the protection afforded by the husk, as in the case of whole oats, and would become burnt and discoloured at such high temperatures. More- over, high-temperature treatment of the groats at low moisture content may lead to the onset of oxidative rancidity. As a result of these limitations, groats are kiln-dried at somewhat lower temper- atures than those used for whole oats, and the products are devoid of the typical oaten flavour. The flavour of oaten products made by the green- shelling process, often described as ‘bland’, seems insipid to palates familiar with products made by the traditional dry-shelling process. The high-temperature kilning of oats facilitates the subsequent inactivation of any residual lipase by steam in the cooking process to which pinhead meal is subjected before flaking, whereas more severe steam treatment is necessary to inactivate residual lipase in the case of products made by the green-shelling process, in which lower kilning temperatures are used. Shelling In the dry-shelling process, the kiln-dried oats are passed between a pair of large circular stones, one stationary the other revolving. The two stones are separated by a distance slightly less than the length of the oat grain: as the grains roll over and up-end the husk is split off in thin slivers. Careful adjustment of the shelling stones, and length grading of the oats before shelling ensure minimum breakage of the groats. In the green-shelling process, which has almost universally replaced stone-shelling, oats are shelled Kiln-drying In the traditional dry-shelling system, the oats, after stabilization, are dried to 68% m.c. Con- tinuous drying is frequently carried out in a Walworth Kiln, in which the hot air is drawn through an annular layer of slowly descending oats. In the modern green-shelling process, the oats are shelled before kiln-drying, and it is therefore the groats that are kiln-dried, often by passing them over a series of radiators at a rate which is controlled to give a final moisture content of approximately 12%. The final section of the kiln is a cooling section, in which fresh air is drawn through the groats to ensure that condensa- tion is not present, as this would could cause deterioration during storage. Cooling is also assisted by a further aspiration to separate slivers of husk loosened during kilning. A modern alternative to kilning is micronizing (see Ch. 9). The purposes of kiln-drying in the traditional dry-shelling system are: 1. To reduce the moisture content to a satisfactory level: 15% for storage, 13% for international trade, about 6% for immediate milling. DRY MILLING TECHNOLOGY 167 they are produced is the granulator, in which hollow perforated drums rotate on a horizontal axis, typically at 3740 rpm. On the inner surface, the circular holes that pierce the walls are counter- sunk, so that the elongated groats, having been fed into the drums, naturally fall lengthwise into the holes. As the drum rotates they continue to progress through the holes and project beyond the outer profile of the drum. Outside the drum, and parallel to its axis a series of stationary knives is placed in such a position to execute a series of equidistant cuts on groats passing progressively through the holes, thus producing uniformly sized ‘pinhead’ granules. The number of groat sections produced depends upon drum speed, but on average, groats are cut into three parts. The holes are kept clear by pins projecting from smaller cylinders mounted parallel to the drums, so that the pins register with the holes at a point on the circumference where the drum is running empty. Not all groats are perfectly divided and, as a consequence, excessively long fragments and whole groats need to be removed, but first the total stock is aspirated to remove husk fragments and floury material. The amount of fines produced depends on the sharpness of the knives. If the knives are sharp, 1.2-1.3% fines may result but if dull up to 10% is possible (Deane and Commers, 1986). The size selection is made with disk separators or trieur cylinders (p. 116); the good product becomes the liftings and it is now ready for flaking. Oversize portions are returned to the granulator for retreatment. The cutting process produces a small amount of flour known as ‘flow meal’; it is separated by sieving and is used for making dog biscuits. F‘aking at natural moisture content, viz 14-18%, on impact machines, such as the Murmac huller. The whole oats are fed into the centre of a high- speed rotor fitted with either blades or fins. The grains are thrown outwards and strike, point- first, a hardened ring, possibly of carborundum, possibly of hardened rubber, attached to the casing of the machine. The combination of high velocity and impact against the ring detach the lemma and palea from the groat. The speed of the rotor is adjustable (typically 140CL2000 rpm) allowing the process