A rice milling system’s goal is to remove the bran and husk from paddy rice in order to create entire white rice kernels that are adequately ground, pure, and have as few broken kernels as possible. The quality of the paddy, the milling machinery utilised, and the mill operator’s skill level all affect the rice’s milling yield and quality.
The rice grain contains properties that are both chemical and physical.
As shown in Figure 1, a rice grain is composed of the endosperm, a bran layer, and an outer husk layer. Lemma and palea, the layers of husk that make about 20% of the weight of rice, aid in shielding the grain kernel from fungal and insect damage. Brown rice is the variety of rice that has had the husk removed. Both the endosperm and the bran layer are present in brown rice. The pericarp and testa, the aluerone layer, and the embryo comprise the bran layer. The milling degree refers to how much of this bran layer is removed. Depending on the nation, different levels of bran removal are desired.
| Physical characteristic | Percentage |
| Paddy | 100 |
| Husk | 20 |
| Brown rice | 80 |
| Meal Pericarp and testa (5-6%) Aluerone (1%) Embryo (3%) | 8-10 |
| White rice | 70-72 |
About 80% carbohydrate and 7% protein are present in rice at 12% moisture content. (Currey, 1984) Protein is found in the endosperm as particles that are located in between the microscopic, multi-sided starch granules. In addition, rice grains have minerals, nutritional fibre, fat, and carbohydrates.
| Component | Brown rice | White rice | Bran |
| Water (%) | 13-14 | 13-14 | 13-14 |
| Starch (%) | 68-70 | 80 | 9 |
| Amylose | 28-30 | 33 | 6 |
| Protein (%) | 6-8 | 6-7 | 14 |
| Fat | 3 | 1 | 20 |
| Fiber | 2-3 | 0.5 | 25 |
| Crude ash | 1-1.5 | 0.5 | 9-10 |
To make white rice, the husk and bran layer are removed by a process called rice milling. You can grind rice in the following ways:
In few isolated locations, paddy is still manually pounded in a mortar using a pestle. The husk and some bran layers are removed from the grain by pounding the paddy, which creates upward and downward forces on the grain versus grain. There are also a lot of cracked kernels as a result of the crushing. By hand, winnowing and gravity separation are used to accomplish the last cleaning.
The “Engleberg” coffee huller has been modified to become the single pass rice mill. This kind of mill is still frequently used for custom milling home rice and is highly popular in many of the less developed rice-growing nations. It is still widely used in Bangladesh and many other African nations for milling parboiled rice. This is a steel friction type mill that polishes the grain and removes the shell under extreme pressure. Due to this, there are a lot of broken kernels, only 50–55% of the milled rice is recovered as white rice, and less than 30% of the rice is yielded as head rice. For animal feed, the fine brokens are frequently combined with ground rice hull and bran.
The Engleberg mill has been replaced in many countries by two stage mills, often known as compact rice mills. The two-stage mill features distinct procedures for polishing and hulling. After the husk is removed using rubber rollers, brown rice is polished using a steel friction whitener akin to the Engleberg. These mills are frequently utilised for custom milling in rural areas and have a capacity of 0.5 to 1 tonne per hour paddy input. With milling recoveries often above 60%, the small rice mill outperforms the single pass Engleberg huller in terms of milling performance.
Larger commercial mills use a combination of processes during the milling process to generate white rice from paddy or rough rice that is both greater yielding and of higher quality. The procedure entails:
Paddy contains foreign materials such as straw, weed seeds, soil, and other inert material when it is brought into the mill. The huller’s efficiency and the milling recovery are compromised if this is not removed before hulling.
The majority of pre-cleaners divide materials into three groups: • Firstly, larger-than-grain objects are removed by scalping them. • The second separation keeps the grains but lets broken grains, tiny stones, and weed seeds pass through. This can be accomplished with either a flat oscillating screen or a rotary drum screen that enables the grain to pass through but holds straw. It’s also possible to include an air aspirator to get rid of the light empty grains and dust.
Based on how they clean, grain pre-cleaners can be divided into different categories. These are the following. Type of oscillating sieve: These pre-cleaners are straightforward and frequently produced locally. Depending on the size and shape of the grain, the machine has two different sized sieves. Both screens can be adjusted to fit the grain size or crop type, and the top sieve has a slotted profile that is wider than the bottom.
Goal combined Oscillation Type: By forcing air through the bulk of falling grain and drawing out light pollutants in the air stream, the aspirator grain cleaner eliminates finer particles like dust, dirt, chaff, and straw. Oscillating sieves are then used to remove impurities from the grain that are not eliminated by the air. This equipment functions similarly to a sieve oscillation cleaner. Magnets are another feature that certain cleaners have to get rid of humorous particles. Single-action or double-action aspiration is available for aspiration style cleaners.
Cleaner Rotating This machine has one or two drums, each equipped with an oscillating sieve and mesh with hexagonal or square perforations of varying sizes. The rice or paddy is filtered through the drums to remove any foreign objects bigger than the grains. After that, paddy is directed onto an oscillating sieve to remove heavy contaminants like stone.
Two varieties of rotary cleaners exist.
