Wheat Structure, chemical composition,

Wheat (Triticum)

Wheat is being cultivated from prehistoric times (5000 BC). Wheat originated in the “cradle of civilization” in the Tigris and Euphrates river valley, near what is now Iraq. The wild wheat types, emmer (Triticum dicoccum) and einkorn (Triticum monococcum), was developed from a type of wild grass. These are husked wheat, i.e, the lemma and palea from the husk, which remains attached to the kernel after threshing. Emmer is generally regarded as one of the ancestors of the wheat grown today. Seeds that are sown in late autumn germinate slowly until the spring. Snow in winter covers the young plants and protects them from frost. This is winter wheat. Seeds that are sown in spring and the crop harvested in early autumn is known as spring wheat. Wheat belongs to the genus Triticum and there are over 30,000 species and varieties. The principle wheat of commerce are the common or bread wheat (T. aestivum), macaroni wheat (T. durum), and club wheat (T. compactum).

Structure of Wheat Flower:

The wheat flower does not have petals or sepals. Each female flower consists of an ovary and The male flowers have three stamens that are generally gold or green in color. Wheat generally self-pollinates. Three to five such flowers, attached alternately to opposite sides of a central axis or rachilla and subtended by two empty scales, make up the spikelet. Each spikelet of wheat is also known as ear. The entire inflorescence, usually consisting of 15-25 spikelets is called a spike or a head of wheat.

Wheat Morphology:

The wheat grain or the wheat kernel is one-seeded fruit, called a caryopsis. A wheat kernel is about 5-8 mm in length, 2.5-4.5 mm in width, and 30-50mg in weight. The wheat kernel is somewhat thicker toward the end where the embryo is located. At the opposite end, the hairs of the brush are located. The grain has a single cotyledon and when dissected can be divided into different parts.

1.      Bran:

1.1. The outer layer or the seed covering is called the bran.

1.2. The bran comprises about 15 percent of the seed weight.

1.3. Bran can be further divided into the pericarp (which consists of epidermis, hypodermis, cross-layer, tube cells, seed coats, and hyaline layer).

1.4. The epidermis is a layer of long thin-walled rectangular cells.

1.5. Next to the epidermis is the hypodermis.

1.6. Sometimes next to the hypodermis tube cells are present but not always.

1.7. The bran is a source of protein, large quantities of the three major B-vitamins, trace minerals, and dietary fiber. Aleurone Layer:

2.      Aleurone Layer:

2.1.   Aleurone contains 7% of the total weight of wheat.

2.2.   It contains 20% each of protein, oil, and minerals & approx 10% carbohydrate.

2.3.   It also contains nicotinic acid. Some researchers found that in some wheat varieties it contains phytic acid too.

2.4.   Synthesizes most enzymes to digest and mobilize nutrients (eg: hydrolytic activity for melting barleys to reduce starch to fermentable sugars). so due to this, it is undesirable in flour making.

3.      Endosperm:

3.1.   The endosperm is surrounded by 2 or 3 layers of the aleurone layer.

3.2.   It is also known as the storehouse of the wheat and stores both starch and protein.

3.3.   The starch molecules are embedded inside the protein matrix.

3.4.   Endosperm is the main part of the seed and it accounts for 80 percent of the seed weight.

3.5.   The highest content of protein is observed in the cells of the subaleurone layer of the endosperm. The closer to the center of the grain, the lower the protein content.

3.6.   It is the most desirable layer in flour making as it contains the highest amount of carbohydrates and proteins.

4.      Embryo (Germ):

4.1.   The germ lies at one end of the seed and is a tiny part of the kernel.

4.2.   The germ is responsible for germination when planted in soil. It is a rich source of B-complex (especially Thiamine )vitamins, oil, vitamin E, and natural plant fat.

4.3.   There is a very fine line between endosperm and embryo which is known as Scutellum due to which many researchers consider endosperm and embryo as one layer.

4.4.   Scutellum immobilizes the stored food reserved in endosperm & transports it into the embryo.

Wheat Structure (Simple)

Chemical composition:

The chemical composition of the wheat kernel varies widely, being influenced by the 
environment, soil, and variety.

