Home preparation and cooking

Contents - Previous - Next

Washing of milled rice prior to cooking is a common practice in Asia to remove bran, dust and dirt from the food, since rice is often retained in open bins and thus exposed to contamination. During washing some water-soluble nutrients are leached out and removed. Table 41 presents the washing and cooking losses of nutrients from various types of rice. It indicates that a significant amount of protein, ash, water-soluble vitamins and minerals and up to two-thirds of crude fat may be removed during washing. Marketing clean packaged rice will reduce or delete washing steps and prevent or reduce loss of nutrients during washing.

Boiling in excess water results in leaching out of water-soluble nutrients including starch and their loss when the cooking liquor is discarded. For example, 0.8 percent of the starch was removed on two washings of three milled rices, but 14.3 percent of the starch by weight was in the rice gruel after cooking for about 20 minutes in 10 weights of water (Perez et al., 1987). Protein removal was 0.4 percent during washing and 0.5 percent during cooking. Boil-in-the-bag parboiled rice in perforated plastic bags makes cooking in excess water simple and convenient. In the rice cooker or optimum-water-level method, the leachate sticks to the cooked rice surface as the water gets absorbed by the rice starch. The bottom layer is more mushy than the top layer.

TABLE 41 - Percent nutrient losses during washing and cooking in excess water

^a Kik & Williams, 1945; Cheigh et al., 1977a; Tsutsumi & Shimomura, 1978: Hayakawa & Igaue, 1979: Perez et al., 1987.
^b Cheigh et al., 1977a. 1977b; Perez et al., 1987.
^c El Bayâ, Nierle & Wolff, 1980.

Source: Juliano, 1985b.

Increasing the proportion of brokers in milled rice from 0 to 50 percent by weight increases loss of solids on cooking of raw rice from 13 to 27 percent (Clarke, 1982). A contributing factor is the shorter cooking time of brokers: the proportionate loss from the experiment was 22 percent for large brokers and 47 percent for small brokers.

Boiling in adequate cooking water also reduces the aflatoxin content of milled rice by 50 percent (Rehana, Basappa and Sreenivasa Murthy, 1979). Pressure-cooking destroys 73 percent of the aflatoxin, and cooking with excess water destroys 82 percent.

Boiling reduces the true digestibility of milled rice protein by 10 to 15 percent but has no effect on other cereal proteins (Eggum, 1973); however, it improves the biological value of the protein such that net protein utilization in rats is not reduced notably because lysine digestibility is not reduced (Eggum, Resurrección and Juliano, 1977), (Table 42). The undigested protein, which passes out of the alimentary system as faecal protein particles, represents the lipid-rich core protein of spherical protein bodies (Tanaka et al., 1978), which is poor in lysine but rich in cysteine (Tanaka et al., 1978; Resurrección and Juliano, 1981), (Table 43). Mutants with reduced levels of minor sulphur-rich fractions of rice prolamin (10 and 16 kd) are being developed to improve the digestibility of the protein of cooked rice, since the minor prolamin fractions are probably in the core fraction. Parboiling further reduces protein digestibility and increases the biological value correspondingly, without any adverse effect on net protein utilization (Eggum, Resurrección and Juliano, 1977; Eggum et al., 1984), (Table 40). The reported true digestibility of cooked milled rice is 88 ± 4 percent in adults and children (Hopkins, 1981), (Table 28).

Tanaka and Ogawa (1988) found greater amounts of large spherical protein bodies (PB-I) in indica rice (30 percent) than in japonica rice (20 percent), (Ogawa et al., 1987) and suggested that the protein of cooked indica rice may be less digestible than that of cooked japonica rice.

TABLE 42 - Mean nutritional properties of various raw and cooked, freeze-dried milled rices at 14 percent moisture

^a IR29 and IR480-5-9 only
^b Eggum, Resurrección & Juliano. 1977.
^c IRRI, 1984a.
^d Eggum et al., 1987.

TABLE 43 - Properties of whole and pepsin-treated cooked IR480-5-9 and IR58 milled-rice protein bodies.

^a Protein content of 10.5% for IR480-5-9 and 11.8% for IR58 milled rice
^b Number of pepsin treatments.

Sources: Resurrección & Juliano, 1981; Resurrección et al., 1992.

However, Tanaka, Hayashida and Hongo (1975) and Tanaka et al. (1978) reported similar in vitro digestibilities for protein bodies from japonica and indica rices.

The low lysine content in the protein of pepsin-treated protein bodies and faecal protein particles (Tanaka et al., 1978) explains the retention of the high lysine digestibility of rice protein on cooking. Its high cysteine content also explains why cysteine has the lowest digestibility among the amino acids of rice proteins (Tanaka et al., 1978).

The FAD/WHO method of protein quality evaluation is based on the amino acid score times true digestibility (TD) in rats (FAO, 1990c). Application of this method to the cooked composite rice diets of preschool and adult Filipinos and to their cooked rice component (Eggum, Cabrera and Juliano, 1992) gave protein quality values 6 to 8 percent lower (56 percent for rice and 89 end 80 percent for the two rice diets) than those based on lysine digestibility (62 percent and 95 and 88 percent, respectively). TD was 88 to 90 percent for the three samples, and lysine digestibility was 95 to 96 percent for the rice diets and 100 percent for cooked rice. Milled rice had higher digestible energy and protein but lower biological value and net protein utilization (NPU) than the rice diets. Amino acid scores and protein quality of the rice diets were as high or higher than their NPU, but the NPU of milled rice was higher than its amino acid score and protein quality. Thus, the new method will underestimate the protein quality of cooked rice, but not that of raw rice with 100 percent protein and lysine digestibilities in growing rats (Eggum, Resurrección and Juliano, 1977).

Contents - Previous - Next