Commercial rice milling systems
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,
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.
Objective of commercial milling
A commercial rice miller will have following objectives:
Types of commercial mills
Traditional commercial mills also consist of different pieces of equipment for the three stages outlined above. They are often made from wood with few metal components and are often driven by a single power source through a system of transmissions. Building such traditional mills was almost considered an art rather than engineering.
Modern commercial milling lines are fully automated and usually consist of one or more components for each stage of the milling process.
The modern rice milling process
Modern rice milling processes consist of:
The indented sheet grader (also called trieur) is a standard piece of equipment in rice processing for length grading of milled rice.
Flow diagram of a modern rice mill
The flow diagram below represents the configuration and flow in a typical modern rice mill (using the IRRI rice mill as an example).
Description of flow of materials and processes
1 – paddy is dumped in the intake pit feeding the pre-cleaner
Examples for outputs from the different stages
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.
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.
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.
Brewer’s rice or small broken grains removed by the screen sifter.