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 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, 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 Al I pll?re lzl Conveying zone 1 zOne I Conveying zone Mixing zone FIG. 11.1 Diagram of an extrusion cooker, showing its components and zones. (Reproduced from Guy, 1989, by courtesy of the A.A.C.C.) Ready-to-eat cereals 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 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 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. 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 anti- oxidants (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 vapor- ized 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 par- Puffed 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. 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 carborun- dum 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 with- out 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 equilibra- tion. 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 shred- ders 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 equilibra- tion 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 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 choles- terol 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 cholesterol- lowering activity of oat bran (not shown by wheat bran), and it has been suggested that a hemi- cellulose , beta-D-glucan, which is the major con- stituent of the soluble fibre, is the cholesterol- lowering 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 progres- sively 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 anti- oxidants by interaction of protein and sugar (non- enzymic browning, or Maillard reaction); such a 252 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 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 3 1600 50 39 43 0.6 2 0.45 11 Ricicles 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 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 sus- ceptibility 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 con- tents 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 extru- sion 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 lipo- protein (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-to- eat 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 2 54 TECHNOLOGY OF CEREALS material with 20-35% m.c., although there was at a number of points which are chosen to avoid residual activity if cooked at lower m.c. Under subjecting the protein material to excessive heat relatively mild conditions of extrusion cooking treatment: by incorporation as a dry supplement some activity of alpha-amylase, lipase and protease at the extrusion stage, or by coating the product could be retained, and such residual activity could with a batter of wheat gluten. influence product keeping quality and shelf life Consumption of breakfast cereals (Linko, 1989a,b). Conversely, wheat flour with abnormally high alpha-amylase activity that would be unsuitable for conventional bread- The consumption of ready-to-eat breakfast making can be processed by extrusion cooking in cereals in the U.K. has shown a steady growth which, provided the conditions are correctly from quite a small figure before World War I1 to chosen, the enzyme is quickly and totally inactiv- about 4.2 kg/head/an. in 1972, increasing further ated, permitting the production, from such flour, to 5.0 kg/head/an. in 1978, and to 6.5 kg/head/ of flat bread, snacks, biscuits, etc. (Cheftel, an. in 1988. The consumption of oat products 1989). for hot cereals in the U.K. was 0.6 kg/head/an. in 1984, but increased to 0.9 kg/head/an. in 1988, possibly in response to the claim that oat bran has a blood-cholesterol lowering effect. Minerals and vitamins The