Cassava (Manihot esculenta): characteristics and uses

Cassava (Manihot esculenta)

Origin and economic value of Cassava to humans:

Brazil and Paraguay are the places where Cassava originated. The other wild forms of these species are known and today it is given as a cultigen category. Cassava is grown yearly and it is a persistent woody shrub. For people living in the moist tropics, Cassava is the main source of low cost carbohydrate. Brazil is the largest producer of Cassava, trailed by Thailand, Nigeria, Zaire and Indonesia. In Africa and Asia the production continues to increase while in the Latin America the production remains constant over the past 30 years. This is found throughout the moist areas and it is the major food in many parts of Western and Central Africa. Since there is a large quantity of other alternate materials available, the global market for Cassava starch and meal is limited. Cassava supplies the daily calorie intake for more than 200 million people in the whole of sub-Saharan Africa. (Etejere, and Bhat, 46)

Cassava is a food crop as well as a good commercial cash crop, which forms as the major source of income for many farming families in Africa as per a study done in 16 African countries. About half of the Cassava grown in Africa is sold out and not retained by the farmer. Cassava makes a huge contribution to national economy, rural income and food security. Cassava plays a vital role during poverty and food crisis, as it is available throughout the year, is easy to grow in poor soil and it bears biological stress conditions and it is appropriate to the available farming methods and systems. Cassava has also got many industrial applications. The Cassava can be retained only for a few days, so the roots needs special cure to retain it for a longer time. To increase the shelf life of Cassava to two weeks leaves are to be removed two weeks before the harvest. The roots can be dipped in paraffin or wax or it can be stored in plastic bags to reduce the formation of vascular patches and this extends the life to three or four weeks. The roots can be removed and iced-up. (Cock; Reyes, 542)

In the traditional methods the roots were packed with damp mulch to increase the shelf life. Flour can be obtained by pounding the dried roots. To make the flour protein rich maize is added at the time of pounding. These flours are used for baking breads. Normally Cassava flour is used as an alternative material for wheat flour in the making of bread. People with sensitivity to wheat flour use the breads made by Cassava flour in the U.S.A. A dish equivalent to potato chips can be obtained if the fresh roots are made as thin slices and deep fried in oil. They can also be shaped as pier-like pieces and made in to a dish similar to french fries. Pasta, breads, crackers and pearls of tapioca can be made from the starch, which is obtained when the roots are peeled, grated and washed with water. Unpeeled roots can be used as animal fodder when the roots are dried and grated. The leaves also make protein to the animal feed. Production and processing procedures of products like alcohol, high fructose syrup, adhesives, textiles, papermaking are the industrial uses of Cassava. (Etejere, and Bhat, 47)

Life History:

Apart from waterlogged soils, all types of soils can be used for Cassava even though a sandy or sandy loam soil is favored. To avert deep penetration of roots that assist harvest, soils with a hard pan (impenetrable layer) about 30-40 centimeters are advantageous. Cassava can be uniformly grown in acidic (pH 5 to 5.5) or alkaline soils (pH 8 to 9). Fertile soil is not required for Cassava. That is why in a slash and burn system of cultivation, Cassava is frequently the last crop. High levels of aluminum and manganese often found in tropical soils are endured by Cassava though for other vegetables these are poisonous conditions. Irrigation is not used as Cassava can endure drought. To get superior yields, a minimum of 20 inches of rainfall is mandatory. To plant a new crop of Cassava, stem cuttings of 20 to 30 centimeters long from the earlier crop are used. Though the plant produce viable seed this is not used. The stem cuttings are planted in a grid pattern with a spacing of 60 to 140 centimeters between cuttings and to a depth of 10 centimeters. Higher yields are attained from older and more mature stems than cuttings from young stems. 10 months to 3 years is the period that the crop remains in the field. For longer grown crops, yields are higher. The yield is 13 to 27 tons per acre after one year. (Hillocks; Thresh, 23)

To evade from failure of other crops, the crop may be left in the ground for as long as six years, in some parts of the world. On account of excessive top growth, high nitrogen content in the soil and irrigation tend to decrease yield. Once they have reached the preferred size, the harvesting of the crop is by simple digging of the roots. Cassava is a humid crop and it needs at least 8 months of tepid weather to grow the crop. Conventionally it is grown in a Savanna climate, but it can also be grown in areas with extreme rainfall. In swamp areas the crop cannot withstand torrential rain. In dry areas it sheds the leaves to preserve the wetness and it starts producing new leaves once the rain continues. Under unfavorable conditions like chill or arid conditions it takes 18 months or more to grow the crop. (Cock; Reyes, 542)

