Modern rice milling
The milling process in large commercial mills combines a number of operations that produce better quality and higher yield of white rice from paddy or rough rice.
The modern milling process
When paddy comes into the mill, it contains foreign materials such as straw, weed seeds, soil, and other inert materials. If these are not removed before hulling, the efficiency of the huller and milling recovery will be reduced. The capacity of the paddy pre-cleaner is normally 1.5 times the milling capacity.
2. Removing the husk (dehusking or dehulling)
Brown rice is produced by removing the husk from rough paddy rice. The husk is removed by friction as the paddy grains pass between two abrasive surfaces that move at different speeds. After dehusking, the husk is removed by suction and transported to a storage dump outside the mill. Husk accounts for 20% of the paddy weight and an efficient husker should remove 90% of the husk in a single pass.
3. Paddy separation
The paddy separator separates unhusked paddy rice from brown rice. The amount of paddy present depends on the efficiency of the husker and should not be more than 10%. Paddy separators work by making use of the differences in specific gravity, buoyancy, and size between paddy and brown rice.
4. Whitening or polishing
White rice is produced by removing the bran layer and the germ from the paddy. The bran layer is removed from the kernel through either abrasive or friction polishers. The amount of bran removed is normally between 8 and 10% of the total paddy weight. To reduce the number of broken grains during the whitening process, rice is normally passed through two to four whitening machines connected in series.
5. Separation of white rice
After polishing, white rice is separated into head rice, large and small broken rice, and “brewers” by an oscillating screen sifter. Head rice is normally classified as kernels that are 75−80% or more of a whole kernel. To attain a higher degree of precision for grading and separation a length or indent grader is used.
6. Rice mixing
A good rice mill will produce 50−60% head rice (whole kernels), 5−10% large broken and 10−15% small broken kernels. Depending on country standards, rice grades in the market will contain from 5−25% broken kernels. If rice mixing is to be done properly, then a volumetric mixer is necessary.
7. Mist polishing
Mixing a fine mist of water with the dust retained on the whitened rice improves the luster of rice (polishes) without significantly reducing milling yield. A friction type of whitening machine, which delivers a fine mist of water during the final whitening process, is used for “final” polishing before sale.
8. Rice weighing
Rice is normally sold in 50 kg sacks which must be accurately weighed and labeled. While most rice mills use a manual mechanical weighing system, very accurate, and fast electronic systems are also available