Cereals are plants which yield edible grains such as wheat, rye, rice, or corn. Cereal grains provide the world with a majority of its food calories and about half of its protein. These grains are consumed directly or in modified from as major items of diet (flour, starch, oil, bran, sugar syrups, and numerous additional ingredients used in the manufacture of other foods), and they are fed to livestock and thereby converted into meat, milk, and eggs.
On a worldwide basis, rice is probably the single most important human food, with wheat not far behind. Nearly all rice grown goes directly to human food. Similar amounts of corn and wheat are grown, but much of the corn is used for feeding livestock, whereas only a small portion of wheat is used in animal feed. Although wheat is produced in many temperature-zone countries, over 90% of the rice is grown in Asia, where most of it is consumed. Much of the world’s corn is grown in the United States. In recent years’ annual world production of wheat, rice, and corn has been about 560, 530, and 470 million metric tons, respectively.
The principal cereal grains grown in the United States are corn, wheat, oats, sorghum, barley, rye, rice and buckwheat. In the United States, corn is by far the largest cereal crop; in recent years’ corn production has averaged about 200 million metric tons, but most of it is used for animal feeding. Wheat –with an annual production of about 66 million metric tons –is the largest U.S. cereal crop used primarily for direct human food.
Legume are flowering plants having pods which contain beans or peas. Oilseeds are seeds which contain a high oil content and are widely grown as a source of oil both are considerably higher in protein than are cereal grains (Table 17.1). Legumes include the various peas and beans, most of which are low in fat, but a notable exception is the soybean. The term oilseed is applied to those seeds, including the soybean, which are processed for their oil. Other oilseeds include the peanut seed, cottonseed, sunflower seed, rapeseed, flaxseed, linseed, and sesame seed. The coconut also is an important oilseed. Cereal grains not only are comparatively low in protein but the proteins have deficiencies in certain essential amino acids, especially lysine. Legumes as well as many oilseeds are rich in lysine, though relatively poor in methionine.
Some oilseeds, such as the soybean, peanut, and coconut, are important foods in addition to being sources of oil. Oilseeds also yield great quantities of oilseed meals; for many years these were used principally to fatten livestock. Modern technology has made it possible to separate high quality proteins from these meals, and today, oilseed proteins in their many forms are used to improve the nutritional properties of cereal products, to extend the meat supply, and to generally increase available protein worldwide.
CEREAL GRAIN
General Composition and Structure
The major constituents of the principal cereal grains are listed in Table 17.2. These grains contain about 10-14% moisture, 58-72% carbohydrate, 8-13% protein, 2-5% fat, and 2-11% indigestible fiber. They also contain about 300-350 kcal/100 g of grain. Although these are typical values, composition vary depending on varieties of the particular grain, geographical and weather conditions, and other factors.
A moisture content of 10-14% is typical of properly ripened and dried grains. When the moisture content of grain from the field is higher than this, they must be dried to this moisture range, otherwise they may mold and rot in storage before they are further processed. Some molds which grow on cereal grains containing excessive moisture produce toxic metabolites which can cause disease in humans and animals consuming the grain. Cereal grains contain about two-thirds carbohydrate, most of which is in the form of digestible starches and sugars. The operations of milling generally remove much of the ingestible fiber and fat from these grains when they are to be consumed for human food.
The nutritional quality of cereal proteins is not as high as that of most animal proteins. Table 17.3 lists the patterns of the essential amino acids lysine, methionine (plus cysteine), threonine, and tryptophan of several cereals compared with whole egg and an FAO recommended standard mixture of these amino acids. Because the first limiting amino acid of these cereals is lysine, the ratio of the lysine concentration in a cereal grain protein to the concentration in whole egg, or the FAO standard mixture, can be used as an index of quality. This ration times 100 gives the chemical score of a cereal, which can be improved by the addition lysine. The lysine limitation also can be overcome by consuming cereals with other foods high in lysine.
There are a few important structural features that the cereal grains have in common of the cereal grains are plant seeds and as such contain a large centrally located starchy endosperm, which also is rich in protein, protective outer layers such as hull and bran, and an embryo or germ usually located near the bottom of the seed. These portions are seen in the diagrams for wheat and corn in Figs. 17.1 and 17.2.
For most food uses, processors remove the hulls, which are largely indigestible by man; the dark-colored bran; and the germ, which is high in oil, is enzymatically active, and under certain conditions would be likely to produce a rancid condition in the grain. Thus, the component of primary interest is the starchy, proteinaceous and endosperm. Since the bran is rich in B vitamins and minerals, it is common practice to add these back to processed grains from which ran has been removed; this is known as enrichment.
Besides indigestibility of hulls, bran color, and possible rancidity from the germ, a further reason for removing these components in many cases is to improve the functional properties of the endosperm in manufactured food use. For example, white bread made from wheat flour would have less acceptable color, flavor, and volume if the bran and germ were not removed before the flour was ground. However, there also are application in which un-milled whole grain –containing hulls, bran, and germ –is used. Grain for animal feed is an example; sprouted barley, used for its malting effect in the brewing industry, is another example. Whole wheat bread, preferred by many, utilizes flour from which the bran and germs have not been removed during milling.
