milling

Commercial rice milling systems

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What is commercial milling?

Commercial milling systems mill the paddy in stages, and hence are called multi-stage or multi-pass rice mills. The objective of commercial rice milling is to reduce mechanical stresses and heat buildup in the grain, thereby minimizing grain breakage and producing uniformly polished grain. Compared to village-level systems, the commercial milling system is a more sophisticated system configured to maximize the process of producing well-milled, whole grains.

The rice milling facility comes in various configurations, and the milling components vary in design and performance. “Configuration” refers to how the components are sequenced. The flow diagram below shows a modern commercial mill catering to the higher end market. It has three basic stages,

  • the husking stage,
  • the whitening-polishing stage, and
  • the grading, blending, and packaging stage.

In modern rice mills, many adjustments (e.g. rubber roll clearance, separator bed inclination, feed rates) are automated for maximum efficiency and ease of operation.  The whitener-polishers are provided with gauges that sense the current load on the motor drives which gives an indication of the operating pressure on the grain. This provides a more objective means of setting milling pressures on the grain.

 

The modern rice milling process

Modern rice milling processes consist of:

Stage Function
Pre-cleaning removing all impurities and unfilled grains from the paddy
Husking removing the husk from the paddy
Husk aspiration separating the husk from the brown rice/unhusked paddy
Paddy separation separating the unhusked paddy from the brown rice
De-stoning separating small stones from the brown rice
Whitening removing all or part of the branlayer and germ from the brown rice
Polishing improving the appearance of milled rice by removing remaining ran particles and by polishing the exterior of the milled kernel
Sifting separating small impurities or chips from the milled rice
Length grading separating small and large brokens from the head rice
Blending mix head rice with predetermined amount of brokens, as required by the customer
Weighing and bagging preparing milled rice for transport to the customer


Flow diagram of a modern rice mill

The flow diagram below represents the configuration and flow in a typical modern rice mill.

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1 -  paddy is dumped in the intake pit feeding the pre-cleaner
2 -  pre-cleaned paddy moves to the rubber roll husker:
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
6 -  de-stoned, brown rice moves to the 1st stage (abrasive) whitener
7 -  partially milled rice moves to the 2nd stage (friction) whitener
8 – milled rice moves to 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
A – straw, chaff and empty grains are removed
B  - husk removed by the aspirator
C – small stones, mudd balls etc. removed by de-stoner
D -  Coarse (from 1st whitener) and fine (from 2nd whitener) bran removed from the rice grainduring the whitening process
E - Small brokens/brewer’s rice removed by the sifter



Examples for outputs from the different stages

Clean paddy after pre-cleaning

Paddy grain after precleaning. Poor quality grain is evident from its darker color. These are immature kernels or half filled grains that are not removed in the precleaner. The presence of poor quality grain lowers the total milling recovery.

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Brown rice at the rubber roller husker

Mixture of paddy grain and brown rice coming out of the rubber roll husker. With uniform size paddy, about 90% of the paddy should be husked after the first pass. This mixture goes through a paddy separator, after which the paddy is returned to the husker,and the brown rice goes to a de-stoner.

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Milled rice after polisher

Milled rice after the 2nd stage friction polisher. Small broken grains are still evident. This product goes to a sifter to remove the small broken grains.

Note: Most mills have several polishing stages for gentle milling. In those mills there is undermilled rice after the 1st stage friction whitener. Not all the bran layers are fully stripped. In times of rice shortage, production of undermilled rice is promoted because of the higher milling recovery.

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Brewers rice from the sifter

Brewer’s rice or small broken grains removed by the screen sifter.
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