content of some of the minerals and The total tonnage of packeted breakfast cereals vitamins in ready-to-eat breakfast cereals is shown marketed in the U.K. in 1988 was 383,758 t, of in Table 11.2. Information about other vitamins which 38% was wheat based, 29% maize based, is meagre. About 50% of the thiamin (vitamin B1) and the remainder based on other cereals or on is destroyed during the manufacture of shredded a mixture of cereals (Business Monitor, 1989). wheat and in extrusion cooking, while nearly In the U.S.A. in 1971 about 0.75 million tonnes 100% is destroyed during puffing and flaking. of breakfast cereal were produced, of which about These processes have little effect on riboflavin 35% was puffed, 35% flaked, 10% shredded and (vitamin B2), niacin, pyridoxin and folic acid. about 20% hot cereal. Between 1980/81 and Extrusion cooking caused a loss of 11-2 1% of 1988/89 the total quantity of hot cereal (excluding vitamin E, while in products enriched with wheat corn grits) sold in the U.S.A. increased from 0.16 germ, extrusion cooking caused losses of 50-66% to 0.20 million tonnes, most of the increase being of vitamin E (Asp and Bjorck, 1989). Many of accounted for by oat-based products, the propor- the ready-to-eat breakfast cereals manufactured tion of which increased from 71.6% in 1980181 in the U.K. are enriched with vitamins, as shown to 81.2% in 1988/89. in Table 11.2; some are enriched with iron, and The average consumption of breakfast cereals some with protein (viz. with the high protein (ready-to-eat plus hot) in the U.S.A. in 1971 was fraction of wheat and oat flour, defatted wheat about 3.4 kg/head/an. Of the cereals used, wheat, germ, soya flour, non-fat dry milk, casein or vital bran or farina comprised about 37%, oatmeal or wheat gluten) and with the vitamins B6 (pyridoxin), oat flour 30%, maize grits 22% and rice 11%. D3, C and E. Some ready-to-eat breakfast cereals However, by 1985, the consumption of ready-to- made in the U.S.A. are enriched also with vitamins eat cereals alone had increased to 4.1 kg/head/an. A and B12. (Anon., 1986), slightly lower than in the U.K., Incorporation of the vitamin supplements may with the consumption of maize-based ready-to- be accomplished in various ways: at the cooking eat cereals in the U.S.A. increasing from 2.72 stage, at extrusion, by surface spraying after to 3.63 kg/head/an. between 1970 and 1980. processing, or by incorporation in a sugar coating, Between 1974 and 1983 domestic consumption the method chosen depending on the relative of ready-to-eat cereals in the U.S.A. grew by stability of the individual vitamins. Incorporation about 2% per annum, and growth increased of the protein supplement may similarly be made further to 3.3% per annum between 1983 and - ?+ B 8 s b 3 4 e g $ 3? u U N;3 ZQ 28 bP K B g u 9 .- E ;$ LI f s BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING ----dd43343-d-d 2 22 ss 2222 M 99 255 2 dd-33343-3-4-4- 83 82 mo r\ u.!z mvl wwwww'9?w'9ww'9p' m zz a9 V V?zzzz??zzzzzz 2 2: kmw m G222222G2222G E In xx 2G c c*c++cccccccc c cc mt VI 000000000000~ - +c+ccc******* e cc .yzs\M & +c+ccccc***+* c e* ov~ m m~~bbmvlmvlbbvlm I.. 00 In c 22 2: 2 e- 2-d --4--434336dmom 8 00 33 mod- .gG e ccccc*****+++*+ .I E R mvlmvlm--m3mmdomm .3 fG c ****++ccccccc I..I..I..I..I..~~I..~I..I..~~ * 00 .4 za 2 33iiiiiiiiiim 2 .El 32 5 ccccccccccccccc m m +e c cc mmmmm 1 mm p'? 