Grains and their structure

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Kernels of sorghum and millets show considerable diversity in colour, shape, size and certain anatomical components (Table 2).

The basic kernel structure is similar in sorghum and different millets. The principal anatomical components are pericarp, germ or embryo and endosperm. In finger, prove and foxtail millets the pericarp is like a sack, loosely attached to the endosperm at only one point. In these utricle-type kernels the pericarp easily breaks away, leaving the seed-coat or testa to protect the inner endosperm. The kernels of sorghum and pearl millet are of the caryopsis type, in which the pericarp is completely fused to the endosperm.

The relative distribution of the three main kernel components varies. In the sorghum kernel the distribution by weight is pericarp 6 percent, endosperm 84 percent and germ 10 percent (Hubbard, Hall and Earle, 1950). In pearl millet, it is pericarp 8.4 percent, endosperm 75 percent and germ 16.5 percent (Abdelrahman, Hoseney and Varriano-Marston, 1984). The ratio of endosperm to germ in pearl millet is 4.5:1, while in the sorghum kernel it is 8.4:1. In finger and prove millets the germ is very small and therefore the endosperm-togerm ratio, 11:1 to 12:1, is much higher than in sorghum.

Pericarp

Pericarp is the outermost structural component of the caryopsis and is composed of three sublayers, namely epicarp, mesocarp and endocarp. The epicarp is further divided into epidermis and hypodermic. In the sorghum caryopsis, the epidermis is composed of thick, elongated, rectangular cells which have a coating of cutin on the outer surface. Often a pigment is present in the epidermis. The hypodermic is composed of slightly smaller cells than the epidermis and is one to three cell layers in thickness. The mesocarp, the middle part, is the thickest layer of the sorghum pericarp, but its thickness varies widely among genotypes. Mould resistance in sorghum is associated with thin mesocarp. Grains with thick mesocarp on a hard endosperm are preferred for dehulling by hand-pounding. The endocarp, the innermost sublayer of the pericarp, consists of cross cells and a layer of tube cells which transport moisture into the kernel. During dry milling of sorghum, the breakage occurs at the cross and tube cell layers.

TABLE 2: Structural features of kernels of sorghum and some millets

Grain Type Shape Colour 1 000-kenel

weight

(g)

Sorghum Caryopsis Spherical White, yellow, red, brown 25-30
Pearl millet Caryopsis Ovoid, hexagonal, globose Grey, white, yellow, brown, purple 2.5-14
Finger millet Utricle Globose Yellow, white, red, brown, violet 2.6
Proso millet Utricle     4.7-7.2
Foxtail millet Utricle     1.86
 

Seed - coat

Alcurone

Grain Number of layers Pigmented Thickness (pm) Number of layers Cell size (pm)
Sorghum 1 Sometimes 0.4 1  
Pearl millet 1 Sometimes 0.4 1 16-30 x 5-15
Finger millet 5 Yes 10.8-24.2 1 18 x 7.6
Proso millet 1 No 0.2-0.4 1 12 x 6
Foxtail millet 1     1  
 

Starch granules

Protein bodies

Grain Diameter (Ám) Peripheral zone (Ám) Corneous zone (Ám) Floury zone (Ám) Type Size (Ám) Location
Sorghum 20-30       Simple 0.3-3 All areas
Pearl millet 10-12 6.4 6.4 7.6 Simple 0.6-0.7 All areas
Finger millet 3-21 8-16.5 3-19 11-21 Simple/ compound 2.0 Peripheral/ corneous
Proso millet 2-10 3.9 4.1 4.1 Simple 0.5-1.7 Peripheral
Foxtail millet 10            

TABLE 2 (continued)

Grain

Germ

  Size (Ám) Endosperm:germ
ratio    
Sorghum   8.4:1
Peart millet 1 420 x 620 4.5:1
Finger millet 980 x 270 11:1
Proso millet 1 100 x 310 12:1
Foxtail millet   12:1

The pericarp of the pearl millet caryopsis consists of an epicarp with one or two cell layers, a mesocarp that varies in thickness because of genetic factors and an endocarp made up of cross and tube cells. The mesocarp layer of pearl millet does not contain starch granules; these are found only in sorghum mesocarp. During decortication or milling, the pericarp of pearl millet breaks at the cross and tube cell layers and fragments of endocarp may remain with the endosperm.

