BREAKFAST CEREALS
INTRODUCTION
It is both convenient and logical to categorize breakfast cereals as (1) those products such as oatmeal, which are served hot and therefore are expected to be cooked before they are served, and (2) fully cooked ready-to-eat cereals such as corn flakes which are rarely, if ever, heated before serving. The former class is probably about as old as civilization, since it is very likely that gruels and porridges made from grains cooked with water were among the first cereal foods of mankind. Prepared breakfast foods have a considerably shorter but much better documented history.
The original motivation for the development of precooked breakfast foods seems to have been the desire of some vegetarians to add more variety to their diets. The Seventh Day Adventist Church, many members of which avoid the consumption of animal foods, was closely connected with the early history of prepared breakfast foods. The association of certain factions in this church with the Battle Creek (Michigan) Sanitarium and the cereal experimentation which they inspired at this hospital gave the city of Battle Creek a head start in the breakfast cereal industry. Today, some of the largest producers of breakfast cereals still have their headquarters in or near Battle Creek.
Present Status
Since the early ventures in Battle Creek and elsewhere, ready-to-eat breakfast cereals have become recognized by persons in all walks of life as economical, convenient, and flavorful foods suitable for daily consumption by all age groups. The original granules and flakes have multiplied into and array of forms, colors, and flavors which staggers the imagination. Addition of vitamins, minerals, and protein as well as fiber supplementation have been developed to give finished products a nutritional adequacy which is equalled by only a few other foods. As a consequence of these developments, breakfast cereals have held or increased their level of per capita consumption in recent years in spite of the overall decline in use of grains in foods. Their performance in this respect is superior to that of all other groups in the category with the possible exception of macaroni products.
Although the consumption of ready-to-eat breakfast cereals in foreign countries has not increased at the same rate as in the U.S., there has been substantial acceptance of these foodstuffs where the local economy and tradition will allow it. We can expect further incereases as more countries recognize the importance of a consumer-driven economy.
At its beginning, breakfast cereal manufacture was largely an art based on home kitchen methods of processing. The larger producers ultimately came to the conclusion could they keep pace with their competitors in new product development, process improvement, and quality control. The relative dearth of publications in the field indicates a considerable desire for secrecy rather than a lack of experimentation. Pressure to secure competitive advantages has increased in recent years and forced an even faster advance in technology. It is interesting that some, at least, of the breakfast cereal marketing approach has come full circle after all these years and now emphasizes as the main selling point the improving effect of (in this case, fiber supplemented) breakfast cereals on the overall health of the consumer.
PROCESSING OF HOT-SERVE CEREALS
There are two processing steps which are common to the manufacture of nearly all uncooked breakfast cereals. One of these is the reduction of particle size and the other is the elimination from the raw material of some of the fibrous substances found in the whole grain. The effect of these operations is to reduce cooking time and to improve the texture and perhaps the digestibility of the cooked food. There is usually no attempt to alter materially the natural flavor of the grain by hydrolyzing its starches or caramelizing its sugars, although it is true the heat treatment often applied to oatmeal changes its flavor somewhat.
Many consumers find the cooking time required for preparing meals using ingredients containing unaltered grain products to be excessive. To increase convenience, and therefore consumer acceptance, it is highly desirable to decrease the time required for kitchen preparation. Ideally, it should be possible to pour boiling water on the cereal, stir the mixture an few times, and then consume it. This ideal has been achieved with some modifications of oatmeal and wheat farina, although, it is true that the texture of the finished product is not quite the same as that obtained by cooking the unmodified granules.
Some hot-serve cereals are made from mixed grains which have been precooked and then dried, or from a combination of grains with other ingredients such as nonfat dry milk. Cereals for infant feeding may be precooked mixtures of ingredients which are dried in thin flake form. These flakes can be quickly rehydrated by adding the proper amount of hot water and stirring briefly. The consistency or texture of such materials is generally not appreciated by most adults, since the cereals as served tend to be somewhat pasty and sticky.
Wheat Cereals
The uncooked wheat cereal having the largest consumption is farina, a typical example being Cream of Wheat. This product is nothing more than the wheat middlings which are described in greater detail in the chapter on Milling. Middlings are chunks of endosperm for all practical purposes free of bran and germ. When reduced in size, middlings become flour. In the manufacture of farina for breakfast cereal, it is necessary to use hard wheat as a raw material since soft wheat yields a product which becomes excessively pasty upon cooking. About 30% of the wheat coming into a mill can be obtained as farina by good milling techniques. It is seldom, if ever, that wheat is milled specifically for the purpose of obtaining farina.
Particle size is thought to be a critical factor influencing consumer acceptance. All the farina should pass through a U.S. Standard No. 20 woven wire cloth sieve; not more than 10.0% should pass through a No. 45 sieve; and not more than 30.0% should pass through a No. 100 sieve. Vitamin and mineral enrichment is usually applied to farina. Vitamins are normally added in the dry state which seems to be satisfactory in practice but does give some opportunity for separation in the package.