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Recent estimates (1987) suggest an even larger growth rate of 4-5%, attributed to an advertizing campaign (Fast, 1987). expansion of, and evaporation of water from, the plasticized mass on exiting from the die, when the pressure drops instantly from about 150 kg/ cm2 at 160°C, resulting in drying from about 18% m.c. to about 8% m.c. The extrudate emerges as a flat strip which is cut into slices, roasted in an Other extrusion-cooked products Extrusion-cooking is a high-temperature, short- oven, and cooled. The exactness of expansion in time (HTST) process in which material is trans- terms of height and weight is most important, ported, mixed and plasticized at relatively high as the slices must fit exactly into packages of temperature, pressure and shear before extrusion prescribed volume and weight. to atmospheric temperature and pressure through Pellets a die. Because the process is able to bring about gelatinization, solubilization and complex forma- tion of starches, polymerization of proteins, partial Pellets, or half-snacks (unexpanded half- or complete inactivation of enzymes (according to products), can be made in various shapes - the severity of the operating conditions), reduc- flat, tube, shell, ring, screw, wheel - and are tion of microbial load, and production of particular generally made in a single screw extrusion cooker, forms of texture, the process can be used to make, using a single die and cooling of the barrel, and from cereals, many food products, besides ready- are dried to 5-10% m.c. The pellets are later to-eat breakfast foods, such as pet foods, flat fried, during which they expand 6-8 fold, lose bread, snacks, croutons, soup bases, drink bases, water, pick up 15-25% of fat, and increase in weight biscuits, confectionery and breadings. Besides (Colonna et al., 1989; Meuser and Wiedmann, these end products, extrusion cooking is also used 1989). to make intermediate products for further proces- Modified starch sing, both for food and also for non-food use (Fichtali and van de Voort, 1989; Linko, 1989a). In 1987, about 3 million tonnes of products Extrusion cooking can be used to treat starch were made by extrusion cooking in the U.S.A. at a relatively low moisture content, e.g. about (Hauck and Huber, 1989). 40% m.c., to bring about derivatization, plastic- ization, and drying to make thin-cooking, pregela- tinized and substituted and chemically modified starches for use in food, and also for non-food Pet foods The largest product group manufactured by uses, e.g. in the paper and textile industries extrusion cooking is pet foods, both dry expanded (Meuser and Wiedmann, 1989). and semi-moist, to replace canned food and dog- Brewing adjunct biscuits (Harper, 1986, 1989). Extrusion cooked cereals have potential use as brewing adjuncts. The starch granule structure 'Flat bread' The extrusion cooker takes over the function is destroyed during extrusion cooking, so that the of the oven, producing expansion of dough pieces, starch is more easily hydrolyzed by enzymes in formation of structure, partial drying, and forma- the mashing process (Smith, 1989). tion of flavour and colour. Thus, the process is suitable for making bread, described as 'flat ,,igh dextrose-eguivalent syrups bread' (Kim, 1987). The flat breads produced by extrusion cooking are imitations of crisp breads These can be made by extrusion cooking (although the word 'bread' is really a misnomer). of starch with the addition of thermostable The typical crumb-pore structure results from alpha-amylase (Smith, 1989). BREAKFAST CEREALS AND OTHER PRODUCTS OF EXTRUSION COOKING 2 57 FICHTALI, J. and van de VOORT, F. R. (1989) Fundamental and practical aspects of twin screw extrusion. Cereal Fds Wld, 34 (1 1): 921-929. GUY, R. c. E. (1991) Structure and formation in snack foods. Extrusion communique, 4 (Jan.