Rice husking

Rice botanically belongs to Oryza sativa L. of Gramineae family. Paddy is a self-pollinated crop. A complete seed of rice is called paddy and contains one rice kernel. Outer layer of rice shell is called husk. The next layer is called rice bran and the innermost part is called rice kernel.

Rice husks are the hard protective coverings of rice grains which are separated from the grains during milling process. Rice husk is an abundantly available waste material in all rice producing countries, and it contains about 30%–50% of organic carbon.

The conventional way to husk rice is to pass it between two rubber rollers that are rotating with a surface speed differential. The resulting normal pressure and shear stress causes the husk to be peeled away from the kernel.

Rice husking operation is one of the most important stages during milling process, which effectively determines the quantitative and qualitative losses of rice. A common rice milling system is a multi-stage process where the paddy is first subjected to husking by using a husker and then to the removal of brownish outer layer, known as whitening.

Husks contain a combination of cellulose, hemicellulose, and lignin, along with appreciable amounts of silica and other minor components.

Cellulose and hemicellulose are both polysaccharides; cellulose is a long straight-chain polymer containing exclusively β-glucose monomers, whereas hemicellulose is a shorter cross linked polymer that contains other sugars, such as xylose, galactose, mannose, rhamnose, and arabinose.
Rice husking