Disodium phosphate has been used (at about the 0.25% level) to increase the rate at which farina cooks. Other steps are required to get an "instant" farina which is ready to eat after about one minute of cooking time. One method of decreasing cooking time is to apply proteolytic enzymes (such as bromelin) or fungal enzymes so that microscopic pathways for water penetration are formed in the granule.
Farina flavored with dried malt syrup or with cocoa is marketed. Generally, these products are simple mixtures of the dry flavoring ingredients with middlings.
Whole wheat meal, cracked wheat, flaked wheat, flaked wheat, and farina with bran and germ are sold as hot cereals to a rather limited extent. The shelf-life of these materials is limited by the tendency of germ oils to become rancid unless the product is specially processed.
Corn Cereals
Corn grits is the maize analogue of farina. For all practical purposes, it is just the endosperm of corn ground into medium coarse granules. Large amounts of grits are consumed in the southern states. They are served with sugar and milk or cream as a breakfast cereal ("mush"). They can also be served as a potato or rice substitute at the noon and evening meals, often being topped with butter or gravy ("grits"). Grits are generally cooked with water, only salt being added. The boiled grits can be allowed to cool and the congealed mass sliced into fairly thick strips and fried (with or without preliminary flouring) to give a culinary delight which is served with syrup or gravy topping.
Corn grits is one of the products obtained in dry milling of corn kernels, a process which is described in considerable detail in a preceding chapter.
Oat Cereals
The typical commercial oat products used as breakfast cereals or breakfast cereal ingredients can be categorized as :
- 1. Rolled oats, produced by flaking whole groats. These are the thickest of standard oat-flake products. Flake thickness varies from 0.020 to 0.030 inch, depending on the intended use. Thicker flakes require longer cooking times and maintain flake integrity during extended holding times.
- 2. Steel-cut groats are produced by the sectioning of groats into several pieces by a kind of cutting action (as contrasted to crushing); they are used in the preparation of flakes and flour and as a speciality ingredient.
- 3. Quick oats are flakes produced from steel-cut groats. In this process, oat groats are typically cut into three or four pieces before the final steaming and flaking processes. Quick oats, which are usually 0.014 to 0.018 inch thick, require less cooking time than whole-oat flakes.
- 4. Baby oats are also produced from steel-cut groats, but the flakes are thinner and have a smaller particle size than quick oats. These smaller, thinner flakes cook more rapidly than quick oats and have a smoother texture.
- 5. Instant oat flakes are produced from "instantized" steel-cut groats. Before cutting, the groats are subjected to a special proprietary process that allows them to acquire a satisfactory eating consistency after a relatively short cooking time. These flakes are typically 0.011 to 0.013 inch thick.
- 6. Oat flour is produced by grinding flakes or groats into flour for use as an ingredient in a wide variety of food products.
- 7. Oat bran is a bran-rich fraction produced by sieving coarsely ground oat flour. "Oat bran is the food which is produced by grinding clean oat groats or rolled oats and spearating the resulting oat flour by sieving, bolting, and/or such other suitable means into fractions such that the oat bran fraction is not more than 50% of the starting material, and has a total b-glucan content of at least 5.5% (dry weight basis) and a total dietary fiber content of at least 16% (dry weight basis), and such that at least one-third of the total dietary fiber is soluble fiber."
Oat processing for food involves at least the steps of cleaning, hull removal, steaming, and flaking. Cleaning requires that weed seeds, dirt, and other unwanted materials be removed; this is accomplished through separation of the rubbish by screening, air flotation, and classification by particle shape. Then, the oats are graded by size and reduced in moisture content to permit efficient removal of the hulls. Hulls are then abraded or knocked off the seed by specialized equipment. Each of these steps will be discussed below.
As oats are received at the country elevator, they are weighed and then given a preliminary cleaning by receiving separators. These will be either the vibrating screen type or the rotating reel type. Typically, on the slanted, vibrating screen type, the grain will first pass over a coarse-screen deck that has openings 1 inch by one-half inch. Large piece of impurities are removed at this point. Oats fall through the openings onto a fine screen having triangular perforations 3.5 mm on a side. The desirable kernels pass along the top of this screen while small particles of any description pass through. The oats are carried by the vibration into an aspiration section to remove light weight (low density) impurities from the rest of the grain. The scalper-reel receiving separators require that the oats be fed along the length of a horizontal rotating screen or reel that is fitted with three-quarter inch wire mesh and rotates at about 20 rpm. Large and/or long objects are carried through the inside of the reel to the scrap exit and eventually to a rescalping reel fitted with wire mesh of about 16 mm openings where any oat kernels which have been carried along on the top of large pieces of debris can be recovered. From the original reel, the grain has passed through the 0.75 inch wire mesh and been carried to an aspirating channel where light impurities are blown off and then carried to a dust settling chamber where they are removed by a mechanical conveyor.
