DRY MILLING TECHNOLOGY of 15%. The fat content of grits and four is extraction from 16-25%to about 6% in the germ 1.5-2.0% cake. The extracted oil is purified by filtering Finer through-stocks pass through a drier followed through cloth, using a pressure of 552-690 kn/m by an aspirator and purifier, to remove bran and (80-100 lb/in). The oil, which is rich in essential germ. Germ may be extracted to produce oil, the fatty acids, has a sp gr. of 0.. 925, and finds remainder being compressed into germ cake. use as a salad oil. Its high smoke point also makes Finer fractions of the through-stock consist of it suitable for use as a cooking oil endosperm heavily contaminated with bran and germ, the mixture being known as ' standard meal,. The various fractions are combined to give Composition of products hominy feed Chemical composition and nutritional value vary among the tissues of the maize grain Impact grinding hence their separation leads to a concentration of components into different products(see Table An alternative to the beall is the European- 14.12) developed system in which impact machines such as entoleter(see Fig 5.7) or turbocrushers used to detach the embryos. Treatment is per Uses for dry-milled maize products formed at natural moisture content 13-15%(cf. The products of maize dry milling, their par- 0% for the Beall). It is relatively economical as ticle size ranges, and average yields are shown in bsequent drying is not necessary. Separation Table 6.2 of germ from endosperm on gravity tables is efficient, but separation of endosperm from bran TABLE 6.2 by aspiration, and of vitreous endosperm from Yield and Particle Size Range of Dry-milled matse Products mealy endosperm, is less effective than with the Particle size range Yield rminator. The following products are btained from the entoleter process Product Maize germs, 1-4.5 mm, with 18-25%fat. Flaking grits Coarse g 2.0-1.4 Maize grits, 1-4.5 mm, with 1.5%fat, 0.8-1.2% Medium grits 1,4-1.0 crude fibre content: about 60% of the original Fine grits 16-2610065 0.65-0 malze Semolina and flour, which may be made by rime 6,7-0.5 size reduction of the grits. The mealy endo- Hominy feed sperm, of higher fat content, reduces to flour particle size more readily than does the vitreous *Tyler Standard Screen Scale sizes Sources: Stiver, Jr(1955); Easter(1969 endosperm; thus, four has a higher fat content -about 3%, and semolina a lower fat content Flaking grits are used for the manufacture of 0.8-1. 3%, than the grits the ready-to-eat breakfast cereal corn fakes'(cf Ch. 11). Grits from yellow maize are preferred Oil extraction from germ Other uses for dry-milled maize products are discussed elsewhere: Solvent extraction is generally used in the dry ready-toasted cereals( Ch. 11), for bread and illing industry, although mechanical pressing, other baked foods( Ch. 8), and for industrial e.g. with a screw press, is sometimes used. The purposes( Ch. 15) germ from the mill is first dried to about 3%m c. Dry milled maize flour is not to be confused with and then extracted while at a temperature of about 'corn flour,, the term used in the u. K. for maize 121C. The oil content of the germ is reduced by starch obtained as a product of wet milling
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 particle 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 Europeandeveloped system in which impact machines such as entoleters (see Fig. 5.7) or turbocrushers are used to detach the embryos. Treatment is performed 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 12-16 1.4-1.0 ] q60 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
140 TECHNOLOGY OF CEREALS Milling of common(bread )wheat (skirt). In western Europe the local soft wheat as ground by a low grinding process, in which Historical the upper stone was lowered as far as possible towards the lower stone, thereby producing a In prehistoric times, the barley and husked heavy grind from which a single type of four was wheat (emmer: Triticum dicoccum) used for made. Attempts have been made, at least as far human food were dehusked by pounding the back as classical Roman times, to make a white grain in mortars. The invention of rotary flour for breadmaking At the time of the norman grain mills, for grinding ordinary bread wheats Conquest(A D. 1066)in England, stone ground (T aestivum)is attributed to the Romans in the flour was being sieved into fractions -a fine econd century B C. Thereafter, until the devel- four called smedma and a coarse four called groth opment of the rollermill in the mid-nineteenth century(see Table 6. 3), wheat was ground by (Storck and Teague, 1952) Where contrasting conditions existed side stone-milling. Even in Industrial countries today by side, as in eighteenth-century france and some stone milling is carried out to meet specialist demand nineteenth-century Austria, the use of flours of more than one quality became possible. For this TablE 6.3 purpose a high - system was used, with Significant Dates in the History of Flour-Milling the upper stone slightly raised, producing a gritty intermediate material from which, by further 1753 Wilkinson patented iron rollermills for grinding flour treatment, flours of diverse quality could be 860 First rollermill system worked in Budapest made Hard wheat, from the Danube basin, was 1877 Lord radford of liverpool and a group of Englishmen ideally suited to the high-grinding system,and 1878 First complete rollermill in England at Barlows, Bilston 1878 Roller milling introduced in America by Washburn TABLE 6. 4 Crosby Co The Ideal relationship Between grain Components and Mill fractions Produced in Milling White Flour from Wheat Stone milling Grain component Mill fraction French burr from La Ferte-sous-Jouarre, Seine- et-Marne, millstone grit from Derbyshire, German over most of grain, but cell lava, Baltic flint from Denmark, and an artificial stone containing emery obtained from the island ascular tissue in crease of Paxos, Greece. The opposing surfaces of the b) Intermediate cells Cross cells terned es, which are in close contact, are pat- Tube cells (Inner epiderm ith series of grooves leading from the ( 2)Seed centre to the periphery. In operation, one stone is stationary while the other rotates. Either the upper stone (upper runner,)or the lower stone Under runner)may rotate, but it is usually the Starchy endosperm (d)Embryo former Embryonic axis fed into the centre ('eye) of the upper stone is fragmented between the two stones ★ In the of durum milling, 'semolina' should be nd the ground products issue at the periphery substituted for"white flour
