《谷物食品技术 Technology of Cereals》课程教学资源（参考书籍，英文版，第四版）11 Breakfast Cereals and Other Products of Extrusion Cooking

Breakfast Cereals and other Products of Extrusion Cooking Breakfast Cereals the starch. Cooking by extrusion at low moisture All cereals contain a large proportion of starch. causes the starch granules to lose their crystallinity In its natural form, the starch is insoluble, but they are unable to swell as in the normal gelatinization process in excess water. However, tasteless,and unsuited for human consumpton. when they are exposed to moisture during con cooked. Breakfast cereals are products that are sumption they hydrate and swell to become consumed after cooking, and they fall into two susceptible to enzymic digestion categories: those made by a process that does not include cooking and which therefore have to be Hot cereals cooked domestically(hot cereals)and those which are cooked during processing and which require Porridge from oats no domestic cooking The first class of products is exemplified by various types of porridge Porridge is generally made from oatmeal or second by products which are described as ' re oatcakes (rolled oats or 'porridge oats), the cture to-eat’ cereals of which was described on p. 167 Besides the distinction regarding the need for The milling process to make oatmeal includes no cooking(unless the oats are stabilized to inactivate domestic cooking as against readiness for con- the enzyme lipase: cf. p. 165), and the starch in according to the form of the product, and accord- oatmeal is ungelatinized; moreover, the particles ing to the particular cereal used as the raw material sequently, porridge made from coarse oatmeal requires prolonged domestic cooking, by boiling with water, to bring about gelatinization of the Cooking of cereals starch. Oatmeal of four fineness cooks quickly, but the cooked product is devoid of the granular If the cereal is cooked with excess of water and structure associated with the best scotch porridge only moderate heat, as in boiling, the starch Rolled oats are partially cooked during manu gelatinizes and becomes susceptible to starch- facture; the pinhead oatmeal from which rolled hydrolyzing enzymes of the digestive system. If oats is made is softened by treatment with steam cooked with a minimum of water or without and, in this plastic condition, is er, but at a higher temperature, as in toasting, flaking rolls. Thus, porridge made from rolled a non-enzymic browning (Maillard) reaction oats requires only a brief domestic cooking time between protein and reducing carbohydrate may to complete the process of starch gelatinize occur, and there may be some dextrinization of The amount of domestic cooking required by

I1 Breakfast Cereals and Other Products of Extrusion Cooking Breakfast Cereals All cereals contain a large proportion of starch. In its natural form, the starch is insoluble, tasteless, and unsuited for human consumption. To make it digestible and acceptable it must be cooked. Breakfast cereals are products that are consumed after cooking, and they fall into two categories: those made by a process that does not include cooking and which therefore have to be cooked domestically (hot cereals) and those which are cooked during processing and which require no domestic cooking. The first class of products is exemplified by various types of porridge, the second by products which are described as ‘ready￾to-eat’ cereals. Besides the distinction regarding the need for domestic cooking as against readiness for con￾sumption, breakfast cereals can also be classified according to the form of the product, and accord￾ing to the particular cereal used as the raw material. the starch. Cooking by extrusion at low moisture causes the starch granules to lose their crystallinity, but they are unable to swell as in the normal gelatinization process in excess water. However, when they are exposed to moisture during con￾sumption they hydrate and swell to become susceptible to enzymic digestion. Hot cereals Porridge from oats Porridge is generally made from oatmeal or oatflakes (rolled oats or ‘porridge oats’), the manufacture of which was described on p. 167. The milling process to make oatmeal includes no cooking (unless the oats are stabilized to inactivate the enzyme lipase: cf. p. 165), and the starch in oatmeal is ungelatinized; moreover, the particles of oatmeal are relatively coarse in size. Con￾sequently , porridge made from coarse oatmeal requires prolonged domestic cooking, by boiling with water, to bring about gelatinization of the starch. Oatmeal of flour fineness cooks quickly, but the cooked product is devoid of the granular Cooking of cereals If the cereal is cooked with excess of water and structure associated with the