Most oats for human consumption are marketed as rolled oats. Some demand also exists for ground oats and steel-cut groats. The initial step in the manufacture of rolled oats is the roasting process. Thoroughly cleaned grains are heated to 212°F for one hour. This roasting procedure reduces the grain's moisture content to about 6% and probably at least partially dextrinizes the starch. In addition, the hulls become fragile instead of tough and are more easily removed during subsequent processing.
After cooling, the toasted grains are separated according to size. The hulls are then removed by passing the kernels between two large circular milling stones mounted horizontally with the grinding surfaces separated by a short distance. The upper, or rotating, member of this pair of stones has a very slightly convex milling surface, while the lower stone, which is stationary, has a flat surface, while the lower stone, which is stationary, has a flat surface. Oats enter at the center of the upper stone and are carried to the periphery by centrifugal force. The distance between the stones is everywhere more than the width of a dehulled kernel, but less than the length of the kernels. As the grains travel outwardly, hulls are removed by the tearing and abrading action of the stones. The mixture of hulls, whole kernels, and broken grains exiting the mill is screened and subjected to other procedures in order to separate these components. The grain with its hull removed is called a "groat". Hulling can also be performed by impact milling, using such devices as the Entoleter.
The next step is flaking. The groats are steamed before flaking to inactivate enzymes and increase the moisture content. Flaking rollers are about 14 inches in diameter and 28 inches wide; one roller in each pair is fixed and the other is movable so the gap between each pair of rollers can be adjusted. There is a scraper knife on each roll to remove the flakes. The cylinders are often made of chilled iron or centrifugal alloy-iron castings.
Whole groats may be flaked, or they may be first cut into pieces by rotary granulators. The smaller the piece size and the thinner the flake, the quicker the cereal can be cooked. For example, so-called "quick" oats, which are flakes made from a particle about one-fourth to one-third the size of a whole groat, will cook in about five minutes, while flaked whole groats ("regular oats") require ten to fifteen minutes boiling before they are soft enough to satisfy most consumer's requirements. The smaller particles do not stand up as well under prolonged heating, however. Thus, regular oats will maintain a satisfactory texture for about three hours in the steam table of a cafeteria, while quick oats become unsatisfactory after about one hour under these conditions. By making an even thicker (than regular oats) flake from the whole groat, oats withstanding six hours of heating can be obtained, and steel-cut oats (not flaked) are even more resistant to overcooking. Of course, the reader may well ask what sort of serving operation would require oats to maintain an acceptable texture for more than three hours. Some consumers do prefer the chewier texture of steel-cut oats.
Instant oats are generally prepared by cooking or gelatinizing flaked oats, then reducing the particle size and thickness. These steps greatly enhance water penetration and reduce the need for kitchen cooking to gelatinize the starch.
PROCESSING READY-TO-EAT BREAKFAST CEREALS
Flakes
General Considerations
Flaking is a relatively simple process, consisting in its most elemental form of cooking fragments of cereal grains (or in some cases whole grains) with water, flattening the soft particles between large steel rollers, and toasting the resultant flake at high temperatures. Apparently, the first commercial production of such a food occurred around the turn of the century when J. H. Kellogg and W.H. Kellogg made whole wheat flakes in a barn behind the Battle Creek Sanitorium. Since that time, many complications have been introduced into the process in attempts to improve the flavor of the product and the efficiency of operations, and to gain the uniformity of flake size and appearance which is so desirable to the manufacturer and perhaps even to the consumer.
Flakes owe their popularity with consumers to their crisp but friable texture, to their sweet but rather bland flavor, and to the ease with which they can be readied for consumption.
In the basic processing steps, the raw material undergoes the following changes: (1) the starch is gelatinized and probably slightly hydrolyzed; (2) the particle undergoes a browning reaction due probably to interaction of protein and sugars; (3) enzymatic processes are stopped, rendering the final product more stable; (4) dextrinization and caramelization of the sugars occur as a result of the high temperatures in the roasting oven; and (5) the flake becomes crisper as a result of reduction of its moisture content to a very low level.
Early methods for making flaked cereals, e.g., wheat or corn flakes, were based on the use of a fairly large chunk of the grain, which retained its integrity throughout the entire process. Conventional corn flakes are still made this way. The technique has some disadvantages, such as the restrictions it places on the size, shape, and composition of the particle. In essence, this invention required the extrusion of a dough piece by a pelleting mill, and the subsequent puffing or flaking of the piece. In the words of the patent, the process involved "gelatinizing hydrated, ungelatinized cereal dough pieces having a major proportion of at least one highly nutritious cereal flour (and at a moisture content of about 22% to about 36%) at temperatures from about 215° to about 265°F at steam pressures of from about 1 to about 23 psig, cooling the gelatinized cereal dough pieces to a temperature whereat they are less plastic and their stiffness is increased, by cooling them to temperatures from above about 150° to about 205°F, pelletizing the cereal dough pieces, partially drying the gelatinized dough pieces to from about 5% to about 21% moisture, mechanically modifying the shape of the dough piece (as by flaking), and drying the flakes to a moisture content of from about 1% to about 3.5% moisture under conditions such that the temperature of the cereal dough pieces does not exceed about 250°F.