1.      Carbohydrate: Wheat contains 60% to 80% total carbohydrate content which is divided into two classes.

Ø  Soluble (Sucrose, Raffinose and maltose).

Ø  Insoluble {Starch(amylose + amylopectin)}

Amylose contributes around 25-30% of cereal starch

Ø  Practically all of the starch is in the endosperm, while the soluble sugars are mostly found in the germ.

Ø  The carbohydrates of bran are largely cellulose and hemicellulose.

Ø  Pentosans although their content is low (2-3%) are important owing to their water absorbing capacity i.e. 10 times of their mass. In combination with other hemicelluloses, they form the basic structure of endosperm cell walls.

2.      Protein: Wheat contains about 11-13% of total protein which is highest among cereals.

Ø  The bran and germ proteins have a higher content of essential amino acids and are more balanced composition than the inner endosperm proteins.

Ø  Biological value (BV) of endosperm proteins are much lower than that of whole wheat protein because bran and germ proteins have higher essential amino acid content.

Wheat proteins

1..1.   Glutenins: These proteins are resilient, prone to rupture, rubbery and resistant to Extention

Ø  It is too elastic in nature.

Ø  35 to 40% of flour proteins

1..2.   Gliadin: These proteins are gluppy, very cohesive, little resistance to Extention.

Ø  Low intrinsic viscosity.

Ø  Hydrophobic interaction and hydrogen bonds in dough formation.

Ø  There are four types: α, β, γ, and ω:

α, β and γ have got intra-chain disulphide bonds (Cys, Met). They hence are made of α helix and β sheets.

ω has got no disulphide bonds {no Cys or Met and low amount of basic amino acids (His, Arg and Lys)}. They are made of β turns.

1..3.   Gluten:

Gliadin + Glutenin = Gluten

Ø  Gluten has properties between gliadin and glutenin.

Ø  Gluten is used as an adulteration material in starch.

Ø  Rheological properties of wheat are due to gluten.

Ø  The gluten proteins are rich in glutamic acid, mainly as glutamine and proline.

2.      Lipids: The total lipid content of Indian wheat varies from about 0.97 to. 2.28 percent.

Ø  The lipid content of endosperm (1-2 percent) is less than that of bran (5-6 per cent) or germ (8-15 percent) but the proportion of compound lipids in the endosperm is much higher.

Ø  Wheat contains triglycerides of palmitic, oleic, and linoleic acids.

Ø  Wheat germ oil is rich in Vitamin E and produced commercially.

Ø  Wheat contains lipoxygenase enzyme which gives off the flavor of lipids & thus it is important to remove germ from wheat.

3.      Minerals: Wheat contains a significant amount of iron, phosphorus, magnesium, manganese, copper, and zinc.

Ø  Sometimes potassium and phosphorus in the phytic form is also present.

Ø  Phytic acid as an anti-nutrient substance and impairs the absorption of  zinc, iron and calcium.

Ø  When seeds sprout, phytate is degraded and the phosphorus released to be used by the young plant. Phytic acid is also known as inositol hexaphosphate or IP6.

Ø  It’s often used commercially as a preservative due to its antioxidant properties.

Ø  Not only is phytic acid an antioxidant, it may also, be protective against kidney stones and cancer.

4.      Vitamins: Whole wheat is a good source of thiamine and nicotinic acid, but is relatively poor in riboflavin.

Ø  Niacin is relatively more in Wheat and Rice than other cereals.

Ø  Vitamin is present in wheat germ.

Ø  scutellum contains the highest amount of thiamine among wheat. Due to this white wheat flour is devoid of a major part of B-vitamins.

5.      Enzymes: Protease, Amylase, Lipase, oxidoreductase are found in the germ of wheat.

Ø  Lipase and oxidoreductase can't retain if the temperature of processing exceeds 50⁰C.

     For sensory characteristics to be attained by wheat it is important that enzyme amylase get 

     activated.

To be continued in next post........

Below are the recommendations of the book if. Click and buy from amazon...