4. Nutritive value

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The main nutritional value of roots and tubers lies in their potential ability to provide one of the cheapest sources of dietary energy, in the form of carbohydrates, in developing countries. This energy is about one-third of that of an equivalent weight of grain, such as rice or wheat, because tubers have a high water content. However, the high yields of most root crops ensure an energy output per hectare per day which is considerably higher than that of grains (see Table 4.1). Sweet potato for example has a tremendous capacity for producing high yields, up to 85 t/ha have been recorded on experimental plots, though most plantation yields do not exceed 20 t/ha. As shown in Table 4. 1, potato is one of the highest calorie-yielding crops in the world. Such root crops are particularly valuable in the tropics where most of the population depends on carbohydrate foods as dietary staples.

Because of the low energy content of root crops compared to cereals on a wet basis, it is often assumed that root crops are not suitable for use in baby foods. This is not necessarily true if their energy density is increased by drying. Tapioca, for instance, is used in a number of commercial baby foods in industrialized countries. Composite flours prepared from root crops and cereals could be used in baby food formulas, if appropriately supplemented. The addition of germinated (malted) cereals to cassava flour increases the energy density of gruels prepared from it, by reducing their viscosity through the action of amylolytic enzymes. However, the use of fresh cassava products as infant weaning foods should be discouraged, because of probable toxicity, low protein content and energy density. Infants and young children, pregnant and lactating women are among the most nutritionally vulnerable people. Their nutrient requirements are specifically higher in order to meet the increased physiological demand for growth and lactation. These requirements are listed in Tables 4.2 and 4.3 together with those for adolescents and adults.

TABLE 4.1 - Comparison average energy and protein production of selected food crops In developing countries (per hectare and per day)

Source: FAO, Production yearbook 1983 (Rome 1984), USDA Composition of foods (Washington, D.C. 1975) and FAO, Report of the agroecological zones project (Rome, 19;8). Production estimates arc 1981-83 averages; price estimates are for 1977.
Adapted and modified from Horton et al., (1984).

Undernutrition is often the outcome of either an insufficient food intake or poor utilization of food by the body, or both simultaneously. Recent surveys show that very few people in tropical countries suffer from a simple protein deficiency. The most prevalent deficiency is protein-energy, in which an overall energy deficiency forces the metabolism to utilize the limited intake of protein as a source of energy. This is an area in which root crops could play a more significant role as additional sources of dietary energy and protein. Increasing the consumption of root crops could help save the much-needed protein provided essentially by other foods such as cereals and legumes. Traditionally, in Africa, root crops such as cassava are eaten with a soup or stew made of fish, meat or vegetables, providing an excellent supplement to cassava meal.

TABLE 4.2 - Average dally energy, protein, vitamin A, folic acid, Iron and iodine requirements for Infants and children

Notes:
¹Values derived from Energy and protein requirements: report of a joint FAO/WHO/UNU expert consultation. WHO Technical Report Series 724. Geneva, 1985.
²Values derived from Requirements of vitamin A, iron, folate and vitamin B12: report of o joint FAO/WHO Expert Consultation (In press)
³Values derived from Recommended dietary allowances: Ninth rev. ea., US National Academy of Sciences. Washington, D.C., 1980.
Source: FAO, 1988b.

TABLE 4.3 - Average daily energy, protein, vitamin A, folic Iron and iodine requirements for adolescents and adults

Notes:
¹Values derived from Energy and protein requirements: report of a joint FAO/WHO/UNU expert consultation. WHO Technical Report Series 724. Geneva, 1985.
²Values derived from Requirements of vitamin A, iron, folate and vitamin B.': report of a joint FAO/WHO Expert Consultations. (In press)
³Values derived from Recommended dietary allwances. Ninth rev. ea., US National Academy of Sciences. Washington, D.C., 1980.
⁴Among pregnant women, dietary supplementation of iron is usually called for because the iron requirement cannot be met through normal dietary intake.
+ In addition to the normal requirement.
Source: FAO, 1988b.

Nutritional composition of roots and tubers

As with all crops, the nutritional composition of roots and tubers varies from place to place depending on the climate, the soil, the crop variety and other factors. Table 4.4 shows the nutritional composition for common roots and tubers and the amino-acid composition of some root crop proteins along with a comparison of suggested amino-acid requirement is shown in Table 4.5.

