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

Because of the low energy content of root crops compared tocereals on a wet basis, it is often assumed that root crops arenot suitable for use in baby foods. This is not necessarily trueif their energy density is increased by drying. Tapioca, forinstance, is used in a number of commercial baby foods inindustrialized countries. Composite flours prepared from rootcrops and cereals could be used in baby food formulas, ifappropriately supplemented. The addition of germinated (malted)cereals to cassava flour increases the energy density of gruelsprepared from it, by reducing their viscosity through the actionof amylolytic enzymes. However, the use of fresh cassava productsas infant weaning foods should be discouraged, because ofprobable toxicity, low protein content and energy density.Infants and young children, pregnant and lactating women areamong the most nutritionally vulnerable people. Their nutrientrequirements are specifically higher in order to meet theincreased physiological demand for growth and lactation. Theserequirements are listed in Tables 4.2 and 4.3 together with thosefor adolescents and adults.

TABLE 4.1 - Comparison average energy and proteinproduction of selected food crops In developing countries (perhectare and per day)

Source: FAO, Production yearbook 1983 (Rome 1984), USDAComposition of foods (Washington, D.C. 1975) and FAO, Report ofthe agroecological zones project (Rome, 19;8). Productionestimates 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 insufficientfood intake or poor utilization of food by the body, or bothsimultaneously. Recent surveys show that very few people intropical countries suffer from a simple protein deficiency. Themost prevalent deficiency is protein-energy, in which an overallenergy deficiency forces the metabolism to utilize the limitedintake of protein as a source of energy. This is an area in whichroot crops could play a more significant role as additionalsources of dietary energy and protein. Increasing the consumptionof root crops could help save the much-needed protein providedessentially by other foods such as cereals and legumes.Traditionally, in Africa, root crops such as cassava are eatenwith a soup or stew made of fish, meat or vegetables, providingan 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 ofa joint FAO/WHO/UNU expert consultation. WHO Technical ReportSeries 724. Geneva, 1985.
²Values derived from Requirements of vitamin A, iron, folate andvitamin B12: report of o joint FAO/WHO Expert Consultation (Inpress)
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 ofa joint FAO/WHO/UNU expert consultation. WHO Technical ReportSeries 724. Geneva, 1985.
²Values derived from Requirements of vitamin A, iron, folate andvitamin B.': report of a joint FAO/WHO Expert Consultations. (Inpress)
³Values derived from Recommended dietary allwances. Ninth rev.ea., US National Academy of Sciences. Washington, D.C., 1980.
⁴Among pregnant women, dietary supplementation of ironis usually called for because the iron requirement cannot be metthrough normal dietary intake.
+ In addition to the normal requirement.
Source: FAO, 1988b.

As with all crops, the nutritional composition of roots andtubers varies from place to place depending on the climate, thesoil, the crop variety and other factors. Table 4.4 shows thenutritional composition for common roots and tubers and theamino-acid composition of some root crop proteins along with acomparison of suggested amino-acid requirement is shown in Table4.5.

The main nutrient supplied by roots and tubers is dietaryenergy provided by carbohydrates. The protein content is low (oneto two percent) and in almost all root crop proteins, as inlegume proteins, sulphur-containing amino-acids are the limitingamino-acids (Tables 4.5, 4.9). Cassava, sweet potato, potato andyam contain some vitamin C and yellow varieties of sweet potato,yam and cassava contain beta-carotene or provitamin A. Taro is agood source of potassium. Roots and tubers are deficient in mostother vitamins and minerals but contain significant amounts ofdietary fibre. Leaves of taro are cooked and eaten as avegetable. They contain betacarotene, iron and folic acid, whichprotects against anaemia. Leaves of sweet potato and cassava arealso commonly eaten.

The dry matter of root crops, banana and plantain is made upmainly of carbohydrate, usually 60 to 90 percent. Plantcarbohydrates include celluloses, gums and starches, but starchesare the main source of nutritive energy as celluloses are notdigested.

Starches are made up of two main polymers, a straight chainglucose polymer called amylose, which usually constitutes about10 to 30 percent of the total, and the branched chain glucosepolymer, amylopectin, which makes up the rest. The principalconstituent of edible carbohydrate is starch together with somesugars, the proportion depending on the root crop.

The physical properties of starch grains influence thedigestibility and processing qualities of root crops. The starchgranules of some varieties of cocoyam are very small, aboutone-tenth those of potato, which improves the starchdigestibility, making these varieties more suitable for the dietsof infants and invalids. For the preparation of certain foodslike fufu, a stiff dough is required and so the rheologicalproperties of the starch paste become significant. The viscosityof starch-water pastes of different yam starches variesconsiderably from a relatively low value for D. dumetorum throughincreasing viscosity in D. esculenta to the highest value in D.rotundata (see Table 4.6). Hence D. rotundata is traditionallythe accepted yam for fufu. Most yams give viscous pastes with amuch higher gel strength than that of other crops. Therefore yamsare traditionally preferred for fufu, a starch paste which isprepared by pounding cooked roots or tubers in a mortar with apestle (Rasper, 1969, 1971). Cassava starch has some specialcharacteristics for food processors. It is readily gelatinized bycooking with water and the solution after cooling remainscomparatively fluid. The solutions are relatively stable and donot separate again into an insoluble form (retrogradation) as isthe case with maize and potato starch.

In addition to starch and sugar, root crops also contain somenon-starch polysaccharides, including celluloses, pectins andhemicelluloses, as well as other associated structural proteinsand lignins, which are collectively referred to as dietary fibre(Table 4.7). The role of dietary fibre in nutrition has aroused alot of interest in recent years. Some epidemiological evidencesuggests that increased fibre consumption may contribute to areduction in the incidence of certain diseases, includingdiabetes, coronary heart disease, colon cancer, and variousdigestive disorders. The fibre appears to act as a molecularsieve, trapping carcinogens which would otherwise have beenrecirculated into the body; it also absorbs water thus producingsoft and bulky stools. Sweet potato is a significant source ofdietary fibre as its pectin content can be as high as 5 percentof the fresh weight or 20 percent of the dry matter at harvest(Collins and Walter, 1982). However, banana, which is also knownto have a beneficial effect in correcting intestinal disorders,appears to contain very little dietary fibre, only 0.84 percentusing traditional methods of analysis. Because of this, Forsythe(1980) carried out some studies on the cell wall materials ofbanana pulp by extracting with ascorbic acid, centrifuging andwashing the sugars away. The residue, comprising 3.3 percent ofthe pulp had a water-holding capacity 17 times its dry weight.Analysis yielded 15.2 percent lignin, 13 percent starch, 9.8percent protein, 4.8 percent cellulose, 3.7 percent lipid, 1.3percent pectin and 0.4 percent ash. There is therefore a need topay more attention to the significance of fibrous components inthese root crops, especially in banana and sweet potato, and todetermine their composition and dietary function. Other rootcrops, particularly yam, contain mucilages, which have aconsiderable influence on their cooking qualities.

crops

TABLE 4.6 - Rheological properties of various yamstarches

Source: Rasper and Coursey (1967).

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