-March): 8-10. GUY, R. C. E. and HORNE, A. W. (1988) Cereals for extrusion cooking processes: a comparison of raw materials derived from wheat, maize and rice. 35th Technology Conference 1988. Biscuit, Cake, Chocolate and Confectionery Alliance, 4549, GUY, R. C. E. and HORNE, A. W. (1989) The effects of endosperm texture on the performance of wheat flours in extrusion cooking processes. Milling, 182 (Feb.): ix-xii. HARPER, J. M. (1986) Processing characteristics of food extruders. In: Food Engineering and Process Applications, Vol. 2, Unit operations, Le MAGUER, M. and JELEN, P. (Eds) Elsevier Appl. Sci. Publ., London. HARPER, J. M. (1989) Food extruders and their applications. In: Extrusion Cooking, MERCIER, C., LINKO, P. and HARPER, J. M. (Eds.). Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A. HAUCK, B. w. and HUBER, G. R. (1989) Single screw vs twin screw extrusion. Cereal Fds Wld, 34 (11); 930-939. HOLLAND, B., UNWIN, I.D. and Buss, D. H. (1988) Cereals and cereal products. 3rd Suppl to McCance and Chem. and Min. Agric. Fish. Food. HOSENEY, R. C. (1986) Breakfast cereals. Principles of Cereal Science and Technology. Ch. 13. Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A. ILLMAN, R. J. and TOPPING, D. L. (1985) Effects of dietary oat bran on faecal steroid excretion, plasma volatile fatty acids and lipid synthesis in rats. Nutr. Res. 5: 839. JULIANO, B. 0. (Ed.) (1985) Rice: Chemistry and Technologv, 2nd edn. Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A. JULIANO, B. 0. and SAKURAI, J. (1985) Miscellaneous rice products. In: Rice: Chemistry and Technology 2nd edn, JULIANO, B. 0. (Ed.) Amer. Assoc. Cereal Chem., St Paul, BRIT. PAT. SPEC. No. 754,771 (sugar coating). MN, U.S.A. BUSINESS MONITOR, PAS 4239. Miscellaneous Foods, 1989. KIM, J. C. (1987) The potential of extrusion cooking for the Business Statistics Office. utilisation of cereals. In: Cereals in a European Context pp. CHEFTEL, J. C. (1989) Extrusion cooking and food safety. 323-331, MORTON, I. D. (Ed.) Chichester, Ellis Horwood. In: Extrusion Cooking, MERCIER, c., LINKo~ p. and KLOPFENSTEIN, C. F. and HOSENEY, R. C. (1987) Cholesterol- HARPER, J. (Eds.). Amer. Assoc. Cereal Chem., St. Paul, lowering effect of !3-glucans enriched bread. Nutr. Rep. MN., U.S.A. Int. 36: 1091. COLONNA, P., TAYEB, J. and MERCIER, c. (1989) Extrusion KRISHNAN, p. G., CHANG, K.C. and BROWN, G. (1987) cooking of starch and starchy Products. In: Extrusion Effect of commercial oat bran on the characteristics and Cooking, MERCIER, C., LINKO, P. and HARPER, J. (Eds.). composition of bread. Cereal Chem. 64: 55. LINKO, P. (1989a) The twin-screw extrusion cooker as a Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A. versatile tool for wheat processing. In: Wheat is Unique, COOPER, H. (1988) Milling moves. Food Processing, April: Ch. 22, POMERANZ, Y. (Ed.) Amer. Assoc. Cereal Chem., 4142. DE GROOT, A. P., LUYKEN, R. and PIKAAR, N. A. (1963) St Paul, MN, U.S.A. Cholesterol lowering effect of rolled oats. Lancet 2: 303. human consumption. Cereal Fds Wld, 32 (3): 241. Exaustion Cooking, Ch. 8, MERCIER, C., LINKO, P. and FAST, R. B. and CALDWELL, E. F. (Eds) (1990) Breakfast HARPER~ J. (Eds)~ Amer. Assoc. Cerea1 Chem.y St Pau1, cereals and how they are made. Amer. Assoc. Cereal Chem., MN, U.S.A. St Paul, MN, U.S.A. MCAULEY, J. A., HOOVER, J. L. B., KUNKEL, M. E. and FAST, R. B., LAUHOFF, G. H., TAYLOR, D. D. and ACTON, J. C. (1987) Relative protein efficiency ratios for GETGOOD, S. J. (1990) Flaking ready-to-eat breakfast wheat-based breakfast cereals. J. Fd Sci. 52 (July-Aug.): cereals. Cereal Fds Wld, 35 (3): 295. 