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 ‘readyto-eat’ cereals. Besides the distinction regarding the need for domestic cooking as against readiness for consumption, breakfast cereals can also be classified according to the form of the product, and according 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 consumption 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. Consequently , 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 manugelatinizes 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 particlesize 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 suggested 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 desirmade. 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 consumed for many years, the development of readyto-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 comercially available ready-to-eat breakfast cereal. A similar product, ‘Granola’, was made by J. H. Kellogg by grinding biscuits made from wheatmeal, 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 control 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 chanThe equipment may consist of single- or twin- nel of the screw, giving a fairly broad residencescrews with spirally-arranged flights for conveying, time distribution. and special kneading and reversing elements for All twin-screw extruders have a positive pumpcreating 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 residencenecessary 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, compresthe dissipation of mechanical energy from the sion and shearing action of the screws. Coscrew caused by frictional and viscous effects, by rotating twin-screw extruders, which have selfthe 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 prebarrel 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. Howpitch, flight angles, etc. and, in the twin-screw, ever, control of the process is simpler in the twinthe 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 backtwin-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. Decooker, 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
BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING Puffed products with the enrichment, and bumped more heavily than for Krispies, then oven-puffed and toasted Cereals may be puffed in either of two ways: The high protein enrichment may be vital wheat by sudden application of heat at atmospheric gluten, defatted wheat germ, non-fat dry milk pressure so that the water in the cereal is vapor- or dried yeast, plus vitamins, minerals and anti ized in situ, thereby expanding the product(oven buffing); or by the sudden transference of the oxidants ( uliano and Sakurai, 1985) cereal containing superheated steam from a high pressure to a low pressure, thereby allowing the Gun-puffed rice water suddenly to vaporize and cause expansion Long-grain white rice or parboiled medium gun puffing). The key to the degree of puffing grain rice, Is generally used for gun-puffing of the cooked grain is the suddenness of change although short-grain, low-amylose (waxy )rice in temperature or pressure(Hoseney, 1986) is used for gun-puffing in the U.S.A. and par- wheat,oats or pearl barley, which are prepared boiled waxy rice may be used in the Philippines by cleaning, conditioning and depericarping(e.g. colour and tends to undergo oxidative rancid by a wet scouring proces etc.) are added as for faster than puffed raw rice, but parboiled rice faked products requires less treatment, viz. lower steam pressure and temperature, than raw rice uliano and Sak 1985) Oven-puffed rice a batch of the prepared grain is preheated to 5210-638C and fed to the puffing gun, a pressure Oven-puffed rice is made from raw or parboiled chamber with an internal volume of 0.5-1.0 ft' milled rice which is cooked, with the adjuncts, which is heated externally and by injection of for 1 h at 15-18 lb/inin a rotary cooker until superheated steam, so that the internal pressure uniformly translucent. It is dried to 30% m.c. rapidly builds up to about 200 lb/in(1 379 MN/ tempered for 24 h, dried again, this time to 20% m")at temperatures up to 242C, and the starch m.c., and subjected to radiant heat to plasticize in the material becomes gelatinized The pressure the outside of the grain. The grain is ' bumped' is suddenly released by opening the chamber of through smooth rolls, just sufficiently to fatten the puffing gun. The material is shot'out, and compress it, and then surface dried to about expansion of water vapour on release of the 15%mc, and tempered again for 12-15 h at pressure blowing up the grains or pellets to room temp The bumped rice then passes to the several times their original size. The puffed