best Scotch porridge. only moderate heat, as in boiling, the starch Rolled oats are partially cooked during manu￾gelatinizes and becomes susceptible to starch- facture; the pinhead oatmeal from which rolled hydrolyzing enzymes of the digestive system. If oats is made is softened by treatment with steam cooked with a minimum of water or without and, in this plastic condition, is flattened on water, but at a higher temperature, as in toasting, flaking rolls. Thus, porridge made from rolled a non-enzymic browning (Maillard) reaction oats requires only a brief domestic cooking time between protein and reducing carbohydrate may to complete the process of starch gelatinization. occur, and there may be some dextrinization of The amount of domestic cooking required by 244

BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING rolled oats is dependent to a large extent on the in approximately equal proportions:(a)ordinary processes of cutting, steaming and flaking, which rolled oats made from small particle-size pinhead are interrelated. The size of the pinhead oatmeal oatmeal, and(b) very thin fakes of a roller-driee influences rate of moisture penetration in the batter of oatfour and water, similar to products steamer; smaller particles will be more thoroughly of this nature used for infant feeding. When this moistened than large particles by the steaming porridge mix is stirred with hot water, the thin process, and hence the starch will be gelatinized flakes form a smooth paste while the rolled oats to a greater degree, and the steamed pinhead meal which do not completely disperse, provide a will be softer. For a given roller pressure at the chewy constituent and give body to the porridge faking stage, this increase in softness will result The preparation of an instant reconstitutable in thinner flakes being obtained from smaller- oatflake is disclosed in U.S. Pat. No 4, 874, 624 sized particles of pinhead meal. During the The product is made by conditioning normal domestic cooking of porridge, the thinner fakes oatcakes with water to 18.5% m.c., extrusion ill cook more rapidly than thicker flakes because cooking them(cf. p. 246)at high pressure for moisture penetration is more rapid 10-120 sec to an exit temperature of 95C, cutting Thin flakes would normally be more fragile the extrudate into pellets, and faking the pellets than thick ones, and more likely to break during on rolls, and then drying the fakes to 2-12%mc. transit. However, thin fakes can be strengthened The flakes so processed may be blended (70: 30 by raising the moisture content of the pinhead with normal oatcakes which have been steamed meal feeding the steamer, thereby increasing the to inactivate enzymes degree of gelatinization of the starch. Gelatinized tarch has an adhesive quality, and quite thin fakes rolled from highly gelatinized small particle- Specification for oatmeal and oatflakes size pinhead meal can be surprisingly strong Quality tests for milled oat products include The average thickness of commercial rolled determination of moisture, crude fibre and free oats is generally 0. 30-0. 38 mm(0.012-0015 in); fatty acid(FFA)contents, and of lipase activity hen tested with Congo Red stain(which colours A recommended specification is a maximum of only the gelatinized and damaged starch granules; 5% acidity(cf p. 164)due to FFA(calculated as cf. p. 185), about 30% of the starch granules in oleic acid, and expressed as a percentage of the fat rolled oats appear to be gelatinized and a nil response for lipase activity. Other sug gested tests are for arsenic(which could be derived Ready-cooked porridge from the fuel used in the kiln), lead and copper which catalyze oxidation of the fat(Anon, 1970) In the search for porridge-like products which Raw oats normally contain an active lipase require even less cooking than rolled oats, a enzyme, and, with the fat content of oats being product called Porridge without the pot has been some 2-5 times as high as that of wheat, it is desir made. Porridge can be made from this material able that the lipase should be inactivated during merely by stirring with hot or boiling water in the processing of oats, to prevent it catalyzing the the bowl: it consists of oatflakes of a special type. hydrolysis of the fat, which would lead to the As compared with ordinary rolled oats, these production of bitter-tasting free fatty acids. Lipase fakes are thinner, stronger, and contain starch is inactivated by the stabilization process,as which is more completely gelatinized. They could described above in Ch. 6, a most important be manufactured by steaming the pinhead oat- safeguard in keeping the quality of the oatmeal meal at a somewhat higher moisture content than normal, rolling at a greater pressure than normal and using heated flaking roll Porridge from other cereals Another type of porridge mix, known as Ready- In Africa, maize grits or hominy grits are used brek, consists of a blend of two types of flakes to make porridge by boiling with water. In Italy

BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 245 rolled oats is dependent to a large extent on the in approximately equal proportions: (a) ordinary processes of cutting, steaming and flaking, which rolled oats made from small particle-size pinhead are interrelated. The size of the pinhead oatmeal oatmeal, and (b) very thin flakes of a roller-dried influences rate of moisture penetration in the batter of oatflour and water, similar to products steamer; smaller particles will be more thoroughly of this nature used for infant feeding. When this moistened than large particles by the steaming porridge mix is stirred with hot water, the thin process, and hence the starch will be gelatinized flakes form a smooth paste while the rolled oats, to a greater degree, and the steamed pinhead meal which do not completely disperse, provide a will be softer. For a given roller pressure at the chewy constituent and give body to the porridge. flaking stage, this increase in softness will result The preparation of an instant reconstitutable in thinner flakes being obtained from smaller- oatflake is disclosed in U.S. Pat. No. 4,874,624. sized particles of pinhead meal. During the The product is made by conditioning normal domestic cooking of porridge, the thinner flakes oatflakes with water to 18.5% m.c., extrusion will cook more rapidly than thicker flakes because cooking them (cf. p. 246) at high pressure for moisture penetration is more rapid. 10-120 sec to an exit temperature of 95"C, cutting Thin flakes would normally be more fragile the extrudate into pellets, and flaking the pellets than thick ones, and more likely to break during on rolls, and then drying the flakes to 2-12% m.c. transit. However, thin flakes can be strengthened The flakes so processed may be blended (70:30) by raising the moisture content of the pinhead with normal oatflakes which have been steamed meal feeding the steamer, thereby increasing the to inactivate enzymes. degree of gelatinization of the starch. Gelatinized starch has an adhesive quality, and quite thin Specification for oatmeal and oatflakes flakes rolled from highly gelatinized small particle￾size pinhead meal can be surprisingly strong. Quality tests for milled oat products include The average thickness of commercial rolled determination of moisture, crude fibre and free oats is generally 0.30-0.38 mm (0.012-0.015 in); fatty acid (FFA) contents, and of lipase activity. when tested with Congo Red stain (which colours A recommended specification is a maximum of only the gelatinized and damaged starch granules; 5% acidity (cf. p. 164) due to FFA (calculated as cf. p. 185), about 30% of the starch granules in oleic acid, and expressed as a percentage of the fat) rolled oats appear to be gelatinized. and a nil response for lipase activity. Other sug￾gested tests are for arsenic (which could be derived from the fuel used in the kiln), lead and copper which catalyze oxidation of the fat (Anon., 1970). Ready-cooked porridge In the search for porridge-like products which Raw oats normally contain an active lipase require even less cooking than rolled oats, a enzyme, and, with the fat content of oats being product called 'Porridge without the pot' has been some 2-5 times as high as that of wheat, it is desir￾made. Porridge can be made from this material able that the lipase should be inactivated during merely by stirring with hot or boiling water in the processing of oats, to prevent it catalyzing the the bowl: it consists of oatflakes of a special type. hydrolysis of the fat, which would lead to the As compared with ordinary rolled oats, these production of bitter-tasting free fatty acids. Lipase flakes are thinner, stronger, and contain starch is inactivated by the stabilization process, as which is more completely gelatinized. They could described above in Ch. 6, a most important be manufactured by steaming the pinhead oat- safeguard in keeping the quality of the oatmeal. meal at a somewhat higher moisture content than Porridge from other cereals normal, rolling at a greater pressure than normal, and using heated flaking rolls. Another type of porridge mix, known as Ready- In Africa, maize grits or hominy grits are used brek, consists of a blend of two types of flakes to make porridge, by boiling with water. In Italy

