The difference that deliberate efforts have been taken to improve the quality of processed rice (Ihedioha and Odoemena 2006)
Harvesting
Harvesting tropical rice is usually at 20 percent or more moisture, when grains will provide optimum total and head role yields. Moisture content at harvest is lower during the dry season than in the wet season because of sun –drying while the grain are in the intact plant. The actual period of dry matter production is not more than 14 to 18 days, after which the grain
undergoes drying. Harvesting is carried out by cutting the stem (Juloiano 1993) by hand sickles or knives combine harvesting are used in large areas as the muda estate in Malaysia and in the united state, Australia, Europe and Latin America. (Juliano 1993).
Threshing
The moisture content of paddy at harvest ranges between 22-26% wet asis in order to prevent deterioration after harvest, paddy should be dried down to a level of water activity that will enable safte storage by reducing respiration, inhibiting mould growth and preventing production of mycotoxins. This corresponds to a moisture content of about 13 -14 percent which is considered adequate for safe storage milling and further storage as milled rice. (Hall. 1970). Drying can be done traditionally by sun –drying if the whether permits drying rice (Paddy ) by Thinly spreading it on a floor or pilling it over to increase the surface area of the grain bulk to air and prevent the build up of heat within the grain bulk, (Alaka, 2005). This method allows drying in open air and under strong sunshine for 1 to 2 days. But the sun-drying method can produce sun –checks which reduces the head rice and increases the rice bran percentage, also this method requires intensive care and labour. Drying in more developed countries is done artificially such as the use of batch dryers in Japan and hot air dryer in United State, this is an alternative to sun –drying and can avoid dehydration, which cause reduce head, rice (Juliano 1993).
Cleaning /winnowing
After drying, the rough rice is winnowed to remove chaff (Juliano 1993). This is important to remove foreign seed and trash. The three major cleaning methods includes air cleaning by using sieves and gravity cleaning in rice mill. All impurities, odds and ends such as stones and peaces of soil and straw must be removed by screening or ridding process and fan. Also because of the subsequent effect of the storability and milling quality, unclean rice increase maintenance requirement on milling machine (FAO 1993).
Parboiling
In some countries a popular form of parboiled rice is subjected to a sterming or parboiling process while still a brown rice gain, the parboiling of rice is an ancient tradition in India and Pakisten but now, is part of traditional milling of rice in many developing countries (Okaka, 2005). The traditional parboiling process involves soaking rough rice over –night or longer in water at ambient temperature followed by boiling or steaming rice at 100% to gelatinize the starch, while the grain expands until the hall’s lemma and palea to separate (GARIBALDI 1984, Bhattacharys, 1985), Pullaiyar 1988). The parboiled rice is then cooled and sun-dried before storage or milling (Juliano 1993). Parboiling had as its original intention the loosing of the hulls, but in addition this enhances the nutritive value of milled rice so rerated because water dissolved the vitamins and minerals concentrated in the bran and hulls and re –distributed these in the endosperm (Okaka, 2005). Parboiling gelatinizes the starch granules and hardens the endosperm making it translucent. Chalky grains, and those with chalky back, belly or core becomes completely translucent on parboiling . A white core or centre indicate incomplete parboiling of the grain (Juliano 1993) results show that water soluble B vitamins (thiamine, riboflarin and niacin ) are higher in milled parboiled rice than in milled raw rice (Kik and Williams, 1945). Oil and protein are reported to diffuse outward during parboiling, based on microscopic observations. They cannot diffuse as readily though cell walls as water, soluble vitamins but the spherosme is destroyed. At similar degree of milling, parboiled milled rice has lower protein content than raw rice but parboiled rice bran has more protein and oil than raw rice bran (Padua and julieno 1974). Parboiling also removes cooked rice volatiles including free fatty acids, in activates enzymes such as lipase and lipoxygenase, kills the embryo and decomposes some antioxidants (SOwbhagya and bhattacharya, 1976) . hence cooked parboiled rice lack the volatiles characteristics of freshly cooked raw rice, hydrogen sulphide, acetaldehyde and ammonia (Obata and Tanaka 1965). The volatiles identified where mainly Addehydes and Ketnes. (Tsugita, 1996). Parboiled rice takes longer to cook than raw rice and may be presoaked in water to reduce the cooking time to be comparable to that of raw rice. The cooked grains are less sticky, do not dump and are resistance to disintegration, the grains are also
Advantages of parboiled rice parboiled rice as earlier started brings about many changes in the property of rice
– Parboiled rice is harder and therefore resists attack during storage
– It improves the amount of vitamin and other nutrient in the rice
– The loss of solid matter into the liquid furing cooking is less than in raw rice
– It reduces grain damage during milling shelling paddy becomes easier because the husk did split during parboiling
Disadvantages
– Parboiled rice may also choke the polisher due to the higher oil content in the bran
– Parboiled rice takes time to cook than raw rice
– The heat treatment during parboiling destroy antioxidant so parboiled becomes rancid more easily than raw rice
– Parboiled rice is more difficult to polish than the raw rice therefore milling is more costly in terms of time and power requirement expand more in girth rather than in length as compared to raw rice (Juliano 1993).
