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%