The Clausi et al. patent disclosed several formulas which could be used for compounded doughs suitable for processing by the invention. Two of these are shown in Table 1.
Table 1: Formulas for Pelletized and Flaked Cereals
<td colspan='2' align='center'>Formula A<td colspan='2' align='center'>Formula B
| Ingredient |
Percent by weight |
Ingredient |
Percent by weight |
| Oat flour |
60-70 |
Soy flour |
80.4 |
| Rice flour |
7-12 |
Sucrose |
8. |
| Soy flour |
5-10 |
Casein |
5. |
| Sucrose |
5-15 |
Malt syrup |
4. |
| Lecithin |
0.05-0.15 |
Salt |
2.5 |
| Salt |
2-4 |
Vanilla flavor |
0.1 |
| Milk casein |
1.5-10.0 |
|
|
Corn Flakes
Hybrid yellow corn is most often used as the raw material for corn flakes, although white corn provides an equally satisfactory ingredient. The corn is broken (milled) so as to yield a No. 4 to No. 5 grit free of germ and bran. These large pieces repreent about half of a corn kernel and they retain their identity throughout the process, each particle eventually emerging as a corn flake. The hulls resulting from the milling operation are used in animal feeds, and the germs are pressed in expellers to yield the corn oil of commerce and a meal or cake which can serve as a protein enrichment in animal feeds.
Into a cylindrical pressure cooker or retort are placed about 1,700 lb of grits and 36 gal of a flavoring syrup made from sugar, nondiastatic malt, salt, and water. Occasionally, niacin will be added at this point. During cooking, the charge accumulates additional water from the steam which is introduced into the retort, and its moisture content rises to about 33%.
The contents of the slowly rotating retort are cooked at 15 to 23 (typically 18) pounds steam pressure for 1 to 2 hr. Different lots of corn can vary appreciably in their cooking time requirements. The end point can be judged by examining a small sample which is blown out through a gate valve. A uniform transparency in the kernels indicates an adequate cook. At the expiration of the cooking time, pressure in the vessel is reduced to atmospheric level, the retort is opened, and its contents are dumped onto a moving belt.
After the lumps from the cooker are broken down to individual particles by a revolving reel, they are distributed to a set of driers. The latter devices are essentially large tubes or tanks extending vertically for several stories. Wet kernels enter the top and are dried by a countercurrent of hot (150°F) air as they travel to the bottom. Another type of drying equipment consists of horizontal rotating cylinders having numerous steam-heated pipes passing longitudinally through them. Louver driers may also be used
.
As they exit the driers, the corn particles contain 19 to 23% moisture, but the water is unevenly distributed, so the material is transferred to tempering bins for several hours (as many as 24) so that the moisture can equilibrate. After tempering, the hard dark brown grits are ready for flaking.
The flaking rolls are steel cylinders weighing over a ton. They revolve at a speed of about 180 to 200 rpm, and are cooled by internal circulation of water. Hydraulic controls maintain a pressure of over 40 tons at the nip of the rollers. The most popular sizes of rolls are 20 inches in diameter Ã- 30 inches wide and 26 Ã- 40, with other standards being 32Ã-40, 24Ã-40, and 20Ã-24.5 for ready-to-eat cereals. The cooked dried grits are pressed into thin flakes as they pass between the rolls. The flake is rather flexible at this time, and lacks the desired crispness and toasted flavor of the finished corn flake.
From the rolls, flakes pass directly to a rotating toasting oven which is usually gas-fired. The moist flakes tumble through the perforated drums and pass within a few inches of gas flames. Treatment may be, for example, 50 sec at 575°F or 2 to 3 min at 550°F. In addition to being thoroughly dehydrated by this process, the flakes are toasted and blistered. They contain less than 3% moisture when they emerge from the oven.
From the ovens, flakes are carried by belts to expansion bins where they are permitted to cool to room temperature. During transfer, the corn flakes are cooled by circulating air and they are usually sprayed with a solution of thiamin and perhaps other B-vitamins.