Seed-coat or testa

Just underneath the endocarp is the testa layer or seed-coat. In some sorghum genotypes the testa is highly pigmented. The presence of pigment and the colour are a genetic character. The thickness of the testa layer is not uniform. It is thick near the crown area of the kernel and thin near the embryo portion. In some genotypes there is a partial testa, while in others it is not apparent or is absent. In pearl millet the testa layer is thin and sometimes pigmented. In other millets the testa is always pigmented and is only a single layer thick. In finger millet the testa is very thick, with five cell layers, and is also pigmented.

Endosperm

The largest component of the cereal kernel is the endosperm, which is a major storage tissue. It is composed of an aleurone layer and peripheral corneous and floury zones. In all the millets and sorghum, the aleurone layer is a single layer of cells which lies just below the seed-coat or testa. The aleurone cells are rich in minerals, B-complex vitamins and oil and contain some hydrolysing enzymes.

The peripheral endosperm is distinguished by long rectangular cells which are densely packed and contain starch granules and protein bodies enmeshed in the protein matrix. The starch in these cells is therefore not easily available for enzyme digestion, unless the protein associated with it is also reduced (Chandrashekar and Kirleis, 1988). The matrix protein in general is alkalisoluble glutelin and the protein bodies are alcohol-soluble prolamins which account for the largest proportion of total protein in the kernel.

The protein bodies in the endosperm of sorghum and millets are spherical and differ in size among species and also within the endosperm of a single kernel. In sorghum the number of protein bodies decreases as the starch content increases from the peripheral zone to the central core where the floury endosperm is located. In pearl millet the protein bodies are more numerous in the floury than in the corneous zone. Adams, Novellie and Liebenberg (1976) have reported the presence of several enzymes, e.g. protease, 3-glucosidases, 3galactosidase and phosphatases, in the protein bodies of sorghum. The protein bodies of sorghum, pearl millet and finger millet also contain phosphorus, calcium, potassium and magnesium.

The starch granules of corneous endosperm are polyhedral and differ in size in different millet species. In floury endosperm the starch granules are spherical and bigger than the starch granules of the corneous zone. The starch in the floury zone is more amenable to enzyme digestion. In pearl and finger millets, the starch granules of the floury endosperm are spherical and big. The starch in pearl millet is hydrolyzed more slowly than that of sorghum by hog pancreatic amylase (Sullies and Rooney, 1977).

The proportions of corneous and floury endosperm determine the texture of the millet kernel. In soft-textured kernels there is more floury than corneous endosperm. In hard-textured kernels, on the other hand, there is more densely packed corneous endosperm than floury endosperm. Foxtail millet contains very little floury endosperm and is of a hard, corneous texture. Finger and prove millet kernels, with the endosperm evenly divided between the corneous and floury zones, are of intermediate texture. In pearl millet and sorghum the kernel texture varies widely, from all floury, very soft endosperm to all corneous, very hard or vitreous endosperm.

Grain texture is one of the most important determinants of the processing and food quality of sorghum and millets (Rooney, Kirleis and Murty, 1986). Hardendosperm sorghum when decorticated gives fewer brokers and more full grains than softer-endosperm sorghum (Desikachar, 1982). In dry milling, the flour yield is higher in corneous than in soft floury types. On the other hand, in wet milling the starch yield is higher in soft-endosperm genotypes. In the preparation of thick porridge, varieties with a higher proportion of vitreous endosperm are preferred. Such varieties are also suitable for popping (Chandrashekar and Desikachar, 1986; Murty, Patil and House, 1982). For preparation of bread, fermented or unfermented, the flour of soft-endosperm sorghum is highly preferred (Rooney, Kirleis and Murty, 1986).

Germ

The embryonic axis and the scutellum are the two major parts of the germ. The scutellum is a storage tissue rich in lipids, protein, enzymes and minerals. In pearl millet the ratio of germ to endosperm is larger than in sorghum and other millet kernels. The oil in the sorghum germ is rich in polyunsaturated fatty acids and is similar to corn oil (Rooney, 1978).


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