- Milling of Rice
Unlike other food grains, rice is mostly cooked and consumed in whole form. Hence the milling operation should provide maximum out turn of milled rice and with a minimum of brokens. Before the paddy is kept ready for milling operation, it is necessary to bring all the grains, preferably to a uniform moisture content level which may vary from 10% to 14% for optimal milling yields and avoid excessive breakage. The different kinds of milling are hand pounding, huller, sheller cum huller.The operations of a Modern Rice Mill are
- Cleaning: Removing foreign matter such as sand, stones, straw, seeds etc.
- Dehusking: Removes husk from paddy with minimum of damage to the grain.
- Husk separation: Removes the husk from the mixture obtained after dehusking.
- Paddy separation: Separates dehusked brown rice from remaining unhusked paddy, the paddy being returned for dehusiing
- Polishing: Removes all or part of the bran layer from the brown rice to produce polished rice
- Grading: Separates brokens from unbroken rice. The brokens are separated into different sizes
- Apart from the above, the mill also use color sorters and length graders.
Cleaning is the first step in rice milling. It enables the production of clean rice and provides protection to other milling machinery, thereby increasing milling capacity. Impurities that are lighter than paddy are removed by an aspirator. Metallic (iron- impurities) are removed by the use of a magnet. Impurities larger or smaller in size but heavier than paddy are removed by sieves. Vibrating sieves are used. Impurities that have the same size as paddy but are heavier than paddy are removed by specific gravity separator namely de stoners. Intake paddy is often subjected to a preliminary partial cleaning prior to storage and prior to the main cleaning in the mill.
- De husking
A rubber- roll sheller consist of two rubber roller rotating in opposite direction at different speeds, both rollers have the same diameter, but one roll rotates about 25% faster than the other. The differences in peripheral speed subject the paddy grains falling between the roll to a shearing action that strip of the husk. One roller is fixed in position and the other is adjustable laterally in order to increasing or decreasing the clearance between the two rolls. Rolls are cooled by blowing air on the roll surface. The shelling i.e., dehusking rate is generally maintained at about 85%, compared to the disc shelling. Some breakage invariably occurs and the fine brokers may be blown off along with the husk. The degree of Shelling with the rubber roll can be raised to about 85% without much of grain breakage, whereas the disk sheller can be operated at not more than 60-70% degree of shelling, to keep breakage at a low level. However, rubber roll wear out fast and have to be replaced often. This, though a disadvantage, is offset by the reduction in breakage and increase in total rice outturn.
- Husk Separation
A mixture of dehusked rice (brown rice), remaining unshelled paddy, some broken rice and husk that has been split off the paddy comes out of the sheller. This mixture is subjected to sieving cum aspiration to separate brokens and husk. Sieving prior to aspiration helps in separating and recovering the small brokens formed during shelling. Light weight paddy husk is separated from the heavier paddy and rice by aspiration.
- Paddy Separation
Shelling is not possible to a level of 100%. The grains differ in size due to which some grains remain unshelled. Therefore, a paddy separator is used to separate the remaining unhusked paddy from husked brown rice. The unhusked paddy is returned to the dehusker while the brown rice is carried forward to the polisher. The separation is accomplished in the separator by taking advantage of the difference in physical density (heaviness), size and surface smootheness (or toughness) of paddy and brown rice.Paddy collected from the separator is called return paddy, for it has to be returned to the sheller: The return paddy grains are shorter or thinner than normal paddy. Hence it is preferable to collect the return paddy in a bin and shell it at the end with closer clearances between the rolls.Alternatively, the return paddy can be shelled in a separate small sheller. This will increase the efficiency of the plant. If return paddy is returned to the original sheller with same setting, it will simply go on circulating, thus lowering the capacity.
- Compartment Type
SeparatorThe oscillating table is-divided into zigzag channels and is inclined from one side to the other along the zigzag channels. The surface of the table is of smooth steel. The table oscillates cross wise, i.e. perpendicular to the direction of the grain flow. The mixture of paddy and brown rice is fed from the hopper to the center of the channels. The impact of the grains on the sides of each channel causes the unhusked paddy grains to move up the inclined slope toward high side of the table. The dehusked brown rice slides down the slope to the lower side of the table. The slope and stroke of the table are adjusted to meet the needs of paddy of different size or condition, to ensure complete separation. Usually there are several decks one above the other to increase capacity.Paddy collected from the separator is called return paddy, for it has to be returned to the sheller. The return paddy grains are shorter or thinner than normally paddy.Hence, it is preferable to collect the return paddy in a bin and shell it at the end with closer clearances between the rolls. Alternatively the paddy can be shelled in a separate, small sheller. This will increase the efficiency of the plant. If return paddy is returned to the original Sheller with same setting, it will simply go on circulating, thus lowering the capacity.
The brown rice is next polished to remove bran layers. Some amount of polishing is essential for easy cooking and storage, although excessive polishing reduces the nutritive value of rice. There are three whiteners, one glaze master and one silky polisher in the unit for effective polishing of brown rice.
After the polishing operation, the milled rice contains, in addition to white grains, broken grains of different sizes as well as some bran and dust. Separation of these materials must be done. Bran and dust particles are removed by aspiration. Broken rice may be separated either by a Plan sifter or by a Trieur.
- Color Sorter
Color sorter is used for sorting out discolored grains from the lot. Optical sensors are adopted here to compare the color of the Individual grains. The discolored grains are blown out of the main stream.
- Weighing and Packing
This can be manual or automatic. An automatic intake weigher receives rice in a continuous flow and releases it in exact predetermined quantities by weight. At the same time, the number of weighments made is recorded by an automatic counter, thus giving an accurate record of the total weight of material processed in the mill. The automatic bagging scale ensures dust free bagging of the finished products and records the number of weighments made, by an automatic counter.