TECHNOLOGY OF CEREALS maize porridge, made from fine maize grits parse maize meal, and flavoured with cheese, is Transition point Water Flour called"polenta Barley meal is used for making a type of porridge in many countries in the Far East, the Middle East, and North Africa(cf p. 13) Wholemeal four made from sorghum or millet m may be cooked with water to make a porridge like food in African countries and in India Porridge made from parched millet grain in the pressure z/or Soviet union is called Kasha IG. 11. 1 Diagram of an extrusion cooker, showing its components and zones.(Reproduced from Guy, 1989, by Ready-to-eat cereals courtesy of the A.A. C.C. While porridge-type cereals have been con- sumed for many years, the development of ready- injected, and the system was a batch process. The to-eat cereals is relatively recent. Ready-to-eat batch cooking process has now been largely cereals owe their origin to the Seventh Day superseded by continuous cooking processes in Adventist Church, whose members, preferring which cooking and extrusion through a die are an entirely vegetable diet, experimented with the both carried out in a single piece of equipment processing of cereals in the mid-nineteenth century. -a cooking extruder or extrusion-cooker(see Fig a granulated product, "Granula, made by J C. 11.1). Extrusion-cooking is a high-temperature Jackson in 1863, may have been the first commer- short-time(hTST) process in which the material cially available ready-to-eat breakfast cereal. a is plasticized at a relatively high temperature similar product, Granola,, was made by J. H. pressure and shear before extrusion through a die Kellogg by grinding biscuits made from wheat- into an atmosphere of ambient temperature and meal, oatmeal and maizemeal. Mass acceptance of pressure(Linko, 1989a) ready-to-eat cereals was achieved in countries such as the U.S.A. by means of efficient advertizing Continuous cooking Doking methods have man advantages over batch methods: for example The stages in the processing of ready-to-eat continuous methods require less floor space and cereals would include the preparation of the cereal less energy in operation; they permit better con by cleaning, and possibly pearling, cutting or trol of processing conditions, leading to improved grinding; the addition of adjuncts such as salt, quality of the products. moreover, batch cooking malt, sweeteners and flavouring materials; mixing methods were usually restricted to the use of with sufficient water to give a paste or dough whole grain or to relatively large grain fragments of the required moisture content; cooking the whereas extrusion cooking can also utilize much mixture; cooling and partially drying, and shaping finer materials, including flor the material by, e.g. rolling, puffing, shredding An extrusion cooker is a continuous processing Batch cooking unit based on a sophisticated screw system rotating within the confines of a barrel. Raw materials are Until recently, rotating vessels

246 TECHNOLOGY OF CEREALS maize porridge, made from fine maize grits or called ‘polenta’. Barley meal is used for making a type of porridge in many countries in the Far East, the Middle East, and North Africa (cf. p. 13). Wholemeal flour made from sorghum or millet may be cooked with water to make a porridge- G F E D c B like food in African countries and in India. Porridge made from parched millet grain in the Soviet Union is called Kushu. coarse maize meal, and flavoured with cheese, is Tronsition point Woter Flour I pll?re lzl Conveying zone 1 zOne I Conveying zone Al Mixing zone FIG. 11.1 Diagram of an extrusion cooker, showing its components and zones. (Reproduced from Guy, 1989, by Ready-to-eat cereals courtesy of the A.A.C.C.) While porridge-type cereals have been con￾sumed for many years, the development of ready￾to-eat cereals is relatively recent. Ready-to-eat cereals owe their origin to the Seventh Day Adventist Church, whose members, preferring an entirely vegetable diet, experimented with the processing of cereals in the mid-nineteenth century. A granulated product, ‘Granula’, made by J. C. Jackson in 1863, may have been the first comer￾cially available ready-to-eat breakfast cereal. A similar product, ‘Granola’, was made by J. H. Kellogg by grinding biscuits made from wheat￾meal, oatmeal and maizemeal. Mass acceptance of ready-to-eat cereals was achieved in countries such as the U.S.A. by means of efficient advertizing. Processing The stages in the processing of ready-to-eat cereals would include the preparation of the cereal by cleaning, and possibly pearling , cutting or grinding; the addition of adjuncts such as salt, malt, sweeteners and flavouring materials; mixing with sufficient water to give a paste or dough of the required moisture content; cooking the mixture; cooling and partially drying, and shaping the material by, e.g. rolling, puffing, shredding, into the desired form, followed by toasting, which also