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 the variety and degree of whiteness required.
The process used to whiten brown rice can be classified as either abrasive or friction.
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 peels off the bran layers from the brown rice and applies less pressure on the grain than a friction process and is therefore better suited for long grain varieties. Abrasive polishers can be either vertical or horizontal in design. The vertical cone whitener is very common in many Asian countries.
|Abrasive laboratory whitener||Older vertical cone whitener||Modern vertical cone whitener in Vietnam|
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 apply more pressure on the grain than an abrasive whitener.
The whitening process applies pressure to the grain, which generates heat and causes cracking and breakage of some 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 43-44°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. The normal arrangement of whitening and polishing long and short grain rice is as follows.
Short grain: Abrasive - Friction - Friction - PolishingLong grain: Abrasive - Abrasive - Polishing
For mills that produce premium or export quality rice, a mist polisher is employed to brush off remaining bran dust and to create a characteristic gloss on the milled rice.
In both the abrasive and friction whitener, provisions are made for a jet stream of air through the cylinder and portholes to cool the grain, and blow off fine bran. This minimizes breakage and improves efficiency of subsequent steps in the milling process.
Adjusting pressure in whiteners and polishers
Adjustment of the pressure in whiteners and polishers is crucial in meeting the objectives of milling. A certain pressure is required to peel off the bran: if the pressure is too low, only energy is converted into heat but no bran is removed. Conversely, too much pressure results in generation of broken rice. Pressure adjustment is often based on a judgment call of the operator. Many advanced models however contain an ampere meter (that shows the electric load on the motor drive) that indicates the pressure inside the mill. In friction-type whiteners, pressure is regulated by changing the flow rate of grain through the mill. Flow Rate is adjusted by a weight that puts pressure against the output valve.
Abrasive stones should be resurfaced regularly to maintain high quality milled rice output. Excessive use of jet air stream can result in a decrease of moisture of milled rice and result in higher grain breakage.