Introduction

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Cassava (Manihot esculenta Crantz) is a perennial vegetatively propagated shrub that is grown throughout the lowland tropics (Figures I and 2). Cassava appears to be one of the earliest crops to have been domesticated and was widespread throughout the New World tropics by the late fifteenth century. Introduction to Africa occurred in the late sixteenth century (Jones, 1959) and to Asia probably not until the eighteenth century. Typically the crop is grown between 30° north and 30° south of the equator, in areas where the annual mean temperature is greater than 18 to 20°C. Cassava, which is believed to have originated in Latin America, has a number of attributes that have made it an attractive crop for small farmers with limited resources in marginal agricultural areas:

• it is one of the most efficient carbohydrate-producing crops;
• it is tolerant of low soil fertility and drought;
• it has the ability to recover from the damage caused by most pests and diseases;
• the roots can be left in the ground for long periods as a food reserve and, thus, provide an insurance against famine;
• the crop is well adapted to traditional mixed cropping agricultural systems and subsistence cultivation in which farmers seek to minimize the risk of total crop failure.

In the tropics, cassava is the most important root crop and as a source of calories for human consumption it ranks fourth after rice, sugar cane and maize. It is a major carbohydrate food for an estimated 500 million people and in tropical Africa it is the single most important source of calories in the diet (CIAT, 1992). The roots are the principle edible portion of the plant and typical ranges of composition given are; water 62 to 65 percent, total carbohydrate 32 to 35 percent, protein 0.7 to 2.6 percent, fat 0.2 to 0.5 percent, fibre 0.8 to 1.3 percent and ash 0.3 to 1.3 percent (Kay, 1987). In nutritional terms, cassava is considered primarily as a source of carbohydrate energy, most of which is derived from starch.

Total world production has increased from 70 million tonnes in 1960 to an estimated 150 million tonnes in 1990 (Table 1). Of this total, 43 percent is produced in Africa, 35 percent in Asia and 22 percent in Latin America. In the Americas during the 1970s and early 1980s there was a decreasing trend in cassava production which, since the late 1980s, has gradually changed into one of slow growth. During the period from 1985 to 1990 cassava production increased by 9.6 percent, from 29.6 million tonnes to 33.7 million tonnes (FAO Yearbooks). Brazil, Paraguay and Colombia, which together represent 92 percent of total cassava production on the continent, have all experienced growth in production.

Cassava production in Asia has risen, almost 1.5 percent above the annual population growth rate, from 48.5 million tonnes in 1985 to 52.0 million tonnes in 1990. The two major Asian cassava-growing countries, Thailand and Indonesia, have shown the largest increases in production. The Thai cassava industry was for several years largely based on the export of cassava pellets to the European Union (EU). Despite the introduction of quotas bythe EU during the mid-1980s, which threatened to limit growth in this market, Thailand's comparative advantages have kept the cassava industry buoyant and other export markets in Asia, Eastern Europe and the Russian Federation have been developed. Thai cassava exports have continued to experience an annual growth rate of 7 percent from 1985 to 1990 (Table 2). Although export volumes from Indonesia are only one-tenth of those from Thailand, the former has experienced an even stronger growth (17.1 percent) during this period. During the period 1985 to 1990 increases occurred in cassava starch production and in Japan investments have been made into plants for producing modified cassava starch and other starch derived products (CIAT, 1992). The apparent decline in cassava production in the People's Republic of China (Table 1) is not substantiated by local figures, which report a significant increase (CIAT, 1992).

TABLE 1 World cassava production (in million tonnes)

Note: Figures are approximations.
Source: FAO Production Yearbooks.

TABLE 2 World trade in cassava (in thousand tonnes)

Source: FAO Production Yearbooks.
Notes: Figures are approximations.

Figures include pellcts. "native'' pellets and dried cassava chips.

Cassava production in Africa increased from 58.2 million tonnes in 1985 to 64.1 million tonnes in 1990, a growth rate of 2 percent per annum. The most significant increase in production was recorded by Uganda, with a growth rate of 6.3 percent per annum. In Nigeria the ban on wheat imports provided a stimulus to cassava production, which rose from 13.5 million tonnes in 1985 to 17.6 million tonnes in 1990.

The crop is principally used as a human food, either fresh (boiled, baked, fried or pounded) or in numerous processed forms (Lancaster et al., 1982). Cassava is of growing importance, however, both for animal feed and as a raw material for producing starch, starch-based products and starch derivatives. Over the last 20 years, there has been mounting recognition of the contribution that cassava can make to increasing incomes and generating employment opportunities in the rural sector (CIAT, 1992).