The processing and utilization of the major cereal grains are discussed in the following sections.
Wheat
As with all cereal grains, there are many varieties of wheat differing in yield, in resistance to weather, insects, and disease, and in composition. Wheats are classified into two types: hard and soft. In comparison with soft wheat, hard wheat is higher in protein, yields a stronger flour, which forms a more elastic dough, and is better for bread-making when a strong elastic dough is essential for high leavened volume. In contrast, soft wheat is lower in protein, yields a weaker flour, which forms weak doughs or batters, and is better for cake-making. Wherever wheat is used for human consumption, the majority of it is first converted to flour.
Conventional Milling
The miller receives the wheat, cleans it of foreign seeds and soil, soaks or conditions the wheat to about 17% moisture to give it optimum milling properties, and then proceeds with the milling.
Milling involves a progressive series of disintegrations followed by sieving’s (Fig. 17.3). The disintegrations are made by rollers set progressively closer and closer together. The first rollers break open the bran and free the germ from the endosperm. The second and third rollers further pulverize the rather brittle endosperm and flatten out the more semi-plastic germ. The flakes of bran and flattened germ are removed by the sieves under these first few sets of rollers. The pulverized endosperm is run through successive rollers still closer together to grind it into finer and finer flour, which also is sifted under each set of rollers to remove the last traces of bran.
From such an operation several flour fractions having finer and finer endosperm particles are collected. These finer fractions also contain progressively lower and lower amounts of ground-up contaminating germ or bran, some of which always gets through the earlier sieves. As a result, as the flour is progressively milled, it becomes whiter in color, better in bread-making quality, but lower in vitamin and mineral content.
The starch and protein composition of flour –no matter how fine it is ground in the milling process depends on the variety and kind of wheat that was ground. Thus, the protein to starch ratio of flour made from hard wheat will be greater than that of flour made from soft wheat. The kind of flour that is produced during conventional milling is largely dependent on the kind of wheat available.
Figure 17.4 is a diagram of two finely milled flours. The endosperm contains both protein (the dark matter) and starch granules (the shite matter in this diagram). In ad dition to the large mixed endosperm agglomerates, there are smaller fragmented starch and protein particles. The fragmented starch and protein particles are too close in size to be further separated from one another by the sieves of the conventional milling operation. If they could be, it would be possible to separate any flour into fractions differing in protein and starch contents. Such a separation could yield both a hard and a soft flour from the same wheat. Further, a naturally hard wheat could be made to yield a soft flour plus a protein fraction, just as a naturally soft wheat could be made to yield a hard flour plus a starch fraction.
Figure 17.3
Turbomilling and Air Classification
Further processing can separate flour into higher protein or higher starch fractions in a process known as Turbomilling. In Turbomilling, flour from conventional milling is further reduced in particle size in special high-speed turbo grinders, which cause the endosperm agglomerates to abrade against each other in a high-speed air vortex.
Figure 17.4
Although the resulting protein and starch particles are too close in size to be separated by sieves, they do differ sufficiently in particle size, shape, and density to b separable in a stream of turbulent air. In this case, the slightly finer protein particles rise and the starch particles settle in the stream of air. The flour and air mixture is blown into a specially designed air classifier, which then may impose centrifugal force on the suspended particles, and two fractions of flour differing in protein and starch concentrations are recovered.
Turbomilling, developed in the late 1950s, probably is the greatest milling advance of the past century since it gives us the ability to separate flour into fractions and then blend the fractions in any desired ration. Thus, Turbomilling makes it feasible to custom-blend flours for bread-making, cake-making, cookie-making, and many other specific applications.
Uses of Wheat Flour and Granules
The uses of wheat flour in the baking industry include the making of breads, sweet doughs, cakes, biscuits, doughnuts, crackers, and the like. Wheat flour is also used in making breakfast cereals, gravies, soups, confections, and other articles. But a principal use of wheat flour, and coarser milled fractions of wheat, is in the preparation of alimentary pastes, such as macaroni, spaghetti, and other forms of noodles and pasts. Alimentary pastes like bakery doughs contain mostly milled wheat and water. The wheat, usually a hard durum granular, and finer durum flour. Alimentary bakery doughs in that alimentary pastes are not leavened.
Figure 17.5
The unleavened dough is formed by mixing the ingredients in the ratio of about 100 parts of the wheat products to 30 parts of water. The dough then may be extruded in a thin sheet (Fig. 17.5), which is cut into flat noodles and dried in an oven to about 12% moisture; or the unleavened dough may be extruded in dozens of other shapes depending on the choice of dies. Figure 17.6 shows a die for extruding macaroni with a hole in the middle; this product also is oven-dried to about 12% moisture. Quick cooking noodles, sometimes referred to as instant noodles, are made by steaming noodle dough and then frying it. Frying removes moisture and the noodle are not further over-dried.
