Rice processing usually involves parboiling and milling. There are two major types of milling- milling of paddy rice to produce the rice grains and milling of rice grains into flour.
Paddy Rice Processing
In this process, paddy rice (harvested rice with its husk) is given hydrothermal treatment (parboiled) to meet certain consumer preferences before milling. There are three major steps in preparing paddy rice before milling. These include; soaking/steeping, steaming/parboiling and drying. These steps bring about certain changes in the physical, chemical and aesthetic qualities of milled rice.
Soaking/Steeping
The main objective of this process is to achieve quick and uniform water absorption. The rice paddy is soaked in hot water at about 70oC for 5 – 6 hours. The grain absorbs moisture and reaches a moisture content of 30 – 36% in 2 to 4 hours
Steaming/Parboiling
This is the heat treatment of soaked paddy to complete the physical and chemical changes in the rice grain. During steaming, the moisture content of the paddy increases to about 38%. Water is boiled to a temperature of 100oC and the soaked paddy lowered into it for about 15 – 20 minutes during which time the husks split. The cracking of 50% of the paddy rice indicates the end of steaming
Drying
Parboiled rice should be dried to a moisture content of 14% for safe storage or milling. The manner in which moisture content is attained is of very great importance. If drying is done too fast, internal stresses develop in the grain and cause breakage during milling. If the moisture is removed at a very slow rate, microorganisms will grow and partially or fully spoil the parboiled rice. . After drying is completed, the paddy should be allowed to stand for at least 1 or 2 days before it is milled to permit internal moisture differences and stresses to equalize.
Milling
Milling can be carried out by traditional or by mechanized means. In traditional milling, parboiled or raw paddy is pounded in a mortar with pestle after drying. The husk is then blown off by the air or wind. Optionally, the dehusked or brown rice is pounded lightly to remove the bran before consumption or marketing. Two models of mechanized mills are available. The small capacity single mills usually operated at the village level. Their capacities range from 45 – 270kg of paddy/hour. They are powered by electric motors, diesel engines or tractors. Such mills combine the dehusking and polishing process into one operation. The other is the larger capacity multi - purpose machines, which use a different machine for each processing step. Bucket elevators are used to move the grain from one machine to the next. The capacity of a standard size is 2 to 4 t/hr.
Direct Milling into Flour
Both parboiled and non-parboiled paddy rice can be processed to produce milled rice that would be milled to rice flour. However, the commonest raw material for rice flour production is broken, non-parboiled rice. Rice flour is mainly used for the extraction of starch used in other industries such as pharmaceutical, food and beverages, textile etc.
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