Post by Trade Coach on Feb 13, 2017 22:50:55 GMT 1
Depending on the requirements of the customer, rice should have a minimum number of broken kernels. Most rice varieties are composed of roughly 20 per cent rice hull or husk, 11 per cent bran layers, and 69 per cent starchy endosperm, also referred to as the total rice.
In an ideal milling process this will result in the following fractions: 20 per cent husk, 8-12 per cent bran, depending on the milling degree, and 68-72 per cent milled rice or white rice depending on the variety. Total milled rice contains whole grains or head rice, and broken kernels. The by-products in rice milling are rice hull, rice germ and bran layers, and fine broken kernels.
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 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 paddy is milled in the village for self consumption or for marking, rice milling systems can be classified into the categories village rice mills and commercial mills.
MEANWHILE, THE OBJECTIVE OF COMMERCIAL RICE MILLING IS TO MINIMISE GRAIN BREAKAGE AND PRODUCE UNIFORMLY POLISHED GRAIN.
Village Milling Method
Hand pounding of paddy in a mortar with a pestle is the commonness traditional milling process in most remote villages. Pounding the paddy induces upward and downward forces on grain against grain that removes the husk and bran layers. The pounding also breaks up fissured grain. The final cleaning is by winnowing in a woven bamboo tray. The winnowing process to separate unmilled paddy grain is an art.
Village-type rice mills can be found in rural communities and are used for service milling paddy of farmers for home consumption. The single pass rice mill is an adaptation of `Engleberg’’ coffee huller from the United States of America, modified for milling rice. In earlier days this type of rice milling was very popular in most rice-growing countries. It is still the main stay technology for milling parboiled paddy in most African countries. The iron ``hullers’’, or ``single pass mills’’ which all refer to the same mill are notorious for breaking the paddy grain. Because of the high breakage, the total milled rice recovery is usually 53-55 per cent, and head rice recovery is in the order of 30 per cent of the milled rice. The fine brokens are mixed with bran and the ground rice hull. This by-product is used for animal feeds. In many rural areas, Engleberg mills are used for custom milling the rice requirements of households. The bran produced is left to the miller as the milling fee.
Single pass, single stage mills are small capacity two-stage rice mills, 0.5 to 1 ton per hour paddy input. They are also used for custom milling services in the rural communities. A typical compact rice mill consists of a 15 cm diameter multiply by 15cm wide rubber roller husker, and a friction whitener. The friction whitener has a very similar design configuration as the Engleberg except that it has no husking knife. The milling performance of the compact rice mill is superior to the single pass Engleberg huller. Milling recoveries are normally above 60 per cent.
Reason For Commercial Milling:
A commercial rice miller will have the following objectives:
1. produce edible rice that appeals to the customer ie, rice that is sufficiently milled and free of husks, stones, and other non-grain materials
2. Maximize the total milled rice recovery out of paddy minimize grain breakages
Timely harvesting, threshing, drying, and storing properly can result in production of good quality milled rice. Mixtures of chalky and immature kernels, mechanically stressed grain during harvesting threshing, delays in drying, and moisture migration in storage can result in broken and discoloured milled rice.
Blending/mixing different varieties with different physic-chemical properties during the post-harvest operations contribute to the lowering of the milled rice quality produced. Purity is related to the presence of dockage in the grain. Dockage refers to material other than paddy and includes chaff, stones, weed seeds, soil, rice straw, stalks, etc. These impurities generally come from the field or from the drying floor. Unclean paddy increases the time taken to clean and process the grain. Foreign matters in the grain reduce milling recoveries and the quality of rice and increases the wear and tear on milling machinery.
It is not possible to produce good quality milled rice with poor milling equipment even if the paddy quality is optimal and the operator is skilled. It is also important to regularly service and maintain the mill properly. The rice mill should always be clean and well maintained and should be operated by a skilled operator.