toasting oven, where it remains for 30-90 sec. product is dried to 3%mc. by toasting, then The temperature in the oven is about 300C in cooled and packaged (Fast, 1987; Fast and the latter half of the oven-cycle - as hot as Caldwell, 1990; Juliano and Sakurai, 1985) ossible short of scorching the grains. due to the For satisfactory puffing, the starch should have bumping, which has compressed the grains, and plastic flow characteristics under pressure, and the high temperature, the grains immediately hence the temperature should be high enoug puff to 5-6 times their original size. The puffed Moreover, the material at the moment before grains are cooled, fortified with vitamins and expansion requires cohesion to prevent shattering minerals, if required, and treated with anti- and elasticity to permit expansion. The balance oxidants(Hoseney, 1986; Juliano and Sakurai, between these two characteristics can be varied 1985). Kellog's Rice Krispies'is a well-known by adding starch, which has cohesive properties. brand of oven-puffed rice Kellogg's "Special K, Extruded gun-puffed cereals can be made from ontaining 20% of protein, is made in a similar oat flour or maize meal, with which tapioca or way to Rice Krispies up to the drying before rye four can be blended. This material is fed pumping The material is then wetted, coated into the cooking extruder and a solution of the
BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 249 with the enrichment, and bumped more heavily than for Krispies, then oven-puffed and toasted. gluten, defatted wheat germ, non-fat dry milk, or dried yeast, plus vitamins, minerals and antioxidants (Juliana and Sakurai, 1985). Gun-puffed rice Puffed products by sudden application of heat at atmospheric pressure so that the water in the cereal is vaporized in situ, thereby expanding the product (oven puffing); or by the sudden transference of the cereal containing superheated steam from a high pressure to a low pressure, thereby allowing the water suddenly to vaporize and cause expansion Cerea1s may be puffed in either Of tWo ways: The high protein enrichment may be vital wheat Long-grain white rice or parboiled medium- (gun puffing)* The key to the degree Of puffing Of the cooked grain is the suddenness Of change grain rice is generally used for gun-puffing, although short-grain, low-amylose (Gwaxy7) rice is used for gun-puffing in the U.S.A., and parPuffed parboiled rice has a darker, less acceptable colour and tends to undergo oxidative rancidity faster than puffed raw rice, but parboiled rice requires less treatment, viz. lower steam pressure and temperature, than raw rice (Juliano and Sakurai, 1985). A batch of the prepared grain is preheated to 521”-638”C and fed to the puffing gun, a pressure 0 ven -p u ffed rice Oven-puffed rice is made from raw or parboiled chamber with an internal volume of 0.5-1 .O ft3, milled rice which is cooked, with the adjuncts, which is heated externally and by injection of for 1 h at 15-18 lblin2 in a rotary cooker until superheated steam, so that the internal pressure uniformly translucent. It is dried to 30% m.c., rapidly builds up to about 200 lb/in2 (1.379 MN1 tempered for 24 h, dried again, this time to 20% m2) at temperatures up to 242”C, and the starch m.c., and subjected to radiant heat to plasticize in the material becomes gelatinized. The pressure the outside of the grain. The grain is ‘bumped’ is suddenly released by opening the chamber of through smooth rolls, just sufficiently to flatten the puffing gun. The material is ‘shot’ out, and compress it, and then surface dried to about expansion of water vapour on release of the 15% m.c., and tempered again for 12-15 h at pressure blowing up the grains or pellets to room temp. The bumped rice then passes to the several times their original size. The puffed toasting oven, where it remains for 30-90 sec. product is dried to 3% m.c. by toasting, then The temperature in the oven is about 300°C in cooled and packaged (Fast, 1987; Fast and the latter half of the oven-cycle - as hot as Caldwell, 1990; Juliano and Sakurai, 1985). possible short of scorching the grains. Due to the For satisfactory puffing, the starch should have bumping, which has compressed the grains, and plastic flow characteristics under pressure, and the high temperature, the grains immediately hence the temperature should be high enough. puff to 5-6 times their original size. The puffed Moreover, the material at the moment before grains are cooled, fortified with vitamins and expansion requires cohesion to prevent shattering minerals, if required, and treated with anti- and elasticity to permit expansion. The balance oxidants (Hoseney, 1986; Juliano