Paddy in its raw form cannot be consumed by human beings. It needs to be suitably processed for obtaining rice. Rice milling is the process which helps in removal of hulls and barns from paddy grains to produce polished rice. Rice forms the basic primary processed product obtained from paddy and this is further processed for obtaining various secondary and tertiary products.
Rice Milling is a crucial step in rice milling process. The basic objective of a rice milling system is to remove the husk and the bran layers, and produce an edible, white rice kernel that is sufficiently milled and free of impurities.
The Rice Kernel Composition
Most rice varieties are composed of roughly 20% rice hull or husk, 11% bran layers, and 69% starchy endosperm, also referred to as the total milled rice.
In an ideal milling process this will result in the following fractions: 20% husk, 8−12% bran depending on the milling degree and 68−72% milled rice or white rice depending on the variety. Total milled rice contains whole grains or head rice, and brokens. The by-products in rice milling are rice hull, rice germ and bran layers, and fine brokens.
The modern rice milling process
1.Pre-cleaning:Paddy cleaner is the most essential equipment in a rice mill, as it separates all the impurities like dust, straw, sand, clay and heavy particles of even an uneven sizes from paddy. The advantages with the paddy cleaner are that increases the life of rubber rollers and the percentage of oil in bran.
2.De-stoning: Separating small stones from paddy.
3.Husking:This dehusker machine is used for dehusking of paddy and removing of husk. The machine is based on centrifugal principle.
4.Husk aspiration:separating the husk from the brown rice/unhusked paddy
5.Paddy separation:separating the unhusked paddy from the brown rice
6.De-stoning:separating small stones from the brown rice
7.Whitening:Removing all or part of the bran layer and germ from brown rice . The whitener is used for whitening (i.e. removal of bran) of brown rice to white rice. Through a smooth flow of rice and the efficient aspiration system inside the machine, the rice is whitened very gently.
8.Polishing:improving the appearance of milled rice by removing remaining ran particles and by polishing the exterior of the milled kernel
Improving the appearance of milled rice by removing the remaining bran particles and by polishing the exterior of the milled kernel.
9.Length Grading : Separating small and large brokers from head rice.
10.Blending: Mixing head rice with predetermined amount of brokers, as required by the customer.
11.Sifting:separating small impurities or chips from the milled rice
12.Length grading:separating small and large brokens from the head rice
13.Blending:mix head rice with predetermined amount of brokens, as required by the customer
14.Weighing and bagging：preparing milled rice for transport to the customer
Flow diagram of a modern rice mill
1-paddy is dumped in the intake pit feeding the pre-cleaner
A-straw, chaff and empty grains are removed
2-pre-cleaned paddy moves to the rubber roll husker:
B-husk removed by the aspirator
3-mixture of brown rice and unhusked paddy moves to the separator
4-unhusked paddy is separated and returned to the rubber roll husker
5-brown rice moves to the destoner
C-small stones, mudd balls etc. removed by de-stoner
6-de-stoned, brown rice moves to the 1st stage (abrasive) whitener
7-partially milled rice moves to the 2nd stage (friction) whitener
D-Coarse (from 1st whitener) and fine (from 2nd whitener) bran removed from the rice grainduring the whitening process
8-milled rice moves to the sifter
E-Small brokens/brewer’s rice removed by the sifter
9a-(for simple rice mill) ungraded, milled rice moves to bagging station
9b-(for more sophisticated mill) milled rice moves to the polisher1
10-Polished rice, will move to length grader
11-Head rice moves to head rice bin
12-Brokens moves to brokens bin
13-Pre-selected amount of head rice and brokens move to blending station
14-Custom-made blend of head rice and brokens moves to bagging station
15-Bagged Rice moves to the market
Guidelines for good rice milling
The best quality rice will be attained if the quality of paddy is good and the rice is milled properly. To improve the quality of the rice, factors such as the paddy quality and milling technology should be considered.
Mill at the right moisture content
A moisture content of 14% is ideal for milling.
If the moisture content is too low, high grain breakage will occur resulting in low head rice recovery. Broken grain has only half the market value of head rice. Use a moisture meter to determine the moisture content. Visual methods are not accurate enough.
Pre-clean paddy before husking
Use of paddy without impurities will ensure a cleaner and higher quality end product.
Do not mix varieties prior to milling
Different varieties of paddy have different milling characteristics that require individual mill settings. Mixing varieties will generally lead to lower quality of milled rice.
Use rubber roll technology for husking
Rubber roll huskers produce the best quality. Engleberg-type or "steel" hullers are no longer acceptable in the commercial rice milling sector, as they lead to low milling recovery and high grain breakage.
Use a paddy separator
Separate all paddy from the brown rice before whitening. Paddy separation after husking will lead to better quality milled rice, and reduce overall wear and tear on the rice mill.
Consider two-stage whitening
Having at least two stages in the whitening process (and a separate polisher) will reduce overheating of the grain and will allow the operator to set individual machine settings for each step. This will ensure higher milling and head rice recovery.
Grade the milled rice
Install a screen sifter to remove small brokens and chips from the polished rice. Rice with a large number of small brokens (or brewer’s rice) has a lower market value. The small brokens can be utilized to produce rice flour.
Monitor and replace spare parts regularly
Turning or replacing rubber rolls, refacing stones, and replacing worn screens regularly will keep milled rice quality high at all times.