The chemical composition of the
rice grain varies considerably depending upon the genetic factor of plant
variety and upon such environmental influences as location and season in which
grown, degree of milling and conditions of storage. According to Ihekoronye and
Ngoddy (1985), a sample of milled rice grain will contain about 80% starch, 8%
protein, 0.5% ash, and 11% water.
The starch is a mixture of amylose
and amylopectin. The proportion of these two starches has much to do with the
cooking and eating qualities of rice. The higher the proportion of amylose, the
drier and more separated the grains after cooking. True glutinous rice, on the
other hand, are essentially 100% amylopectin.
The protein content of polished rice
is somewhat lower than that of wheat, maize and sorghum. Lysine, the most
important limiting amino acid, constitutes about 4% of the protein of rice
twice the level in wheat flour or dehulled maize. Nevertheless, rice protein
does not contain enough lysine, threonine, or methionine. Consequently, for
proper nutrition, supplementary foods such as grain legumes, meat, and fish
should be part of the diet of those who consume large amounts of rice.
The lipids of rice are concentrated
in the bran, germ and polish fractions. In the tropics however, oil recovery
from rice for food use is hardly practiced. Like other cereals, rice is lacking
in vitamins A, D. and C, it does contain small amount of thiamine, riboflavin
and niacin. The levels of these vitamins are considerably higher in brown rice
than in polished rice, because the B- complex vitamins are concentrated largely
in the bran and germ, which are removed by milling.
Home-pounding, which is still the
common way rural people in the tropics dehull their rice, leaves the grain
higher in the B- complex vitamins than milled rice because bran and germ are
not completely removed. Polished rice that is parboiled also tends to provide
larger amounts of these vitamins than are available in the non parboiled grain
(Table 2.2).
Table 2.2: Vitamin and Mineral content of
rice grains, rice germ, rice bran, and rice polishing
|
Component
|
Whole
Rice
|
Milled
Rice
|
Parboiled Rice
|
Rice
Bran
|
Polished
Rice
|
Rice
Germ
|
|
Vitamins
(micrograms per gram of dry material)
|
||||||
|
Thiamine
(B1)
|
3.90
|
0.55
|
2.30
|
24.00
|
22.00
|
65.00
|
|
Riboflavin
(B2)
|
0.66
|
0.30
|
0.60
|
2.00
|
2.20
|
5.00
|
|
Pyridoxine
|
6.20
|
0.37
|
1.00
|
25.00
|
20.00
|
16.00
|
|
Pantothenic
acid
|
15.20
|
7.66
|
13.70
|
27.70
|
33.30
|
30.00
|
|
Nicotinic
acid
|
41.00
|
10.00
|
30.00
|
336.00
|
330.00
|
33.00
|
|
Inositol
|
1194.00
|
125.00
|
250.00
|
4627.00
|
4536.00
|
3725.00
|
|
Choline
|
1124.00
|
452.00
|
982.00
|
1700.00
|
1020.00
|
3000.00
|
|
Biotin
|
0.12
|
0.06
|
0.10
|
0.60
|
0.57
|
0.58
|
|
Minerals (percent dry matter)
|
||||||
|
Calcium
|
0.04
|
0.027
|
0.061
|
0.13
|
0.09
|
0.275
|
|
Phosphorus
|
0.27
|
0.09
|
0.130
|
1.
48
|
2.44
|
2.100
|
|
Iron
|
0.0027
|
00.011
|
0.0015
|
0.019
|
0.012
|
0.013
|
Source: Ihekoronye and Ngoddy (1985).