Wheat History and Kernel Composition

1. Wheat History
K. Brunckhorst ( The Future of Flour )

Common wheat belongs to the Triticum genus of the grass-like subfamily (Pooideae). Scientists have traced its origin back to the Middle East region, particularly the valley of the Tigris and Euphrates rivers. This area was then called Mesopotamia and is now part of Iraq. A form of the grass grew in the Euphrates valley as early as 7000 BC. The Assyrians and Babylonians mentioned wheat in stone ruins dating from 3000 BC. The Chinese are recorded as cultivating wheat in 2700 BC and had developed elaborate rituals to honour it.

Today wheat covers more of the earth's surface than any other grain crop and it is the staple
grain food for much of the earth's population. Even in areas where there is a long tradition of rice eating, as in North and South East Asia, there is extensive use of wheat flour for making noodles, steamed bread and other foods.

The different wheat species are classified according to their ploidy level, i.e. the number of chromosomes. Since the basic number for wheat and also barley, rye and oats, is seven, diploid wheat has 14 chromosomes (e.g. Triticum monococcum); tetraploid wheat has 28 (e.g. the cultivated form Triticum durum), and hexaploid wheat such as we know it in the form of our common wheat (Triticum aestivum) has 42 chromosomes.

The discovery of how this wheat originated is now regarded as one of the most important examples of the successful use of genome analysis to answer questions of evolution. By crossing diploid and tetraploid wild forms and observing the mating behaviour of the chromosomes under the microscope as evidence of homology it was possible to prove that our modern common or bread wheat has three diploid ancestors.

Along the eastern shore of the Mediterranean, through Anatolia and Mesopotamia to the Persian Gulf – the Fertile Crescent – the hybridization (crossing) of two diploid species (with 14 chromosomes each) resulted in a tetraploid species (emmer) without the intervention of man. Further hybridization of this emmer with diploid goatgrass (Aegilops tauschii) in about 6000 BC resulted in today's common wheat, which spread from the Arabian peninsula to the whole world.

2. Wheat Kernel Composition
J.A. Gwirtz, M.R. Willyard and K.L. Mc Fall (The Future of Flour )

Fig. 1: Longitudinal view of a wheat kernel (modified from Berghoff, 1998)

Wheat, like other members of the grass family, produces a one-seeded fruit that does not split open at maturity. The seed consists of germ and endosperm enclosed by a nucellar epidermis and a seed coat. A fruit coat, or pericarp, surrounds the seed and adheres closely to the seed coat. This type of fruit is commonly called a kernel or grain but is known as a caryopsis to the botanist. The longitudinal and cross-section views of a wheat kernel are presented in Fig. 1. The pericarp, or fruit coat, surrounds the entire seed and acts as a protective covering. It is composed of several layers, which are, in order, from the outside to the inside towards the centre of the kernel: epidermis, hypodermis, remnants of thin-walled cells, intermediate cells, cross-cells and tube cells. In the crease of the wheat kernel, the seed coat joins the pigment strand and together they form a complete coat around the endosperm and germ. The seed coat is firmly joined to either the cross or tube cells on the outside and to the nucellar epidermis on the inside. Three layers are distinguishable in the seed coat: a thick outer cuticle, a coloured layer containing pigment and a very thin inner cuticle. The nucellar epidermis, commonly called the hyaline layer, is a compressed cellular layer between the seed coat and the aleurone layer and closely united to both. Botanically, the aleurone layer is the outer layer of the endosperm but to most millers it is considered part of the outer structure of the wheat kernel commonly called bran. Bran (pericarp plus aleurone layer) makes up about 17% of the kernel weight and contains about 9% ash, almost 20 times more than the ash content of the endosperm (Tab. 1).

Tab. 1: Typical composition of wheat kernel fractionsa (% d.b.)
The starchy endosperm is composed of three cell types. Peripheral cells are located just inside the aleurone layer and are equal in diameter in all directions. Prismatic cells, located inside the peripheral cells, are radially elongated towards the centre of the kernel. Central cells are located inside the prismatic cells and are irregular in size and shape. The starchy endosperm is the source of flour and is estimated to constitute 74.9 - 86.5% of the kernel weight. Its cells are packed with starch granules embedded in a protein matrix. Two types of endosperm protein have been identified. The first is salt-soluble albumans and globulins, equated with functional cytoplasimic and membrane protein. The second is gluten-forming gliadin and glutenin, which are storage proteins. The storage proteins in wheat are unique in the plant world; upon wetting, these proteins yield a viscoelastic substance called gluten. The gluten produces the strong elastic dough that is required for yeast-leavened products such as bread and rolls. A decrease in both the endosperm protein and the mineral (ash) content is observed from the outside to the centre of the kernel.

The germ is the embryo or sprouting section of the seed, which is composed of two major parts. The first is the embryonic axis, which is made up of the rudimentary root and shoot. The second is the scutellum that functions as a storage, digestive and absorbing organ. The germ makes up approximately 2 - 3% of the kernel weight and is usually removed in the milling process because it contains lipids that limit the keeping qualities of flour. The structure and composition of the wheat kernel is responsible for its use as human food. The bran layer provides the protection needed for storage purposes. The low amount of lipids and a limited amount of lipid-splitting enzymes also contributes to storage stability. Wheat contains large amounts of starch, which represents a significant source of energy in the human diet and contributes to the 6 - 11 servings1 identified at the base of the food pyramid (Fig. 2).
Fig. 2: Food guide pyramid
Although the recommendation on the composition of the diet is actually being modified, starch will still be a major part of it. The kernel structure itself has determined the methods of commercial separation into its component parts. Specifically, the light-density, tough bran and the soft, pliable germ are separated from the more dense and friable endosperm by the modern milling process.

Note :
1 Serving sizes using USA conventional and metric measurements are as follows:
1 slice (50 g) bread
1/2 cup (125 mL) cooked rice or pasta
1/2 cup (125 mL) cooked cereal
1 oz (28 g) ready-to-eat cereal


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