on a vertical axis. Types of stone used include Beeswing ’ French burr from La Ferte-sous- Jouarre, Seineet-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 patterned 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 when steam power became available, Hungary TABLE 6.5 became the centre of the milling industry Flour Extraction Rates in Various countries Wheat flour has been defned as the product Norway (90 x 24 italy(1990199r G\ Country Country Milling of white flour The majority of flour sold in the U. K. is Austria(1990 Canada(198889)* That is to say, it consists almost entirely of s endosperm. The ideal dispersal of other components is shown in Table 6.4 Malta(1988)* Morocco(1988) prepared from grain of common wheat(Triticum Spain(1990)t aestivum L. or club wheat(Triticum aestivum ssp S K.(1990 986)86 Zamibia (1988/89 which the bran and embryo are partly removed Sources * International Wheat Council, t NABIM 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 quantity: the weight of clean wheat required to produce 100 lb(I cwt U.S. 45 kg)of four described as white flour would pass through a The wheat grain contains about 82% of white Hour sieve having rectangular apertures of starchy endosperm which is required for white 140 um length of side. Some fiour is produced flour, but it is never possible to separate it exactly in the milling of durum wh at (triticum durum from the 18% of bran, aleurone and embryo, and but it is not the main product thereby obtain a white flour of 82% extraction rate The objectives in milling white four are (products basis). In spite of the mechanical limita 1. To separate the endosperm, which is required tions of the milling process, extraction rates well for the flour, from the bran and embryo, which in excess of 75% are now achieved. Flour extrac are rejected, so that the flour shall be free tion rates prevailing in various countries are rom bran specks, and of good colour, and so shown in Table 6.5. They are calculated from that the palatability and digestibility of the values for wheat milled and flour produced product shall be improved and its storage provided by official bodies in the respective lengthened countries 2. To reduce the maximum amount of endo- Mill capacities in the U. K. were formerly sperm to flour fineness, thereby obtaining the expressed in terms of" sacks/h,, a sack being 280 maximum extraction of white flour from the Ib, 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 Extraction rate wheat) per 24 h The accounting unit in the flour- duced per 100 parts of wheat milled is known as capacity is expressed as cwt of flour per &s z milling industry is 100 kg. Bran and fine w The number of parts of flour by weight pro- feed are quoted in metric tonnes. In the U.s the four yield or percentage extraction rate Flour yield is generally synonymous with extrac Principles Is C percentage of the products of milling of clean Were it not for the ventral crease in the wheat wheat, although it may also be expressed as the grain it is likely that an abrasive removal of the percentage of flour as a proportion of the dirty pericarp, embryo and aleurone tissue would be wheat received. It is prudent to check which a stage in the conventional milling of wheat, system is in use, wherever possible. In the U.S. A. as it is in rice milling. The inaccessibility of the the term ' yield'is used to express a different outer tissues in the crease region, however, has
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 aestivum L.) or club wheat 80 Zambia (1988/89)* 75 (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 limitations of the milling process, extraction rates well in excess of 75% are now achieved. Flour extraction 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 flourmilling industry is 100 kg. Bran and fine wheatfeed 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 produced per 100 parts of wheat milled is known as the flour yield, or percentage extraction rate. Flour yield is generally synonymous with extraction 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
142 TECHNOLOGY OF CEREALS Break Scratch Reduction Flour dressing F FIG 6.8 A simple schematic diagram of the common- wheat flour-milling proces generally precluded this approach, leading to the complex arrangement of modern flour milling development of the modern roller milling process systems, and for the particular design of the which solves the problem of the crease by a specialized machinery used combination of shearing, scraping and crushing The main stages in the milling of wheats to exploit the differences in mechanical proper- whether durum or common wheats, are grinding ties between the starchy endosperm, bran and on roller mills, sieving and purifying It is interesting that attention is now Details of different rollermilling possibilities being given to the possibility of applying advanced are given earlier in this chapter. In bread wheat pearling technology as an alternative to this It is milling a break, scratch and reduction system essential, in effecting the required separation, to are included The breaking system serves to open minimize the production of fine particles of bran, the grain and progressively scrape endosperm and this basic requirement is responsible for the from the bran, the scratch system gently scrapes conditioning process described in Ch. 5, the bran from larger endosperm particles, and the
Scratch Scalping grading dusting Semolina and middCings 1 1 1111 Purifying Reduction