to be optimized for a particular feedstock. Both type of oat and moisture content will dictate the conditions required. A variation of the green-shelling process is the wet-shelling process, invented by Hamring (1950). In this process the whole oats are first damped to 22% m.c. or higher, before shelling by impact. It is claimed that the preliminary moistening decreases the breakage of groats and increases the efficiency of shelling, in comparison with the green-shelling process. After shelling, the husk slivers are separated from the kernels and unshelled oats by aspira- tion. The few unshelled oats are then removed on a ‘paddy’ or inclined table separator (cf. p. 119), on which groats and unshelled oats move in oppo- site directions. The machine is designed to exploit differences in specific gravity and resiliency (or ‘bounce’) between oats and groats. The unshelled oats are returned to the shelling stage. After separation of the husk slivers by aspira- tion, the groats pass to a ‘clipper’ or scourer, the abrasive action of which removes surface hairs (‘dannack’) and any remaining fragments of husk. Further aspiration separates the cleaned groats from the ‘oat dust’, as the by-product is named. The groats are graded so that particular size fractions can be directed to further processing appropriate to their sizes, thus large groats are used for flaking into ‘jumbo’ oatflakes, medium groats are Cut to produce quick-cookng flakes, Or even for Oat bran production and sma11 groats are ground into flour. Cut groats Cut groats are the highest volume oat product for human consumption. The machine by which This is carried OUt by passing steam-cooked groats or parts of groats between a pair of flaking rolls. In the steamer the temperature is raised to 99”-104“C and moisture content rises from 8-10% to 10-12%. Steaming performs two functions at this stage: it completes the inactivation of lipase and prepares the groats for flaking: they enter the roll nip while they are still hot, moist and plastic. 168 TECHNOLOGY OF CEREALS Flaking rolls are much heavier and larger than standard flour milling rolls, typically they are 305-71 1 mm in diameter and 762-1321 mm long, depending on capacity requirements. Rolls run at zero differential, and at 250450 rpm, roll pressure and roll gap are maintained hydraulically. Flakes are dried, and cooled and passed over a final sifter and metal detector before packaging at a moisture content of about 10%. The size of the groat or granule that is flaked determines the amount of domestic cooking that the product requires. The smaller the feed to the steam cooker, the greater the proportion of water absorbed and the greater the proportion of starch granules that become gelatinized. The more gelatinization, the less domestic cooking is required. Cooking require- ments can also be controlled by varying roll pressures and steam cooking as thinner flakes are more rapidly cooked. Instant or quick-cooking flakes are about 0.25-0.38 mm thick while tradi- tional rolled oats may be 0.5-0.76 mm thick. The oats processed. The composition of oat flakes is much the same as that of the whole groat as no separation is made of the endosperm, pericarp and embryo during processing. The method of flour production has an important influence on the viscosity of flourlwater slurries produced. white groats This product, used for black puddings and haggis, is made by damping groats and subjecting them to a vigorous scouring action in a barley polisher, pearler or blocker before the moisture has penetrated deeply into the grains. Alternatively) the groats may be scoured without any preliminary damping. A proportion of the pericarp is removed by this process. There is also a certain amount of breakage, with the release of oat flour; the latter becomes pasted over the groats (no attempt being made to remove it), which thus acquire a whitened appearance. References product represents 5040% w/w, of the whole ANON. (1988)MillProcesses I. Module 8 in Workbook Series. Incorporated National Association of British and Irish Millers. London, ANON. (1990) Special Report. Milling. July: 30. BEECH, G. A. and CRAFTS-LIGHTLY, A. F. (1980) Energy use in flour production. J. Sci. Fd. Agnc. 31: 830. BHATTACHARYA, K. R. (1985) Parboiling of rice. In: Rice: Chemistry and Technology. 2nd edn, JULIANO, B. 0. (Ed.) Amer. Assoc. of Cereal Chemists Inc. St. Paul MN. U.S.A. BIZZARRI, 0. and MORELLI, A. (1988) Milling of durum wheats. In: Durum: Chis@ and Technology, pp. 161-188, FABRIANI, G. and LINTAS, C. (Eds.) Amer. Assoc. of Cereal Chemists Inc., St. Paul, MN. U.S.A. BREKKE, 0. L. and KWOLEK, W. F. 