By: Rupesh P. Dati*
The objective of polishing is to remove, to a greater extent, the outer layers of brown rice viz. the pericarp, tegmen and aleurone layers, as well as germ, collectively called bran. The principle of polishing is quite different from that of dehusking.
Husk is the material which encloses the caryopsis quite tightly, but does not adhere to the grain. Bran layers, on the other hand, are wrapped around the endosperm, and deeply embedded and fused together. Also, rice has a unique cono-ellipsoid shape withridges and furrows present on its surface of varying depths of bran layers covering it. Therefore, bran cannot be removed layer-wise on all sides at any degree of milling. The extent to which these layers are removed is indicated by ‘degree of milling’ of rice. This is calculated as the percentage by weight of brown rice removed as bran during milling.
Methods of Polishing
There are two types of polishers – one made of emery and the other made of metal. The abrasive (emery) polishers (called whiteners) polish the grains by abrasion with emery, while the friction (metal) polishers employ a process which induces friction between rice grains for achieving the polishing effect. In the former, the abrasive roller acts as a blade that cuts and removes small bits of the bran layer from the brown rice. The process is similar to cutting off an orange skin little by little in small pieces using a sharp razor blade. In friction type machines, however, bran is removed in big flakes, akin to peeling off the skin of an orange. Friction, cutting, grinding and impact forces are processes present in varying degrees in all types of polishing machines.
1. Abrasive type of polishers
In these units, brown rice is scraped against an abrasive surface and the outer part is cut away as if scraped off by a knife; meanwhile, each grain is forced to rub against each other as well as against other parts of the milling chamber which help remove the bran layers. Abrasive polishers are of two types – vertical and horizontal.
1.1 Vertical type abrasive polishers
1.1.1 Cone polisher
The whitening cone consists of an inverted truncated cast iron conical rotor covered with an abrasive material like emery. Mounted on a vertical spindle, the cone revolves inside a crib. The crib is lined with a steel wire cloth or perforated metal sheets, and is provided with vertically and equally spaced lubber brakes which protrude into the interstitial gap between the cone and the crib. As the rotor shape is conical, this allows for gap adjustment between the cone and the crib by moving the cone up and down. Brown rice that enters at the top of the rotor moves outward, dueto the centrifugal force, towards the annular space and is dragged along by the rough surface of the rotating cone. The rubber brakes tend to stop it and cause it to pile up against their sides. While pressed up against the brakes, grains undergo a strong swirling and revolving movement because of their oval shape and smooth surface.
Grains are scoured by the abrasive surface of the cone, and also by the friction caused as grains are rubbed against the surrounding walls and lining of the crib. The grains revolve around the cone in the gap until their own weight causes them to sink lower and lower. Finally, the rice is discharged at the bottom of the cone. The bran layers are allowed to pass through the openings in the crib.
1.1.2. Vertical polisher
In this polisher, the rice grains are pushed upwards. The rice grains are transported through a horizontal conveying screw and pushed up with a screw roll into the abrasive section. In the vertical down flow type machines, rice is forced down the machine using gravity. The grains are then polished by a cylindrical emery grindstone. The pressure on the grains are controlled by hanging different weights on the discharge gate. The grindstone (abrasive cylinder) is formed in the same way as that used in the horizontal abrasive mill. Air is sucked through the mill stock as the grains are polished. This prevents heating, reduces breakage, and keeps dust out of the mill.
1.2 Horizontal type abrasive polisher
This consists of an abrasive cylinder disc attached to a steel shaft which rotates in a perforated cylindrical metallic screen mounted horizontally. This polisher is also called a primary polisher. Brown rice that is fed into the system passes through the clearance between the silicon carbide abrasive roller and the perforated steel cylinder, towards the discharge end. As the grain passes through the space between the roll and the perforated screen, bran layers are peeled off from the grain. Bran passes out through the screen and the polished rice is discharged through the outlet.
2. Friction type polishers
2.1 Vertical friction polisher
The machine consists of a cylindrical steel roller rotating inside a perforated screen. Semi-polished rice is fed into the milling chamber by the feed conveyor. The pressure inside the milling chamber (degree of milling) is adjusted by putting loads at the outlet gate. Rice passes from the bottom to the top and is whitened using friction. Polished rice is discharged at the outlet. The bran which is removed is sucked through as it comes out. The high-pressure air lowers the operating temperature at the milling chamber, makes removal of bran easier and reduces breakage.
2.2 Horizontal friction polisher
This machine consists of a cylindrical steel roller rotating inside a hexagonal perforated screen. The cylinder has a long slit along its length and a hollow shaft for passing air. The clearance between the screen and cylinder is adjustable by opening or closing the screen. The pressure on the rice is further controlled by hanging weights on the discharge gate. A strong stream of air is blown by a centrifugal blower through the hollow shaft and long slit of the cylinder. The air helps in separating the bran and removing the heat generated by the friction between each grain of rice.
3.Water jet polisher
This polishing unit is very similar to the friction type polisher. The main differences are with regard to the milling action. The milling chamber in the former is elongated to almost twice the length of those present in the friction
units. Water and compressed air tubing are fitted to the hollow shaft. Milled or semi-milled rice is fed into the machine. Rice is forced to move quickly in a circular motion around its feeding axle, with grains rubbing against each other at a high pressure and very high speed. As the rice grains are rubbing against each other at very high speeds, breakage is minimal. Due to high pressure and friction, grain temperature increases rapidly. This cause the surface of the rice grains to gelatinize.
During polishing, some of the surface dust and bran is rubbed off the rice, providing cleaner grains. The gelatinization of the rice grain surface results in a thin layer of shiny dried gelatinized starch on each grain. This thin layer has a smooth and glossy touch, lending the grains a shiny appearance.
*Research Scholar, ICAR, SRS, Bangalore