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 com￾requirements from millet (Varriano-Marston and position changes, reflecting the concentration of Hoseney, 1983) The problem of storage of flour fibre in the outer layers, and of protein and fibre does not arise, and this is just as well as products in the aleurone layer and the peripheral (subaleu￾may have a high moisture content. Further, rone) starchy endosperm (cf. p. 37). Oil and protein the continued pounding expresses oil from the contents of the pearlings are at a maximum when embryo and incorporates it into the flour, leading about 12% of the grain has been abraded. to rancidity on oxidation. Sorghum endosperm used as a brewing adjunct leaves the mill as coarse grits: they may be produced by impaction following decortication. Embryos are removed during the process (Rooney Industrial milling of sorghum As urban drift accelerates in Tropical coun- and Serna-Saldivar, 1991). Removal of embryos tries, strain is increasingly imposed on the domes- after milling is difficult as they are the same size tic production system and industrialization of as some of the grits. They can be separated, flour production becomes more attractive. however, by virtue of their different densities; In Africa the relationship between domestic floatation on water or use of a gravity table are processing and mechanized milling is a delicate suitable methods. balance that is affected by a number of changes, Reduction to flour particle size may be achieved occurring on that continent. Processing is justi- by roller milling, impaction or pin milling and fied only if it prolongs the storage period, increases may or may not include degerming. Maintenance convenience and preserves the nutritional quality and correct setting of roller mills can be a problem of the product (Chinsman, 1984). and the use of easily maintained and adjusted Processing methods may include adapted special mills is advocated by some. The United wheat flour milling methods and specifically Milling System (now Conagra) two stage process designed abrasive methods. Most begin with a is one example. The decortication stage employs a decortication stage using mills with abrasive discs vertical rotor which hurls grains against each or carborundum stones (Reichert et al., 1982). other and against a cylindrical screen through Wholemeals are produced by use of stone, which the fragments produced by the impacts hammer, pin or roller mills. pass for collection in a cyclone. This stage was In some cases traditional methods of decortica- designed to resemble the pounding typical of tion are combined with ‘service’ milling. Such domestic processing. The disc mill that follows combinations can improve on the traditional has sawblade elements that are cheap and simple methods alone by as much as 20% extraction rate. to replace. A two tonnes installation in the Sudan An experimental milling operation using a produces flour or grits of 80% extraction rate with laboratory Biihler mill gave best results with ash yield of 0.7-1.1%. sorghum conditioned to 20% m/c. Even broom- If it is required to remove the embryo before yard sorghum (which has pales attached to the milling, this may be done using machines designed grain, and which cannot be milled by other for degerming maize. Alternatively, special methods) was successfully processed by this method machines have been produced for sorghum itself