Contents - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
Food and nutrition policy
implications of projects giving high priority to the production
of staples of low nutritive quality: The Case for Cassava
(Manihot esculenta, Crantz) in the Humid Tropics of West Africa
Strategies for the prevention of vitamin-a deficiency
Determining the prevalence of calorie deficient diets in developing countries by alternative methods of measurement
Nutritional implications of projects giving high priority to the production of staples of low nutritive quality: The Case for Cassava (Manihot esculenta, Crantz) in the Humid Tropics of West Africa
Bede N. Okigbo
International Institute of Tropical Agriculture, Ibadan, Nigeria
Cassava (Manihot esculenta, Crantz) is emerging as a dominant staple of primary or secondary importance in many developing countries of the humid and sub-humid tropics in Africa and elsewhere. Since it can withstand drought, it is sometimes a nutritionally strategic famine reserve crop in areas of unreliable rainfall. This paper reviews reasons for the spread of, and increasing interest in, cassava, the possible cyanide toxicity associated with it, the nutritional problems of cassava dependency, and the need for consideration of the total food/nutrition system in the planning of interventions to ensure balanced diets or attainment of acceptable nutritional status in cassava-dependent cultures. It also emphasizes the need to ensure that cassava, which is not usually fed to young children in tropical Africa, does not replace the traditional cereal-based and more protein-rich weaning foods.
CASSAVA AS A MAJOR STAPLE IN THE WORLD AND IN WEST AFRICA IN PARTICULAR
Cassava (Manihot escutenta, Crantz), variously designated as manioc, yucca, or tapioca, is native to South America and southern and western Mexico. It was one of the first crops to be domesticated, and there is archaeological evidence that it was grown in Peru 4,000 years ago and in Mexico 2,000 years ago. Cassava is adapted to the zone within latitudes 30 north and south of the equator, at elevations of not more than 2,000 m above sea level, in temperatures ranging from 18 to 25 C, to rainfall of 50 to 5,000 mm annually, and to poor soils with a pH from 4 to 9.0 (1 - 3). Within the cassava belt are located most of the developing countries of the world. They account for only 13 per cent of the world's GDP, but have 47 per cent of the world's population and 46 per cent of the arable land.
From Mid and South America, cassava spread to other parts of the world in post-Columbian times and was introduced into the West Coast of Africa and Zaire in the late sixteenth century, probably in slave ships. It was introduced into East Africa (Madagascar and Zanzibar) via Réunion by the end of the eighteenth century, and by 1800 it had reached India. It was widely grown in Africa and Southeast Asia by the 1850s.
Importance of Cassava
Cassava is a major subsistence staple, with an annual world production estimated at over 100 million tons in 1972. This made cassava the sixth major source of staple foods in the world, amounting to 57 per cent of the tropical root and tuber production in 1972 (4,5). Cassava supplies 38.6 per cent of the caloric requirements in Africa, 11.7 per cent in Latin America, and 6.7 per cent in the Far East (3). In 1970, it was estimated that about 200 million people relied upon cassava for most of their calorie requirements. However, the number of people relying on cassava for satisfying 50 per cent or more of their calorie requirements amounted to 420 million in 1972 (3).
Although cassava is of New World origin, 40 per cent of the current world production takes place in Africa. According to FAO (6), Brazil accounts for 30 million tons annually, Zaire 10.5 million, Indonesia 10.5 million, and Nigeria 7.5 million tons. Other major producing countries that make up the bulk of the world's market supplies of cassava products are Thailand, Madagascar, Indonesia, and Benin.
In the humid and sub-humid areas of tropical Africa, cassava is either a primary staple or a secondary co-staple. About 80 per cent of Africa's cassava production, equivalent to 42 million tons in 1972, was produced in West Africa. Nigeria's production of cassava in 1977 was 10.6 million tons on 1.1 million ha. Cassava comprised about 25 per cent of all food crops consumed in Nigeria in 1968 (7). Cassava is not only used in many food preparations for human consumption, but is of importance in industries (starch, textiles, fuel, confectionery, etc.), and in animal feeds. A list of cassava products for these purposes is given in table 1.
