RICE: INTRODUCTION, ANATOMY, CHEMICAL COMPOSITION AND NUTRITIVE VALUE

INTRODUCTION

Rice (Oryza sativa,) belongs to the Gramineae or grass family and the tribe Oryzeae. Rice is a semi-aquatic plant which can thrive under flooded soil condition. Rice plant possesses the roots of a dry land crop, which are able to pass moisture from roots to stem and oxygen from leaf through stem to roots. There are different varieties of rice suited for growth under different environmental conditions. It is estimated that there are over 1,00,000 different varieties of rice in the world.

Rice is one of the oldest and most important food crops of the world. It is staple food for more than half of the World’s population and about 92% of the World’s rice crop is produced in the Asian continent (FAOSTAT).It is the predominant dietary energy source providing 20% of the world's dietary energy supply, while wheat supplies 19% and maize (corn) 5%.The total area of rice cultivation varied between 350 – 360 million acres globally during the last few years. India ranks second in rice production in the world after China with production of 116.42 MT in the year 2018-19.

In India, several types of dishes are prepared from rice flavoured with spices and other ingredients. Other rice preparations include parched rice, puffed rice, and fermented preparations like ‘idli’ and ‘dosa’. By products of rice, such as bran, rice polishing and paddy straw are used for feeding livestock.

ANATOMY OF THE PADDY GRAIN

A combined knowledge of the physical properties and anatomical composition of the rice grain is a prerequisite in gaining a closer understanding of what happens to the grain in the different postharvest operations. The understanding of the anatomy of the rice grain will clarify the reasons why rice kernels break so easily on mechanical impact during the physical operations of threshing and milling, and under thermal stress during drying. methods on the surface tissue of the grain kernel and the kernel itself, will indicate the importance of the correct adjustment of hulling machines in order to prevent breakage, and ensure higher milling recovery.

The anatomy of the rice grain consists of a brown rice kernel enclosed by the husk. The brown rice grain consists or a bran layer, a germ and the starchy centre of the grain endosperm (Fig-2).

Structure of Rice grain

The Husk

The most visible part or a rough rice grain is the husk. This is also known as the hull. This is formed from the two leaves of the spikelet namely the palea covering the ventral part of the seed and the lemma covering the dorsal portion. Both parts are longitudinally joined together by an interlocking fold. This fold is a weak point in the hull and easily breaks up when a twisting force is applied to the grain. The upper end of the two hull sections transfer into the apiculus sections and finally ends in the pointed awn.

At the lower part, where the grain is fixed on the panicle is a tiny leaf-shaped part called the

sterile lemma and then the rachilla. Normally the panicle breaks off during threshing, however a

small part of the pedicel frequently remains attached to the grain. The husk is formed mostly of

cellulose and fibrous tissue and is covered with very hard glass-like spines or trichomes. The

present of this make the husk abrasive and very hard thus, they give the grain a good protection

against insects, microorganism, moisture and gases

The caloric value to the hulls is rather high and ranges from 3000 to 3500 kcal/kg making hull an

important source of energy in agriculture.

However, the most disturbing presence in rice hull is high proportion of silica which causes

considerable damage to processing equipment through excessive wear of machine parts and

interconnecting transfer facilities.

The Pericarp

When the hull is removed, a thin fibrous layer can be seen called the pericarp, frequently known as the "silver skin". The layer is usually translucent or greyish in colour. When the pericarp is not translucent, but reddish in colour the gain is referred to as red rice. It is considered as an integral part of the brown rice kernel (caryopsis) but is easily removed in the or whitening process. The main function of this layer is to serve as an additional protective layer against moulds and quality deterioration through oxidation and enzymes due to the movements of oxygen, carbon dioxide and water vapor.

The pericarp actually consists of three layers namely epicarp, mesocarp & cross layer immediately under the pericarp layer is the testa or sometimes called tegmen layer which is only a few cells in thickness but with less fibrous than the pericarp layer. This layer is rich in oil and protein but its starch content is very low.Sometimes this layer is considered as part of the seed coat but because of its oil content, it is normally considered as the outermost layer of the bran.

The Bran Immediately under the testa or tegmen layer is the bran layer or aleurone layer (Fig- 2). This part is the main constituent removed in the whitening stage during milling. It has a very low starch content but has a high percentage of oil, protein, vitamins and minerals. Because of its high oil, content, the bran is easily affected by oxidation when the oxygen in the air comes in contact with oil.

In the milling process, the higher milling degree indicates a greater percentage of bran removed. Table 1 shows the degree of milling as determined by the quantity of the outer layer removed from the brown rice kernels. When rice is fully milled the vitamins (B complex), protein, mineral, and oil contents are lessened. This explains why persons with beri-beri (thiamine deficiency) are advised to eat brown rice. This also probably explains why persons who eat well milled rice are prone to be protein deficient or even malnourished. Thus, it is not surprising that some dieticians recommend the eating of regularly milled or even undermilled rice.In the processing Industry, the vitamins in the grain can be retained by parboiling before milling.

This allows the movement of nutrients from the bran layer to the inner part of the grain thus, making the vitamins available in the milled rice.

The Embryo

The embryo is located at central bottom portion of the grain, where the grain has been attached to the panicle of the rice plant. This is the living organism in the grain which develops into a new plant. The embryo respires by taking in oxygen in the air, consumes food which comes from the starch in the grain itself while simultaneously releasing moisture and heat. This explains why grains during storage have the tendency to decrease in weight as a result of the loss in moisture and dry matter content in the endosperm. During milling, the embryo is removed resulting in an indented shape at one end of the milled rice grain.

The Endosperm

When the husk, the pericarp, the bran and the embryo are removed, what remains is the endosperm. It mainly consists of starch with only a small concentration of protein and hardly any minerals, vitamins or oil. Because of its high percentage of carbohydrates, its energy value is high. In the central core of the grain the starchy cells are somewhat hexagonal in shape, but between the centre and outside they are elongated with the long walls radiating outwards from the centre

CHEMICAL COMPOSITION AND NUTRITIVE VALUE

The chemical composition of rice is influenced by genetic and environmental factors. The nutritional value of rice depends on the strain of rice, such as white, brown, red, and black (or purple) varieties having different prevalence across world regions.The germ, the pericarp and aleurone layers are richer than endosperm in nutrients like protein, minerals and vitamins however, get separated from the grain during milling along with the husk.

Carbohydrate:

The major carbohydrate of rice is starch which is 72- 75 per cent.The amylase content of starch varies according to the grain type. The longer grain and superior types containing upto 17.5 per cent amylase while some coarse type are completely devoid it. Rice also contains free sugars like glucose, sucrose, dexrine, fructose and raffinose

Protein:

The protein content of rice is 7 per cent. Oryzenin is the principal protein of rice. The proteins of polished rice have a lower biological value but a higher digestibility than those of rice bran and rice polishing. Parboiling has no effect on the biological value or digestibility of the proteins. Rice is deficient in lysine and threonine.

Minerals:

Most of the minerals present in the rice are located in the pericarp and germ. Polished rice is poor in calcium and Iron. Coloured types of rice contain more iron than the white rice. The phosphorus content is high, about 4 per cent of which is present as phytic acid.

Enzymes:

Rice contains amylases, proteases, lipases, oxidases, peroxidises and phenolases. On storage the amylase, lipase, peroxidise activities decrease. In fresh rice α amylase is responsible for its sticky consistency after cooking.

Pigments:

Coloured rice contains anthocyanins and carotenoids.The chemical composition of rice obtained by dehusking by different methods is given in Table-5.1Table 5.1 Chemical composition of rice obtained by different dehusking .