and Sakurai, between these two characteristics can be varied 1985). Kellog’s ‘Rice Krispies’ is a well-known by adding starch, which has cohesive properties. brand of oven-puffed rice. Kellogg’s ‘Special K’ , Extruded gun-puffed cereals can be made from containing 20% of protein, is made in a similar oat flour or maize meal, with which tapioca or way to Rice Krispies up to the drying before rye flour can be blended. This material is fed bumping. The material is then wetted, coated into the cooking extruder and a solution of the in temperature or pressure (Hoseney, 1986). wheat, oats or pearl barley, which are prepared by cleaning, conditioning and depericarping (e.g. by a wet scouring process). Flavouring adjuncts (sugar, malt syrup, salt, etc.) are added as for flaked products. The preferred grains for puffing are riceY boiled waxy rice may be used in the Philippines
TECHNOLOGY OF CEREALS djuncts-sugar, malt syrup, salt -is added tion. During this time, the kernels firm up with more water. The dough is cooked in the because of retrogradation of the starch: this cooking extruder and is transferred to a forming firming is essential for obtaining shreds of adequate extruder in which a non-cooking temperature strength. The conditioned grain is fed into shred- below 71oC-is maintained. The extruded collets ders consisting of a pair of metal rolls- one is are dried from 20-24%m c. to 9-12% m c and smooth, the other has grooves between which the then gun-puffed at 260-427oC and 100-200 lb/ material emerges as long parallel shreds. the in pressure as previously described(Fast and shreds are detached from the grooves by the teeth Caldwell, 1990; Rooney and Se Saldivar, 1987).a of a comb and fall onto a slowly travelling band 10-16-fold expansion results a thick mat being built up by the superimposition of several layers. The mat is cut into tablets Puffed wheat by a cutter which has dull cutting edges: the squeezing action of the cutter compresses the a plate wheat called Tagenrog is the type of shreds and makes them adhere to one another wheat preferred for puffing on account of its The tablets are baked at 260oC in a gas-heated puffs, but durum or CWRS wheat may alo ge revolving oven or a conveyor-belt oven, taking large grain size which gives high yields of lar e about 20 min The major heat input is at the feed y The wheat is pretreated with about 4% of a is lost in the middle section, while colour is end; the biscuits increase in height as moisture saturated brine solution (26% salt content) to developed in the final section. The moisture toughen the bran during preheating and make it content of the biscuits is about 45% entering the cohesive, so that the subsequent puffing action oven, about 4% leaving the oven. The biscuits blows the bran away from the grain, thereby may be further dried to 1%m c, passed through improving its appearance Alternatively, the bran a metal detector, and then packaged(Fast, 1987; can be partly removed by pearling on carborun- Fast and Caldwell, 1990) dum stones. The puffing process is similar to that Shredded products may also be made from the described for rice(Fast and Caldwell, 1990) four of wheat, maize, rice or oats which would be cooked in batches or by continuous extrusion Continuous cooking Flavouring and nutritional adjuncts may be added After cooking, cooling and equilibra Using a steam- pressurized puffing chamber, tion for 4-24 h, the material is shredded and the prepared grain is admitted through valves and baked as described above. When using maize or subsequently released through an exit pore with- rice to make a shredded product, however, it is out loss of pressure in the chamber(U.S. Pat. desirable to produce a degree of puffing to avoid No.3,971,303) hardness. This is achieved by using a lower emperature in the first part of the baking Shredded products followed by an extremely high temperature in the Wheat is the cereal generally used, a white, starchy type, such as Australian, being preferred The whole grain is cleaned and then cooked in Granular products min until the centre of the kernel changes from A yeasted dough is made from a fine wholemeal starchy white to translucent grey, and the grain or long extraction wheaten four and malted is soft and rubbery. The moisture content is 45- barley flour, with added salt. The dough is 50%, and the starch is fully gelatinized. The fermented for about 6 h and from it large loaves cooked grain is cooled to room temperature and are baked. These are broken up, dried and rested for up to 24 h to allow moisture equilibra- ground to a standard degree of fineness