The main nutrient supplied by roots and tubers is dietary energy provided by carbohydrates. The protein content is low (one to two percent) and in almost all root crop proteins, as in legume proteins, sulphur-containing amino-acids are the limiting amino-acids (Tables 4.5, 4.9). Cassava, sweet potato, potato and yam contain some vitamin C and yellow varieties of sweet potato, yam and cassava contain beta-carotene or provitamin A. Taro is a good source of potassium. Roots and tubers are deficient in most other vitamins and minerals but contain significant amounts of dietary fibre. Leaves of taro are cooked and eaten as a vegetable. They contain betacarotene, iron and folic acid, which protects against anaemia. Leaves of sweet potato and cassava are also commonly eaten.

Carbohydrates

The dry matter of root crops, banana and plantain is made up mainly of carbohydrate, usually 60 to 90 percent. Plant carbohydrates include celluloses, gums and starches, but starches are the main source of nutritive energy as celluloses are not digested.

Starches are made up of two main polymers, a straight chain glucose polymer called amylose, which usually constitutes about 10 to 30 percent of the total, and the branched chain glucose polymer, amylopectin, which makes up the rest. The principal constituent of edible carbohydrate is starch together with some sugars, the proportion depending on the root crop.

The physical properties of starch grains influence the digestibility and processing qualities of root crops. The starch granules of some varieties of cocoyam are very small, about one-tenth those of potato, which improves the starch digestibility, making these varieties more suitable for the diets of infants and invalids. For the preparation of certain foods like fufu, a stiff dough is required and so the rheological properties of the starch paste become significant. The viscosity of starch-water pastes of different yam starches varies considerably from a relatively low value for D. dumetorum through increasing viscosity in D. esculenta to the highest value in D. rotundata (see Table 4.6). Hence D. rotundata is traditionally the accepted yam for fufu. Most yams give viscous pastes with a much higher gel strength than that of other crops. Therefore yams are traditionally preferred for fufu, a starch paste which is prepared by pounding cooked roots or tubers in a mortar with a pestle (Rasper, 1969, 1971). Cassava starch has some special characteristics for food processors. It is readily gelatinized by cooking with water and the solution after cooling remains comparatively fluid. The solutions are relatively stable and do not separate again into an insoluble form (retrogradation) as is the case with maize and potato starch.

In addition to starch and sugar, root crops also contain some non-starch polysaccharides, including celluloses, pectins and hemicelluloses, as well as other associated structural proteins and lignins, which are collectively referred to as dietary fibre (Table 4.7). The role of dietary fibre in nutrition has aroused a lot of interest in recent years. Some epidemiological evidence suggests that increased fibre consumption may contribute to a reduction in the incidence of certain diseases, including diabetes, coronary heart disease, colon cancer, and various digestive disorders. The fibre appears to act as a molecular sieve, trapping carcinogens which would otherwise have been recirculated into the body; it also absorbs water thus producing soft and bulky stools. Sweet potato is a significant source of dietary fibre as its pectin content can be as high as 5 percent of the fresh weight or 20 percent of the dry matter at harvest (Collins and Walter, 1982). However, banana, which is also known to have a beneficial effect in correcting intestinal disorders, appears to contain very little dietary fibre, only 0.84 percent using traditional methods of analysis. Because of this, Forsythe (1980) carried out some studies on the cell wall materials of banana pulp by extracting with ascorbic acid, centrifuging and washing the sugars away. The residue, comprising 3.3 percent of the pulp had a water-holding capacity 17 times its dry weight. Analysis yielded 15.2 percent lignin, 13 percent starch, 9.8 percent protein, 4.8 percent cellulose, 3.7 percent lipid, 1.3 percent pectin and 0.4 percent ash. There is therefore a need to pay more attention to the significance of fibrous components in these root crops, especially in banana and sweet potato, and to determine their composition and dietary function. Other root crops, particularly yam, contain mucilages, which have a considerable influence on their cooking qualities.

TABLE 4.4 - Nutritive values of tropical root crops (per 100 9 edible portion)

TABLE 4.5 - Comparison of suggested amino-acid requirement patterns with amino-acid composition of tropical root crops

TABLE 4.6 - Rheological properties of various yam starches

Source: Rasper and Coursey (1967).

TABLE 4.7 - Fibre as percentage of dry matter in raw sweet potato and banana

TABLE 4.8 - Calorie and protein contribution of starchy staples to diets in developing country regions, (in percentage of regional total) 1979-81¹

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