Cereals are the members of monocotyledonous family Gramineae. They are fruits of cultivated grasses. The principal cereal crops are maize, sorghum and the millets. Wheat, rice and corn together contribute three-fourths of the world’s grain production. In general cereal grains provide the bulk energy consumed on earth. The cereal crop that are grown for their edible fruit are generally called grain and are botanically referred as caryopsis. Two major components of cereal seeds are i) endosperm and ii) embryo or germ. The endosperm that forms bulk of seed is the stored food of grain and is source of energy for animals.The outer wall of grain is called pericarp that develops from the wall of ovary. A semipermeable layer under pericarp which surrounds the embryo and is derived from inner ovary wall is called ‘testa’. The testa layer is permeable to water and is important for germination. The third layer which contains thick walled cells that are free of starch is called ‘aleurone’ layer. The pericarp, testa and aleurone layer are collectively called bran. The hulled kernels of various cereal grain such as oat, rye, wheat and barley are called groat.
Rice: Rice is annual grass crop that is grown at different climatic condition throughout the world. Oryza sativa L. is the predominant species of rice cultivated [1]. Globally, there is increase in rice production after World War II. Extensive production of rice with high yield is possible because of increased use of fertilizers, effective control of pests and widespread adoption of improved varieties of rice. Rice is harvested when ripen.
Rice is harvested as rough rice or paddy in which caryopsis is enclosed in tough hull or husk that contains silica. The hull provides protection to caryopsis against insect, pests and unfavorable climatic conditions. The embryo or the germ is very small and is located on the central side at the base of the grain. Typical structure of rice grain is as shown below.
Wheat: it is generally agreed that wheat was one of the first cereal grain to be cultivated. It was cultivated particularly in Persia (Iran, Egypt, Greece and Europe) over period of 1957 – 1970. Wheat is staple food for many countries and is very important due to its use to make wide variety of food product such as biscuits, cookies, cakes, pasta, noodles, bread and many more. Sometimes it is also called king of cereals. Wheat contains more protein than rice. The color of wheat (red to white) is due to the pigment present in seed coat and the color is largely dependent on species and other factors.
Wheat is classified on basis of physical characteristics such as red or white, soft or hard and spring or winter. Winter wheat has larger growing period than spring wheat. It is normally higher yielding than spring sown wheat. The wheat kernel is composed of pericarp, testa any hyaline layer, aleurone layer and endosperm.
Pericarp: Wheat grain are ovoid in shape and rounded at both ends. The germ is prominent at one end and other end is fine hairs. Ventral side of the grain is an indentation of the crease in the folding of the aleurone layer and all covering layer. At the bottom of the crease is the deeply pigmented vascular strands. In wheat, the whole pericarp is thin and papery in ripe grain. The outer layer upon splitting off during cleaning, conditioning, and milling are known as ‘bees wings’.
Testa and hyaline layer: The testa or seed coat is a thin, single or double layer with the cellular structure almost obliterated. The inner layer of the testa of wheat is often deeply pigmented and gives the grain its characteristics color. The testa is impermeable to water. The hyaline layer is colorless and devoid of any cellular structure.
Aleurone layer: The aleurone layer in wheat germ consists of single layer of thick walled cubical cells and comprise of 7 % of grain weight. About 20 % of dry matter is oil, 20 % minerals and 20 % protein and 10 % sugar. Aleurone layer consists mainly of minerals phytic acid, compound of phosphorous in the form of K, Mg salt together with some protein.
Endosperm: The endosperm of wheat consists of a thin cell wall which varies in size, shape, and composition at different parts of endosperm itself. These cells are small and cubical. The cells consists mainly of starch and protein. The starch are in the form of ventricular or spherical granules highly packed together with protein filling inter-granular space.
Maize: Botanical name of corn is Zea mays L. This is the important cereal crop in North America. It apparently originated Mexico. Seed of corn is much larger than other cereal. The corn seed is a single fruit called the kernel. In includes an embryo, endosperm, aleurone and pericarp. The embryo of maize is relatively larger (10 – 13 % of grain) than any other cereal grain. The pericarp is a thin layer that has protection role for the endosperm and embryo. Pericarp adheres tightly to the outer surface of aleurone layer and thought to impart semipermeable properties to the corn kernel. The pericarp is composed of dead cells that are cellulosic tubes. Here, pericarp and testa are fused to form hull. Part of germ overlaying the germ is called tip cap. Industrially, maize is used for manufacture of starch, sugar syrup, glucose, beer and alcohol.
Barley: Botanical name of barley is Hordeum Vulgare L. Barley belongs to the grass family and is one the ancient world’s crop. It ranks the top ten crops in the world and is fourth among cereals after rice wheat and maize. It is widely used as human food and also to produce malt products. In barley husk is usually pale yellow whose cells are dead when barley is matured. The caryopsis is located in the husk and the pericarp is fused to the seed coat or testa. Within the seed coat, the largest tissue is the starchy endosperm that is bonded to the aleurone layer. The embryo is located at the base of the grain.
Oat: Botanical name of oat is Avena sativa. Oat is grown for both grain as well as forage needs. The hull contains leaf like structure that tightly enclose the groat and provide protection during seed growth. The hull contributes about 30 % of total weight of kernel. At early stage of growth, the hull assists in nutrient transport for groat. The hull contains 30 – 40 % hemicellulose and is regarded as good source of dietary fiber [2].
After removing the hull, the groat is extensively covered with hairs and is longer and more cylinder than wheat and barley. The groat consists of three distinct components; bran, germ and starchy endosperm.
Rye: Two popular species of rye are botanically named as Secale cereale and Secale cereale. Rye is mostly used in bread making. The ripe grain is normally grayish yellow, ranging from 6 -8 mm in length and 2 – 3 mm in width. The seed consists of any embryo attached through a scutelum to the endosperm and aleurone tissues. These are enclosed by fragments of epidermis, testa or seed coat and the pericarp or fruit coat. The starchy endosperm is surrounded by aleurone layer and merges into scutullum located between endosperm and embryo. The starchy endosperm represents the bulk of the bulk of kernel and is composed of three types of cells; peripheral or subaleurone, prismatic and central.
Sorghum: Botanical name of sorghum is Sorghum bicolor L. In developing countries of Asia and Arica, it is a major source of energy and protein. The sorghum kernel is roughly spherical and is composed of seed coat, embryo and endosperm. The seed coat consists of fused pericarp and testa. The pericarp forms extreme outer layer that is surrounded by waxy cuticle. The embryo consists of large scutellum, an embryonic axis, a plumule and primary root. The mature sorghum grain comprises of about 10 % embryo, 8 % pericarp or bran layer and 80 % endosperm [3]. The embryo is rich in protein, lipid, mineral and B vitamins.
References:
1. Owens, G. (2001). Cereals processing technology. CRC Press.
2. Chakraverty, A., Mujumdar, A. S., & Ramaswamy, H. S. (2003). Handbook of postharvest technology: cereals, fruits, vegetables, tea, and spices (Vol. 93). CRC press.
3. Kulp, K. (2000). Handbook of Cereal Science and Technology, revised and expanded. CRC Press.
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.