As a crop cassava is one of the most efficient producers of starch, which constitutes about 85 percent of the storage root tissue dry-matter content. However, cassava starch represents only a small percentage of internationally traded starch. Annual production of cassava starch for industrial uses is about 800 000 tonnes, and originates mainly from Brazil (for the national market) and Thailand (for export to Japan and the EC). Native and modified starches are important raw materials for many industrial uses such as in food processing, paper, textile and adhesive manufacturing and in the oil drilling industry. Starch is also a raw material for producing many derived sugar products, such as glucose, fructose, maltodextrins and mannitol, each of which has specific properties and uses in food, chemical or pharmaceutical industries (Balagopalan et al., 1988).

Many small-scale cassava starch industries exist in tropical countries where the product has specific uses in traditional food industries corresponding to a specific market niche; eg krupuk in Indonesia, sago in India, pandebono in Colombia, biscoicho in Brazil and chipa in Paraguay. In Colombia and Brazil a naturally fermented starch is produced ("sour" starch) with specific functional properties that are irreplaceable in the manufacture of traditional breads. These small-scale industries have a high socio-economic importance in specific regions of these countries.

The roots of cassava are more highly perishable compared to those of the other major temperate or tropical root crops. This may be associated with the fact that, unlike other root crop storage organs, cassava roots exhibit no endogenous dormancy, have no function in propagation and possess no bud primordia from which regrowth can occur (Coursey and Booth, 1977; Passam and Noon, 1977). It is common knowledge that cassava roots senesce and deteriorate extremely rapidly after being detached from the plant. In general, due to physiological and pathological deterioration (see Chapters Two and One respectively), cassava roots cannot be kept in a satisfactory condition for more than a few days.

Studies on the deterioration of fresh cassava roots were conducted by the International Centre for Tropical Agriculture (CIAT) and the Natural Resources Institute (NRI) during the late 1970s. Research led to the development of a simple storage system based on root curing in polyethelene bags and treatment with a thiabendazole-based chemical to prevent the onset of deterioration (Wheatley, 1989). Thiabendazole is a permitted agent for post-harvest use in many fruits and vegetables, and residues in cassava tissues have been analysed at less than 20 percent of permitted levels. The storage system was successfully field-tested in several different ecosystems in Colombia.

Traditionally, the problem of storage has usually been overcome by leaving the roots in the ground until they are needed and, once harvested, consuming, marketing or processing immediately. Cassava is processed into a wide variety of products with a long shelf-life, which are traditionally prepared using a number of often fairly complex and time consuming processes (Lancaster et al., 1982). The roots can be left in the ground for several months after reaching optimal root development. The disadvantages of this system are that large areas of land are unavailable for further production, the roots loose some of their starch content, palatability declines as the roots become more fibrous (Rickard and Coursey, 1981) and cooking time increases (Wheatley and Gomez, 1985).

The post-harvest properties of cassava prohibit the holding of stocks of fresh roots at processing plants and restricts the supply area from which cassava can be obtained. The migration of rural populations to urban centres has exacerbated the problems of marketing fresh cassava by increasing the distance, and therefore time, between producers and consumers. Urbanization has been the dominant demographic force in all regions of the developing world since the 1950s (Table 3). The rapid post-harvest perishability of the fresh roots results in an inconvenient, often poor quality foodstuff for urban consumers (Wheatley and Best, 1991). Large marketing margins caused by the risks of commercializing a highly perishable product, together with the effects of subsidized imports, frequently make cassava more expensive than competing carbohydrate sources in the urban environment. These price, quality and convenience disadvantages are not relevant in rural areas, as freshly harvested cassava is more readily available and marketing chains shorter.

TABLE 3 Urban population as a percentage of total population 1960 to 1990

Source: World Resources 1990-1991: A guide to the global environment.

TABLE 4 Urban and rural consumption of fresh cassava in Latin America

Source: Janssen and Wheatley, 1985.
Note: Year of estimation in parenthesis.

In many countries, especially in Latin America, urban consumers are changing from cassava to other more stable carbohydrate sources, such as rice and wheat-based products, and consequently consumption of cassava is lower in urban than in rural areas (Table 4). The production advantages of cassava, however, justify its development as an urban food. Cassava is one of the principal crops grown by small farmers in marginal areas of the developing world. In these areas environmental growth conditions are limiting factors and in many cases cassava is the only crop that can be grown in sufficient quantities to generate income.

Overcoming the post-harvest problems of cassava has therefore become an important factor for the small farmer. Research to make cassava more suitable for the urban consumer will provide direct benefits to the rural poor.

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