Parboiling temperature and duration had significant influence on the hardness. Parboiling of dehusked paddy by soaking at 70°C for 1.5 h and steaming for 12 min, soaking at 75°C followed by natural cooling soaking at 70–100°C and cooling at room temperature for 2 h followed by open steaming for 20 min have been documented. The last method reduced cooking time by 30% and saved 40 % energy. They further reported an increase in hardness of parboiled dehusked rice' samples with the increase in steaming time. A cooking time of 11.6 min was employed which resulted in less sticky rice than raw rice and less hard than traditional parboiled rice . Steaming of parboiled rice increases the hardness thus increasing the yield of head rice. Study on reducing the cost of parboiling equipment by precluding the need for large boilers and rice polisher with elaborate steam distribution systems, in addition to improvements in the quality of the parboiled rice was done by Adhikaritanayaka and Noomhorm.
A modern rice mill consists of a paddy pre-cleaning system which sieves and aspirators, efficient paddy separators, improved rice whiteners, graders and auxiliary equipment such as elevators, conveyors, automatic weighing machine. Parboiling is a pre-milling most-heat treatment given to raw paddy in order to improve its milling quality, nutritive value, cooking quality and storability.
The term milling, in connection with the traditional method of processing or huller mills means removal of husk, bran and a part of endosperm in a single operation. This results in a mixture containing husk, bran and brown rice. In a huller mill, a metallic roller is used as a milling unit cleaned by a fixed or vibratory screen and/or fan. The huller mills are low capacity units handling 300–500 kg/h of paddy and in these, it is difficult to regulate the degree of polish and rice is generally over-polished. Broken percentage is 20–30 %. Also husk gets powdered alongwith the bran and thus rendering it uneconomical for extraction of bran oil.
In the processing Industry, the vitamins in the grain can be retained by parboiling before rice whitener. 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.