Rice
Rice is the stable food of billions of people world-wide. Whereas whet for the most part is ground into flour, most of the world’s rice is consumed as the intact grain, minus hull, bran, and germ. Therefore, the milling process must be designed not to disintegrate the endosperm core of the seed.
Milling
Rice milling begins with whole grains of rice being fed by machine between abrasive disks or moving rubber belts. These machines, known as hullers, do not crush the grains but instead rub the outer layer of hull from the underlying kernels. The hulls are separated from the kernels by jets of air, and the kernels, known as brown rice, move to another abrasive device called a rice-milling machine. Here, remaining inner layers of bran and germ are dislodged by the rubbing action of a ribbed rotor. The endosperms with bran and germ removed can now be further polished to a white, high glossy finish.
As in the case of wheat, the higher the degree of milling or polishing, the lower are the remaining vitamin and mineral contents.
Figure 17.6
This is particularly serious in the case of rice because entire populations depend on rice as the principal item of diet.
Enrichment
The two major to enrich rice differ from the simple admixture of vitamins and minerals in powder from that may be done in the case of flor. One method is to coat the polished rice with the enrichment mixture and then to further coat the grains with a waterproof edible film material. Upon hardening, the film material prevents the enrichment ingredients from dissolving away when the marketed rice is washed, as is common practice.
The second important method involves parboiling or steeping he whole rice grains in hot water before removal of hulls. Bran, and germ in milling. Parboiling may be for about 10 h at 70C, although several other time –temperature combinations can be used. This causes the B vitamins and minerals from the hulls, bran, and germ to leach into the endosperm. The rice is then dried, milled, and polished as before. Parboiled rice, processed for enrichment and other desirable changes in the rice kernels has also been referred to as converted rice.
The principal nutrients used to enrich rice are thiamin, niacin, and iron: thiamin is particularly effective in reducing the incidence of beriberi where polished rice is a major item of diet. Legislation requires all rice sold in Puerto Rico to be enriched. Most of the rice sold in United States is enriched. To be called enriched rice in the United States, the product must meet the standards indicated in Table 17.4.
Improved Varieties
Plant breeders are continuously at work improving the yields and properties of cereal grains. This includes considerations of soil types, water conditions, response to fertilizer application, resistance to disease and insect attack, nutritional quality, storage stability, milling properties, cooking and processing characteristics, and other factors.
The development of a high-yielding strain of rice, designated IR-8, by the International Rice Research Institute in the Philippines, kindled hopes that the continued world shortage of this important food grain could be relieved. IR-8 has been universal, however, and other high-yield varieties with better milling and cooking characteristics have replaced much of the IR-8 in several countries. Meanwhile, it is of interest to note that the importance of rice as a dietary staple in some countries may be reduced as wheat becomes available in the form of bread and pasta. This tendency is now being seen in Japan and in parts of Indonesia.
Rice Products
Rice can be made quick-cooking or almost instant in terms of preparation time. This is done by precooking to gelatinize the starch, and then drying under conditions that will give the rice an expanded internal structure for quick absorption of water during subsequent preparation. Many patents exist.
Rice is also the basis of several prepared foods and dried mixes. Typically, these products contain quick-cooking rice and other ingredients such as spices, noodle products, and starch-based sauces.
Rice may be ground into flour and as such is used by people allergic to wheat flour. Rice is a source of starch. It is the grain that is used preparing the Japanese fermented alcoholic beverage sake. Rice hulls, bran, and germ also are used as animal feed.
Corn is consumed as human food in many forms. In its harvested wet form, it is consumed as a vegetable. The kernels of a special variety may be dried and consumed as popcorn. Popcorn pops because, on heating, moisture trapped in the center of kernels turns to steam and this escapes with force sufficient to explode the kernels. Popcorn might therefore be considered the original puffed cereal.
But the majority of corn consumed as human food has undergone milling and is consumed as a specific or modified fraction of the original cereal grain. Like the other cereal grain, corn is milled to remove hulls and germ. Both are fed to livestock. In addition, the germ is an important source of corn oil. Corn is milled in two basic ways; dry milling and wet milling.
Dry Milling
Corn kernels are first conditioned to about 21% moisture and then passed between special rotating cones that loosen the hulls and germ from the endosperm. The entire mixture is next dried to about 15% moisture to facilitate subsequent roller milling and sieving. The hulls may now be removed by jets of air. From here on, corn milling is much the same as wheat milling. The endosperm and loosened germs are passed through rollers that flatten the germ and crush the more brittle endosperm. Sieving now easily separates the flattened germ from the endosperm particles. The endosperm may be recovered in the form of coarse grits or corn meal, or it may be passed through finer rollers and reduced to corn flour.
Wet Milling
After the corn kernels are cleaned, the first step in wet milling is steeping the kernels in large tanks of warm water that generally contain acid and sulfur dioxide as a mild preservative. The softened kernels are next run through an attrition mill to break up the kernels. The pasty mass from this mill is then pumped to water filled settling troughs. Here the lighter-density rubbery germ floats to the top and is skimmed off to be pressed for oil. The slurry now contains the hulls and the protein and starch fractions of the endosperm. The water slurry is passed through screens that remove the hulls.