Aspirator and oscillating sieve on a single drum. A single drum is used by the single drum aspirator to remove heavy, light, and big pollutants. Suction aspiration is used to separate lighter pollutants, whereas oscillation sieves are used to separate heavier impurities like sand.
two drums and an aspirator. This machine has two rotating drums with varying-sized holes in the wire mesh on each drum. It separates light pollutants using an aspirator. Usually, this machine is used to clean newly harvested paddy.
Desiring to de-stoner This machine includes an additional specific arrangement for separating stones that have the same physical dimensions as paddy, otherwise it is identical to a single drum with aspiration and oscillating sieves. The paddy’s flow direction in relation to the stone movement’s direction is especially significant.
De-husking or de-hulling is the term for the process of removing the husk layer from the paddy by friction. In contemporary rice mills, the paddy grains are dehusked by passing them between two abrasive surfaces that are running at different speeds. Traditionally, this was done using a mortar and pestles. The husk is extracted by suction (aspirated) and sent to a storage dump outside the milling plant once the husk and paddy have been separated.
Hulling efficiency is the proportion of paddy that is dehulled during this procedure to generate brown rice. Ninety percent of the husk may be removed in one pass by an effective husker. Following the removal of the husk, brown rice is fed through a paddy separator. After being separated, the kernels that weren’t dehusked during the initial pass will be put back in the dehusker.
Steel Huller: In one pass, the steel huller whitens the rice and removes the husks. The machine feeds paddy rice through a mesh screen with a cylindrical shape and a revolving steel shaft. These devices are easy to use and typically run on an engine of 15 to 20 horsepower. They’re not too expensive.
Under-runner disc sheller: In Asia, under-runner huskers are highly prevalent. The two steel discs of this machine are coated in emery. The cast iron housing holds the upper disc in place, making it immobile. Between the abrasive surfaces of the rotating lower disc and the stationary upper disc, there is a hopper in the middle from which paddy flows. The husk is removed from the brown rice kernel by resistance between the paddy grains and the emery surface of the discs. After that, brown rice and husks are released through an outlet and around the rotating disc. This machine has a hulling efficiency of approximately 85–90%, produces a moderate amount of cracked or broken grain, and is incredibly energy-efficient.
Rubber roller huller: The most effective hulling device is the rubber-roller huller. To extract the husk from the paddy, two rubber rollers with the same diameter are turned at various speeds, as the name implies. While one roller is fixed in place, the other can be adjusted to get the required clearance. Compared to the fixed roller, the adjustable roller rotates a little more slowly. An aspirator located in the machine’s base helps rubber-roll hullers separate the hulls from the brown rice. There is a range of 150 to 250 mm for the roll diameter and 60 to 250 mm for the roller width. The breadth and length of the paddy, as well as the varietal characteristics, determine the proper clearance.
A mixture of paddy rice, brown rice, husk, broken paddy, and occasionally bran are produced by the huller. Husk, bran, and tiny brokens are among the lighter materials that are removed by the huller aspirator. The remaining rice is fed into a paddy separator, which separates brown rice from unhulled paddy rice. The efficiency of the husker determines how much paddy is present; it shouldn’t be more than 10%. Utilising the variations in specific gravity, buoyancy, and size between paddy and brown rice, paddy separators function. Compared to brown rice, paddy rice is physically larger, longer, and wider, and it has a lower specific gravity and more buoyancy.
By eliminating the germ and bran layer from brown rice, white rice is created. By rubbing the grains against an abrasive surface or against one another, friction is applied to the grain surface in order to remove the bran layer from the kernel. Typically, 8–10% of the total weight of the rice is removed as bran; however, this can vary depending on the variety and level of whiteness needed.
Some kernels break and split as a result of the pressure applied to the grain during the whitening process, which also produces heat. In order to lower the temperature of the rice and minimise the amount of broken grains during the whitening process, rice is often run through two to four series-connected whitening and polishing machines. During any process, rice temperatures shouldn’t be higher than 43–440C. The physical properties of the rice grains determine how the machinery is arranged to process the rice during rice whitening. Reducing the quantity of cracked kernels during whitening and polishing can be achieved by properly sequencing the machines.
A sifter is used to separate the white rice into large and small broken rice as well as head rice after it has been polished. Head rice is typically categorised as kernels, which comprise 75–80% of the entire kernel. The rice is passed through a sequence of oscillating screens that make up the sifter. The output known as “brewers” comes from the bottom screen and consists of extremely tiny broken tips.
An additional tool used for grading and separation is a length or indent grader, which increases precision to a higher level. One to three spinning indented cylinders make up this machine. The complete rice kernels, or head rice, are left behind when the smaller and broken rice pieces are removed by falling into the indentations on the revolving roller surface. Depending on the size of the grain, several indent sizes are employed.
50–60% head rice (whole kernels), 5–10% large broken, and 10-15% little broken kernels are the output of a good rice mill. The percentage of cracked kernels in various rice grades available in the market varies based on national norms. In order to correctly mix rice, a volumetric mixer is required.
Without appreciably lowering the milling yield, the lustre of the rice (polishes) can be enhanced by blending a fine mist of water with the dust left on the whitened rice. The “final” polishing step before sale involves using a friction type-whitening equipment that mists the product with fine water during the whitening process.
Usually supplied in 50 kilogramme bags, rice needs to be precisely weighed and labelled. Although the majority of rice mills have a mechanical, manual weighing method, there are also electronic ones that are incredibly quick and precise.
Credit-Rice Knowledge Bank.