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U.S.A. Oat bran and oat flours In recent years, particularly in North America, a market for Oat bran has deve1oped On account of its high soluble fibre content (see Ch. 14). The response to the demand has cOme in several different forms, Some Idlers using grinders, others using stones to remove as much as possible of the endosperm from the pericarp wherein the fibre is concentrated* The by-product from the process is ‘stripped’ flour; it is used in production of extruded products, which are popular as ready- to-eat breakfast cereals. A more traditional oat flour is produced by grinding stabilized groats, usually in a hammer mill with a screen chosen to suit the specification, and sieving the product to ensure no large pieces. Oat flour produced in this way or by grinding oat flakesy is a1so used as a feed to the extrusion process. DRY MILLING TECHNOLOGY 169 ROONEY, L. W., KIRLEIS, A. W. and MURTY, D. S. 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Today 11: 438440. VAN RUITEN, H. T. L. (1985) Rice milling: an overview. In: Rice: Chemistry and Technology, 2nd edn, JULIANO, B. 0. (Ed.) Amer. Assoc. of Cereal Chemists. St. Paul, MN. U.S.A. VARRIANO-MARSTON, E. and HOSENEY, R. C. (1983) Barriers to increased utilization of pearl millet in developing countries. Cereal Foods World 28: 392. DESIKACHAR, H. S. R. (1977) Processing of sorghum and millets for versatile food uses in India. Proc. Symp. Sorghum and Millets for human food. ICC. Vienna 1976. DICK, J. W. and YOUNGS, V. L. (1988) Evaluation of durum wheat, semolina, and pasta in the United States. In: Durum: Chemistry and Technology, pp. 237-248, FABRIANI, G. and LINTAS, C. (Ed.) Amer. Assoc. of Cereal Chemists Inc. St. Paul, MN. U.S.A. EASTER, W. E. (1969) The dry corn milling industry. Bull. Ass. oper. Millers, p. 3112. GREER, E. N., HINTON, J. J. C., JONES, C. R. and KENT, N. L. (1951) The occurrence of endosperm cells in wheat flour. Cereal Chem. 28: 58-67. HAMRING, E. (1950) Wet shelling process for oats. Getreide Mehl Brot. 4: 177. HUTCHINSON, J. B., MARTIN, H. F. and MORAN, T. (1951), Location and destruction of lipase in oats. Nature, Lond. 167: 458. JOHNSON, L. A. (1991) Corn: production, processing, and utilization. In: Handbook of Cereal Science and Technology, pp. 55-131, LORENZ, K. J. and KULP, K. (Eds.) Marcel Decker, Inc. NY. U.S.A. KENT, N. L. (1965) Effect of moisture content of wheat and flour on endosperm breakdown and protein displacement. Cereal Chem. 42: 125. KENT, N. L. (1966) Subaleurone endosperm cells of high protein content. Cereal Chem. 43: 585-601. KENT, N. L. (1983) Technology of Cereals. 3rd Edn. Pergamon Press, Oxford. KUPRIC, J. N. (1954) Malomipari Technologia. Cited by ROZSA, T. A. Rye milling. In: Rye, Production, Chemistry Assoc. of Cereal Chemists. St. Paul, MN. U.S.A. LOCKWOOD, J. F. (1960) Flour Milling. 4th edn. Northern ANON. (1987-90) Modules of Workbook Series: 4. Instru- Publn. Co. Ltd, Liverpool. mentation and Process Control; 8. Mill Processes I & II; 9. MACRI, L. J., BALLANCE, G. M. and LARTER, E. N. Mill Machinery; 10. Product handling, Storage & Distribu- (1986) Factors affecting the breadmaking potential of four tia; I I. Wheat IntakelMill PerfomzanelQuality Control; 15. secondary hexaploid triticales. Cereal Chem. 63: 263-267. Air, Conveying& Power; 16, U.K. Flour Milling Industry. MATTERN, P. J. (1991) Wheat. In: Handbook of Cereal Science National Association of British and Irish Millers, London. and Technology, pp. 1-54, LORENZ, K. J. and KULP, K. FLORES, R. A., POSNER, E. S., MILLIKEN, G. A. and DEYOE, (Eds.) Marcel Dekker Inc, NY. U.S.A. C. W. (1991) Modelling the milling of hard red winter NATIONAL ASSOCIATION OF BRITISH AND IRISH MILLERS wheat: estimation of cumulative ash and protein recovery. (1991) Facts and Figures 1991. NABIM, London. Trans. ASAE, Sept.-Oct., 2117-2122. NATIONAL ASSOCIATION OF BRITISH AND IRISH MILLERS HOSENEY, R. C., VARRIANO-MARSTON, E. and DENDY, D. (1992) Facts and Figures 1992. NABIM, London. A. V. (1981) Sorghum and millets. Advances in Cereal PERTEN, H. (1977) UNDPIFAO sorghum processing project Science and Technology 4: 71-144. in the Sudan. Proc. Symp. Sorghum and Millets for Human SHELLENBERGER, J. A. (1980) Advances in cereal technology Food, pp. 53-55, ICC, Vienna,1976. Advances in Cereal Science and Technology 3: 227-270. REICHERT, R. D., YOUNGS, C. G. and OOMAH, B. D. (1982) WINGFIELD, J. (1989) Dictionary of Milling Terms and Equip- Measurement of grain hardness and dehulling quality ment. Association of Operative Millers. U.S.A. with a multi-sample tangential abrasive dehulling device YAMAZAKI, W. T. and GREENWOOD, C. T. (Eds.) (1981) (TADD). In: Proceedings of the International Symposium Soft Wheat. Amer. Assoc. of Cereal Chemists Inc. St. Paul on Sorghum Grain Quality, 1981, ROONEY, L. W. and MN. U.S.A. MURTY, D. S. (Eds.) International Crops Research Institute YOUNCS, V. L., PETERSON, D. M. and BROWN, C. M. (1982) for Semi-Arid Tropics. Patancheru, India. Oats. Advances in Cereal Science and Technology 5: 49-106. and Technology, pp. 111-125, BUSHUK, W. (Ed.) Amer. Further Reading