6 ry Milling Technology Introduction In milling processes involving both separation The single term milling,,applied in the con and size reduction the two operations may be carried out in two distinct phases, as in sorghum text of cereals, covers a wide range of processes. milling, or, to some extent, combined, as in In general they are methods of transforming bread- and soft-wheat milling, where the two whole grains into forms suitable for consumption or for conversion into consumable products processes continue throughout the multi-stage Milling processes peration(although e ei phasis changes as the intentional heating, although in some cases, as n process proceeds). In rice milling, two stages es involve oat processing, a heating phase precedes the occur but neither seeks to fragment the endo- milling, and in maize dry milling a drying phase sperm. The first stage removes the husk and the is included second removes the bra Characteristic(but not essential) features of Even the above distinctions are not absolute as milling processes are processes are in use for decorticating wheat grains before reduction of size and some rice is milled 1. Separation of the botanical tissues of the grain Into flour. It is clear that few generalizations can (e.g. endosperm from pericarp, testa and be made about cereals milling, most milling embryo technologies depend upon a series of individual 2. Reduction of the endosperm into four or grits. processes through which stocks pass in sequence Some milling systems include both operations Milling processes (e. g. white flour milling from wheat), while others The processes are of three types, they may involve only one(e.g. rice milling comprises only separation, and wholemeal wheat milling seeks 1. Change the shape and size of the feedstock only to reduce particle size 2. Separate fractions produced by (1)-type Milling schemes are conveniently classified treatments 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 of the stocks is used in almost all separations. Damping or tempering' features even in 'dry' milling; it is The processes are described below considered in detail in Ch 5 as it is a pre-milling Treatments that change shape and size treatment. This chapter is concerned with so called dry milling; wet milling processes are dealt Abrasion with in Ch. 12. Emphasis is placed on preparation for human consumption but the term milling Effects depend upon severity, thus also applies to production of animal feeds. A brief 1. Surface abrasion is a relatively gentle process description of feed-milling is given in Ch. 15 which removes all or part of the fruit coats
6 Dry Milling Technology Introduction The single term ‘milling’, applied in the context 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 endosperm. 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 socalled 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
TECHNOLOGY OF CEREALS (pericarp) and possibly the embryo. greater severity, or retreatment of already abraded tocks, removes part of the endosperm also Grains are brought into contact with an abra ive 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 present context abrasion is used to include the similar process of attrition. There is a nice Diagonal rolls distinction between the two terms, depending on the roughness of the surfaces involved but it is insufficient to warrant separate considera- tion. When used specifically to remove the outer tissues of caryopses either process may OOO be described as decortication 2. Severe abrasion includes heavy or protracted grinding between surfaces such as those of a pestle and mortar and of grinding stones; it Horizontal rolls features therefore in many of the simple FIG 6. 1 Disposition of rolls in roller mill stands. The fast and historical methods used for preparing roll of each pair is indicated by the broader arrow ground meals. Such grinding may reduce grains to a range of particle sizes including Break rolls that of four Ro∥ er milling Whole grains or partially milled stocks are Roll fluting assed between rotating rollers for several reasons including 1. Grinding -a process in which grains are British reduced to smaller particle siz stages are performed within rollermills, each of which is equipped with a pair of rolls Rollstands usually consist of two mills back- to-back, within the same housing but operat ing independently and on different feedstocks The rollers may be disposed diagonally, verti- FIG 6 of break rolls. In the enlarged view of the ally or horizontally. They are aligned in nip' olls are disposed in the dull-to-dull configura parallel and rotate in opposite directions. One tion. Details of typical British and U.S. roll flutings of the rolls rotates faster than the other so that a speed differential exists(see Fig. 6.1) The rollers may be smooth surfaced resembles an italic v, with one side shorter futed, but when used in a grinding mode they than the other (fig 6.2) rotate at different speeds As the flutes are asymmetric the rolls On fluted rolls the profile of each flute be run with either the steep(sharp)
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, selectively permitting passage of particles. In the \A/ be described as decortication. 2. Severe abrasion includes heavy or protracted
DRY MILLING TECHNOLOG 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. TheThe 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 pearling haracteristics, as in porridge oats from decorticated grains Multiple factors Grains or milled stocks are thrown against a hard and possibly abrasive surface. This is usually The paddy separator(see p. 120)is an example achieved by feeding stocks into the centre of a of a machine in which several grain characteristics very high-speed rotor. The process is very versatile are exploited in effecting their separation. Specific It be used means of dehulling (as of gravity, surface roughness and shape all combine ats), abrading, size-reduction, (as in pin milling), to direct grains into appropriate streams on a or disinfestation, destroying all stages in the life- tilted vibrating table with a cunningly sculptured cycle of insect pests found in grain and four surface Fractionating processes Changes