The remaining water slurry containing the starch and protein fractions is now passed through high-speed centrifuges to separate the heavier starch from the lighter protein. The starch fraction is finally dried to yield the familiar corn starch. The protein fraction is also dried to yield corn gluten, which is rich in the corn protein known as zein. Corn gluten is commonly used in animal feeds. Separated zein has industrial uses including some as a food ingredient. Corn starch can be used as such in manufactured foods or be further converted into corn syrup by the hydrolytic action of acid or starch-splitting enzymes. The relationships between these various products from the wet milling of corn are shorn in Fig. 17.7. Processes for the wet milling of wheat, rye, and oats also have been developed.
Corn Sugars
The corn syrup resulting from hydrolysis of starch may be used as a sweetener. It contains varying proportions of dextrin, maltose, and glucose, depending on the method and degree of hydrolysis. More extensive hydrolysis yields a greater proportion of glucose, also known as dextrose. Since glucose is sweeter than dextrin or maltose, more extensive hydrolysis also yields a sweeter syrup. The glucose may further be enzymatically converted to fructose, which is still sweeter. Such a conversion is known as isomerization. Through hydrolysis and isomerization many sweeteners can be produced from corn starch; these include corn syrups, high-glucose (dextrose) syrups, glucose –fructose syrup, and high-fructose syrups. These syrups may be dehydrated to produce corn syrup solids, or they may be used to yield highly purified glucose or fructose by crystallization.
Blends of glucose and fructose can be made to equal the sweetness of cane or beet sugar (sucrose). Further, corn sugars and syrups in various proportions can yield a wider range of functional properties than is possessed by sucrose. The properties, ready availability, and favorable costs of corn-derived sweeteners in recent years have resulted in enormous quantities of these products replacing part or all of the sucrose in many food formulations.
Alcohol Form Corn
Conversion of plant materials by fermentation to ethanol has long been known, and corn has long been an important ingredient in the manufacture of alcoholic beverages. In recent years, increased fuel oil costs have focused research on more efficient conversion of biomass into ethanol as a partial replacement for gasoline. A 90% gasoline -10% ethanol mixture (gasohol) has been produced and used in the United States and higher ethanol fuels are used in Brazil. In the United States, one source of fermentable sugar for fuel ethanol has been corn starch.
Sine U.S. corn is a major human food and animal feed grain at home and abroad, its increased demand for ethanol production could increase the cost of feed and food throughout the world. This has created concern among some that a valuable food commodity should not be sacrificed to fuel for unessential purposes while millions experience hunger. One bushel of corn (25.4 kg) can yield 9.7 liters of anhydrous ethanol plus useful by-products. The cost of this conversion relative to conversions of other fuel-producing raw materials, as well as political considerations, will determine the future for this additional use of corn.
Barley, Oats, Rye
Barley, oats, and rye are used for animal feed. Barley and rye also provide sources of fermentable carbohydrate in the production of fermented beverages and distilled liquor. The flour of rye, after removal of bran and germ, is used in mixture with wheat flour in the production of rye bread. Rye flour cannot be used alone for this purpose since its protein would not from films sufficiently strong to support and expanded bread structure. Most oats for human consumption are marketed as rolled oats or as an ingredient in breakfast cereals. Recent findings that oat bran can lower serum cholesterol levels in humans have created a growing market for oats in many forms.
Barley is also used to produce barley malt. In this case, the whole barley seed is steeped in water to allow the live germ to sprout. The sprouted barley, having initiated growth, becomes much increased in enzymatic activity, especially starch-digesting amylase activity. The sprouted barley is next dried under mild heat so as not to inactivate its enzymes. The sprouted dried barley, now known as malt, is used in the brewing industry to help digest starchy material into sugars for rapid yeast fermentation. Malt also has a distinctive flavor which contributes to the flavor of brewed beverages. Malt syrup also find used in various bakery operation where amylase activity is desired.
Breakfast Cereals
The cereal grains find an important use in the manufacture of breakfast cereals. Most breakfast cereals are made from the endosperm of wheat, corn, rice, or oats. The endosperm may simply be broken or pressed, with or without toasting to yield such uncooked cereals as farina and oatmeal.
But far more popular are the so-called ready-to-eat cereals. For these, the endosperm may be broken or ground into a mash, and then converted into flakes by squeezing the broken grits or mash between rollers. The mash also can be extruded into numerous shapes; or the endosperm may be kept intact as kernels to b puffed, as in the case of puffed rice. But in all cases, the flaked, formed, or puffed cereals must be oven-cooked and dried to develop toasted flavor and to obtain the crisp, brittle textures desired. This crispness requires that many ready-to-eat breakfast cereals be dried to about 3-5% moisture.
In Fig. 17.8 is a set of flaking rolls that may be used for producing corn flakes. The coarse pieces of corn endosperm or grits are cooked and then partially dried to a firm plastic consistency. The grits are then passed through these rolls, which squeeze them into individual flakes. The flakes are oven-toasted and dried to about 3% moisture.
