Principles Of Cereal Science And Technology Pdf
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Bert Lagrain kuleuven. Tim Belien Head of Department Zoology, pcfruit pcfruit. Jane Ward Rothamsted Research rothamsted.
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Insight into the three-dimensional architecture of cereal tissues and the compartmentalization of the various cereal constituents is, in these contexts, of prime importance. In general, members of the grass family Gramineae , which include the cereal grains, produce dry, one-seeded fruits. This type of fruit is commonly called a "kernel" or "grain.
The wheat caryopsis or for that matter, the grain, see Fig. The fruit coat adheres tightly to the seed coat, which surrounds the remainder of the seed.
The seed itself consists of the embryo or germ , the endosperm, the nucellar epidermis, and the seed coat. The nucellar epidermis and the seed coat enclose the endosperm. In general, all cereal grains have these same parts in approximately the same relationship to each other. Their caryopses develop within floral envelopes, which are actually modified leaves. These are called the "chaffy parts" or "glumes. In wheat, rye, maize, grain sorghum, and pearl millet, the grain and hull separate readily during threshing, and the grains are said to be "naked" i.
The chemical constituents of cereal grains are often separated from each other by cell walls or other barriers. Such compartmentalization, along with the relatively low water activity, is largely responsible for the stability of the grain during storage. The grains themselves often contain both degrading enzymes and the substrates of these enzymes.
Certainly, if the two come in contact and a proper water activity threshold is passed such as in germination , degradation processes can easily start. However, if enzyme and substrate are protected from coming in contact with each other, the system is stable. WheatWheat is grown on more land than is any other food crop. The reasons for this are probably twofold. First, the wheat plant is quite hardy and can grow under a wide variety of environmental and soil conditions.
Second, significant parts of the world population like wheat-based products. The speciesT. In North American terminology, the T. The terms hard and soft refer to the force required to crush the kernels. Generally, the North American soft cultivars, which are easy to crush, are used for cookies biscuits , while the hard cultivars, which are more difficult to crush, are used in breadmaking. In contrast, in some European countries, the term soft e.
To avoid confusion, this book uses the North American terminology. In this section, all of these wheats are discussed as a group, and possible reasons for the obvious differences in hardness are commented on. Wheat types and the types of products made from them. Note that the figure is valid for applications of North American wheats and that the terminology of soft, hard, and durum wheats is that used in North America.
In Europe and other areas, wheats of softness comparable to that of the North American soft wheats are not available. Reprinted, with permission, from Moss Figure 1. The kernels of North American wheats average about 8 mm in length and weigh about 35 mg. European wheats weigh an average of about 55 mg. Their sizes vary widely depending upon the cultivar and their location in the wheat head or spike.
Wheat kernels are rounded on the dorsal side the same side as the germ and have a longitudinal crease over the length of the ventral side opposite the germ. The crease, which runs nearly the entire length of the kernel, extends nearly to its center. The two cheeks may touch and thus mask the depth of the crease. The crease not only makes it difficult for the miller to separate the bran from the endosperm with a good yield but also forms a hiding place for microorganisms and dust.
Wheat kernels vary widely in endosperm texture i. The variation in texture, which appears to be related to binding forces in the endosperm, is discussed later in this chapter. The color of the seed, usually white or red, is related to pigment in the seed coat.
Purple and even black seeds are known but are not common. The type and presence of the pigments is under genetic control and thus can be manipulated by the plant breeder. The outer pericarp is what millers call the "beeswing. Because of their lack of continuous cellular structure, they form a natural plane of cleavage. Thus, the beeswing is often lost before milling.
The cross cells are tightly packed, with little or no intercellular space. The tube cells are of the same general size and shape as the cross cells but have their long axis parallel to the long axis of the kernel.
They are not packed tightly and thus have many intercellular spaces. Seed Coat and Nucellar EpidermisThe seed coat is firmly joined to the tube cells on their distal outer side and to the nucellar epidermis on its proximal inner side. It consists of three layers: a thick outer cuticle, a layer that contains pigment for colored wheats , and a thin inner cuticle.
