Rice in human nutrition - Grain structure, composition and consumers' criteria for quality

Chapter 3 - Grain structure, composition and consumers' criteria for quality

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The rice grain (rough rice or paddy) consists of an outer protective covering, the hull, and the rice caryopsis or fruit (brown, cargo, dehulled or dehusked rice), (Juliano and Bechtel, 1985), (Figure 2). Brown rice consists of the outer layers of pericarp, seed-coat and nucellus; the germ or embryo; and the endosperm. The endosperm consists of the aleurone layer and the endosperm proper, consisting of the subaleurone layer and the starchy or inner endosperm. The aleurone layer encloses the embryo. Pigment is confined to the pericarp (Juliano and Bechtel, 1985).

The hull (husk) constitutes about 20 percent of the rough rice weight, but values range from 16 to 28 percent. The distribution of brown rice weight is pericarp 1 to 2 percent, aleurone plus nucellus and seed-coat 4 to 6 percent, germ 1 percent, scutellum 2 percent and endosperm 90 to 91 percent (Juliano, 1972).

The aleurone layer varies from one to five cell layers; it is thicker at the dorsal than at the ventral side and thicker in short-grain than in long-grain rices (del Rosario et al., 1968). The aleurone and embryo cells are rich in protein bodies, containing globoids or phytate bodies, and in lipid bodies (Tanaka et al., 1973; Tanaka, Ogawa and Kasai, 1977).

The endosperm cells are thin-walled and packed with amyloplasts containing compound starch granules. The two outermost cell layers (the subaleurone layer) are rich in protein and lipid and have smaller amyloplasts and compound starch granules than the inner endosperm. The starch granules are polyhedral and mainly 3 to 9 ,Ám in size, with unimodal distribution. Protein occurs mainly in the form of spherical protein bodies 0.5 to 4 Ám in size throughout the endosperm (del Rosario et al., 1968; Bechtel and Pomeranz, 1978), (Figure 3), but crystalline protein bodies and small spherical protein bodies are localized in the subaleurone layer. The large spherical protein body corresponds to PB -I of Tanaka et al. ( 1980) and the crystalline protein body is identical to PB-II. Both PB-I and PB-II are distributed throughout the rice endosperm.

FIGURE 2 Longitudinal section of rice grain

Non-waxy rice (containing amylose in addition to amylopectin) has a translucent endosperm, whereas waxy (0 to 2 percent amylose) rice has an opaque endosperm because of the presence of pores between and within the starch granules. Thus, waxy grain has about 95 to 98 percent the grain weight of non-waxy grain.

Rice classification

There is no international standard for brown rice grain size and shape. IRRI uses the following scale for size: extra long, >7.50 mm; long, 6.61 to 7.50 mm; medium, 5.51 to 6.60 mm; and short, <5.50 mm. Grain shape is characterized based on length-to-width ratio: slender, >3.0; medium, 2.1 to 3.0; bold 1.1 to 2.0; and round, < 1.0.

FIGURE 3 Schematic diagram of various protein bodies and compound starch granule in the endosperm subaleurone layer

The Codex Alimentarius Commission committee considering the draft standard for rice proposed the following classification of milled rice based on length-towidth ratio; long grain, <3.1; medium grain, 2.1 to 3.0; and short grain, <2.0 (Codex Alimentarius Commission, 1990).

Proposed tolerances for defects for milled rices are 0.5 percent each for organic and inorganic extraneous matter, 0.3 percent for rough rice, 1.0 percent each for brown rice and waxy rice, 2.0 percent for immature grains, 3.0 percent each for damaged and heat-damaged grains, 4.0 percent for red grains, 8.0 percent for redstreaked grains and 11.0 percent for chalky grains (Codex Alimentarius Commission, 1990). The proposed tolerances for milled parboiled rices are identical to those for milled rices except for no tolerance for chalky grains, 6.0 percent for heat-damaged grains and additional tolerances of 2.0 percent each for raw milled rice and pecks (grains with >25 percent of the surface coloured dark brown to black). A more detailed description of milling is given in Chapter 4.


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