Primary processing of Rice
Rice is the staple food for the majority of the world's population and is cooked in boiling water and eaten mostly with cooked pulses, vegetables, fish or meat. It is also used in many food preparations like idli and dosa. Rice with the husk is called paddy. Primary processing of rice consists of cleaning, grading, dehusking (shelling) and milling (polishing). Dehusking and polishing are traditionally accomplished by hand pounding, using pestle and mortar. In modern rice milling, the two main steps involved are dehusking and polishing.
(i) Cleaning and grading: The paddy as received will contain foreign matter such as stones, clay particles, straw, chaff and dirt. These have to be separated in a paddy cleaner.
(ii) Dehusking (shelling): After cleaning, the outer husk is first removed by shelling process exposing the grain covered by a brown bran layer. It is sold in this condition as brown rice.
The shelling is carried out normally using two different types of shellers:
(a) Disc sheller: It consists of two discs. The inside surface of the discs is covered with a mixture of emery and hard cement. The clearance between the discs is adjusted close to the length of the paddy grain to be shelled. One plate is stationary, while the second plate revolves. The husk is removed by aspiration.
(b) Roller type Sheller: It consists of two horizontally set rubber rollers rotating in opposite directions, the differential rotation between the two being kept at about 200 per min. Dehusking is effected by the grain hitting the rotating rollers. The resulting brown rice contains the pericarp and germ almost intact. The breakage of rice is minimum with this machine.
(iii) Rice milling (polishing): The brown rice obtained by shelling can be milled (polished) further in a stage known as "pearling" using either a cone-type polisher or a horizontal-type polisher to remove the coarse outer layers of bran and germ, leaving a white grain. Sometimes, the polished rice is further treated with mineral substances such as talc or sugar to give the grain a bright shining surface.
Subsequently, a simple machine like huller came into existence. Hullers achieve both dehusking and polishing in one step. It is estimated that there are over 1, 30,000 hullers in operation throughout the country. They are largely located in the rural areas. More than 30% of paddy produced is processed in hullers.
(iv) Parboiled rice: Parboiling is an ancient process of India. More than 50% of paddy produced in the country is parboiled. Parboiling means partial boiling and cooking of rice in a limited water environment. For this reason, prior to milling, the paddy is fully soaked in water and then the drained paddy is cooked by steaming or by dry heat. The process gelatinizes the starch in the grain aiding the retention of much of the natural vitamin and mineral content. Surprisingly, parboiled rice takes longer to cook, but has the advantage of taking up more water during cooking and therefore increasing the yield.
(v) By-products of rice: The important by-products obtained in rice milling are rice bran oil, bran or polishing (good source of protein and fat), husk (fuel, insulating material, paper making, production of furfural).
(vi) Rice products: Of the 100 million tones of paddy, about 10% is converted to various products like flaked rice (Aval, Chewda or beaten rice), expanded rice (Puri, Murmura) and popped rice (Aralu, Kheel). The other rice products are instant rice (quick cooking rice), rice flour, rice starch etc.