1111. High alpha-amylase activity flour Flour having an abnormally high alpha-amylase activity, unsuitable for making bread by conven- tional baking methods, can be processed by extrusion cooking - which causes rapid and complete inactivation of the enzyme - to make flat bread, snacks and biscuits, while indigenous raw materials can be processed by extrusion cooking in developing countries to make stable and nutritionally balanced foods, such as biscuits and pre-cooked flours for preparation of gruels, porridges and infant foods (Cheftel, 1989; Linko, 1989b). Other uses for extrusion cooking Other suggested uses for extrusion cooking include the texturizing of vital wheat gluten; the pre-treatment of wheat bran for the extraction of hemicellulose; the treatment of wheat flour, as ratio sponge cake (Kim, 1987). an alternative to chlorination, for use in high- Widdowson’s The Composition of Foods, 4th edn. R. SOC. References ANON. (1970) Cereal specifications. Milling, Oct.: 16. ANON. (1986) Ready-to-eat cereal industry report. Investment report No. 609910. Kidder, Peabody and Co. Inc. ASP, N.-G. and BJORCK, I. (1989) Nutritional properties of extruded foods. In: Exmion Cooking, MERC*ER, c., LINKO, P. and HARPER, J. (Eds). Amer. Assoc. Cereal Chemistry, St Paul, MN, U.S.A. FAST, R. B. (1987) Breakfast cereals: processed grains for LINKO, p. (1989b) Extrusion cookkg in bioconversions. In: 2 58 TECHNOLOGY OF CEREALS MEUSER, F. and WIEDMANN, W. (1989) Extrusion plant Further Reading design. In: Extrusion Cooking, Ch. 5, MERCIER, C., LINKO, P. and HARPER, J. (Eds) Amer. Assoc. Cereal Chem., St BROCKINGTON, S. F. and KELLY, V. J. (1972) Rice breakfast Paul, MN, U.S.A. cereals and infant foods. In: Rice: Chemistry and Technology, MIDDEN, T. M. (1989) Twin screw extrusion of corn flakes. 1st edn, pp. 400-418, HOUSTON, D. F. (Ed.), Amer. Cereal Fds Wld, 34 (1 1): 941. Assoc. Cereal Chem., St Paul, MN, U.S.A. NESTEL, P. J. (1990) Oat bran, rice bran. Food Australia, FAST, R. B. and CALDWELL, E. F. (Eds.) (1990) Breakfast 42 (7): 342. Cereals and How They Are Made. Amer. Assoc. Cereal NEWMAN, R. K., NEWMAN, C. W. and GRAHAM, H. (1989) Chem., St Paul, MN, U.S.A. The hypocholesterolaemic function of barley beta-glucans. GUY, R. C. E. (1986) Extrusion cooking versus conventional Cereal Fds Wld, 34 (10): 883-886. baking. In: Chemistry and Physics of Baking, pp. 227-235. OAKENFULL, D. (1988) Oat bran. Does oat bran lower plasma Spec. Pub. 56, R. SOC. Chem., London. cholesterol and, if so, how? CSZRO Food Res. Q., 48: GUY, R. C. E. (1989) The use of wheat flours in extrusion 37-39. cooking. In: Wheat is Unique, Ch. 21, POMERANZ, Y. (Ed.) PAUL, A. A. and SOUTHGATE, D. A. T. (1978) McCanee Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A. and Widdowson’s ‘The Composition of Foods’, 4th ed., JOHNSON, I. T. and LUND, E. (1990) Soluble fibre. Nutr. H.M. S .O., London. and Fd Sci. 123: 7-9. ROONEY, L. W. and SERNA-SALDIVAR, S. 0. (1987) Corn- LUH, B. S. and BHUMIRATANA, A. (1980) Breakfast rice based ready-to-eat breakfast cereals. In: Corn: Chemistry cereals and baby foods. In: Rice: Production and Utilization, and Technology, WATSON, S. A. and RAMSTAD, P. E. pp. 622-649, LUH, B. S. (Ed.) Avi Publ. Co. Inc., (Eds). Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A. Westport, CT, U.S.A. SEIBERT, S. E. (1987) Oat bran as a source of soluble dietary MILLER, R. C. (1988) Continuous cooking of breakfast fibre. Cereal Fds Wld, 32 (8): 552-553. cereals. Cereal Fds Wld, 33 (3): 284-291. SMITH, A. (1989) Extrusion cooking: a review. Food Sci. MORTON, I. D. (Ed.) (1987) Cereals in a European Context. Technol. Today 3 (3): 156161. Chichester, Ellis Horwood. U.S. PAT. SPEC. Nos. 3,971,303 (continuous puffing); POMERANZ, Y. (Ed.) (1987) Modern Cereal Science and 4,497,840 (cooked oat bran cereal); 4,501,759 (aspartame Technology, Ch. 20: Extrusion Products. VCH Publishers sweetener); 4,540,587 (aspartame sweetener); 4,594,252 Inc., New York. (dipeptide sweetener); 4,608,263 (dipeptide sweetener); POMERANZ, Y. (Ed.) (1989) Wheat is Unique. Amer. Assoc. 4,614,657 (dipeptide sweetener); 4,874,624 (reconstitutable Cereal Chem., St Paul, MN, U.S.A. oatflakes). WATSON, S. A. and RAMSTAD, P. E. (Eds.) (1987) Corn: Chemistry and Technology. Amer. Assoc. Cereal Chem., St Paul, MN, U.S.A.