Gross nutrient composition

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Among the milling fractions of rice, the bran has the highest energy and protein content and the hull has the lowest (Table 14). Only the brown rice fraction is edible. Abrasive or friction milling to remove the pericarp, seed-coat, testa, aleurone layer and embryo to yield milled rice results in loss of fat, protein, crude and neutral detergent fibre, ash, thiamine, riboflavin, niacin and a-tocopherol. Available carbohydrates, mainly starch, are higher in milled rice than in brown rice. The gradients for the various nutrients are not identical as evidenced from analysis of successive milling fractions of brown rice and milled rice (Barber, 1972), (Figure 4). Dietary fibre is highest in the bran layer (and the hull) and lowest in milled rice. Density and bulk density are lowest in the hull, followed by the bran, and highest in milled rice because of the low oil content. The nutritional properties of the rice grain are discussed further in Chapter 4.

The B vitamins are concentrated in the bran layers, as is a-tocopherol (vitamin E), (Table 15). The rice grain has no vitamin A, vitamin D or vitamin C (FAO, 1954). The locational gradient in the whole rice grain is steeper for thiamine than for riboflavin and niacin, resulting in a lower percent retention of thiamine (vitamin B1) in milled rice (Table 15). About 50 percent of the total thiamine is in the scutellum and 80 to 85 percent of the niacin is in the pericarp plus aleurone layer (Hinton and Shaw, 1954). The embryo accounts for more than 95 percent of total tocopherols (of which a-tocopherols account for one-third) and nearly one-third of the oil content of the rice grain (Gopala Krishna, Prabhakar and Sen, 1984). By calculation, 65 percent of the thiamine of brown rice is in the bran, 13 percent in the polish and 22 percent in the milled rice fraction (Juliano and Bechtel, 1985). Corresponding values for riboflavin are 39 percent in the bran, 8 percent in the polish and 53 percent in the milled rice fraction. Niacin distribution is 54 percent in the bran, 13 percent in the polish and 33 percent in the milled rice fraction.

FIGURE 4 Distribution pattern of major constituents of brown rice determined using a tangential abrasive mill

The minerals (ash) are also concentrated in the outer layers of brown rice or in the bran fraction (Table 15). A major proportion (90 percent) of the phosphorus in bran is phytin phosphorus. Potassium and magnesium are the principal salts of phytin. The ash distribution in brown rice is 51 percent in the bran, 10 percent in the germ, 10 percent in the polish and 28 percent in the milled rice fraction; iron, phosphorus and potassium show a similar distribution (Resurrección, Juliano and Tanaka, 1979). However, some minerals show a relatively more even distribution in the grain: milled rice retained 63 percent of the sodium, 74 percent of the calcium and 83 percent of the Kjeldahl N content of brown rice (Juliano, 1985b).

TABLE 14 - Proximate composition of rough rice and its milling fractions at 14 percent moisture

Sources: Juliano, 1985b; Eggum. Juliano & Maniñgat, 1982; Pedersen & Eggum, 1983.

TABLE 15 - Vitamin and mineral content of rough rice and its milling fractions at 14 percent moisture

Sources: Juliano, 1985: Pedersen & Eggum, 1983.

TABLE 16 - Amino acid content of rough rice and its milling fractions at 14 percent moisture (9 per 16 9 N)

^a Based on 5.8 g lysine per 16 g N as 100% (V/HO, 1985).

Sources: Juliano, 1985b; Eggum. Juliano & Maniñgat, 1982; Pedersen & Eggum, 1983.

The amino acid content of the milling fractions is given in Table 16.

Starch

Starch is the major constituent of milled rice at about 90 percent of the dry matter. Starch is a polymer of D-glucose linked a -( 1-4) and usually consists of an essentially linear fraction, amylose, and a branched fraction, amylopectin. Branch points are a -(1-6) linkages. Innovative techniques have now shown rice amylose to have two to four chains with a number-average degree of polymerization (DP_n) of 900 to glucose units and a ß-amylolysis limit of 73 to 87 percent (Hizukuri et al., 1989). It is a mixture of benched and linear molecules with DP_n of 1100 to 1700 and 700 to 900, respectively. The branched fraction constitutes 25 to 50 percent by number and 30 to 60 percent by weight of amylose. The iodine affinity of rice amyloses is 20 to 21 percent by weight.