Corn flakes can also be made by cooking the corn in an extruder, cooling the cooked mass to perhaps 25°C (to eliminate post-extrusion expansion) then passing it through a die to form strands which are cut into short lengths (e.g., 5 mm), A finer grind of corn may be used in this method, as compared to the raw material required in the traditional method described previously, since the size of the flake is not dependent on the dimensions of the corn grit. The cooked pellets are then flaked and toasted in equipment very similar to that used for "old-fashioned" corn flakes. Flaking is done by pairs of large steel cylinders operating at a speed differential that shapes the strands into thin flakes. Adjustments of the rollers can be used to control the thickness, texture, and surface characteristics of the finished flake. Final drying and toasting can be conducted in any convenient type of hot air unit which does not subject the flake to excessive attrition. Belt dryers or fluid bed units are possible choices. In any case, the moisture content of the flake must be reduced from about 20% to about 3 or 4%. Applying the hottest air to the entry point causes blistering of the flake, which is generally considered desirable. It is said that extruded flakes can be completely processed in 30 minutes, or even less, as compared to an 8 hour cycle time for conventional processing.
Wheat flakes
Plump kernels of soft wheat are frequently used as the raw material for wheat flakes. After cleaning and sorting according to size, the kernels are tempered in steel bins of small diameter by adding moisture and holding at about 80°F for about 24 hr. The wheat may be transferred one or more times during this period if such a procedure is necessary in order to keep the temperature within reasonable limits. After tempering, the wheat is steamed at atmospheric pressure until it reaches 203°F and 21% moisture.
The steamed wheat is bumped between smooth steel rollers set considerably farther apart than are flaking rollers. This treatment flattens the grain slightly, and ruptures the bran coat in several places making the kernel more permeable to the moisture which will be added during the cooking step. Next, the flattened kernels are transferred to pressure cookers, which are similar to those used for corn flakes, and the other ingredients are added. These ingredients normally include sugars, salt, malt, and sometimes a coloring substance such as caramel.
The retort contents are cooked at 20-psi steam pressure for 90 min while the vessel is slowly rotating. After cooking, the grains are soft, translucent, and brown. They contain about 45 to 50% moisture. Their starch has been completely gelatinized, of course. The retort is now opened and rotated so that its contents fall onto a moving belt which transfers the cooked mass to a chute leading to a "wiggler".
The wiggler consists of a horizontal perforated disc and a rotating arm carrying vertically oriented rigid fingers around its upper surface. The clumps of slightly adhering grains are dropped onto the center of the perforated disc, through which warm air is being blown in an upward direction. The moving fingers break up the lumps and move individual grains outwardly until they fall from the edge of the disc into a pneumatic conveyor and are transferred to a horizontal rotating cylinder fitted with internal louvres. In this drier, air at 250° to 300°F is passed over the grain, eventually reducing it to 28 to 31% moisture content. At this point the grains are still intact and are rather tough and chewy in texture. Holding bins are used to store this material until it can be transferred to the presses.
Additional processing is needed to secure the desired crispness and flavor. First, the equilibrated or tempered wheat pieces travel through a drier. This could be a Proctor and Schwarz drier composed of three sections, the first at 280°F, the second at 290°F, and the third unheated. Rate of movement of the material is adjusted so as to yield an emerging product of about 21% moisture content. A spray of B-complex vitamins is applied at this stage.
Screw conveyors or drag chain conveyors transport the partially dried pellets to the flaking rollers. Just before they fall into the flaking rolls, the pellets are heated to about 180° to 190°F, making them more plastic in consistency. The large steel flaking rollers are practically identical with those used for making corn flakes. The pressure they apply to the pellets increases the latter's diameter several times and decreases their thickness proportionately.
After they pass through the rolls, the flakes contain 10 to 15% moisture and are still slightly flexible. To obtain the necessary crispness, the flakes are toasted and dehydrated to less than 3% moisture content in a drier provided with a perforated metal conveyor belt. There are typically four temperature zones in the oven; these may include, for example, heated sections at 310,300°, and 280°F, and an unheated section to partially cool the flakes. The decreasing temperature pattern is said to promote the development of desirable curling and blistering.
Bran Flakes
Bran flakes constituted a rather minor part of the breakfast cereal market until the recent fiber craze, when they began to assume a much more important role. Old-fashioned bran flakes were manufactured by combining a dried portion containing wheat and sufficient amounts of bran to yield flakes containing about four grams of fiber per ounce of cereal. Use of larger amounts of bran, with the aim of providing consumers with a product having greater fiber content, leads to grave difficulties in flaking and soft (not crisp) products. Most cereals of very high bran content are extruded so as to yield, typically, the shreds sold as "All Bran".
To make bran flakes, the dry wheat and bran are combined with a flavoring syrup containing sugar, corn, syrup, malt, and salt, then cooked until the starch is completely gelatinized. The cooked particles are partially dried, tempered, flaked by previously described methods, and finally, toasted to give finished flakes having a thickness of 0.005 to 0.040 inch.