in temperature and/or moisture Water can be added with or without substantial hich involve grinding, sieving features at some physical conditions, as in stabilization of oats,or tage, to separate stocks as final products or for to change the mechanical properties of the grain further appropriate treatment components, as in wheat and rice milling, when the temperature is of less importance Shape Stocks produced from grains or intermediates processes suc h as oat milling and rice milling milled at very high moisture need to be dried to grains are graded on a shape(and size)basis, before permit proper processing or safe storage. Drying dependent; and during the milling process, as small been heated may subsequently require cooling grains which escape treatment need to be re-fed Shape-sensitive fractionating machines include Fine grinding and air classification disc separators and trieur cylinders The contents of cells comprising the bulk of Specific gravity storage tissues of many legume cotyledons and cereal endosperms consist essentially of starch Particles differing in density may be separat granules embedded in a protein matrix. In oats on a fuid medium such as air or water. In the and some legumes an appreciable amount of oil
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 specifically 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 lifecycle of insect pests found in grain and flour. Fractionating processes Size This is an important criterion by which particles 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 pherical or near spherical starch granules are wedge shaped and where protein occupies these spaces It is compressed into 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) embedded in them Microns When wheat endosperm is fragmented by grinding it is usually reduced to a mixture of particles, differing in size and composition( Greer 1951). These may be classified into three main fractions ○○ 1. Whole endosperm cells(singly or in clumps) segments of endosperm cells, and clusters of starch granules and protein(upwards of 35 2\ Hm in diameter). This fraction has a protein content similar to that of the parent flour. 2. Large and medium sized starch granules FIG 6.3 Above: the two main types of endosperm cell rismatic(lef), polyhedral (right)-showing large and sma some with protein attached (15-35 um in starch granules(white)embedded in protein matrix(black) diameter). This fraction has a protein content Below: exposed endosp of further breakdown (night): 1. detached large starch granules 3 one half to two thirds that of the parent flour (about 25 Hum diameter ); 2. 'clustersof small starch granules Small chips (wedges)of protein, and detached and protein matrix(about 20 um diameter);3. detached sma small starch granules (less than 15 um in starch granules (about 7 um diameter); 4. fragments of free diameter). This fraction has a protein content wedge protein(less n eh aperture width of a typic approximately twice that of the parent flour flour bolting cloth (Redrawn from C.R. Jones et al., y (Fig.6.3) Biochem MicrobioL. Technol. Engng. 1959, 1: 77 and repro- duced by courtesy of Interscience Publishers. The proportion of medium-sized and small particles(below 35 um) in flour milled conven- The reduction in particle size due to fine tionally from soft wheat is about 50% by weight, grinding further separates the components, as but in hard wheat flours it is only 10%. The previously described, allowing increased propor proportion of smaller particles can be increased tions of starch and protein to be concentrated into at the expense of larger ones by further grinding different fractions n, for example, a pinned disc grinder, which Particles below about 80 um are considered to consists of two steel discs mounted on a vertical be in the sub-sieve range, and for making separa axis, each disc being studded on the inward- tions at 15 um and 35 um, the four as ground facing surface with projecting steel pins arranged or after fine-grinding, is fractionated by air in concentric rings that intermesh. One disc, the classification. This process involves air elutria stator, remains stationary while the other rotates tion, a process in which particles are subjected at high speed. Feedstock enters the chamber to the opposing effects of centrifugal force and between the discs at the centre, and it is propelled air drag Smaller particles are infuenced more by centifugally by the air current created. The the air drag than by centrifugal force, while the particles impact against the pins and against each reverse is true of the larger particles. The size at other, as a result of which, they are fragmented. which a separation is made is controlled by
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 pm in diameter). granules and protein (upwards This fraction has a protein of 35 * 4 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 conventionally 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 inwardfacing 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 reproduced by courtesy of Interscience Publishers.) The reduction in particle size due to fine grinding further separates the components, as previously described, allowing increased proportions 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 separations at 15 pm and 35 pm, the flour as ground, or after fine-grinding, is fractionated by airclassification. This process involves air elutriation, 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 Table 6.1 Yield and Protein Content of Air-Classified fractions of Flours With or Without Pinned-Disc grinding* Fine Medium Coarse (0-17pm) >35μm) Protel Yield ontent Yield Protein %片 Hard whea 13.6 17.1 13.8 7.6 14.5 5.3 89 9.5 Source: Kent(1965). 14%m.c. basis.N×5.7 varying the amount of air admitted or by ac uniform particle size and granular nature are ing the pitch of baffles which divide or cut advantageous airborne stream of particles practised commercially, air classification Classification can be continued into is generally carried out in the mill. It is customary fractions by cuts corresponding to larger to effect separations into a protein-rich fraction As Fig. 6. 