Wheat or rice endosperm to be puffed are first cooked and then partially dried as individual kernels. These, are next placed in a puffing gun which heats the kernels under pressure, converting moisture within the kernels into steam. When the gun is opened suddenly, the steam under pressure within the kernels expands explosively and puffs the kernels. In some cases, mashes of cereal doughs are extruded into moist pellets, which may be puffed in the same manner. The puffed cereals are toasted, often sugar coated, and dried.
Figure 17.8
SOME PRINCIPLE OF BAKING
Wheat flours find their principal applications in the production of bakery products. Most bakery products –unlike other wheat products such as alimentary pastes or noodles and un-puffed breakfast cereals –are leavened; that is, they are raised to yield baked goods of low density.
The term baking strictly refers only to the operation of heating dough products in an oven. But since there are many steps that must take place before the oven if baking is to be successful, baking has come to mean all of the science and technology that must precede the oven as well as the oven-heating step itself. We will consider baking in this broader sense.
Although there are a great many bakery products which grade one into another in terms of their formulas, methods of preparation, and product characteristics, it is possible to classify bakery products according to the way in which they are leavened. This classification, though not perfect, is useful. Four categories may be defined:
· Yeast-raised goods include breads and sweet doughs leavened by carbon dioxide from yeast fermentation
· Chemically leavened goods include layer cakes, doughnuts, and biscuits raised by carbon dioxide from baking powders and chemical agents
· Air-leavened goods include angel cakes and sponge cakes made without baking powder
· Partially leavened goods include pie crusts, certain crackers, and other items where no intentional leavening agents are used yet a slight leavening occurs from expanding steam and other gases during the oven-baking operation.
This classification refers to the intended source of the leavening gas; however, the intended source is not the only source of leavening, as well be seen shortly. Leavening gas can produce leavening only if it is trapped in a system that will hold the gas and expand along with the gas. Therefore, much of cereal science related to baking technology is really the engineering of food structures through the formation of correct doughs and batters to trap leavening gases, and then the coagulation or fixing of these structures by the application of heat. This brings in the need to understand further some of the properties of flour and certain other baking ingredients.
Major Baking Ingredients and Their Functions
Gluten and Starch of Wheat Flour
The principal functional protein of wheat flour is gluten. Gluten has the important property that when it is moistened and worked by mechanical action, it forms an elastic dough. It does this by forming linkages between protein molecules. These linkages form a three-dimensional structure which provides strength to the dough. The longer the dough is worked, the more linkages are formed. This is the reason the dough is kneaded when a strong structure is required. The resulting dough may be stretched in two direction and form sheets or films, or it may be stretched in all directions under the pressure of expanding gas and form bubbles as does bubble gum. However, gluten films weaken and then break down under excessive mechanical action such as over mixing of the dough. Additionally, on exposure to sufficient heat, the gluten coagulates and forms a semi-rigid structure. If the gluten has been expanded by gas prior to by gas prior to being heated, then this fairly rigid structure will be of a cellular character such as the inside of a loaf of bread.
The gluten of wheat flour has starch associated with it. Wheat starch does not form elastic films as does gluten; rather, the moistened starch, when heated, forms a paste and stiffens, or more correctly gelatinizes.
Thus, these two constituents of wheat flour together are capable of forming a batter or dough depending on the amount of water employed; and both the gluten and starch contribute to the semi-rigid structures resulting when such batters or doughs are heated.
The character of a dough or batter depends considerably on the type of flour used. As indicated earlier, strong flours containing more gluten, and gluten of a quality that will stretch farther before tearing, are the kind chosen for making bread because bread dough must be able to expand to a great degree and yield baked products of especially light density. Weaker flours generally contain less gluten and their films tear more readily; further, such films are less tough, and when baked, they yield structures that are less chewy and more tender. This is the kind of flour selected for making cakes and related products in which more tender and friable structures are desired. Figure 17.9 shows unbaked and baked doughs that were made from gluten separated from the same weight of cake flour, and an intermediate flour known as all-purpose flour.
Leavening Agents
Yeast and baking powders are not the only effective leavening agents. Water in doughs or batters turns to steam in the oven, and the expanding steam contributes to leavening. Air in a dough or a batter similarly expands when heated in the oven and contributes to leavening. In yeast-leavened or chemically leavened goods, although carbon dioxide from fermentation or from baking powder is the major leavening gas, it is supplemented with expanding steam air from oven heat. Not only are the amounts of gas leaven produce important but also the rates of gas production and the time of gas production.
Yeast. Two forms of yeast are used in baking –moist pressed cakes and dehydrated granules. Both form consist of billions of living cells of Saccharomyces cerevisiae. When rehydrated, the yeast begins metabolism and fermentation, of which carbon dioxide is a by-product. In the bread-making process and related sweet dough processes, yeast ferments simple sugars and produce carbon dioxide and alcohol. This fermentation is gradual, beginning slowly and increasing in rate with time. This increase in rate with time is due to two conditions in a dough:
Yeast cells are multiplying and their enzymes are becoming more active while the dough is prepared and held.