Drying Of Parboiled Rice
Unlike most cereals, rice is consumed primarily as unbroken kernel’s is much greater than that of broken kernels. Therefore, more care is needed to avoid breakage in drying rice than in drying other cereals. Moisture reduction takes place rapidly during the first drying from 36-18 percent moisture level, but it is low from 18-14% . as the kernels is dried the other portion shrinks setting up stress and strain and when moisture is removed too rapidly, checking or shattering of kernel results. Frying should stop at 18 percent moisture to allow paddy to equilibrate for several hours before continuing the drying to 14%. Paddy drying is one of the important steps in post harvest handling of rice. It determines storage time, milling out turn and appearance of rice . (Alaka 2005)
Milling
Milling in the rice industries can either be referred as the overall operation that includes cleaning, hulling pealing, polishing and grading or simply the removal of the rice bran or outer layers. Typically, the percicap accounts for 1 to 2% of the weight of the whole rice caryopsis seed coat and aleuronic 5% embryo 2 to 3% and endosperm 89-91% (Okaka 2005 ). The edible part of rice grain is enclosed in the glumes, which need to be first separated by hulling. The most primitive implement for hulling is the system with wood mortar and pestle or treadle or water mill. These hulling method are still used in some of rice grouting region in Asia and African. Now mechanical hulling or milling is popular in most of the rice growing countries. The hulled gain are then winnowed in order to remove the chaff and bran.
Rice with the hulls removed but the bran left on, is called “Brown rice” the pearling / milling is whitening or securing process in which the remaining layer, of pericap, tasti and aleuronic materials on the hulled rice are removed by passing it through services of pearling cone milled rice refers to the rice after removing all hulls, bran layer and germ (Okaka, 2005)
Polishing operation similar to pearling operation further whiten, and smoothens the rice gain to obtain fine white rice. Polishing is necessary for better appearances to meet the market needs with sacrifice of the flavor and health constituent in the rice grain (Okaka 2005, Francois et al; 1997) during grading operation the broken grains are separated and sorted to meet market standards. This separation can be accomplished by passing through a series of staves or cellular cylinder. These broken grains can be reduced to rice flourgrits and put to several useful ways as rice fufu or as raw material for manufacturing of several convenience
foods. (Osaretin 2006). Effect of milling on vitamins and minerals content of Rice.
Extraction rate(%)
|
Calcium (mg/g) |
phosphorus (mg/g) |
zinc (ppm) |
Iron (ppm) |
Copper (ppm) |
100(Rough Rice) |
0.3 |
3.1 |
24 |
38 |
2.8 |
82 (brown rice) |
0.1 |
3.2 |
33 |
88 |
2.7 |
72 (milled rice)
|
0.2 |
1.5 |
18 |
18 |
2.2 |
Extraction rate (%)
|
Thiamin mg/100 |
Riboflavin mg/100 |
Niacin microgram |
Pyridoxine microgram |
Folate (m/g) |
Biotin ng/g |
100(Rough Rice) |
2.8 |
0.5 |
29.6 |
51 |
0.5 |
91 |
82 (Brown Rice) |
2.4 |
0.3 |
29 |
5.1 |
0.3 |
48 |
72 (Milled Rice)
|
1.6 |
0.2 |
6.0 |
16 |
0.1 |
243 |
Soruce. Baurent friend and Derither (1991)
A lot of losses are encountered during milling of brown rice to white or polished rice. Milled or white rice represent 40 to 70% extraction of rough rice. The B. vitamins and iron are found primarily in the germ and bran layer, and are therefore removed in the cause of milling it has been estimated that in the cause of milling brown rice to white rice, approximately 80 percent of the thiamins is removed Baurent fiend and Dertter (1991)
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