4 shows, this does not lead, as h-rich fraction of be expected, to many fractions varying in si 15-35 um, and a fraction over 35 um 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 um is yields and characteristics of fractions derived that between 44 and 55 um, in which are concen from fine-ground and unground fours 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 as a percentage of total protein in the material 22A fractionated Applications for which commercial classified o 16 fractions might be used ar Fine fraction: increasing the protein content of bread flours, particularly those milled from low or medium protein wheats, and in the manu- Cut size (um facture of gluten-enriched bread and starch Medium fraction: use in sponge cakes and FIG 6.4 Results of air-classification of flour into actions of varying particle size. The flour, milled from pre-mix Hours milled one pass, and classified at 10 35,44,and55μn minal cut sizes. Protein nt of the Coarse fraction: biscuit manufacture where the parent flour was 13
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 concenfrom 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 manufacture 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%
TECHNOLOGY OF CEREALS Air classification has been applied to other cereal and legume fours with some success. A Whole grain rye four of 8.5% protein was separated into a fine fraction with 14.4% protein and a coarse fraction with 7. 3%. Sorghum flour fractions between Pounding 5% and 16.6% protein were prepared from parent fours of 5.7-7.0% protein. Starting with grits(which are derived from the higher protein horny parts of the grain) of 9.2-11.9% protein content, fractions between 6.8 and 18.9% protein fLotation Husk were obtained(Stringfellow and Peplinski, 1966) Protein shifting by air-classification has not met with success when attempted in rice. Three factors are held responsible for this: the smallness of the starch granules, the intimate dispersions of the protein bodies and the extreme vitreousness of the endosperm(Deobold, 1972) Sorghums and millets are considered together Flour and meal as they are both tropical cereals, the majority of FiG 6. 5 Schematic diagram of domestic processing of sorghum the processing of which remains in the hands of subsistence farmers. The grinding of flour is Water may be used during the continuing grind- performed by traditional manual methods which ing, as appropriate to the type of grain used. The occupy much of the day of the women. In the decorticated grain is separated from the bran by case of sorghum particularly, several industrial winnowing, after drying in the sun; or water may milling methods are in use. Because industrial be used to separate the components. Further scale operations are relatively new, these may pounding follows to reduce the size of the particles be regarded as somewhat experimental, no of decorticated grains Sieving is used to separate single method having yet been established as material that has been reduced sufficiently, from the standard. Several methods are described that needing further treatment in domestic processing several principles are schematically in Fig. 6. Drocess is illustrated the following text. It is noticeable that, even The simplicity of the involved, and these are similar to those used in The principles illustrated are the industrial systems 1. Use of attrition to break open the grain 2. Separation of endosperm from the surrounding Domestic processing 3. Use of water to aid the separation The techniques used have been in continuous 4. Sieving to select stocks fo use for hundreds of years. Although simple, the or appropriate processes are instructive, both in their own right, and because they reflect the methods from which Another principle, common to all milling pro- modern cereal processing has evolved. They are cesses, and illustrated by hand milling, is the hand operations in which wooden pestle and dependence of the method adopted on the nature mortar are used, the abrasive action of pounding of the varieties processed. variations include the on the washed grain freeing the outer pericarp hardness of the endosperm and the thickness of from the remainder of the grain (cf. Ch. 13). the pericarp Soft grains break into pieces during
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. AO.(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 four 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 pearling 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- ay 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 pearling 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 Industrial milling of sorghum produced by impaction following decortication Embryos are removed during the process rooney 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 four particle size may be achieved ccurring 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( Chinaman, 1984) and the use of easily maintained and adj 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 Wholemeal 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%o extraction rate with laboratory Buhler 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
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 comrequirements 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 (subaleumay 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