Sugar for fermentation is gradually being liberated from starch in the dough by the action of natural flour enzymes.
Figure 17.9
This gradual production of carbon dioxide is preferable to an immediate burst of gas because the film-forming property of gluten also develops gradually as the dough is being hydrated and mechanically kneaded. If the gas were to evolve before the film-forming property became developed, it would escape entrapment and there would be no leavening. Further, in the operations involved in converting the large dough mass into individual loaf-size pieces, there is considerable rough handling which tends to knock gas out of the rising dough pieces. Gradual and continued production of carbon dioxide up to the point of oven baking replenishes the lost gas and maximizes leavening.
The distribution of gas bubbles in the elastic dough prior to baking, the delicate structure of the leavened dough, and the origin of the cellular structure of the baked loaf can be seen in Fig. 17.10. the amounts of leavening gases and their rates of production must be balanced against the rate of development of the film-forming structure and its strength to hold the gas prior to and during baking.
Figure 17.10
The heat of the baking operation kills the yeast and inactivates its enzymes; thus, fermentation and the release of carbon dioxide ceases. However, the bubbles already formed enlarge under the influence of heat due to expansion of carbon dioxide, expansion of entrapped air, and conversion of water into steam. As the temperature of the leaf rises, starch gelatinizes and gluten coagulates, resulting in semi-rigid, less fragile structure.
Baking Powders.
Baking powders used in making cakes and relate goods contain particles of sodium bicarbonate as a source of carbon dioxide, and particles of an edible acid to generate the carbon dioxide when water and heat are supplied. The simplified overall reaction in the case of baking powder containing sodium bicarbonate as the carbon dioxide source and mono-calcium phosphate as the baking acid is as follows:
Formula
Such a reaction takes place too rapidly and so its speed and time of occurrence must be controlled. Various baking powders differ in the times and rates of reactions, and baking powders are formulated to produce controlled release of gas for specific bakery product applications.
For example, in making cakes, all ingredients may be mixed together and then deposited as a fluid batter into pans from a large bakery hopper. The fluid batter with its weak cake flour has very little gluten development or other means to hold evolved carbon dioxide. Thus, if there is a major carbon dioxide evolution during mixing or holding of batters, the gas will largely escape from the batter, and leavening power will be lost. However, when the batter is placed in the oven, starch gelatinizes, gluten coagulates, and egg proteins, if present, coagulate. If gas is produce while this is taking place, the gas will be trapped and expand the solidifying mass, giving the desired volume increase and cellular structure.
On the other hand, too much gas may evolve in the oven due to an excessive amount of baking powder. This tends to over expand the gas cells, which become weakened and collapse. The result is a coarse-grain structure with lowered volume. It also is possible to produce gas in the oven too slowly. When this happens, the gluten, starch, and eggs set the structure and the crust is formed before all the gas is released. The late gas can then rupture the crumb structure and produce cracks in the surface crust.
The times and rates of gas evolution from baking powders can be regulated by the selection of different baking acids that react faster or slower with sodium bicarbonate. These acids may be used also in different particle sizes or they may be coated with various materials to control their rates of solution, thereby further controlling their rates of reaction with sodium bicarbonate.
Baking powders, contain both a fast and a slow-reacting acid in combination with sodium bicarbonate. Double-acting baking powders are compounded to give a quick burst of carbon dioxide in the batter stage to lighten the batter and make mixing easier, especially for the home baker who may mix by hand, and then to liberates additional carbon dioxide in the oven when the structure is being set.
Eggs
In addition to their nutrient, flavor, and color contributions; eggs can function as a principal structure builder in cakes. Like gluten, egg white is a mixture of proteins. It forms films and entraps air when it is whipped, and on heating, it coagulates to produce rigidity. The proteins of egg yolk have similar properties. This is particularly important when eggs are combined with relatively low levels of a weak flour, as is the case in the preparation of angel cakes and sponge cakes.
In these cakes the eggs are whipped and gently folded together with the other ingredients. The entrapped air in the egg foam is the primary leavening system since, generally, no baking powder is used. In the oven, the gluten, starch, and egg stiffen, and the subdivided air bubbles expand from heat. Steam generated from water enters the air bubbles and further serves the expand them. This is one reason why the whipping quality and foam stability of eggs are so important to the baker.
Shortening
Unlike flour and eggs, which are structure builders and toughness, shortening is a tenderizer. But in many recipes, additionally, the beating of shortening is called for to entrap air prior to the incorporation of other ingredients to finish the batter. When the batter is baked in the oven, the shortening melts and releases the air bubbles which contribute to the leavening action of baking powder and expanding steam. The melted shortening then deposits around the cell walls of the coagulating structure to contribute a tenderizing effect and lubricate the texture.
