Modern
Rice Milling
The milling process in larger commercial mills combines a
number of operations that produces better quality and higher yields of white
rice from paddy or rough rice.
The Modern Milling Process
1. Pre-cleaning
When paddy comes into the mill it contains foreign material
such as straw, weed seeds, soil and other inert material. If this is not
removed prior to hulling the efficiency of the huller and the milling recovery reduced. The
capacity of the paddy pre-cleaner is normally 1.5 times milling capacity.
2. Removing the husk
(dehusking or dehulling)
Brown rice is produced by removing the husk from the
paddy rice. The husk is removed by friction as the paddy grains pass between
two abrasive surfaces that are moving 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.
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3. Paddy
separation
The paddy separator separates unhusked
paddy rice from the brown rice. The
amount of paddy present depends on the efficiency of the husker and should not
be more 10%. Paddy separators work by
making use of the differences in the specific gravity, buoyancy and the size
difference 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 either abrasive or friction polishers. The amount of bran removed is normally between
8-10% 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
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After polishing, the 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 also 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 the countries standards, rice grades in the market
will contain from 5-25% broken kernels. If rice mixing is to be done properly 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 the rice
(polishes) without significantly reducing the milling yield. A friction
type-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 as 50kg sacks which must be accurately weighed and labeled. While most rice mills use manual mechanical weighing system very accurate and fast electronic systems are also available.