The production process includes the following
- Paddy Pre-Processing
- Paddy Cleaning
Paddy after harvesting and thrashing contains some foreign matter depending upon harvesting, thrashing and handling methods. The foreign matter may be other seeds, straw, chaff, sand; stones, dust, soil and iron particles. Paddy received in the mill must be first cleaned to remove these foreign matters before it can be properly stored. Otherwise they may cause deterioration of the paddy during storage or may damage or obstruct the conveying and milling machinery. The first cleaning operation of paddy after thrashing is called ‘Scalping’. It removed the bulk of the foreign matter. But the paddy is not completely cleaned. The small amounts of impurities that still remain are later removed during the next cleaning operation during the rice milling process. Scalper cleaners use both sieves and aspirators.
- Paddy Drying
Intake paddy also generally contains more moisture than is safe for storage and has to be dried. Freshly harvested paddy normally has moisture content of 18% to 25%. This moisture must be brought down by drying to ensure a good storage quality. A moisture content of 14% is considered safe for short periods of storage. For long storage, the grain should be dried to 13% moisture or less. During the process of drying, cracking of paddy may occur if drying is not proper. This will lead to breakage of rice during milling. Therefore, drying process must be so adjusted that cracks to not develop in the grains. The high moisture gradient between the grain center and the surface causes cracking. Drying should not be continuous, but in stages, with the rest periods between drying stages. These periods are called tempering. This avoids cracking of paddy while drying. It is said that not more than 2 to 4% moisture should be removed in one drying stage in the case of mechanical drying with hot air. Paddy may be dried for, say half an hour then tempered for 6 hours, then dried again. This cycle is followed till the paddy is dried to required moisture. Several methods of drying are adopted viz., Sun Drying, Mechanical Drying, Batch dryers or continuous flow dryers. Paddy husk is the most common fuel used for drying paddy.
- Paddy Storage
Paddy is harvested twice or thrice in a year. But rice is consumed throughout the year. Therefore, paddy is stored to meet the need between harvests. Storage must keep the paddy safe and also maintain its quality. It should provide protection against weather, insects, pests, birds, micro organisms, moisture and any type of contamination. Care should also be taken to store different types of paddy separately to avoid mix up. Paddy dried to a moisture level (less than 14%) should be maintained dry and cool during storage.
Paddy can be stored in bags or in bulk. The choice between the two systems depends on a number of local factors including cost of local construction, bags, operating and handling equipment, transport system, labour and investments. Either method can provide safe storage as long as scientific storage practices are observed. Storage bins can be either of steel or concrete.
- Paddy Cleaning
- 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.