TABLE 1. Cassava Products for Human Consumption, Livestock Feed, and Industrial Use
|Human consumption||Raw cassava|
|Cooked cassava slices|
|Fried cassava slices|
|Composite flour, bread|
|Cassaripo or tucupa|
|Livestock feed||Cassava pellets|
|Cassava slices (fresh or boiled)|
|Glues and pastes|
|Bodying agent (caramel)|
|Dusting agent (chewing gum)|
Reasons for the Rapid Spread of Cassava
The reasons for the rapid spread of cassava and why it is often the preferred crop in parts of West Africa include the following.
a. It adapts to poor soils on which most other crops fail.
b. It resists drought, except at planting time, and it resists locust damage, making it a good famine crop.
c. Cassava is very easily propagated by stem cuttings that, unlike the case with yams, are not used for food.
d. Cassava is a relatively high yielder and an excellent source of calories. It can produce more carbohydrate per unit area than is provided by other staples. According to DeVries (8), its potential yield may reach 75 tons/ha and up to 250,000 calories/ha/day.
e. Cassava is relatively inexpensive to produce and (i) requires very little weeding when planted in optimal plant populations; (ii) has no critical planting date, provided there is enough moisture at planting; and (iii) its roots can be left stored in the ground and harvested when required.
Despite these obvious advantages, cassava remained for some time a neglected crop in agricultural research and development activities to an extent not commensurate with its importance as food. However, some developments within the past 15 years have enhanced interest in the crop and research priority has been given to research on its improvement, increased production, and utilization.
First, the International Society for Tropical Root and Tuber Crops was founded in 1967 to encourage research, increased production, and utilization and exchange of information on tropical root and tuber crops, including cassava, yams, sweet potatoes, and aroids. Second, among the International Agricultural Research Networks are two institutes (International Centre for Tropical Agriculture - IITA- in Nigeria, and the International Centre for Tropical Agriculture- CIAT- in Colombia) that have programmes giving high priority to research on the improvement, production systems, storage, and utilization of cassava, and other related training.
Cassava in Tropical Africa
Since its introduction into West Africa in the sixteenth century, cassava has spread rapidly to various parts of Africa, where it is grown alone in sequence or in different inter-cropping systems with other staples Cassava is now an important staple in southern Nigeria, parts of Ghana, the Ivory Coast, Sierra Leone, Liberia, Guinea, Senegal, the United Republic of Cameroon, Gabon, Zaire, the Central African Republic, Madagascar, Tanzania, Uganda, the southern Sudan, Angola, Mozambique, and Kenya (fig. 1) (9). In countries such as Sierra Leone and Zaire, cassava leaves are popular leaf vegetables. A more detailed account of the importance and production of cassava in tropical Africa is presented in Jones (10).
FIG. 1. Areas of Cassava Production in Tropical Africa (Adapted from ref. 9)
NUTRITIONAL VALUE OF CASSAVA
Cassava is a starchy staple whose roots are very rich in carbohydrates, a major source of energy. In fact, the cassava plant is the highest producer of carbohydrates among crop plants with perhaps the exception of sugarcane. It has been reported that cassava can produce 250 x 103 calories/ha/day compared to 176 x 103 for rice, 110 x 103 for wheat, 200 x 103 for maize, and 114 x 103 for sorghum (3,11 - 13). The chemical composition of cassava varies in different parts of the plant, and according to variety, location, age, method of analysis, and environmental conditions (table 2).
