BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 24 nd pressures to undergo a melt transition and zone on the screw but the use of barrel heaters form a viscous fuid The extruder develops the and steam injection in preconditioning units can fuid by shearing the biopolymers, particularly help to induce sharper and earlier melt transitions he starch( Guy and Horne, 1988; Guy, 1991)and or to increase the throughput(Harper, 1989) shapes the fuid by pumping it through small dies. Considerable back-mixing may occur in the chan The equipment may consist of single-or twin- nel of the screw, giving a fairly broad residence screws with spirally-arranged fights for conveying, time distribution heading All twin-screw extruders have a positive pump creating high pressure shearing and kneading ing action and can convey all types of viscous zones. In order to achieve the high temperatures materials with efficiency and narrow residence- necessary for the melt transition, the raw materials time distributions. Special zones can be set up require large heat inputs. These are achieved by along the screw to improve the mixing, compres the dissipation of mechanical energy from the sion and shearing action of the screws. Co- screw caused by frictional and viscous effects, by rotating twin-screw extruders, which have self- the injection of steam into the cereal mass, and wiping screws and higher operating speeds than by conduction from the heated sections of the counter-rotating machines, are currently the pre barrel or screw, using heating systems based on dominant choice of extruders for use in the food electrical elements, steam or hot fluid industry(Fichtali and van de voort, 1989). The In extrusion cookers with twin screws, the physical changes to the raw materials occurring screws may be co-rotating or counter-rotating vithin the single- and twin-screw extruders are Further, there are many variations possible in basically the same and have the same relationships screw design relating to physical dimensions, to temperature, shearing forces and time. How the extent to which the separate screws on screw machines because the output is not affected each shaft intermesh(Fichtali and van de voort, by the physical nature of the melt phase being 1989). The main difference between single- and produced within the screw system, and the back twin-screw extruders rns the conveying mixing can be more tightly controlled, givin characteristics of the screws better overall control and management of the facture ready-to-eat breakfast cereals in the 1960s but they had probl slippery or gummy materials because they rely Flaked products from n the drag flow principle for conveying the Maize(for corn flakes), wheat or rice are the materials within the barrel. The problems of cereals generally used for faking slippage can be overcome to some degree by using In the traditional batch process for making grooves in the barrel walls(Hauck and Huber, cornflakes, a blend of maize grits hunks of 989). The single-screw extruder has a continuous about 0.. 33 of a kernel in size-plus favouring channel from the die to the feed port, and materials, e.g. 6%(on grits wt) of sugar, 2% of herefore its output is related to the die pressure malt syrup, 2%of salt, possibly plus heat-stable and slippage The screw is usually designed to vitamins and minerals, is pressure-cooked for compress the raw materials by decreasing the about 2 h in rotatable batch cookers at a steam fight height, thereby decreasing the volume pressure of about 18 psi to a moisture content of available in the flights. At relatively high screw about 28% after cooking. The cooking is complete speeds the screw mixes and heats the flour mass, when the colour of the grits has changed from and a melt transition is achieved permitting the chalky-white to light golden brown, the grits have ftened starch granules to be developed by the become soft and translucent, and no raw starch This transition usually occupies a fairly broad remains shearing action of the screw The cooked grits are dried by falling against aBREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 247 and pressures to undergo a melt transition and zone on the screw but the use of barrel heaters form a viscous fluid. The extruder develops the and steam injection in preconditioning units can fluid by shearing the biopolymers, particularly help to induce sharper and earlier melt transitions the starch (Guy and Horne, 1988; Guy, 1991) and or to increase the throughput (Harper, 1989). shapes the fluid by pumping it through small dies. Considerable back-mixing may occur in the chan￾The equipment may consist of single- or twin- nel of the screw, giving a fairly broad residence￾screws with spirally-arranged flights for conveying, time distribution. and special kneading and reversing elements for All twin-screw extruders have a positive pump￾creating high pressure shearing and kneading ing action and can convey all types of viscous zones. In order to achieve the high temperatures materials with efficiency and narrow residence￾necessary for the melt transition, the raw materials time distributions. Special zones can be set up require large heat inputs. These are achieved by along the screw to improve the mixing, compres￾the dissipation of mechanical energy from the sion and shearing action of the screws. Co￾screw caused by frictional and viscous effects, by rotating twin-screw extruders, which have self￾the injection of steam into the cereal mass, and wiping screws and higher operating speeds than by conduction from the heated sections of the counter-rotating machines, are currently the pre￾barrel or screw, using heating systems based on dominant choice of extruders for use in the food electrical elements, steam or hot fluids. industry (Fichtali and van de Voort, 1989). The In extrusion cookers with twin screws, the physical changes to the raw materials occurring screws may be co-rotating or counter-rotating. within the single- and twin-screw extruders are Further, there are many variations possible in basically the same and have the same relationships screw design relating to physical dimensions, to temperature, shearing forces and time. How￾pitch, flight angles, etc. and, in the twin-screw, ever, control of the process is simpler in the twin￾the extent to which the separate screws on screw machines because the output is not affected each shaft intermesh (Fichtali and van de Voort, by the physical nature of the melt phase being 1989). The main difference between single- and produced within the screw system, and the back￾twin-screw extruders concerns the conveying mixing can be more tightly controlled, giving characteristics of the screws. better overall control and management of the Single-screw extruders were first used to manu- process. facture ready-to-eat breakfast cereals in the 1960s, but they had problems with the transport of F,aked products from maize slippery or gummy materials because they rely on the drag flow principle for conveying the Maize (for ‘corn flakes’), wheat or rice are the materials within the barrel. The problems of cereals generally used for flaking. slippage can be overcome to some degree by using In the traditional batch process for making grooves in the barrel walls (Hauck and Huber, cornflakes, a blend of maize grits - chunks of 1989). The single-screw extruder has a continuous about 0.5-0.33 of a kernel in size -plus flavouring channel from the die to the feed port, and materials, e.g. 6% (on grits wt) of sugar, 2% of therefore its output is related to the die pressure malt syrup, 2% of salt, possibly plus heat-stable and slippage. The screw is usually designed to vitamins and minerals, is pressure-cooked for compress the raw materials by decreasing the about 2 h in rotatable batch cookers at a steam flight height, thereby decreasing the volume pressure of about 18 psi to a moisture content of available in the flights. At relatively high screw about 28% after cooking. The cooking is complete speeds the screw mixes and heats the flour mass, when the colour of the grits has changed from and a melt transition is achieved permitting the chalky-white to light golden brown, the grits have softened starch granules to be developed by the become soft and translucent, and no raw starch shearing action of the screw. remains. This transition usually occupies a fairly broad The cooked grits are dried by falling against a