2 50 TECHNOLOGY OF CEREALS adjuncts - sugar, malt syrup, salt - is added with more water. The dough is cooked in the cooking extruder and is transferred to a forming extruder in which a non-cooking temperature - below 71°C - is maintained. The extruded collets are dried from 20-24% m.c. to 9-12% m.c. and then gun-puffed at 26Oo-427"C and 100-200 Ib/ in2 pressure as previously described (Fast and Caldwell, 1990; Rooney and Se Saldivar, 1987). A 10-16-fold expansion results. Puffed wheat A plate wheat called Tagenrog is the type of wheat preferred for puffing on account of its large grain size which gives high yields of large puffs, but durum or CWRS wheat may also be used. The wheat is pretreated with about 4% of a saturated brine solution (26% salt content) to toughen the bran during preheating and make it cohesive, so that the subsequent puffing action blows the bran away from the grain, thereby improving its appearance. Alternatively, the bran can be partly removed by pearling on carborundum stones. The puffing process is similar to that described for rice (Fast and Caldwell, 1990). Continuous puffing Using a steam-pressurized puffing chamber, the prepared grain is admitted through valves and subsequently released through an exit pore without loss of pressure in the chamber (U.S. Pat. No. 3,971,303). Shredded products Wheat is the cereal generally used, a white, starchy type, such as Australian, being preferred. The whole grain is cleaned and then cooked in boiling water with injection of steam for 30-35 min until the centre of the kernel changes from starchy white to translucent grey, and the grain is soft and rubbery. The moisture content is 45- 50%, and the starch is fully gelatinized. The cooked grain is cooled to room temperature and rested for up to 24 h to allow moisture equilibration. During this time, the kernels firm up because of retrogradation of the starch: this firming is essential for obtaining shreds of adequate strength. The conditioned grain is fed into shredders consisting of a pair of metal rolls - one is smooth, the other has grooves between which the material emerges as long parallel shreds. The shreds are detached from the grooves by the teeth of a comb and fall onto a slowly travelling band, a thick mat being built up by the superimposition of several layers. The mat is cut into tablets by a cutter which has dull cutting edges: the squeezing action of the cutter compresses the shreds and makes them adhere to one another. The tablets are baked at 260°C in a gas-heated revolving oven or a conveyor-belt oven, taking about 20 min. The major heat input is at the feed end; the biscuits increase in height as moisture is lost in the middle section, while colour is developed in the final section. The moisture content of the biscuits is about 45% entering the oven, about 4% leaving the oven. The biscuits may be further dried to 1% m.c., passed through a metal detector, and then packaged (Fast, 1987; Fast and Caldwell, 1990). Shredded products may also be made from the flour of wheat, maize, rice or oats which would be cooked in batches or by continuous extrusion cooking. Flavouring and nutritional adjuncts may be added. After cooking, cooling and equilibration for 4-24 h, the material is shredded and baked as described above. When using maize or rice to make a shredded product, however, it is desirable to produce a degree of puffing to avoid hardness. This is achieved by using a lower temperature in the first part of the baking, followed by an extremely high temperature in the last part. Granularproducts A yeasted dough is made from a fine wholemeal or long extraction wheaten flour and malted barley flour, with added salt. The dough is fermented for about 6 h and from it large loaves are baked. These are broken up, dried and ground to a standard degree of fineness
BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING Sugar-coated products reaction is known to occur, for example, in the ked or puffed cereals, prepared as described. roller-drying of milk, and may be the explanation are sometimes coated with sugar or candy. The of the improved antioxidant activity of oat products process described in BP No. 754, 771 uses a inactivation- stabilization(cf sucrose syrup containing 1-8% of other sugars 165,244) Similarly, enzyme-inactivated, stabilized wheat (e. g. honey)to provide a hard transparent coating bran has a long shelf life: this material can be conditions. The sugar content of corn flakes was used for breakfast cereals, snack foods. extruded products which need a high fibr raised from 7 to 43%by the coating process, extended shelf-life( Cooper, 1988) that of puffed wheat from 2 to 51%. As an The addition of synthetic antioxidants, such as alternative to sugar, use of aspartame as a sweetener bhA or bhT, to the prepared breakfast cereal for breakfast cereals is described in U.s. Pat. N 4, 501, 759 and U.S. Pat. No. 4, 540, 587, while or to the packaging material as an impregnation, the use of a dipeptide sweetener is disclosed as practised in the U.S.A., is not at present in U.S. Pat. Nos. 4, 594, 252, 4, 608, 263 and permitted in the U. K.