The cellular structure of a cake (i.e., whether it is fine or coarse grain) and cake volume are affected by the number and size of air bubbles and water droplets trapped in the beaten shortening. These in turn are determined by the plasticity of the shortening and the use of emulsifiers. The state of emulsion is affected also by the other ingredients present and the sequence in which they are incorporated into the batter. The photomicrographs in Fig 17.11 are of two-layer cake batters with the shortening stained by a fat-soluble dye. The spheres are mostly air bubbles within fat globules.
Figure 17.11
Such differences between batters may be produced, for example, by creaming the shortening and sugar together prior to mixing the remaining ingredients, in contrast to beating all of the ingredients together in a single a step. Such modification in mixing procedure can easily give differences in grain structure and cake volume from the same ingredient formulation.
Sugar
Sugar, like shortening, is a tenderizer in baked goods. It also adds sweetness and, in the form of sucrose, provides additional fermentable substrate in yeast-raised goods. Bakers’ yeast cannot ferment sucrose directly, but hydrolyzes it first by means of the enzyme inverts into glucose and fructose. The yeast then immediately ferments the glucose; after the glucose is consumed, it proceeds to ferment the fructose. Sugar also has moisture-relating properties in baked goods. In this respect the hydrolytic products of sucrose, namely, glucose and fructose which together are referred to as invert sugar, usually are superior to sucrose. This is one reason why invert sugar syrups are frequently used in addition to sucrose in various baked goods made without yeast. Corn syrups from the hydrolysis of starch, which contain glucose, maltose, and dextrin, also have this moisture-retaining property. Sucrose, fructose, glucose, maltose, and dextrin further contribute to the different kinds of browning that baked goods develop in the oven.
The Baking Step
Baking is a heating process in which many reactions occur at different rates. Some of these reactions include the following:
1. Evolution and expansion of gases;
2. Coagulation of gluten and eggs and gelatinization of starch;
3. Partial dehydration from evaporation of water;
4. Development of flavors;
5. Changes of color due to Maillard browning reaction between milk, gluten and egg proteins with reducing sugars, as well as other chemical color changes;
6. Crust formation from surface dehydration and
7. Crust darkening from Maillard browning reactions and caramelization of sugar.
The rates of these different reactions and the order in which they occur depend to a large extent on the rate of heat transfer through the batter or dough. If the crust forms before the center of the mass is baked, because of too high top heat compared to bottom heat or too hot an oven, then the center of the baked item may remain soggy or late escaping gas may crack the crust. Quite apart from the temperature distribution in the oven, the rate of heat transfer is affected also by the nature of the baking pan.
Shiny pans reflect heat and slow heat transfer into the pan contents. Dull and dark colored pans absorb heat more rapidly and speed heat transfer. The shape of pans also is obviously important: a shallow pan with the batter or dough in a thin layer will develop different thermal gradients and give different results than a smaller deeper pan containing the same weight of material to be baked.
Were all of the preceding factors not enough to provide causes for variations in baked goods, there also is the effect of altitude. Unless otherwise indicated, most bakery formulas were developed for use at altitudes near sea level. At elevations of about 900 m(3000ft) and higher, excessive expansion of leavening gases under reduced atmospheric pressure causes stretching and weakening of the cellular structure being formed in the oven. the result can be collapsed items of coarse and irregular gains. Corrective measure at high altitudes, therefore, call for cake formulas with less baking powder, more or stronger flour as toughness, or decreased levels of tenderizers such as shortening and sugars. Because of their tougher doughs, bread formulas are less sensitive to altitude than cake formulas.
Much of what has been said regarding the baking step applies principally to conventional baking in ovens where conduction, convection, and radiation heating contribute to the end result. Consumer products are being developed for baking in home microwave ovens. This involves special product formulation to overcome differences in water binding and browning, which can be achieved through the use of various starches, hydrocolloids, reducing sugars and other Maillard-producing ingredients.
The varieties of breads, cakes and other bakery items can run into the thousands as ingredients, formulas; and preparation methods are changed. Today, the principles of cereal chemistry and baking technology are well understood and the many possible variables can be kept under fairly rigid control in large modern bakeries. This permits automated high-speed operation with uniform production rates of tens of thousands of units per hour. In smaller bakeries and in the home, however, baking remains more of an art then a science.
LEGUMES AND OILSSEEDS
General Composition
As stated earlier, legumes and oilseeds are considerably higher in protein than cereal grains, and oilseeds also are much higher in fat. Whereas different cereal grains may contain about 7-14% protein and about 2-5% fat. Various mature dry legumes and oilseeds contain about 20-40% protein; fat levels in peas and beans are low but are 20-50% in oilseeds. These compositions are reflected in the meals and flour derived from legumes and oilseeds. Table 17.5 gives the compositions of some de-hulled legume flours as well as data on protein yields that can be obtained from them. The high fat content of soybean is why this legume is also commonly listed among the oilseeds. If fat is removed from the de-hulled soybean, the flour that can then be produced world be even more concentrated in protein than is indicated Table 17.5.
Protein Supplementation and complementation
Although cereal grains are relatively low in total protein and generally low in lysine and certain other amino acids, these shortenings can be overcome by appropriate blending with legume or oilseed products.
