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