Rice amylopectins have ß-amylolysis limits of 56 to 59 percent, chain lengths of 19 to 22 glucose units, DP_n of 5 000 to 15 000 glucose units and 220 to 700 chains per molecule (Hizukuri et al., 1989). The iodine affinity of rice amylopectin is 0.4 to 0.9 percent in low- and intermediate-amylose rices but 2 to 3 percent in high-amylose rices. Isoamylase-debranched amylopectins showed more longest chain fractions (DP_n >100) (9 to 14 percent) in high-amylose samples with higher iodine affinity than in low- and intermediate-amylose samples (2 to 5 percent) and waxy rice amylopectin (0 percent), (Hizukuri et al., 1989).

Based on colorimetric starch-iodine colour absorption standards at 590 to 620 nm, milled rice is classified as waxy ( 1 to 2 percent), very low amylose (2 to 12 percent), low amylose (12 to 20 percent), intermediate (20 to 25 percent) and high (25 to 33 percent), (Juliano, 1979, 1985b). Recent collaborative studies showed that the maximum true amylose content is 20 percent and that additional iodine binding is due to the long linear chains in amylopectin (Takeda, Hizukuri and Juliano, 1987). Hence colorimetric amylose values are now termed "apparent amylose content".

The waxy endosperm is opaque and shows air spaces between the starch granules, which have a lower density than non-waxy granules. The structure of the starch granule is still not well understood, but crystallinity and staling are attributed to the amylopectin fraction.

Protein

Protein is determined by first carrying out micro Kjeldahl digestion and ammonia distillation and then using titration or colorimetric ammonia assay of the digest to determine nitrogen content, which is converted to protein by the factor 5.95. [The factor, based on a nitrogen content of 16.8 percent for the major protein of milled rice (glutelin), may be an overestimation; reappraisals have suggested values of 5.1 to 5.5 (5.17 + 0.25) (Mossé, Huet and Baudet, 1988; Mossé, 1990), 5.24 to 5.66 (mean 5.37) (Hegsted and Juliano, 1974) and 5.61 (Sosulski and Imafidon, 1990).]

Endosperm (milled rice) protein consists of several fractions comprising 15 percent albumin (water soluble) plus globulin (salt soluble), 5 to 8 percent prolamin (alcohol soluble) and the rest glutelin (alkali soluble), (Juliano, 1985b). Using sequential protein extraction, the mean ratio for 33 samples was found to be 9 percent prolamin, 7 percent albumin plus globulin and 84 percent glutelin (Huebner et al., 1990). The mean prolamin content of seven IRRI milled rices was 6.5 percent of their total protein (IRRI, l991b). The lysine content of rice protein is 3.5 to 4.0 percent, one of the highest among cereal proteins.

Rice bran proteins are richer in albumin than endosperm proteins and are found as distinct protein bodies containing globoids in the aleurone layer and the germ. These structures are different from endosperm protein bodies. Tanaka et al. ( 1973) reported the presence of 66 percent albumin, 7 percent globulin and 27 percent prolamin plus glutelin in aleurone protein bodies. Ogawa, Tanaka and Kasai (1977) reported the presence of 98 percent albumin in embryo protein bodies.

The endosperm protein is localized mainly in protein bodies (Figure 4). The crystalline (PB-II) protein bodies are rich in glutelin, and the large spherical protein bodies (PB-I) are rich in prolamin. Ogawa et al. (1987) estimated that endosperm storage proteins were composed of 60 to 65 percent PB-II proteins, 20 to 25 percent PB-I proteins and 10 to 15 percent albumin and globulin in the cytoplasm.

Rice starch granule amylose binds up to 0.7 percent protein that is mainly the waxy gene protein or granule-bound starchy synthase, with a molecular mass of about 60 kilodaltons (kd), (Villareal and Juliano, 1989b).

Rice glutelin consists of three acidic or a subunits of 30 to 39 kd and two basic or ß subunits of 19 to 25 kd (Kagawa, Hirano and Kikuchi, 1988). The two kinds of subunits are formed by cleavage of a 57-kd polypeptide precursor (Sugimoto, Tanaka and Kasai, 1986). Prolamin consists mainly (90 percent) of the 13- cd subunit plus two minor subunits of 10 and 16 kd (Hibino et al., 1989).