The most obvious result of such blending is that the mixture is higher in protein than the cereal component alone. Beyond this, however, legumes and various oilseeds improve the quality of cereal proteins by supplementing them with limiting amino acids such as lysine (sometimes tryptophan or threonine). This is called protein supplementation. On the other hand, legumes and some oilseeds, which are deficient in methionine, can be supplemented by cereal grains, which are not deficient in this amino acid. Such mutual balancing of each other’s amino acids is known as protein complementation. An example of this is shown in Fig. 17.12 where mixtures of corn
Flour and soybean flour were fed to rats and their weight gains per gram of protein consumed (protein efficiency ratio) were measure. Optimum results were obtained with the 40% corn/60% soybean protein ratio. With less soybean, lysine became limiting; with more soybean, methionine was limiting. Much progress has been made over the past 30 years in using such mixtures of local crops to improve human nutrition.
Soybean Technology
Of the various legumes and oilseeds is the outstanding source of protein due to its high protein content and the relative ease of its extractability. The soybean has been intensively studied and many processes have been developed to obtain and modify its protein for special food uses. Some of the more important processing operations and resulting products are indicated in Fig. 17.13. A food-grade flour of about 50% protein is obtained by de-hulling and low-temperature extraction of the oil. Partially defatted flours also are available. The defatted flour and be further concentrated in protein by acid washing starch and other components from the acid precipitated protein, or it can be still further concentrated to the “isolate” stage by dissolving the defatted flour in alkali, filtering, re-acidifying, and centrifuging the precipitated protein from the whey. The protein isolate can then be modified by enzymes and other treatments to affect its solubility, whip ability, and other properties and then spray dried; or the isolate can be dissolved in alkali, forced through the holes of a spinneret, and re-coagulated into fibers in an acid bath. In this way, meat like texture can be given to soy protein and used to manufacture meat analogs. The equipment for fiber production and the fibers being gathered and drawn from the bath are shown in Fig. 17.14. But soybean protein can be texturized in other less costly ways, including extrusion cooking directly from soy flour. In this process, flour of about 50% protein is wetted to about 30-40% moisture and heat coagulated under pressure. Further texturizing occurs as the dough expands and becomes oriented passing through the extrusion orifice. The dough is then cut into chunks and dried. Upon reconstitutions and cooking, its texture is appearance are remarkably like meat (Fig. 17.15). such products are less costly than meat and are increasingly being used as partial replacements for meat in meat-containing mixtures.
Peanuts
Like the soybean, the peanut, or groundnut, is both a legume and an oilseed. The shelled whole nuts contain about 25% protein and about 50% oil peanut flours, protein concentrates, and protein isolates can be produced, but their use as human food is limited. The protein of peanut is not as high in lysine as that of soybean.
Figure 17.13.
The principal uses of peanuts today are as the whole nut, as a source of peanut oil with the peanut meal going largely to livestock feeding, and in the Unites States, as ground nuts in the form of peanut butter. About two-thirds of the world’s peanuts are pressed for oil and supply about one-fifth of all edible oil production. Somewhat over half of the U.S crop is made into peanut butter. Its basic manufacture involves shelling the nuts, roasting, removing the skins and “hearts” with heat followed by rubbing (this is called blanching), grinding, adding salt and sugar for flavor, emulsifiers to keep the oil suspended, and packaging.
SOME SPECIAL PROBLEMS
Legumes contain certain anti-nutritional and toxic factors that must be inactivated if their full value is to be realized. Raw soybeans contain an anti-trypsin factor or trypsin inhibitor. Other legumes contain hemagglutinins. These factors interfere with normal growth of animals and humans but fortunately can be inactivated by the heat of cooking or by controlled heating during processing.
Peanuts, because of their moisture content at harvest, may support mold growth and development of toxic metabolites of molds such as aflatoxins. Today, peanuts are stored under conditions to control mold growth and are carefully inspected to minimize this hazard. Aflatoxins also have been removed from peanut meal by solvent extraction and been inactivated by oxidizing agents, ammonia, and other treatments. Due to the ubiquitous nature of molds, aflatoxins and other mycotoxins can never be completely eliminated from feeds and foods, although they can be decreased to insignificant levels. Dry and wet milling of corn, for example, removes a major portion of any aflatoxin which may have been present initially. Heat processing and cooking further reduce remaining aflatoxins. Currently, maximum permissible levels in the parts per billion range are enforced in many countries.
Cottonseed endosperm has pigment glands that contain the toxic pigment gossypol. Any gossypol that gets into the oil is largely removed during oil refining. The presence of gossypol in the meal has impeded acceptance of cottonseed flour and cottonseed protein for human food. It is possible to remove un-ruptured pigment glands by controlled disintegration of the seeds in hexane and centrifugal separation of the lighter glands from the rest of the endosperm. Glandless varieties of cotton seeds that are free of gossypol also have been developed by plant breeders.
Problems such as these, which generally yield to research and controlled processing, must always be considered when less common sources of food are proposed for use in technologically underdeveloped regions.
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