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The usual value assigned to the protein content of milled rice is 7 percent, based on a Kjeldahl conversion factor of 5.95. However, in nutritional studies the factor 6.25 is used to make the diets isonitrogenous with the standard proteins. The true digestibility of cooked rice protein in humans is 88 + 4 percent (WHO, 1985), (Table 28). Its amino acid score is about 65 percent based on 5.8 percent lysine as 100 percent (WHO, 1985). The NPU of milled rice in rats is about 70 percent (Eggum and Juliano, 1973, 1975). Biological value in growing rats is about 70 percent for raw rice and about 80 percent for cooked rice (Eggum, Resurrección and Juliano, 1977).
Raw rice protein is 100 percent digestible in growing rats (Eggum and Juliano, 1973, 1975). Although cooking reduces true digestibility in growing rats to 89 percent, lysine digestibility remains close to 100 percent (Eggum, Resurrección and Juliano, 1977; Eggum, Cabrera and Juliano, 1992). Thus the NPU of cooked rice is also about 70 percent. The effects of cooking are discussed in more detail in Chapter 5.
TABLE 29 Comparative protein utilization and faecal dry weight for Peruvian preschool children fed cooked cereals and potato
Feeding trials in growing rats and a study of growth rate data (Blackwell, Yang and Juliano, 1966), determinations of protein efficiency ratio and nitrogen growth index (Bressani, Elias and Juliano, 1971), net protein utilization studies (Eggum and Juliano, 1973,1975; Murata, Kitagawa and Juliano, 1978) and values for relative nutritive value (Hegsted and Juliano, 1974) showed that an increase in milled rice protein from 7 to 9 percent has nutritional advantages, based on utilizable protein (protein content x protein quality), (Tables 30 and 31). The lysine content of rice protein drops only slightly with an increase in the protein content of milled rice to 10 percent and then becomes constant above 10 percent protein (Cagampang et al., 1966; Juliano, Antonio and Esmama, 1973).
These rat trials were verified by isonitrogenous N balance studies in preschool children in Peru (MacLean et al., 1978) and the Philippines (Roxas, Intengan and Juliano, 1979), (Table 32). Although apparent N retention was somewhat lower for the high-protein rice, the decrease was just a fraction of the increase in protein content. Short-term N balance studies also showed that with the replacement of average-protein rice (7.5 to 7.8 percent) by an equal weight of high-protein rice (11.4 to 14.5 percent) apparent N retention increased from 3.6 to 11.7 percent in adults on rice diets (Clark, Howe and Lee, 1971), from 27.7 to 29.8 percent in adults on rice/ fish diets (Roxas, Intengan and Juliano, 1975) and from 21.6 to 31.6 percent in children on rice/mung bean diets (Roxas, Intengan and Juliano, 1976), (Table 33).
Long-term feeding trials in children's institutions in India and the Philippines demonstrated that replacement of average-protein (6 to 7 percent) milled rice with an equal weight of high-protein (10 percent) milled rice in children's diets improved growth, provided that other nutritional factors, such as zinc, did not become limiting (Pereira, Begum and Juliano, 1981; Roxas, Intengan and Juliano, 1980). The absence of height or weight response by the Indian children who were without a vitamin and mineral supplement may have resulted from a deficiency in zinc and other minerals and in vitamins at the higher protein intake.
TABLE 30 - Relation of protein content and protein quality of milled rice based on NPU and various slope ratio assays (weight gain) and reference proteins In growing rats
|Rice protein source||Protein
|Relative nutritive value (%)|
a Based on 5.8% lysine as 100% (WHO, 1985).
b Eggum & Juliano, 1973, 1975; Murata, Kitagawa & Juliano, 1978.
c Based on 0, 28, 56 and 84% rice diets and lactalbumin slope as 100% (Hegsted & Juliano, 1974).
d Based on 0, 1, 2, 3, 4 and 5% protein diets and casein slope as 75% (Bressani, Elias & Juliano, 1971).
e Based on 2, 5 and 8% protein diets and casein slope as 75% (B.E. McDonald, personal communication, 1974).
f Based on 0, 4, 8,12 and 15% protein diets end egg slope as 100% (Murata, Kitagawa& Juliano, 1978).
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