Shreds
Shredded Wheat Biscuits
The most popular representative of this class is the pillow-shaped shredded wheat biscuit manufactured by Nabisco. It differs from most other ready-to-eat breakfast cereals in that it is made from whole grain without the addition of any flavor and without removal of the germ or bran. Cooking is typically done at atmospheric pressure in boiling water. After one hour or more of cooking, moisture content of the wheat kernels will have reached 50 to 60% and the grain will be very soft. Some preliminary drying in louvred ovens may follow, but the whole wheat is not brough much below 45 to 50% moisture content. The cooked and slightly dried wheat is transferred to stainless steel bins and tempered for many hours before it goes to the shredding rolls.
The shredding rolls are from 6 to 8 inches in diameter and as wide as the finished biscuit is to be, thus much smaller than flaking rolls. One of the pair of rolls has a series of 20 shallow corrugations or ridges running around its periphery. In cross section, these corrugations may be rectangular, triangular, or a combination of these shapes. The other roll of the pair is smooth-surfaced. Moist, softened wheat is drawn between these rollers as they rotate, and it issues as continuous strands of dough. By cutting grooves perpendicular to the circumferential corrugations, shred layers having a net-like effect can be obtained.
Biscuits are built up by layering strands on a moving belt which passes under sets of rolls positioned in tandem. Ten to 18 rolls may be used for circular biscuits, while 22 rolls is a common number for rectangular biscuits. In the latter case, layered strands are separated into biscuits by passing them below blunt "knives" which press a thin line of the dough into a solid mass at regular intervals.
Since the biscuits are formed from dough of relatively high moisture content, they are quite tender and fragile, and must be handled very carefully to prevent distortion. In practice, this means that the transfer steps must proceed more slowly than is necessary with flakes or puffed products.
The wet biscuits are placed on a metal belt moving through a high temperature gas-fired oven. After 10 to 15 minutes, the outside of the product is dry and toasted but the interior is still wet. Then, the biscuits are transferred to another hot air oven (or to a different section of the same oven) where they are dried at 250°F for 30 to 60 minutes through time, depending on the size of the biscuit and the rate of airflow. Finished moisture content should be about 11%. The combination of heat treatments causes the biscuit to assume the familiar oval cross section (pillow shape) as a result of differential shrinkage of the layers.
A triple shredding mill is used for producing bite-size breakfast cereals. Dough made from wheat, corn, or rice is fed to long, water-cooled shredding rolls. These rolls deposit a shredded dough sheet onto a constant speed conveyor to form a wide, three-layer ribbon. The rolls on the first and third shredding mills extrude dough sheets with a laced pattern, due to the use of combinations of smooth and grooved rolls. The middle set of rolls revolves at higher speed than the other two pairs. As a result, the dough sheet in the middle folds as it falls onto the relatively slow moving conveyor belt which has been covered by the first dough sheet. Sugar is sprinkled over the middle dough sheet, and the top sheet is then added. The combined structure passes between scoring rolls, and the baked cereal is finally broken along scored lines to form individual bite-size pieces.
Puffed Cereals
General Considerations
All puffed cereal manufacturing process are based on the rapid generation of steam within a plasticized mass which then expands. Puffing may be conducted at atmospheric pressure, as in the preparation of popcorn, or it may involve sudden pressure changes in which a product heated above the boiling point of water in some sort of retort is rapidly transferred to an area of lower (e.g., atmospheric) pressure. In both cases, puffing results from the quick conversion of liquid water to vapor in the interstices of the cereal particle or dough. The cereal is fixed in its expanded state by the dehydration which results from diffusion of water vapor out of it and also by the cooling. Gun puffing may result in an increase of apparent volume (bulk density decrease) of 8-fold to 16-fold for wheat and 6-fold to 8-fold for rice. Oven puffing leads to a smaller increase in volume for corn, about 3-fold to 4-fold.
Puffed products must be maintained at about 3% moisture or less in order to have satisfactory crispness. Even at 5% moisture a definite toughness becomes evident. These levels are most critical and hardest to maintain in foods which have been gun-puffed.
Oven-puffed Rice
This product is prepared from whole kernels of domestic short-grain milled rice. Frequently the rice is parboiled and pearled before being introduced into the puffing plant. Typically, a batch of 1,400 lb of rice is weighed into cookers such as are used in the preparation of corn or wheat flakes. About 53.5 gal of sugar syrup with salt are added, and the mixture is cooked for 5 hr under 15 lb steam pressure. Sometimes non-diastatic malt syrup and enriching ingredients are added before cooking
.
The lumps of cooked rice coming from the retorts are broken up and dried to approximately 25 to 30% moisture content in rotating louvred driers. Then, the moisture is allowed to distribute uniformly in the grain by storing the partially dried product in stainless steel bins for about 15 hours. Lumps form during the tempering process and must be broken up before the rice is sent to the flaking rolls.