The essential amino acid contents of the glutelin and prolamin subunits (Table 17) showed lysine as limiting in these polypeptides except in the IEF3 fraction of the 13-kd prolamin subunit, which has 5.5 percent lysine and is limiting in methionine plus cysteine. Thus, glutelin has a better amino acid score than prolamin except for the 16-kd prolamin subunit. The 10-kd prolamin subunit has a high (6.8 percent) cysteine content.

Lipid

The lipid or fat content of rice is mainly in the bran fraction (20 percent, dry basis), specifically as lipid bodies or spherosomes in the aleurone layer and bran; however, about 1.5 to 1.7 percent is present in milled rice, mainly as non-starch lipids extracted by ether, chloroform-methanol and cold water-saturated butanol (Juliano and Goddard, 1986; Tanaka et al., 1978). Protein bodies, particularly the core, are rich in lipids (Choudhury and Juliano, 1980; Tanaka et al., 1978). The major fatty acids of these lipids are linoleic, oleic and palmitic acids (Hemavathy and Prabhaker, 1987; Taira, Nakagahra and Nagamine, 1988). Essential fatty acids in rice oil are about 29 to 42 percent linoleic acid and 0.8 to 1.0 percent linolenic acid (Jaiswal, 1983). The content of essential fatty acids may be increased with temperature during grain development, but at the expense of reduction in total oil content (Taira, Taira and Fujii, 1979).

TABLE 17 - Aminogram (9/16 g N) of the acidic and basic subunits of rice glutelin and the mayor and minor subunits of prolamin

^a S-cyanoethyl glutelin subunits.
^b Only the IEF3 fraction of the 13-kd, 10-kd and 16-kd prolamin subunits had cystine. All glutelins had substituted cysteine residues
^c Based on 5.8% lysine as 100% (WHO, 1985).
^d AIternative value is 34% based on 2.5% methionine + cysteine as 100% (WHO, 1985).

Sources: Juliano & Boulter, 1976; Villareal & Juliano, 1978 (glutelin subunits); Hibino et al., 1989 (prolamin subunits).

Starch lipids are mainly monoacyl lipids (fatty acids and lysophosphatides) complexed with amylose (Choudhury and Juliano, 1980). The starch lipid content is lowest for waxy starch granules (<0.2 percent). It is highest for intermediatearnylose rices ( 1.0 percent) and may be slightly lower in high-amylose rice (Choudhury and Juliano, 1980; Juliano and Goddard, 1986). Waxy milled rice has more non-starch lipids than non-waxy rice. Starch lipids are protected from oxidative rancidity, and the amylose-lipid complex is digested by growing rats (Holm et al., 1983). However, starch lipids contribute little to the energy content of the rice grain. The major fatty acids of starch lipids are palmitic and linoleic acids, with lesser amounts of oleic acid (Choudhury and Juliano, 1980).

TABLE 18 - Yield and composition of defatted and protease-amylase treated cell wall preparations obtained from different histological fractions of milling of brown rice

Source: Shibuya, 1989.

Non-starch polysaccharides

Non-starch polysaccharides consist of water soluble polysaccharides and insoluble dietary fibre (Juliano, 1985b). They can complex with starch and may have a hypocholesterolaemic effect (Normand, Ory and Mod, 1981; Normand et al., 1984). The endosperm has a lower content of dietary fibre than the rest of brown rice (Shibuya, 1989), (Table 18). Reported values for neutral detergent fibre are 0.7 to 2.3 percent (Juliano, 1985b), (Table 14). In addition, the endosperm or milled rice cell wall has a low lignin content but a high content of pectic substances or pectin. Endosperm pectin has a higher uronic acid content but a lower arabinose-to-xylose ratio than the other grain tissues. The hemicellulose of endosperm also has a lower arabinose-toxylose ratio than the three other grain tissues.

Volatiles

The volatiles characteristic of cooked rice are ammonia, hydrogen sulphide and acetaldehyde (Obata and Tanaka, 1965). Upon cooking, all aromatic rices contain 2-acetyl-1-pyrroline as the major aromatic principle (Buttery et al., 1983). Volatiles characteristic of fat rancidity are aldehydes, particularly hexanal, and ketones.

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