Before milling, paddy is cleaned to remove small and large, heavy impurities. In dehulling and milling coarse outer layer of bran and germ are removed.
Paddy is milled in India either by home pounding or in mechanised rice mills. In home pounding there are more broken rice.
The process of milling involves the following steps:
Rice is passed through two stone or rubber discs rotating at different speeds and by shearing action on the grain, the hull is pulled away. The whole kernel from which the hulls have been removed is known as brown rice.
This is then milled in a machine called pearler to remove coarse outer layers of bran and germ by a process of rubbing, resulting in unpolished milled rice. Some amount of breakage of rice occurs in this milling.
Unpolished rice is liable to develop rancidity and so it is next polished in a brush machine which removes the aleurone layer and yields “ polished rice”.
Sometimes the polished rice is further treated in a device known as ttrumbol to give a coating of sugar and talc to produce a brighter shine on the grains.
Rice is separated from the broken kernals. Large kernals are called as second heads, medium ones are called screenings, smallest ones are called the brewers rice.
Most varieties of coarse rice are not highly polished. As the Indian Government regulations the extent of polishing should not be exceed 5 per cent. If rice is milled beyond 10 per cent then most of the thiamine is lost. The percentage of losses of different nutrients during milling is protein 15 per cent, fat 82 per cent, thiamine 85 per cent, and riboflavin 70 per cent, pyridoxine (vitamin B6) 50 per cent.
The degree of milling determines the amounts of nutrients removed.
Losses during milling can be compensated by the following processes.
By under-milling or unpolishing rice, the loss of nutrients can be reduced. Rice that is under-milled do not have the customary white luster and they are more subject to insect infestation and flavour deterioration than white rice is.
A second method is that of increasing vitamin retention by processing the rough rice prior to milling. This is done by parboiling which is commercially known as converted rice. Another means of remedying the losses occurring in the milling of rice is artificial enrichment of the grain. A premix has been developed in which the rice is wetted with a solution of thiamine and niacin, and then dried. A second coating of iron pyro phosphate is distributed on the rice. The rice premix is highly resistant to washing, cooking and storage losses.