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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