THE PRODUCTION PROCESS OF RICE (ORIYZA SATIVA) IN NIGERIA

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   harder, they also tend to expand more  in  girth rather than in length as  compare to raw rice (Juliano  1993).
 
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 flour\grits 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.

Table1: Mineral content.
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
           


Vitamins content
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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