1. Wheat History
K. Brunckhorst ( The Future of Flour )
Triticeae of the Poales order of the plant family, which contains wheat, barley, and rye (Pooideae). As scientists have discovered, the beginning of this behaviour may be linked to the Middle East, namely to the valley of the Tigris and Euphrates rivers. Before this area was known as Mesopotamia, it was known as Sumer and is now part of Iraq. Grass appears in the Euphrates Valley between 7000 and 6500 BC. For the Assyrians and Babylonians, wheat was written about in stone ruins that date back to around 3000 BC. Wheat has been cultivated in China as far back as 2700 BC, and it is clear from the records that they established complex rites to honour it.
Wheat is now grown on almost half of the earth's surface, and it provides the primary diet for billions of people. Noodles, steamed bread, and many other food products made with wheat flour are famous throughout Asia, even in locations where rice has long been eaten. Ploidy level refers to the number of chromosomes in a species. For example, with 28 chromosomes, tetraploid wheat is the source of the diploid variation (the 14-chromosome diploid wheat). Wheat that has a genome sequence known as wheat (Triticum aestivum) contains 42 chromosomes, just like hexaploid cereal (such as wheat), which has an extensively cultivated gene pool.
The success of using genomic research to answer issues of evolution is today seen as one of the most important discoveries. For example, with today's technology, by combining naturally occurring tetraploid and diploid wheat varieties and the two tetraploid kinds found in nature, it was possible to determine that the two contemporary wheat types are among 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 )
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| 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).
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| 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).
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| 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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