Although cassava roots are rich in calories, they are grossly deficient in proteins, fat, and some of the minerals and vitamins. Consequently, cassava is of lower nutritional value than are cereals, legumes, and even some other root and tuber crops such as yams (table 3; 14,15). The cassava root contains carbohydrates, 64 to 72 per cent of which is made up of starch, mainly in the form of amylose and amylopectin. About 17 per cent sucrose is found in sweet varieties, and small quantities of fructose and dextrose have been reported (14). The lipid content of cassava is only 0.5 per cent. Cassava is poor in proteins (1 to 2 per cent), and the aminoacid profile of the cassava root is very low in some essential amino acids, particularly lysine, methionine, and tryptophan. The peel of cassava roots contains slightly more protein than is found in the flesh. Therefore, peeling results in loss of part of the valuable protein component of the root. However, fermentation of the roots results in protein enrichment by a factor of some 6 to 8 (14). Cassava is reasonably rich in calcium and vitamin C, but the thiamine, riboflavin, and niacin contents are not as high. Large proportions of these nutrients have been reported to be lost during processing. All of this should be taken into account in cassava-processing in order to retain as much as possible of these nutrients.
Cassava leaves are much richer in proteins than the roots are. Although the leaves contain far less methionine than the roots, the levels of all other essential amino acids exceed the FAO's recommended reference protein intake. For this reason, cassava-leaf protein is claimed to be superior to soybean protein. Supplementation of Cassava products such as leaf-meal with methionine or any other of the nutrients it lacks serves to improve its biological value significantly and has been widely practiced in industry for the processing of food for human consumption and animal feeds (16).
TABLE 2. Average Chemical Composition of Cassava on a Per Cent Dry Matter Basis
Source: V.A. Oyenaga, personal communication.
TABLE 3. Average Nutrient Composition (per 100 gm Edible Portion) of Cassava Compared to That of Some Staple Food Crops Found in West Africa
Source: Refs. 14 and 15.
TABLE 4. Concentration of Cyanogenic Glucosides in Tissues of Sweet and Bitter Cultivars of Cassava (M. esculenta)
|Cultivar||Tissue||Cyanogenic glucosides (mg HCN/kg fresh wt. tissue)|
Source: Ref. 13.
CYANIDE CONTENT AND TOXICITY OF CASSAVA
Cassava contains cyanogenic glucosides in the form of linamarin (93 per cent), and to much less extent, lotaustralin (7 per cent). The amount of cyanogenic glucosides varies with the part of the plant, its age, variety, and environmental conditions such as soil, moisture, temperature, etc. (table 4) (13). Certain varieties of Cassava have long been designated as sweet or bitter, purportedly in relation to their cyanogenic glucoside content. The sweet varieties are supposed to be much lower in HCN content than the bitter varieties. However, results of chemical analysis of various parts of the cassava plant at different stages of development indicate that, at times, no significant differences exist between comparable parts of sweet and bitter varieties. Nartey (13) observed that the phelloderm of sweet varieties may contain cyanogenic glucoside, while the fleshy cortex may contain none. Also, the seeds of sweet varieties contain no HCN, though seeds of bitter ones do.
Cassava Toxicity and Nutritional Problems Associated with a High Cassava Diet
Circumstantial evidence, epidemiological studies, and laboratory studies with experimental animals have linked Cassava consumption with certain pathological conditions and diseases such as tropical ataxic neuropathy and endemic goitre (13, 17 - 24). Only a brief review of the problem will be presented here.
Cassava-eating American Indians have known of toxic properties in the roots for centuries. This led to the development of methods for detoxification. Consumption of cassava and other foodstuffs high in cyanide can cause acute cyanide poisoning and death in man and other animals. Coursey (23) has suggested the following as a rough guide to cyanide toxicity:
1. Innocuous: less than 50 mg HCN/kg of fresh, peeled root
2. Moderately poisonous: 50 - 100 mg HCN/kg of fresh, peeled root
3. Dangerously poisonous: over 100 mg HCN/kg fresh, peeled root
He also observed that, in relation to organoleptic quality, sweet-cassava products usually contain 40 to 130 ppm cyanide, non-bitter ones 30 to 180 ppm, bitter 80 to 412.5 ppm, and very bitter 280 to 490 ppm of cyanide. It is interesting to note the overlap in cyanide content among different bitter varieties of cassava.