(cf. p. 178). In Japan 4,614,657 where addition of antioxidants is not permitted use is made of oxygen absorbers to restrict the onset of oxidative rancidity (Juliano, 1985) Keeping quality of breakfast cereals Another form of deterioration of breakfast cereals after processing and packaging is moisture The keeping quality of the prepared product uptake which causes loss of the distinctive crisp depends to a large extent on the content and texture moisture uptake is prevented by the use keeping quality of the oil contained in it. Thus, of the correct type and quality of moisture products made from cereals having a low oil vapour-proof packaging materials(Fast, 1987) content (wheat, barley, rice, maize grits: oil content 1. 5-2.0%)have an advantage in keeping quality over products made from oats(oil content: Oat bran 4-11%, average 7%). Whole maize has high oil a boost has been given to the use of oat bran content(4.4%), but most of the oil is contained in breakfast cereals following the discovery that in the germ which is removed in making grits this material has a hypocholesterolaemic effect in the human( de groot et al., 1963), that is, it causes The keeping quality of the oil depends on its a lowering of the concentration of plasma choles- degree of unsaturation, the presence or absence terol in the blood. As high blood cholesterol has of antioxidants and pro-oxidants, the time and been associated with the incidence of coronary temperature of the heat treatment, the moisture heart disease for each 1% fall in plasma content of the material when treated, and the cholesterol, coronary heart disease falls by 2% conditions of storage (Nestel, 1990)-a dietary factor that will reduce Severe heat treatment, as in toasting or puffing, blood cholesterol is to be welcomed may destroy antioxidants or induce formation of The content of soluble fibre is much higher in pro-oxidants, stability of the oil being progres- oat bran(10.5%)than in wheat bran(2.8%); this sively reduced as treatment temperature is raised, may be an important factor in the cholesterol treatment time lengthened or moisture content lowering activity of oat bran(not shown by wheat of the material at the time of treatment lowered bran), and it has been suggested that a hemi- On the other hand, momentary high-temperature cellulose, beta-D-glucan, which is the major con- treatment, as at the surface of a hot roll in the stituent of the soluble fibre, is the cholesterol- roller-drying of a batter produce new anti- lowering agent, acting by increasing the faecal oxidants by interaction of protein and sugar(non- excretion of cholesterol(Illman and Topping enzymic browning, or Maillard reaction); such a 1985; Oakenfull, 1988; Seibert, 1987)
BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 251 reaction is known to occur, for example, in the roller-drying of milk, and may be the explanation made from oats after steam treatment for lipase Similarly, enzyme-inactivated, stabilized wheat bran has a long shelf life: this material can be used for breakfast cereals, snack foods, extruded products which need a high fibre content and extended shelf-life (Cooper, 1988). The addition of synthetic antioxidants, such as BHA or BHT, to the prepared breakfast cereal, or to the packaging material as an impregnation, permitted in the U.K. (cf. p. 178). In Japan, where addition of antioxidants is not permitted, use is made of oxygen absorbers to restrict the onset of oxidative rancidity (Juliano, 1985). Another form of deterioration of breakfast cereals after processing and packaging is moisture uptake which causes loss of the distinctive crisp texture. Moisture uptake is prevented by the use of the correct type and quality of moisture vapour-proof packaging materials (Fast, 1987). Oat bran A boost has been given to the use of oat bran in breakfast cereals following the discovery that this material has a hypocholesterolaemic effect in the human (de Groot et al., 1963), that is, it causes a lowering of the concentration of plasma cholesterol in the blood. As high blood cholesterol has been associated with the incidence of coronary heart disease - for each 1% fall in plasma cholesterol, coronary heart disease falls by 2% (Nestel, 1990) - a dietary factor that will reduce blood cholesterol is to be welcomed. The content of soluble