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........

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Cereal structure, classification and microbiology

CEREALS

Introduction:

⮚     Cereal grains provide the world with a majority of its food calories and about half of its protein.

⮚      The cheapest source of energy.

⮚      The word “cereal” is derived from the most important grain deity, the Roman Goddess Ceres.

⮚      Botanically, however, the seed and the grain are not exactly the same despite their interchangeable use.

⮚      All the cereal grains are angiosperms, monocots, and belong to the monocotyledonous family, Gramineae or Grass family.

⮚      Also used as animal feed and for industrial purposes.

⮚      As a source of carbohydrates related to land use, Maize ranks first among the cereal grains, followed by rice, sorghum, and wheat.

⮚      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.

⮚      Over 90% of the rice is grown in Asia, where most of it is consumed too.

⮚      Although with similar uses, the pseudocereals or pseudograins are not grasses.
Ex. Amaranth, Buckwheat, Quinoa, Chia seeds.

  

Composition:

The average nutritional composition of Cereals is as follows.

Although these are typical values, compositions vary depending on varieties of the particular grain, geographical and weather conditions, and other factors. The moisture content of 10-14% is typical of properly ripened and dried grains. When the moisture content of grains 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.

Carbohydrates: Carbohydrates are the major constituents comprising about 80% of the dry matter of the cereals. The carbohydrates are customarily considered in two parts: the "crude fiber," and the "soluble carbohydrates". Knowledge of the proportion of fiber and soluble carbohydrates is of importance in relation to nutritional and digestibility studies.

Protein: The protein content of the different cereals varies and that of rice is lower than that of all other cereals. The protein content of different varieties of the same cereal also varies. Proteins are found in all the tissues of cereal grains, the higher concentration occurring in the embryo, scutellum, and aleurone layers than in the starchy endosperm, pericarp, and testa. Within the endosperm, the concentration of protein increases from the center to the periphery. The types of proteins present in cereal are albumins, globulins, prolamines (gliadins), and glutelins.
They are generally deficient in lysine content. The biological value of the proteins in germ and aleurone is higher than that of the endosperm proteins.

Lipids: More lipids are present in germ and bran than in other parts of the grain. The lipids are mostly the triglycerides of palmitic, oleic, and linoleic acids. Cereals also contain the phospholipid, lecithin.
The lipids in milled cereal products undergo two types of deterioration: hydrolysis due to the action of the enzyme lipase present in the grain and oxidation by the action of the enzyme lipoxygenase or non-enzymatically in the presence of oxygen. These changes give rise to unpleasant flavors. When the germ is separated from the endosperm as in the milling of cereals, the keeping quality of the milled products is improved.

Minerals: The husks of the cereal caryopsis of rice, barley, and lye are rich in minerals. A considerable part of phosphorus in cereals is present in the form of phytin, the calcium, magnesium salt of phytic acid (inositol hexaphosphoric acid).

→ Phosphorus and calcium present in phytin are not available for absorption.

→ Some mineral elements like copper, zinc, manganese, and iron are also present in very small quantities in cereals, oats being particularly rich in iron.

→ Rye and oats have the highest calcium content of all cereals.

Vitamins: B-group vitamins are present in all cereals, more or less to the same extent, except niacin, which is more in wheat, rice, barley, and sorghum. The distribution of the vitamins in different grains and in different parts of the same grain is not uniform. Oils from cereal grains are rich in vitamin E.

Enzymes: Cereal grains contain many enzymes and of these the amylases, proteases, lipases, and oxidoreductases. The lipases of cereals are responsible for the fatty acids appearing during the storage of cereals and their products.

General structure:

⮚      Cereal grains develop from flowers (florets) and these florets are enclosed in bracts (leaves).

⮚      Bracts are of two types Palea & Lemma.  Ovary or Ovule are developed inside the bracts into a grain.

Depending upon the structure grains are of two types.
(1) Covered Caryopsis: It is the coated grain. The seed comprises the Seed coat, Germ,& Endosperm and in addition fused palea and lemma which constitute the husk outside the fruit for ex. rice, oats, some varieties of barley, and sorghum. Thus, these grains are covered or coated caryopsis.

(2) Naked Caryopsis: These grains consist of fruit coat (pericarp) & seed only. In wheat, rye, common varieties of maize, some varieties of sorghum and barley, the lemma and palea become free from the grain at threshing and form chaff. Thus, wheat, oats, maize, and sorghum are said to be naked grains called a caryopsis.

      1.            Pericarp: It is known as the part of the fruit formed from the wall of the ripened ovary surround the seed coat (the testa). Pericarp sometimes consists of 3 layers: the Epicarp, Mesocarp, & Endocarp.
→ The innermost layer of the pericarp tears during the ripening of the seed & in mature grain they are represented by a layer of branching hyphae-like cells known as tube cells.