TECHNOLOGY OF CEREA Such a device consists of a wire brush rotating(75%in the U.s.A. of maize is processed to within a perforated cylinder. produce starch. The product with the highest value, coming from dry milling is 'grits. Two Industrial milling of millets important characteristics of the maize grain influence the production of grits, viz. the large Industrial processing of millets is even less embryo and the presence of horny and mealy mon and less developed than that of sorghum. endosperm in the same grain(see Fig 6.6) While there are reports of experimental attempts The significance of the large embryo lies not to adapt technology appropriate to other cereals, only in its failure to contribute to the grits yield the concept of industrial scale millet milling is but also in its high oil content. Inclusion of this not well established. Industrial production of oil in the product, either as a component of four inevitably imposes a need for distribution embryo chunks or through its expression on to facilities, and for storage. The inclusion of the surface of grits, reduces the shelf-life through embryo parts, or even oil expressed from the its oxidation and consequent rancidity. variation embryo in flour, reduces storage life, as in in endosperm texture is important because grits sorghum. Any successful process should there- are essentially derived from the horny parts of fore include a degerming stage. The limited the endosperm; softer parts too readily breaking amount of small-scale industrial processing that down to four. In the industrialized world, grits is carried out consists of abrasive decortication are used mainly in production of, or consumption followed by reduction of endosperm with as, breakfast cereals. They are also used for hammer mills or similar devices dependent on making fermented beverages. In Africa a fine grit attrition meal ( mealy meal)is an important staple Maize dry-milling exploits most of the prin Dry milling of maize ciples used in grain milling(cf. p. 134), but not all are involved in the same process. Several Q Dry milling is a relatively minor industry combinations are described in the text below. mpared with wet milling, by which the majority Historical Dry milling techniques were in use by north nd south american indians in ancient times Endosperm Hand-held stones were used initially, but a later development was one hand-held stone ground CotyLedon against a concave bedstone. a further develop- 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 Plumule Embryo 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 th milk today, applied to Radicle some of the products of modern maize dry milling, e.g. 'hominy feedand hominy grits Later devel malze included the quern, a device common to the Ro FiG 6.6 Diagram of the maize grain showing the relative sizes times. Querns consisted of two stones; the of the main anatomical componen upper'capstone' being rotated over the stationary
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
DRY MILLING TECHNOLOGY 137 Cleaned, tempered grain Embryo rich fraction Drying, coolin spination cooLing aspiration Sifter Break roLls Sifter Sifter dr ying, cooLing and filte Flaking grits Reduction Sifter Corn meal Brewers grits FIG 6.7 Simple tic diagram of the tempering-degerming process. ( Based on Johnson, 1991. netherstone,. Grain was introduced through a grist mills. Because it includes the embryo the ole in the centre of the capstone and it was meal has a limited shelf life ground by the abrasive action of the two stones as it was worked towards the outside to be Today, dry milling is carried out in several The tempering-degerming (TD) system coarse wholemealof 85-95%extraction, in small degerming process is shown in Fig. 6 /2 perin ways, the simplest being the production of a A schematic summary of the ten
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
TECHNOLOGY OF CEREALS The beall degerminator finished product. Finer stocks are combined with coarser fractions of the through-stocks from the Possibly the most important innovation in dry Beall, for treatment in the milling system maize milling was the introduction of degerming stages. The Beall degerminator is unfortunately named, as it neither reverses the process of Milling germination, nor totally or exclusively removes The feed to the milling system, viz. large fU S dry milling plants today, medium and fine hominy, germ roll stock, and (TD)system. Its virtue lie ering -degerming embryo They are separated by a series of roller as an essential stage in the tem large particle size grits dried and emerging as a diverse range of final with low fat content and low fibre content(about 0.5%), suitable for manufacture of corn flakes products. They are fed to the mill, each entering The favoured feed stock to the TD system in at an appropriate point: the large and medium the U. S A is No. 2 yellow dent corn. In Africa hominy at the first break, the fine hominy and white maize is used. After cleaning and tempering to 20% moisture content, it passes to the Beall The milling is carried out on roller mills, using This machine consists of a cast iron cone, rotating futed rolls, a traditional flow containing up to at about 750 rev/min on a horizontal axis, within sIxteen distinct stages. The grindings with fluted a conical, stationary housing, partly fitted with of 1. 25:1 or 1.5: 1) fatten the embryo fragme llowing them to be removed by sieving. The outer surface. The maize is fed in at the small products are sifted on plansifters and are aspirated end and it works along to the large end, between the two elements. The protrusions on the rotor rolls. The break system releases the rest of the abrasive action, also breaking the endosperm into embryo as intact particles, and cracks the larger particles of various sizes and degrees of purity. nilling system for maize bears some resemblance The Beall discharges two types of stock: the to the earlier part of the wheat milling system tail stocks which are too large to pass through the screens, consisting mainly of fragmented (which is described more fully below--p. 141) endosperm,and the through-stock consisting as far as B2 reduction roll (2nd quality roll,in largely of bran and embryo, ne proportions of the U.S. A)viz. the break, coarse reduction and different sized particles can be controlled by the the scratch systems, but is extended and modified in comparison with this part of the wheat milling setting of the Beall(Brekke and Kwolek, 1969). to make a more thorough separation of the large Drying, cooling and grading quantity of germ present. Modern practice is to 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%-70oC and cooled to order to minimize damage to the germ while 320-38C 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 four products are dried to 12-14% grits, originating from the vitreous parts of the moisture content on rotary steam tube driers aspirator and drier-cooler, before emerging as a s.The germ concentrate consists largely of endosperm. They may pass through a fu avily damaged embryos with an oil content
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