After the individual kernels are separated and again dried so that moisture content of 18 to 20% is reached, they are passed under a radiant heater which brings the external layers of the rice to a temperature of about 180°F. This plasticizes the outer layers of the kernel so that they do not split when the grain is run through the flaking rolls.
Rollers used in the preparation of oven-puffed rice are set relatively far apart so that the tremendous compression effect necessary in corn flakes manufacture is not achieved. In fact, the rolls contact, only the widest part of the kernel. The "bumped" grains are again tempered, this time for about 24 hr. To secure the puffed effect, the cooled and tempered, this time for about 24 hr. To secure the puffed effect, the cooled and tempered rice is passed for about 30 to 45 sec through toasting ovens held at 450° to 575°F
.
Puffing by Extrusion
In a typical process, a cereal premix containing on the order of 60 to 75% expandable starch base is moistened with water or steam. The resultant mash is compacted by a screw revolving inside a barrel which may be heated by steam. The thread of the screw has a progressively closer pitch as it approaches the discharge end. Pressurizing and steam heating bring the dough to a temperature of around 300° to 350°F and a pressure of 350 to 500 psi at the die head. Under these conditions the dough is quite plastic and easily adapts to complex orifice configurations.
The die head will ordinarily contain several or many orifices, and pieces of correct length will be sliced from the emerging strands by rotating blades which rest on the exterior die surface. Adjustment of the speed of rotation of the knife assembly (relative to the rate of extrusion) controls the piece size. Dough pieces expand very rapidly as they leave the die orifice but the expansion may continue for a few seconds after they emerge since the dough is hot still flexible, and steam continues to evolve. Even so, the final moisture content will be too high for satisfactory stability, and the pieces must be further dried on vibrating screens in hot-air ovens. Fines and agglomerates are removed at the same time, and the products cooled and packaged.
The following raw materials can be expanded satisfactorily by appropriate types of extrusion equipment:
- Rice flour-excellent expander; white and bland tasting products; accepts colors and flavors well.
- Corn meal or flour-expands well; texture good; retains corn flavor.
- Oat flour-high moisture content required if satisfactory expansion is to be obtained; high temperature also needed.
- Wheat flour-high moisture and high temperature required for satisfactory results.
- Potato flour-high moisture and high temperature needed.
- Tapioca flour-needs high temperature and moderate moisture content; bland taste.
- Defatted soy flour-requires high temperature and moderate to high moisture.
- Full fat soybeans-should have 3 to 5 minutes preconditioning with steam prior to extrusion; extrude at 250°F.
- Plain and acid-modified corn and wheat starches-need medium to high temperatures; use either steam or water as moisturizer.
As the fat content of the cereal mixture increases, the expansion tends to decrease, but the pieces become more uniform and their surface becomes smoother and brighter, while the cell size becomes smaller and more uniform. Monoglycerides seem to increase these effects. Sugars modify the flavor and texture, and may help to control shape and size of tough doughs.
Sugar-coated Products
Items which are approximately spherical or disc-shaped, such as puffed wheat and puffed rice, can be coated by a technique very similar to the pan-coating process used in confectionery manufacture. The usual apparatus somewhat resembles a cement mixer in having an open bowl rotating about an axis inclined slightly from the horizontal. The very dry cereal particles are placed in the bowl and, as it rotates, a molten (250°F) sugar syrup is slowly dripped on the bowl's contents. A small amount of coconut oil may be added to decrease foaming of the sugar syrup and to promote separation of the coated particles. The tumbling action of the particles results in each of them remaining separate and becoming uniformly coated with a thin glaze of sugar which hardens upon cooling. From 25 to 60% of the weight of the finished product is glaze. A stream of hot air is usually directed into the coating reel to assist in removal of moisture. Some authorities have suggested a syrup formula of 86% sucrose, 13% corn syrup, and 1% salt. Sometimes 0.01 to 0.05% sodium acetate may be added to prevent crystallization of the coating.
Other methods of coating breakfast cereal pieces with sweet syrups include stirring the granules or flakes with the syrup in a mixing bowl or spraying the viscous material on a bed or falling stream of the cereal. In all cases, the moisture content of the syrup is kept as low as possible to avoid causing changes in the structure and texture of the cereal. In practice, this generally means applying a molten sugar mixture with less than five percent of water content, and, preferably about 1% moisture content. Formation of agglomerates of flakes stuck together with the syrup is a recurring problem, and various methods have been suggested to overcome it. An obvious expedient is to keep the pieces widely separated until the coating hardens and loses much if its stickiness, but this creates difficulties in processing. Other methods include applying a non-sticky powder as the coated flakes are cooling.
Hygroscopic coatings lead to sticky surfaces which cause agglomeration of pieces into clumps of serveral pieces. King (1990) patented a process which leads to the absorption into the interior of the puffed piece of most of the added sugar. It is particularly useful with honey coatings. According to the invention, a grain cereal is enrobed with honey, then heated in an oven at a temperature which will cause the particle to swell and give off moisture. The honey seeps into the pores of the swollen grain. This removes most of the sweetener from the surface, reducing the stickiness and the clumping problem.
