Milling is a crucial step in post-production of rice. The basic objective of a rice milling system is to remove the husk and the bran layers, and produce an edible, white rice kernel that is sufficiently milled and free of impurities.
Depending on the requirements of the customer, the rice should have a minimum number of broken kernels.
Most rice varieties are composed of roughly 20% rice hull or husk, 11% bran layers, and 69% starchy endosperm, also referred to as the total milled rice.
In an ideal milling process this will result in the following fractions: 20% husk, 8−12% bran depending on the milling degree and 68−72% milled rice or white rice depending on the variety. Total milled rice contains whole grains or head rice, and brokens. The by-products in rice milling are rice hull, rice germ and bran layers, and fine brokens.
The best quality rice will be attained if (1) the quality of paddy is good and (2) the rice is milled properly. To improve the quality of the rice mill, the following factors should be considered:
- Mill at the right moisture content (MC)
A moisture content of 14% MC is ideal for milling.
If the MC is too low, high grain breakage will occur resulting in low head rice recovery. Broken grain has only half the market value of head rice. Use a moisture meter to determine the moisture content. Visual methods are not accurate enough.
Read: Moisture content for milling
- Pre-clean paddy before husking
Use of paddy without impurities will ensure a cleaner and higher quality end product.
- Do not mix varieties prior to milling
Different varieties of paddy have different milling characteristics that require individual mill settings. Mixing varieties will generally lead to lower quality of milled rice.
Open in new window: Paddy quality
- Use rubber roll technology for husking
Rubber roll huskers produce the best quality. Engleberg-type or "steel" hullers are no longer acceptable in the commercial rice milling sector, as they lead to low milling recovery and high grain breakage.
- Use a paddy separator
Separate all paddy from the brown rice before whitening. Paddy separation after husking will lead to better quality milled rice, and reduce overall wear and tear on the rice mill.
- Consider two-stage whitening
Having at least two stages in the whitening process (and a separate polisher) will reduce overheating of the grain and will allow the operator to set individual machine settings for each step. This will ensure higher milling and head rice recovery.
- Grade the milled rice
Installl a screen sifter to remove small brokens and chips from the polished rice. Rice with a large number of small brokens (or brewer’s rice) has a lower market value. The small brokens can be utilized to produce rice flour.
Open in new window: Milling technology
- Monitor and replace spare parts regularly
Turning or replacing rubber rolls, refacing stones, and replacing worn screens regularly will keep milled rice quality high at all times.
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 removing 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 the paddy is milled in the village for local consumption or for the marketing rice milling systems can be classified into the categories village rice mills and commercial mills.
Hand pounding of paddy in a mortar with a pestle is the traditional milling process in remote villages. This type of milling can be found in rural communities and are used for service milling paddy of farmers for home consumption.
Read: What is village milling?
Commercial milling systems mill the paddy in stages. This is also called multi-stage or multi-pass rice mills. Compared to village-level systems, the commercial milling system is a more sophisticated system configured to maximize theprocess of producing well-milled, whole grains.
The main byproducts of rice milling are rice hulls or husk, rice bran, and brewer’s rice.
- Rice hulls are generated during the first stage of rice milling, when rough rice or paddy rice is husked.
- Rice bran is generated when brown rice moves through the whiteners and polishers. When paddy is hand-pounded or milled in a one-pass Engleberg steel huller, rice bran is not produced separately but mixed with rice hulls.
- Brewer’s rice is separated produced when milled rice is sifted.
One hundred kilogram (100 kg) of paddy rice will generate approx 20 kg of husk. The bulk density is 100−150 kg/m3. Rice husk contains 16 to 22% ash, which is high in silica. The ash composition and structure give rice hulls an abrasive character. Metal surfaces in frequent contact with rice hulls will wear out and eventually puncture.
Common use of rice husks are as bedding materials, and as source of energy. In the modern rice milling industry, rice husks are increasingly being used as a fuel source for grain drying and parboiling, and for electricity generation. In Bangladesh, rice hulls are the preferred fuel for parboiling, and rice hulls are widely used for grain drying in the larger rice mills in Northern India. Rice hulls, once ground, are also used as ingredient in animal feeds.
Using rice husk in gasifyers or furnaces that are used in small and medium size plants Asia produces black ash which still contains around 20% carbon. This so called carbonated rice husk is often used as soil conditioner for poor soils, as an ingredient for bio fertilizers and currently research is elaborating it's potential for carbon sequestration in soils to reduce greenhouse gas emissions.
Fact sheet: What is rice husk
Open in new window: Rice husk
One hundred kilogram (100 kg) of paddy rice will generate approximately 5−10 kg of bran. Rice bran is a mixture of substances, including protein, fat, ash, and crude fiber. In many cases, bran contains tiny fractions of rice hull, which increases the ash content of bran. Bran composition is largely dependent on the milling process.
In modern rice mills, several different kinds of bran are produced: coarse bran (from the first whitening step), fine bran (from second whitening step) and polish (from the polishing step). Polish consists of part of the endosperm and is often referred to as meal.
Rice bran has a high nutritive value. Besides proteins, rice bran is an excellent source of vitamins B and E. Bran also contains small amounts of anti-oxydants, which are considered to low cholesterol in humans. Rice bran contains 10—23% bran oil. The oily nature makes bran an excellent binder for animal feeds. Bran oil, once stabilized and extracted, is a high quality vegetable oil for cooking or eating. The conventional use of rice bran is as ingredient for animal feeds, in particular ruminants and poultry. In recent years however, advances in stabilization techniques have been made which has led to new uses for bran and its derivatives, most notably bran oil for cooking and waxes for cosmetic products. In the developing countries, rice bran is underutilized due to a lack of suitable stabilization techniques.
Open in new window: Rice bran
Brewer’s rice is often used as ingredient for beer brewing, hence the name. In rural areas, brewer’s rice has a variety of uses including ingredient for rice flour and rice noodles.
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I have drawing of Rice Mill RM 150. The drawing seems to come from IRRI, Agricultural Engineering Division, Los Banos, Laguna, Philippines. Could you inform me more about the performance of this type of machine before we begin to fabricate it as a model for our local rice farmer? If you have better one, can we get the drawing?
The Rice Mill RM 150 (micro mill) is indeed an old IRRI design based on the Engleberg principle and adapted from Chinese mills for production by local workshops. The major modification is that the rotor is not made from cast iron but instead it can be assembled from local materials using welding technology only. The design has not been updated since it was released so the drawings you have are the current ones.
The Micro Mill is a single stage mill which means that the de-hulling and the polishing are done by the same rotor. The head rice recovery (based on paddy weight, note that this is different than the info you might have which is based on total white rice weight) is therefore relatively low.
The Micro Mill can be nevertheless useful for farmers groups located in remote areas like the mountains of Tanah Toraja in South Sulawesi, Indonesia. In those mountains, where there is no access to commercial mills and farmers have to walk for hours to get their rice milled or have to use manual milling techniques, the micro mill was well accepted because it was reducing drudgery. In that case the milled rice is used for home consumption and high head rice recovery is not so important. The farmers used the byproducts (bran and small brokens) for feeding their own animals.
However, for rice traded in the markets we think there are better solutions like smaller two stage mills with rubber roller de-husker because it has better head rice recovery and thus lower losses.