Pre-Cleaning: When paddy comes into the mill it contains foreign material such as straw, weed seeds, soil and other inert material. If this is not removed prior to hulling the efficiency of the huller and the milling recovery are reduced. Most pre-cleaners separate three groups of materials. The first separation is done by scalping or removing the objects that are larger than the grain. Either a flat oscillating screen or a rotary drum screen that allows the grain to pass through but retains straw can do this. The second separation retains the grains but allows broken grains, small stones and weed seeds to pass through. An air aspirator may also be incorporated to remove the dust and the light empty grains The capacity of the paddy pre-cleaner is usually based on the capacity of the rice mill. A pre-cleaner for a 3-ton/hr rice mill would normally have a 5 ton/hr cleaning capacity.
De-Husking: The husk layer is removed from the paddy by friction and the process is called either de-husking or de-hulling. De-husking was traditionally done using mortar and pestles but, in modern rice mills, it is done by passing the paddy grains between two abrasive surfaces that are moving at different speeds. After separating the husk and paddy, the husk is removed by suction (aspirated) and then transported to a storage dump outside the milling plant. The percentage of paddy that is de-hulled to produce brown rice during this process is called the hulling efficiency An efficient husker will remove 90% of the husk in a single pass. After the husk has been removed the brown rice goes to a paddy separator. The kernels that were not de-husked in the first pass will be separated and then returned to the de-husker.
Whitening or Polishing Process: White rice is produced from brown rice by removing the bran layer and the germ. The bran layer is removed from the kernel by applying friction to the grain surface either by rubbing the grains against an abrasive surface or against each other. The amount of bran removed is normally between 8-10% of the total paddy weight but this will vary according to whiteness required.
The process used to whiten brown rice can be classified as either:-
Abrasive: In this process the grain is whitened by the abrasive action of the rice kernel passing between a moving abrasive surface and stationary screen. The hard rough surface is usually stone or a carborundum type material. The abrasive process applies less pressure on the grain and is better suited for long grain varieties. Abrasive polishers can be either vertical or horizontal in design.
Friction: In the friction whitener the grain kernels are forced against each other and a metal screen by a steel-ribbed cylinder rotating inside a metal-plated cylinder. The frictional forces created between individual rice grains and between the grains and the metal screen surface remove the bran layer from the grain. Friction polishers are always horizontal in design and whitener. The whitening process applies pressure to the grain, which generates heat and causes cracking and breakage of some Rice Milling kernels. To reduce the number of broken grains and the grain temperature during the whitening process, rice is normally passed through two to four whitening and polishing machines connected in series. Rice temperatures should not exceed 43-440 C during any process. The arrangement of machines to process the rice during rice whitening is dependent on the physical characteristics rice grains. Proper sequencing of the machines will help reduce the amount of broken kernels during whitening and polishing.
Final, The process of white raw rice is completed. Any manufacturer may use cylindrical grader or tray separator to segregate broken brown rice and full grain white raw rice.