The seed coat of white wheat has two compressed cell layers of cellulose containing little or no pigment. EndospermThe endosperm consists of the outer aleurone layer and the starchy endosperm.
The Aleurone Layer. This layer, which is generally one cell layer thick, completely surrounds the kernel, covering both the starchy endosperm and the germ. From a botanical standpoint, it is the outermost layer of the endosperm. Milling removes the aleurone, the nucellar epidermis, the seed coat, and the pericarp together to form what the miller calls "bran. The aleurone cells contain a large nucleus and a large number of aleurone granules Fig.
The structure and composition of the aleurone granules are complex. The aleurone layer is relatively high in enzyme activity and in ash, protein, total phosphorus, phytate phosphorus, and lipid contents. In addition, the vitamins niacin, thiamin, and riboflavin have higher concentrations in the aleurone than in the other parts of the bran.
Over the embryo, the aleurone cells are thin-walled and may not contain aleurone granules. Scanning electron micrographs of a cross section of a hard winter wheat kernel. Reprinted, with permission, from Hoseney and Seib 1.
Pericarp P , aleurone layer A , and endosperm E. Endosperm cells. The Starchy Endosperm. When reduced to appropriate particle size, the contents and cell walls of the endosperm produce either flour, farina, or semolina.
The starchy endosperm is composed of three types of cells that vary in size, shape, and location within the kernel. They are referred to as "peripheral," "prismatic," and "central. Next are several rows of elongated prismatic starchy endosperm cells Fig. Then come the central starchy endosperm cells.
They are more irregular in size and shape than the other cells are. The wheat endosperm cell walls are mainly composed of arabinoxylans in older literature referred to as "pentosans".
The thickness of the cell walls varies with location in the kernel, being thicker near the aleurone. The cells are packed with starch granules embedded in a protein matrix. The protein is mostly, but not entirely, gluten, the storage protein of wheat.
During maturation, gluten is synthesized and deposited as protein bodies. However, as the grain matures, the protein bodies are compressed together into a matrix that appears mud-or claylike, and the bodies are no longer discernible.
In actuality, one can find granules of all sizes between these extremes, but these two size-shape combinations are preponderant. Although these small granules are numerous, they account for only a very small percentage of the mass of starch. Germ, or EmbryoThe wheat germ makes up 2. As detailed in Fig. It contains no starch but is rather high in B vitamins and contains many enzymes. The germ is quite high in vitamin E total tocopherol , with levels of up to ppm.
The sugars are mainly sucrose and raffinose. However, major differences in the hardness texture found in soft, hard, and durum wheats seem to be attributable to the presence, absence, or sequence polymorphism of the proteins puroindolin a and puroindolin b. One theory is that these proteins control the interaction of gluten with the surface of starch granules and that this interaction controls the hardness. Cell wall thickness varies among cultivars and between hard and soft T.
Cereals and Cereal Products
Journal of agricultural and food chemistry 46 8 , , Critical reviews in food science and nutrition 42 3 , , Critical reviews in food science and nutrition 51 2 , , Critical reviews in food science and nutrition 46 6 , , Annual review of food science and technology 3, ,
Isaac Newton to Robert Hooke, February 6, Fifteen years have elapsed since one of us R. The book has been used around the globe as a textbook for students beginning their studies of cereal science. It has also been useful to many industry professionals, particularly those who came to the industry with a limited background in cereal science. In a discussion of what a next version would need to bring, the two of us came to the conclusion that so much had changed in cereal science and technology that a total reediting, development of current views, and additions on specific hitherto uncovered topics would be necessary.
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Cereals and Cereal Products
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Food Chemistry pp Cite as. Cereal products are amongst the most important staple foods of mankind. Moreover, cereal products are also a source of minerals and trace elements.
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