fibre is much higher in oat bran (10.5%) than in wheat bran (2.8%); this may be an important factor in the cholesterollowering activity of oat bran (not shown by wheat bran), and it has been suggested that a hemicellulose , beta-D-glucan, which is the major constituent of the soluble fibre, is the cholesterollowering agent, acting by increasing the faecal excretion of cholesterol (Illman and Topping, 1985; Oakenfull, 1988; Seibert, 1987). Sugar-coated products are sometimes coated with sugar or candy. The sucrose syrup containing 1-8% of other sugars (e.g. honey) to provide a hard transparent coating that does not become sticky even under humid conditions. The sugar content of corn flakes was raised from 7 to 43% by the coating process, that of puffed wheat from 2 to 51%. As an alternative to sugar, use of aspartame as a sweetener for breakfast cereals is described in U.S. Pat. No. the use of a dipeptide sweetener is disclosed in '.'e Pat* Nos* 4y594y252y 4y608y263 and 4,614,657. F1aked Or puffed ceredsY prepared as described, of he improved antioxidant activity of oat products process described in BP No* 754y771 uSeS a inactivation - stabilization (cf. pp. 165, 244). 4,501Y759 and u*s. Pat. No' 4,540,587, whi1e as practised in the U.S.A., is not at present Keeping quality of breakfast cereals The keeping quality of the prepared product depends to a large extent on the content and keeping quality of the oil contained in it. Thus, products made from cereals having a low oil content (wheat, barley, rice, maize grits: oil content 1.5-2.0%) have an advantage in keeping quality over products made from oats (oil content: 4-11%, average 7%). Whole maize has high oil content (4.4%), but most of the oil is contained in the germ which is removed in making grits (cf. p. 138). The keeping quality of the oil depends on its degree of unsaturation, the presence or absence of antioxidants and pro-oxidants, the time and temperature of the heat treatment, the moisture content of the material when treated, and the conditions of storage. Severe heat treatment, as in toasting or puffing, may destroy antioxidants or induce formation of pro-oxidants, stability of the oil being progressively reduced as treatment temperature is raised, treatment time lengthened, or moisture content of the material at the time of treatment lowered. On the other hand, momentary high-temperature treatment, as at the surface of a hot roll in the roller-drying of a batter, may produce new antioxidants by interaction of protein and sugar (nonenzymic browning, or Maillard reaction); such a
TECHNOLOGY OF CEREALS Oat bran is obtained by milling oat flakes that U.S. Pat. No. 4, 497, 840. It can also be incorpo- have been made from stabilized oat kernels rated into bread: addition of 10-15% of oat bran (groats) as described elsewhere(cf. p. 168). It to white wheat four yielded bread of satisfactory an be used as an ingredient in both hot and cold quality(Krishnan et al., 1987) breakfast cereals. A method for making a read Rice bran, and in particular the oil in rice bran to-eat cereal from cooked oat bran is disclosed in has also been shown to have a plasma cholesterol TABLE 11.1 Chemical Composition of Ready-to-eat Breakfast Foods(per 100 g as sold) Carbohydrates Food gy Starch Sugars Prote Fat Ash of data Grape Nuts 3.5 67.8 6.2910 Corn Flakes B0856454320790598 07 Frosties 000332215 969 3232254 0.651 Bran Buds 5225 Bran Flakes 1.5 ountry Store 4.5 21322 Puffed Wheat 1.251.6 77513 Oat Krunchies gar Puffs 3.5 71.8 B Weetabix 1427 11.232.72.2 12.915 853 Toasted Farmhouse bran 3.0 1293 10.2 633.0 20.0 Weetaflake 51.7 18.9 3.1 4N×6.31 7 Total 8 Enriched to this level 1l Data courtesy of kellond the Cop osition of Foods(4th edn; 3rd Supp )1988, reproduced with the permission of the Royal o McCance and Widdowson's 14 Data courtesy of Quaker Oats Ltd(19 i5 Data courtesy of Weetabix Ltd(1990)
252 TECHNOLOGY OF CEREALS Oat bran is obtained by milling oat flakes that U.S. Pat. No. 4,497,840. It can also be incorpohave been made from stabilized oat kernels rated into bread: addition of 1615% of oat bran (groats) as described elsewhere (cf. p. 168). It to white wheat flour yielded bread of satisfactory can be used as an ingredient in both hot and cold quality (Krishnan et al., 1987). breakfast cereals. A method for making a ready- Rice bran, and in particular the oil in rice bran, to-eat cereal from cooked oat bran is disclosed in has also been shown to have a plasma cholesterol TABLE 11.1 Chemical Composition of Ready-to-Eat Breakfast Foods (per 100 g as sold) Dietary Source Food Water Energy Starch Sugars Protein Fat Ash fibre of data Grape Nuts 3.5 1475 67.8 12.1 10.5 0.5 6.29 10 Kellogg's