dries the material to a safe m.c. for packaging. Batch cooking Until recently, the cooking was carried out in rotating vessels, ‘cookers’, into which steam was injected, and the system was a batch process. The batch cooking process has now been largely superseded by continuous cooking processes in which cooking and extrusion through a die are both carried out in a single piece of equipment - a cooking extruder or extrusion-cooker (see Fig. 11.1). Extrusion-cooking is a high-temperature, short-time (HTST) process in which the material is plasticized at a relatively high temperature, pressure and shear before extrusion through a die into an atmosphere of ambient temperature and pressure (Linko, 1989a). Continuous cooking Continuous cooking methods have many advantages over batch methods: for example, continuous methods require less floor space and less energy in operation; they permit better con￾trol of processing conditions, leading to improved quality of the products. Moreover, batch cooking methods were usually restricted to the use of whole grain or to relatively large grain fragments, whereas extrusion cooking can also utilize much finer materials, including flour. Extrusion cookers An extrusion cooker is a continuous processing unit based on a sophisticated screw system rotating within the confines of a barrel. Raw materials are transported into a cooking zone where they are compressed and sheared at elevated temperatures

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 a

BREAKFAST 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

counter-current of air at about 65C under con- slightly narrower than the width of the grain trolled humidity conditions, to ensure uniform Without fragmenting the grain, bumping disrupts drying, to a moisture content of about 20%o, a the bran coat, assisting the penetration of wate process taking 2.5-3 h, and are then cooled and Flavouring adjuncts-sugar, malt syrup, salt rested to allow equilibration of moisture. The are then added and the grain is pressure cooked resting period was formerly about 24 h, but at about 15 psi for 30-35 min. The cooked wheat, is considerably less under controlled humidity at 28-30%m c, emerges in big lumps which have drying conditions. The dried grits are then faked to be'delumped, and then dried from about 30% on counter-rotating rollers, which have a surface m.c. to 16-18%mc. After cooling to about 43.C, temperature of 4346C, at a pressure of 40 t at the grain is binned to temper for a short time the point of contact, and the flakes thus formed and faked (as for maize). Just before faking, the are toasted in tunnel or travelling ovens at 300c grain is heated to about 88c to plasticize the for about 50 sec. The desirable blistering of the kernels and prevent tearing on the faking rolls surface of the flakes is related to the roller The fakes leave the rolls at about 15-18%mc surface temperature and to the moisture content and are then toasted and dried to 3% m of the grits, which should be 10-14% m c. when rolled. After cooling, the fakes may be sprayed with solutions of vitamins and minerals before Rice flakes packaging To make rice flakes using the traditional pr the preferred starting material is head rice(whole Extruded升akes de-husked grains) or 2nd heads (large broken kernels). Flavouring adjuncts are similar to those These, made from maize or wheat, are cooked used with maize and wheat. The blend of rice in an extrusion cooker, rather than in a batch plus adjuncts is pressure-cooked at 15-18 psi for pressure cooker, and can be made from fine meal about 60 min. The moisture content of the cooked or flour rather than from coarse grits. The dry material should not exceed 28% m.c., otherwise material is fed continuously into the extrusion it becomes sticky and difficult to handle. De cooker,and is joined by a liquid solution of the lumping, drying (to about 17% m.c. at lower flavouring materials- sugar, malt, salt, etc. moisture contents the particles shear; at higher These are mixed together by the rotation of the moisture contents the flaking rolls become screw and conveyed through the heated barrel, gummed up), cooling, tempering(up to 8 h)and thereby becoming cooked faking are as for wheat flakes The material is extruded through the die in the In the toasting of the rice fakes, more heat is form of ribbons which are cut to pellet size by a required than for making wheat flakes. The rotating knife The pellets are then dried, tem- moisture content of the feed and the heat of the pered, faked and toasted as described for the oven are adjusted so that the fakes blister and traditional method (Fast, 1987; Fast and Caldwell, puff during toasting; accordingly, the discharge 1990; Hoseney, 1986; Midden, 1989; Rooney