Consumption of cassava products containing non-toxic levels of cyanide over long periods of time results in chronic cyanide toxicity and associated pathological conditions. Osuntokun (21) reported the occurrence of tropical ataxic neuropathy (TAN) consisting of lesions of the skin, mucous membranes, optic and auditory nerves, spinal cord, and peripheral nerves. Patients exhibited myelopathy, bilateral optic atrophy, bilateral hearing loss, and polyneuropathy. About 35 per cent of the patients had stomato-glossitis and motor-neurone disease, Parkinson's disease, cerebellar degeneration; psychosis and dementia were also found to be associated with the disease.
It was concluded that, although the pathological conditions in Nigerian cases of tropical ataxic neuropathy are similar to those observed elsewhere, it is not justifiable to assume that these represent clinical variants of the same disease, since, when a diet is poor, multiple nutritional deficiencies usually occur together, although one single factor may exercise overriding influence in association with others that combine to produce the final picture. TAN was observed by Osuntokun (21) to be prevalent in areas of intense cultivation of cassava, high frequency of cassava consumption, and high thiocyanate levels. The disease was rare among 1- to 10-year olds, and, although it tended to run in some families, there was no evidence to indicate it was genetically inherited. In one village, the average incidence was 3 per cent, but was 8 per cent among those 50 to 60 years old.
Goitre was observed to be 2 to 5 per cent higher among patients with TAN. Osuntokun (21) observed that certain local cassava preparations such as purupuru may contain 50 mg of cyanide per 3 kg of product compared to a lethal dose of 60 mg. Cyanide contents of other local Nigerian cassava preparations such as gari or eba and purupuru were found to be higher than in other food items in areas where TAN occurs. Similar cases have been reported in Tanzania.
Ekpechi (22) reported marked variation in incidence of endemic goitre from village to village, but no inverse relationship was observed between iodine content of drinking water and incidence of goitre. In the village of Eha Amufu, visible goitre was present in 38.2 per cent of the population, though drinking-water iodine content was 2.75 ± 0.04 mg/litre, compared to Nsukka village, where goitre incidence was 9.3 per cent and drinking-water iodine content was 0.86 ± 0.09 mg/litre. A survey revealed a high correlation between intake of dry, smoked, unfermented cassava and goitre incidence. A subsequent laboratory animal experiment in which rats were fed dry, unfermented cassava indicated that the diet had adverse effects on thyroid function- an action comparable to that of thion-amide goitrogen. Delange et al. (17) also reported endemic goitre on Idjwi Island in Lake Kivu that was not related to iodine deficiency alone. Experiments with laboratory animals confirmed that cassava in the diet interfered with iodine uptake by the thyroid.
Other conditions that may result from cassava dependency include kwashiorkor among children following weaning because of an imbalance of protein relative to calorie intake, An incidence of between 2 and 9 per cent of cases of kwashiorkor among children one to four years old has been reported by Sai in Ghana. Mere reduction of the amount of cassava consumed will only serve to aggravate the situation, since seasonal short-falls in available food supplies often reduce intake to only 70 to 80 per cent of the recommended calorie intake in parts of Nigeria and Ghana. Because there is heavy reliance on cheap, starchy staples as sources of energy, there is need in the food system for increasing the amounts of protein-rich foods such as legumes and animal proteins (meat, fish, eggs, milk, etc.).
The major problem with feeding protein-rich foods in the rural areas is that the amount available depends on incomes, and even when incomes are high, nutritional ignorance and certain food habits make it difficult for adequate nutritional status to be attained. Thus, although meat in soups used to make fufu constitutes a protein enrichment measure in the diet, local customs that deny meat to children and certain family members may result in protein malnutrition. As emphasized below, it must be recognized that there is a need to adopt a systems approach in understanding the role of the farming system in the interaction between the food/nutrition status in the population as a basis for designing effective interventions.
Contents - Next