Corn Flakes 3 1550 75 8 83 0.7 3 15 11 Frosties 3 1600 48 40 53 0.5 2 0.65 11 Rice Krispies 3 1600 75 10 63 0.9 3.5 0.75 11 Ricicles 3 1600 50 39 43 0.6 2 0.45 11 Carbohydrates (€9 (kJ) (g) (g) (9) (g) (€9 (g) coco Pops 3 1600 48 39 53 0.9 2.5 15 11 All-Bran 3 1150 28 18 1 43 3.5 5 245 11 Bran Buds 3 1200 27 23 133 3 4.5 225 11 Bran Flakes 3 1350 45 18 123 2 4 135 11 Sultana Bran 7 1300 37 27 103 2 3.5 115 11 Fruit 'n Fibre 6 1500 43 22 93 5 2.5 75 11 Toppas 5 1450 52 21 io3 1.5 1 75 11 Country Store 8 1500 47 22 93 4.5 2.5 65 11 Start 3 1550 51 28 83 2 2.5 55 11 Smacks 3 1600 36 49 73 1.5 1 2.S5 11 Special K 3 1550 60 15 1S3 1 3 2.S5 11 Nabisco Quaker Oats Shredded Wheat 9 1525 75.3 2.7 10.3l 2.3 1.5 11.4 12 Puffed Wheat 2 1360 68. 86 1.37 13.1' 1.25 1.6 7.75 13 Oat Krunchies 2.5 1600 71.16 15.97 10.5' 7.3 4.5 nla 13 Sugar Puffs 2 1554 886 51 6.0 1.2 0.8 3.5 13 Corn Flakes 1565 71.8 9.1 8.33 1.3 n/a 9.3 14 Morning Bran 1180 48.1 14.0' 3.8 n/a 18.5 14 Weetabix 5.6 1427 60.6 5.2 11.23 2.7 2.2 12.9 15 Bran Fare 4.0 962 31.5 nil 17.13 4.8 5.3 37.5 15 Toasted Farmhouse Bran 3.0 1293 36.7 10.2 12.63 3.0 4.8 20.0 15 Ryvita Weetabix Weetaflake 3.7 1490 51.7 18.9 9.73 3.1 da 10.5 15 ' N x 5.7. 2 N x 5.95. 3 N x 6.25. 4 N x 6.31. 5 Non-starch polysaccharides, soluble plus insoluble. 6 As available monosaccharides. ' Total sugars as sucrose. * Enriched to this level. 9 Southgate method. lo McCance and Widdowson's Composition of Foods (4th edn; 3rd Supp.) 1988, reproduced with the permission of the Royal I' Data courtesy of Kellogg Co. of Great Britain Ltd (1993). l2 Data courtesy of Nabisco Ltd (1982). l3 Data courtesy of Quaker Oats Ltd (1990). l4 Data courtesy of The Ryvita Co Ltd (1990). l5 Data courtesy of Weetabix Ltd (1990). Society of Chemistry and the Controller of HMSO
BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 253 lowering effect. Rice bran was not so effective as of available lysine as maillard reaction products oat bran in lowering plasma total cholesterol, but (McAuley et al., 1987). However, lysine deficiency rice bran favourably altered the ratio of high is of less importance in ready-to-eat cereals than in density lipoprotein(HDL) to low density lipo- bread because the former are generally consumed protein(LDL), a sensitive lipid index of future with milk, which is a good source of lysine coronary heart disease(Nestel, 1990) Moreover, some ready-to-eat breakfast cereals Preliminary work indicates that the beta-glucan have a protein supplementation in the soluble fibre of a waxy, hull-less barley ultivar also has hypocholesterolaemic effects and the extracted beta-glucans from barley have Carbohydrates possible use as a fibre supplement in baked The principal carbohydrate in cereals is starch products(Klopfenstein et aL. 1987; Newman et the complete gelatinization of which is desirable al.,1989) in processed foods, such as ready-to-eat cereals Whereas ordinary cooking at atmospheric pressure Nutritive value of breakfast cereals requires the starch to have a moisture content of 35-40% to achieve complete gelatinization, th The nutritive value of breakfast cereals, as same occurs at feed moisture levels of less than compared with that of the raw materials from 20% in extrusion cooking at 110-135C(Asp and which they were made, depends very much on Bjorck, 1989; Linko 1989a). Extrusion cooking the processing treatment involved, remembering increases the depolymerization of both amylose and that all heat treatment processes cause some amylopectin by random chain splitting. The sus- modification or loss of nutrients. Thus, while ceptibility of starch to the action of alpha-amylase extrusion cooking may cause the loss of essential increased in the following sequence: steam cooking amino acids, it also inactivates protease inhibitors,(least), steam flaking, popping, extrusion cooking hereby increasing the nutritional value of the drum drying(most)(Asp and Bjorck, 1989) proteins. The chemical composition of some ready-to- eat breakfast cereals manufactured in the U. K. Calorific value s shown in Table ll The calorific value of most ready-to-eat ceres Shredded wheat, made from low protein, soft as eaten is higher than that of bread(975 kJ/100 g wheat has a protein content considerably lower 233 Cal/100 g), largely on account of the relatively than that of puffed wheat, which is made from lower moisture content of the former. Compared a high-protein hard wheat, such as durum at equal moisture contents, the difference in CWRS wheat calorific value is small. Fat and cholesterol con- tents may be lower than those of some other cereal Proteins and amino acids The processes involved in the manufacture of All cereal products