Rice is the most popular grain global, supplying energy, complex carbohydrate, fiber, vitamin and mineral Rice is staple food on the earth sustain two thirds of the worlds population. 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 post-harvest operations. The anatomy of the rice grain will clarify the reasons why rice kernels break so easily on mecharicenical impact during the physical operations of threshing and milling. Methods on the surface tissue of the rice kernel will indicate the importance of the correct adjustment of hulling machines in order to prevent breakage, as an energy food, supplying nutrient rich complex carbohydrate that fuel the body’s physical activity. Rice is gluten free and most non allergenic of all grain. There is no question of additives or preservatives. Rice is low in fat, salt and has no cholesterol.
Rice hulls are the hard protecting coverings of rice. This is the most visible part of rough rice. Grain is the husk i.e. 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 T 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 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 cellulosic and fubrous 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 hulls 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.
Pericarp is another anatomy of rice grain when the hull is removed, a thin fibrous level can be seen, this is called the pericarp, frequently known as the silver skin. The layer is usually translucent but reddish in color the referred or greyish in color. It is considered as an integral part of the brown rice kernel, easily removed in the rice. The main function of the layer is serve as an additional protection layer against mold and quality deterioration through oxidation and enzymes due to the movement of oxygen, carbon dioxide and water vapor. The pericarp actually consists three layers namely epicarp, mesocarp and 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 then the pericarp layer. This layer is rich in oil and protein but its starch content is very low. Sometimes this layer is considered as the outermost layer of the brain.
Rice bran is a byproduct of the rice milling process and it contains various antioxidants that impart beneficial effect on human health, this part is the main constituent removed in the whitening stage during milling. It has very low starch content but has high percentage of oil protein, vitamins and mineral. Because of its high oil the bran is easily affected by oxidant when the oxygens in the air comes in contents. with oil in the process of milling grater percent of bran removed. When rice is fully milled the vitamins, protein, minerals and oil contents are lessened. This explains why persons with beri beri are advised to eat brown rice. This also probably explains why persons who eats 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. When the husk, the pericarp, the bran and the embryo are removed. What remain is the endosperm. It mainly consists of starch with only a small concentration of protein and hardly any mineral 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 out wards from the Centre.
The embryo is located at central bottom portion of the grain, where the grain has been attached to the panic of the rice plant. This is the living organism in the grain which develops in to a new plant. The embryo respires by taking in oxygen in the air consumes food which comes from the starch in in grain itself.
When the husk, the pericarp, the bran and the embryo are removed, what remains 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.
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.