Of course, there is nothing to prevent coating of cereal pieces with artificial sweeteners such as saccharin or aspartame. These substances are not inherently sticky, however, so a problem arises in getting them to adhere to the substrate. In an invention patented in 1976 by Baggerly, aspartame was mixed with water containing a relatively high concentration of dextrins, and the solution applied to the cereal pieces by atomization. Other patents achieve the same purpose, but yield a frosty coating as the result of using maltodextrins as the adhering agent. Also, the use of gums and of water-soluble vegetable and protein isolates as adhesives has been described.
Crystallization in the coating may be deliberately caused in order to give a frosted, or somewhat snowlike appearance to the cereal pieces and to reduce sticking. Crystallized sugar is not sticky, has reduced hygroscopicity, and has an opaque white or frosted appearance. Noncystalline or glassy sugar is hygroscopic, sticky, and has a transparent glossy appearnace. In one patented method for producing frosted pieces, the breakfast cereal is coated with seed crystals of dextrose and sucrose; an aqueous solution of dextrose and sucrose is then uniformly applied to the surface; finally, the moisture content of the cereal is reduced by drying at a temperature below that which can cause browning. Another invention describes mixing coarse crystalline sugar granules with cereal pieces which have been sprayed with an edible binding agent; subsequently, the coated pieces are dried.
The following formula for a glaze to be applied to puffed wheat: sucrose 54.5%, water 27.2%, invert syrup 7.8%, honey 1.6%, and dextrins 8.9%. This mixture is to be cooked to 325°F and coated on a equal portion of the puffed cereal. Cereal with a non-sticky, non-glassy coating is said to result when 10 parts of cereal are mixed with 12 parts of crystalline sucrose and 4 parts of a spray containing 7.5% gelatin.
A quantity of edible fat is added to an aqueous sugar solution to make up a syrup emulsion having moisture content of 9 to 34%. After emulsification at 115° to 155°F, the mixture is heated to about 180°F and used to enrobe cereal particles. The coating is then dried, giving crisp, nonhygroscopic cereal with a softer, less frangible texture than conventional candy coated cereals. Two examples of coating formulas used in this process are shown in Table 2.
Ovens
Several times in the preceding discussion, it has been mentioned there is a need for drying and/or toasting as one of the final steps in the manufacture of breakfast cereals. Many kinds of ovens have been used for this operation. Some of them were simple adaptations of ovens that had been designed for baking or other food processes. Fast (1990) divided cereal toasting ovens into three major types: rotary toasting ovens, conveyorized or flat band forced-convection ovens, and high velocity fluidized-bed dryers or ovens. A rotary toasting oven will consist of an insulated other shell, an inner revolving cylinder, gas burners, a waste hopper with screw conveyor, and a drive mechanism. In these ovens, wheat and oat cereals, are generally toasted at between 350° and 450°F, rice and corn flakes at 450 to 600°F. Usually, the feed end of the oven is held at a higher temperature than the exit end. Band ovens are similar in many respects to the band ovens used to bake cookies or crackers in large volume factories. They can be either direct-fired or use burners located outside the main oven chamber. Corn or rice strands which are to be puffed will first be dried in this type of equipment, typically dropping in moisture content from 30% to 11% in 30 to 40 sec. The product is then fluidized and heated to 550° 600°F to puff it.
Table 2: Sweet Coatings for Ready-to-Eat Cereals
<th colspan='2' align='center'>Formula A <th colspan='2' align='center'>Formula B
| Ingredient |
Percentage by Weight |
Ingredient |
Percentage by Weight |
| Water |
16 oz |
Water |
32 oz |
| Icing sugar |
64 oz |
Dark brown sugar |
48 oz |
| Soybean oil |
7 oz |
Coconut oil |
15 oz |
| Distilled monoglycerides |
1 oz |
Distilled monoglycerides |
2 oz |
| Honey |
12 oz |
Honey |
12 oz |
| Artificial color |
0.5gm |
|
|
Air impingement ovens are based on the jet-tube effect, in which high velocity streams of hot air are directed into the bed of product. When conditions are properly adjusted, the creal pieces will be suspended in the turbulent currents of gas maintained at, say, 600°F. These ovens are satisfactory for raw or intermediate particles such as pellets, flakes, and fancy shapes but are normally not adaptable to larger pieces such as shredded wheat biscuits. Sequential zones with different temperatures can be used to achieve special effects. Typically, for puffing intermediate products, the first zone(s) will be maintained at 400° to 500°F and puffing will occur. Temperature in the next zone will be lowered to about 300° to 400°F to prevent burning while the product continues to toast.