are deficient in the amino ready-to-eat cereals cause partial hydrolysis of acid lysine, but the deficiency may be greater in phytic acid (cf p. 295); the degree of destruction ready-to-eat cereals than in bread because of the increases at high pressures: about 70% is destroyed changes that occur in the protein at the high in puffing, about 33% in flaking temperature treatment. The protein efficiencies of wheat-based breakfast cereals (relative to casein= 100), as determined by rat-growth trials, have been reported as: -15.3 for extrusion puffed Enzymes, which are proteins, are generally 1.8-16.3 for flaked-toasted; 2.8 for extrusion inactivated partially or completely during extru 69.9 for extruded, lightly roasted. sion cooking. Thus, peroxidase was completely The d ces were partly explained by the loss inactivated by extrusion cooking at 110%-149Cof
BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 253 of available lysine as Maillard reaction products (McAuley et al., 1987). However, lysine deficiency is of less importance in ready-to-eat cereals than in bread because the former are generally consumed with milk, which is a good source of lysine. Moreover, some ready-to-eat breakfast cereals have a protein supplementation. Carbohydrates The principal carbohydrate in cereals is starch, the complete gelatinization of which is desirable in processed foods, such as ready-to-eat cereals. Whereas ordinary cooking at atmospheric pressure requires the starch to have a moisture content of 3540% to achieve complete gelatinization, the same occurs at feed moisture levels of less than 20% in extrusion cooking at 1 1Oo-135"C (Asp and Bjorck, 1989; Linko 1989a). Extrusion cooking increases the depolymerization of both amylose and amylopectin by random chain splitting. The susceptibility of starch to the action of alpha-amylase increased in the following sequence: steam cooking (least), steam flaking, popping, extrusion cooking, drum drying (most) (Asp and Bjorck, 1989). calorific value The calorific value of most ready-to-eat cereals as eaten is higher than that of bread (975 kJ/100 g; 233 Ca1/100 g), largely on account of the relatively lower moisture content of the former. Compared at equal moisture contents, the difference in calorific value is small. Fat and cholesterol contents may be lower than those of some other cereal foods. The processes involved in the manufacture of ready-to-eat cereals cause partial hydrolysis of phytic acid (cf. p. 295); the degree of destruction increases at high pressures: about 70% is destroyed in puffing, about 33% in flaking. Enzymes Enzymes, which are proteins, are generally inactivated partially or completely during extrusion cooking. Thus, peroxidase was completely inactivated by extrusion cooking at 1 10"-149°C of lowering effect. Rice bran was not so effective as oat bran in lowering plasma total cholesterol, but rice bran favourably altered the ratio of high density lipoprotein (HDL) to low density lipoprotein (LDL), a sensitive lipid index of future coronary heart disease (Nestel, 1990). Preliminary work indicates that the beta-glucan in the soluble fibre of a waxy, hull-less barley cultivar also has hypocholesterolaemic effects, and the extracted beta-glucans from barley have possible use as a fibre supplement in baked products (Klopfenstein et al., 1987; Newman et al., 1989). Nutritive value of breakfast cereals The nutritive value of breakfast cereals, as compared with that of the raw materials from which they were made, depends very much on the processing treatment involved, remembering that all heat treatment processes cause some modification or loss of nutrients. Thus, while extrusion cooking may cause the loss of essential amino acids, it also inactivates protease inhibitors, thereby increasing the nutritional value of the proteins. The chemical composition of some ready-toeat breakfast cereals manufactured in the U.K. is shown in Table 11.1. Shredded wheat, made from low protein, soft wheat has a protein content considerably lower than that of puffed wheat, which is made from a high-protein hard wheat, such as durum or CWRS wheat. Proteins and amino acids All cereal products are deficient in the amino acid lysine, but the deficiency may be greater in ready-to-eat cereals than in bread because of the changes that occur in the protein at the high temperature treatment. The protein efficiencies of wheat-based breakfast cereals (relative to casein = loo), as determined by rat-growth trials, have been reported as: - 15.3 for extrusion puffed; 1.8-16.3 for flaked-toasted; 2.8 for extrusion toasted; and 69.9 for extruded, lightly roasted. The differences were partly explained by the loss
