Rice is the grain with the second
highest worldwide production after maize (Boumas, 1985). The domesticated rice
comprises two species of food crop in the Poaceae
(true grass) family Oryza sativa and Oryza glaberrima (Linscombe, 2006).
These plant are native to tropical and subtropical Southern Asian and South
Eastern Africa, respectively (Linares, 2002).
Rice is grown as a monocarpic annual
plant, although in the tropical areas it can survive as a perennial and
can produce a ration crop and survive
for up to 20years (Boumas, 1985). It is also an important crop in sub tropical
and temperate zones, the yield being higher in temperate areas than in the
tropics (Boumas, 1985). It can grow up to 1- 1.8 m tall, or more depending on
the variety and soil fertility. As a member of the grass family, rice has long,
slender leaves between 50- 100cm long and 2-2.5 cm broad. The small mind
pollinated flowers are produced in a branched arching to the pendulous
inflorescence 30- 50 cm long. The edible part of the rice plant is the rice
grain which is a caryopsis 5–12 mm long and 2-3 mm thick, and which includes
glumes, endosperm, and embryo (Boumas, 1985). Some varieties even have awns at
the tips of the grain. The awn is some times very long on certain varieties, so
that special machines are required to break off and remove the awns prior to
the de-husking of the paddy (Belsnio, 1980).
According to Li (2003), rice grain
is described as rough rice or paddy, consisting of brown rice (or caryopsis)
and the hull. Brown rice consists of the endosperm, embryo and several thin
layers of differentiated tissues – the pericarp (the ovary wall), the seed coat
and the nucleus. Li (2003) further suggested that the seed coat consist of six
layers of cells, with the aleuronic layer, being the innermost. The embryo contains
the embryonic leaves (plumule), enclosed by a sheath (coleoptiles) embryonic
primary root (radical), unsheathed by the coleorhizae, and the joining part
(mesocotyl). According to Ihekoronye and Ngoddy (1985), the grains of rice
consist of the endosperm, the main starchy portion, and the embryo or germ,
which are separated by the scutellem. These are shown in Figure 2.1. Rice
endosperm consists mostly of starch granules in a proteinaceous matrix,
together with sugar, fats crude fibre and organic matter. The proximate compositions
of rice endosperm are;
Table 2.1: proximate composition of
rice endosperm
|
Component
|
Amount
|
|
Starch
|
80%
|
|
Moisture
|
11%
|
|
Fibre
|
0.2%
|
|
Protein
|
6.8- 8%
|
|
Fat
|
2%
|
|
Ash
|
0.5%
|
|
Food energy
|
398 calories
|
Source: Ihekoronye and
Ngoddy 1985
Hull weight is
about 20% of the total grain weight. The hull of some rice grains has the
palea, lemmas, and richilla, while others have rudimentary glumes and perhaps a
portion of the pedicels. The lemma is usually tough, archmenlike, sometime
awned, and bigger than the palea. Grain ripening stage (15-65 days) can be
subdivided into milky, dough, yellow- ripe, and maturity stages based on the
texture and color of the growing grains. According to Hammer- Moister (2008),
knowledge about grain quality starts with knowing the anatomy of a single
grain, whether the grain is to be used for feed or for human consumption.