      2.            Seed Coat: (Testa) It can be a thin single or double layer. The inner layer of the testa of wheat is deeply pigmented which gives the grain its characteristics color.

      3            Aleurone: It is a protein found in protein granules in matured seeds.
→ The term aleurone refers to the aleurone layer. It is the outermost layer of endosperm.
→ It contains about 20% each of protein, oil, and minerals & 10% of the total sugars present in the grain.
→ The cells are rectangular with thin cell walls consisting of one or three cell layers.

      4.            Endosperm: It is the storage house of grain and consists of starch and protein.
→ Starch having spherical granules are embedded in a matrix of protein.
→ The granules in Wheat, Rye, Barley, Maize, & Sorghum are simple whereas those in rice are compound i.e., about 60 granules are compound together.
→ If the cereal grain germinates, the seedling uses the nutrients provided by the endosperm until the development of green leaves that allow photosynthesis to begin.

      5.            Embryo: (Germ) The embryo (or germ) is a thin-walled structure, containing the new plant. It is separated by the scutellum (which is involved in the mobilisation of food reserves of the grain during germination) from the main part of the grain, the endosperm.

Fig.: The general structure of a grain

Contamination of Cereals:

⮚      Freshly harvested grains contain a few thousand to millions of bacteria per gram and from none to several hundred thousand mold spores. Bacteria are mostly in the families Pseudomonadaceae, Micrococcaceae, Lactobacillaceae, and Bacillaceae.

⮚      Most of the microorganisms are removed with the outer portions of the grain during milling. The milling processes, especially bleaching, reduce numbers of organisms, but there then is a possibility of contamination during other procedures, such as blending and conditioning.

⮚      Bacteria in wheat flour include spores of Bacillus, coliform bacteria, and a few representatives of the genera Achromobacter *, Flavobacterium, Sarcina, Micrococcus, Alcaligenes, and Serratia.

⮚      Mold spores are chiefly those of aspergilli and penicillia, with also some of Alternaria, Cladosporium, and other genera.

Spoilage:

⮚      The major factors involved in the spoilage of stored grain by molds include microbial content, moisture levels above 12 to 13 percent, physical damage, and temperature. Numerous different molds can be involved, but the most common are species of Aspergillus, Penicillium, Mucor, Rhizopus, and Fusarium.

Preservation:

⮚      Cereals usually have lower water activity (a_w) so it is not very difficult to prevent the growth of microbes as long as these products are kept dry.

⮚      The recommended storage temperature is  4.4 to 7.2^o C for dry products.

⮚      Insecticides and fumigants, ammonia (2 percent) and propionic acid (1 percent) reduce mold growth in high-moisture corn.

Storage of cereal grains:

After harvest, correct storage of the grain is important to prevent mold spoilage, pest infestation, and grain germination. If dry grains are held for only a few months, minimum nutritional changes will take place, but if the grains are held with a higher amount of moisture, the grain quality can deteriorate because of starch degradation by grain and microbial amylases (enzymes). Cereal grains, although stored in the dormant state, continue to respire producing heat, water, and carbon dioxide. This facilitates the growth of molds which are invariably present in the grain. The growth of molds produces many enzymes that cause chemical deterioration of grains.

Processing Aspect of Cereals: 

Slightly different milling processes are used for the various grains, but the process can generally be described as grinding, sifting, separation, and regrinding. The final nutrient content of cereal after milling will depend on the extent to which the outer bran and aleurone layers are removed, as this is where the fiber, vitamins, and minerals tend to be concentrated.

Toxicity:  Recently, acrylamide (described as a probable carcinogen) has been found in starchy baked foods.

No link between acrylamide levels in food and cancer risk has been established and based on the evidence to date, the UK Food Standards Agency has advised the public not to change their diet or cooking methods. However, the Scientific Committee on Food of the European Union (EU) has endorsed recommendations made by the Food and Agriculture Organisation/World Health Organization which include researching the possibility of reducing levels of acrylamide in food by changes in formulation and processing.

Health benefits:

There is evidence to suggest that regular consumption of cereals, specifically whole grains, may have a role in the prevention of chronic diseases such as coronary heart disease, diabetes, and colorectal cancer. The exact mechanisms by which cereals convey beneficial effects on health are not clear. It is likely that a number of factors may be involved, e.g. their micronutrient content, their fiber content, and/or their glycaemic index. As there may be a number of positive health effects associated with eating whole grain cereals, encouraging their consumption seems a prudent public health approach. To increase the consumption of wholegrain foods, it may be useful to have a quantitative recommendation. Additionally, a wider range of wholegrain foods that are quick and easy to prepare would help people increase their consumption of these foods.

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