milling

Rice milling

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Importance of milling

Milling is a crucial step in post-production of rice. 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. Depending on the requirements of the customer, the rice should have a minimum of broken kernels.

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.
See also: diagram with the different rice fractions.



Milling systems

A rice milling system can be a simple one or two step process, or a multi stage process. In a one step milling process, husk and bran removal are done in one pass and milled or white rice is produced directly out of paddy. In a two step process, removing husk and removing bran are done separately, and brown rice is produced as an intermediate product. In multistage milling, rice will undergo a number of different processing steps. Depending on whether the the paddy is milled in the village for local consumption or for the marketing rice milling systems can be classified into the categories village rice mills and commercial mills.

 

Village milling

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Traditional hand pounding Single pass, single stage  rice mill for home use Single pass, two stage rice mill


Commercial rice mills

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Small commercial multi-stage mill State of the art modern multi-stage rice mill


Objective of commercial milling

A commercial rice miller will have following objectives:

  • produce edible rice that appeals to the customer- i.e. rice that is sufficiently milled and free of husks, stones, and other non-grain materials
  • maximize the total milled rice recovery out of paddy minimize grain breakage


Terminology

To understand the different objectives of rice milling, please review the following terminology.

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Rough rice

Also called paddy rice. Rice as it comes from the field. Rice kernels are still encased in their inedible, protective hull.

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Brown rice, husked rice or cargo rice

the least processed form of rice. It has the outer hull removed, but still retains the bran layers that give it a characteristic tan color and nut-like flavor. Brown rice is edible but chewier texture than white rice. Cooking time of brown rice is longer than milled rice

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Milled rice

Also called white rice, or rice after milling which includes removing all or part of the bran and germ from the rough rice.

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Milling recovery

total milled rice obtained out of paddy; expressed as weight percentage of milled rice (including brokens) obtained from a sample of paddy. The maximum milling recovery is 69-70% depending on rice variety, but because of grain imperfections and the presence of unfilled grains, commercial millers are happy when they achieve 65% milling recovery. Some village type rice mills have 55% or lower milling recovery.

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Milling degree

a measure of the amount of bran removed from the brown rice

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Head rice

milled rice with length greater or equal to three quarters of the average length of the whole kernel. It is often expressed on a % paddy or rough rice basis (on 14% Moisture content basis).

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Head rice recovery

weight percentage of head rice (excluding brokens) obtained from a sample of paddy. Under controlled conditions head rice recovery can be as high as 84% of the total milled rice or 58% of the paddy weight. Commercial rice mills turn out 55% head rice on average, whereas head rice recovery of village type rice mills is in the order of 30%.

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Whole kernel

a milled rice grain without any broken parts

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Broken kernel

or brokens. Depending on the rice mill this can be only one fraction or it can contain several fractions of different size:

  • Large broken kernel: 50-75% of the whole kernel size
  • Medium broken kernel: 25-50% of the whole kernel size
  • Small broken kernel: less than 25% of the kernel size, can not pass through a sieve with 1.4mm diameter holes
  • Chips: fragments of a kernel which pass through a sieve with 1.4mm diameter holes

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Impurities

materials in the rice that are not part of the milled rice kernel. May include stones, husk, chaff, weed seeds, etc.

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