Nutrient composition and protein quality of rice relative to other cereals

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Comparison of the nutrient content of staple cereals at 14 percent moisture and higher-moisture tuber foods (Tables 24 to 27) shows a somewhat higher energy content in cereals (Table 24), but a higher ascorbic acid content in tubers (Table 25). Because tubers contain more moisture they have lower nutrient and energy density than cereals. Cassava has an extremely low protein content (Table 24) even after correction for moisture differences.

The protein level of rice is similar to those of potato and yam on a dry weight basis but is the lowest among the cereals. Rice also has the lowest dietary fibre content.

Amino acid analysis (Table 26) showed lysine to be the first limiting essential amino acid in cereal proteins, but lysine content was highest in oats and rice among cereal proteins (Eggum, 1979), (Table 26). In contrast, tuber proteins are adequate in lysine but deficient in sulphur amino acids cysteine and methionine particularly at high protein levels (Eppendorfer, Eggum and Bille, 1979; Food and Nutrition Research Institute, 1980).

TABLE 24 - Proximate composition of cereal and tuber staple foods (per 100 g)

Nitrogen-free extract by difference.

Sources: Souci, Fuchmann & Kraut, 1986; Eggum, 1969,1977,1979.

Whole-grain maize meal had protein quality comparable to that of wheat because of its large germ which is high in lysine-rich protein. Calculated amino acid scores based on the WHO/FAO/UNU pattern (WHO, 1985) showed tuber proteins to be superior to cereal proteins but do not take into consideration actual digestibility.

Rice has the highest protein digestibility among the staples (Table 27). Potato protein had a higher biological value than cereal proteins, consistent with its high amino acid score, but its net protein utilization (NPU) was lower than that of rice. Utilizable protein was comparable in brown rice, wheat, maize, rye, oats and potato but was lower in sorghum and higher in millet. Rice has the highest energy digestibility, probably in part because of its low dietary fibre and tannin content (Tables 24 and 26).

Cereal proteins are less digestible by children and adults than egg and milk protein, except for wheat endosperm (WHO, 1985), (Table 28). Digestibility values for cooked milled rice proteins were lower than those for raw milled rice (almost 100 percent) when tested on growing rats but were close to the values for other cereal proteins, except for the low value for sorghum. Based on the mean true digestibility of egg, milk, cheese, meat and fish protein of 95 percent, the relative digestibility of milled rice is 93 percent (WHO, 1985). The protein of cooked rice has a lower true digestibility in humans than the protein of raw rice in growing rats (Table 28). Cooked rice protein also has a true digestibility of 89 percent in growing rats (Eggum, Resurreción and Juliano, 1977).

TABLE 25 - Vitamin and mineral content of cereal and tuber staple foods (per 100 g)

^a Zinc level of cassava and yam from Bradbury & Holloway (1988).

Sources: Souci, Fuchmann & Kraut, 1986: Eggum, 1969,1977, 1979.

Nitrogen balance studies in Peruvian preschool children fed cooked cereals (Graham et al., 1980; MacLean et al., 1978, 1979, 1981) and potato (Lopez de Romaña et al., 1980) showed the highest apparent N absorption for wheat noodles but the highest apparent N retention for peeled potato and the highest protein quality, based on apparent N retention of casein control diets, for potato and milled rice (Table 29). Utilizable protein is highest for wheat and rice. High-lysine or opaque-2 maize is inferior to milled rice in protein quality but better than normal maize. Energy digestibility, indexed by faecal dry weight, was lowest for sorghum, probably because of its high tannin content (see Table 26).

TABLE 26 - Amino acid and tannin content in whole-grain cereals and tubers

^a All based on 5.8% lysine as 100%, except based on 1.1% tryptophan as 100% for cassava (WHO, 1985).

Sources: Eggum, 1969, 1977, 1979; Food and Nutrition Research Institute, 1980.

TABLE 27 - Balance data of whole-grain cereals and potato in five rats

Sources: Eggum, 1969, 1977, 1979.

TABLE 28 - Calculated true digestibility by adults and children of various cereal proteins as compared to egg, milk and meat protein

^a Mean true digestibility of 95%.

Sources: Hopkins, 1981; WHO, 1985.

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