and end of the oven is hotter than the feed end Serna-Saldivar, 1987 The moisture content of the final product is 1-3% Flaked products from wheat and rice o The process for making rice flakes by extrusion sembles that described for maize and wheat, Wheat flakes except that a colouring material is added to offset the dull or grey appearance caused by mechanical These are traditionally made from whole wheat working during extrusion. The lack of natural grain, which is conditioned with water to about colour is emphasized if the formulation is low in 21%mc and then'bumped by passing through sugar or malt syrup as sources of reducing sugars a pair of smooth rollers set so that the roll gap is that could participate in a maillard reaction

248 TECHNOLOGY OF CEREALS counter-current of air at about 65°C under con- slightly narrower than the width of the grain. trolled humidity conditions, to ensure uniform Without fragmenting the grain, bumping disrupts drying, to a moisture content of about 20%, a the bran coat, assisting the penetration of water. process taking 2.5-3 h, and are then cooled and Flavouring adjuncts - sugar, malt syrup, salt - rested to allow equilibration of moisture. The are then added and the grain is pressure cooked resting period was formerly about 24 h, but at about 15 psi for 30-35 min. The cooked wheat, is considerably less under controlled humidity at 28-30% m.c., emerges in big lumps which have drying conditions. The dried grits are then flaked to be ‘delumped’, and then dried from about 30% on counter-rotating rollers, which have a surface m.c. to 16-18% m.c. After cooling to about 43”C, temperature of 43”-46”C, at a pressure of 40 t at the grain is binned to temper for a short time, the point of contact, and the flakes thus formed and flaked (as for maize). Just before flaking, the are toasted in tunnel or travelling ovens at 300°C grain is heated to about 88°C to plasticize the for about 50 sec. The desirable blistering of the kernels and prevent tearing on the flaking rolls. surface of the flakes is related to the roller The flakes leave the rolls at about 15-18% m.c. surface temperature and to the moisture content and are then toasted and dried to 3% m.c. of the grits, which should be 10-14% m.c. when Rice flakes rolled. After cooling, the flakes may be sprayed with solutions of vitamins and minerals before packaging. To make rice flakes using the traditional process, the preferred starting material is head rice (whole de-husked grains) or 2nd heads (large broken kernels). Flavouring adjuncts are similar to those Extruded flakes These, made from maize or wheat, are cooked used with maize and wheat. The blend of rice in an extrusion cooker, rather than in a batch plus adjuncts is pressure-cooked at 15-18 psi for pressure cooker, and can be made from fine meal about 60 min. The moisture content of the cooked or flour rather than from coarse grits. The dry material should not exceed 28% m.c., otherwise material is fed continuously into the extrusion it becomes sticky and difficult to handle. De￾cooker, and is joined by a liquid solution of the lumping, drying (to about 17% m.c.: at lower flavouring materials - sugar, malt, salt, etc. moisture contents the particles shear; at higher These are mixed together by the rotation of the moisture contents the flaking rolls become screw and conveyed through the heated barrel, gummed up), cooling, tempering (up to 8 h) and thereby becoming cooked. flaking are as for wheat flakes. The material is extruded through the die in the In the toasting of the rice flakes, more heat is form of ribbons which are cut to pellet size by a required than for making wheat flakes. The rotating knife. The pellets are then dried, tem- moisture content of the feed and the heat of the pered, flaked and toasted as described for the oven are adjusted so that the flakes blister and traditional method (Fast, 1987; Fast and Caldwell, puff during toasting; accordingly, the discharge 1990; Hoseney, 1986; Midden, 1989; Rooney and end of the oven is hotter than the feed end. Serna-Saldivar, 1987). The moisture content of the final product is 1-3% m.c. The process for making rice flakes by extrusion resembles that described for maize and wheat, Flaked products from wheat and rice except that a colouring material is added to offset the dull or grey appearance caused by mechanical Wheat flakes These are traditionally made from whole wheat working during extrusion. The lack of natural grain, which is conditioned with water to about colour is emphasized if the formulation is low in 21% m.c. and then ‘bumped’ by passing through sugar or malt syrup as sources of reducing sugars a pair of smooth rollers set so that the roll gap is that could participate in a Maillard reaction