Cassava cannot withstand sub-zero conditions. It is most fruitful in full sun and it bears a wide range of soil pH 4.0 to 8. The growth of Cassava is reduced by famine, weeds, insect pests and bacterial and viral diseases. Due to a speedy post-harvest weakening and insufficient starch and protein content in the roots, the farmers find it difficult to market their products. The global Partnership for Cassava Genetic Improvement will work out on the new methods and use advanced biotechnologies such as Genomic to produce planting materials that integrates the preferred qualities which includes improved pest control and disease, customized starch quality for better commercial use and to increase the levels of protein and starch so that it will make the crop more wholesome. (Cock; Reyes, 556)

Cassava is grown for its puffy starch filled roots, which contains the maximum quantity of starch on a dry weight basis amongst food crops. There is very little of protein content and 30% of starch in the fresh roots. Just like the potatoes the roots are prepared; they can be fried, baked, boiled and peeled. Cassava cannot be taken uncooked as highly toxic concentrations of cryogenic glycosides are condensed in innocuous levels through cooking. The roots are grated and juice is obtained through crushing or pressing in the customary locale of Americas. To get a meal or ferment and to cook the Cassava is roasted over fire. This meal can be added to soups or stews and dehydrated with water. Roots are preserved in different ways in Africa. They are first fermented with water. They are then made in to dough by either sun drying it or grating and then cooked. From the roots alcoholic drinks can be made. Young tender leaves that contain high levels of protein (8-10% F.W) can be used as a potherb. (Rogers, 261) Proper care is to be given to remove poisonous compounds during the cooking process and it can be prepared in the same way as spinach. A plant resembling it with multicolored leaves is planted as a decoration.

Taxonomy

Primitive text on Cassava explained the genus with two edible species M.Ultissima Phol or sweet and M. aipi Phol, outlining the species that have high and low cyanogenic glucoside concentration. This is one of the 98 species in its group that is fairly produced for food consumption. Cassava is evenly 2n=36. Other ploidy levels are not used, but have been formed on experimental basis. In the tropical and subtropical Americas many closely related species are found that can be intersected with M. esculenta. Milkweed and castor bean are also included in the family of Cassava. Cassava, which grows three to six feet tall, is a dicotyledonous perennial shrub. The root is the edible portion. The color of flowers ranges from greenish purple to light greenish yellow and is borne at the end of the stems. These plants have unconnected male and female flowers borne on the same plant and hence are called monoecious. Crossing with other plants happens through insect pollination as the female flowers ripen first. As bitter and sweet cultivars can be found, Cassava is regularly classified by the amount of bitter substances in the root. Cyanogenic glucoside present in the roots is the reason for bitterness. Like one would do in chewing, upon crushing the roots, cyanide is set free. A natural form of defense for the plant is the presence of cyanide in the roots. Quantity of cyanide present in the roots is determined by the soil and climatic conditions. A bitter cultivar in one place may be sweet in another place. (Hillocks; Thresh, 26)

Morphology

Cassava is a persistent woody shrub botanically; but farmers harvest the tuberous roots and leaves during the first of second year. Cassava is mainly breed through stem cutting. Certain agronomic and genetic factors are responsible for the number of shoots that develop from a stem cutting. Male and female flowers are present in the same plant thus making Cassava monoecious. Fibrous root systems are formed b the adventitious root systems at the base of the stem. The tuberous roots are formed when the fibrous roots increase in diameter. Breeding by the seed is frequent in plant than breeding under natural conditions. Within a week, the stem cuttings begin to shoot and roots grow. The plants that grow by planting seedlings take a longer time to grow and are found to be small and weaker than the plants breed by stem cutting. Stem cuttings from the same plant are genetically constant while seedlings are found genetically varied. The shoots increase and the roots began to extend within the first few weeks of appearance or coming out. After 8 weeks of planting tuberisation or root hardening begins. (Cock, 17)

In a maximum time of 4-5 months the leaf comes out. The main feature of Cassava plant is the plant height i.e., the height at the first branch and the position between the branches and main stem. A fully-grown Cassava plant normally ranges form 1 to 2m, though some may reach up to 4m. Ecological and hereditary features restrict the height at first branching. (Tan; Cock, 285) Soil lushness and planting intensity and time are some of the agronomic methods that can be cited as some of the examples of ecological features that affect the height at first branching. Depending on the time of first branching, the types can be treated as early or late branching. In some varieties the branching height will be as low as 20cm, while some never branch at all, thus leading to a state where they never flower. Ecological and hereditary factors effect the angles between the main stem and decumbent or they may be straight and vertical. The Cassava plant can be split into two systems: shoot system and root system; shoot system-stems, leaves and inflorescences and root system-fibrous roots and tuberous roots. Cassava plant can be classified into five major types based on the following features: number of nodes that occur at first branching, branching angle, height at first branching, absence or presence of branches. (Tan; Cock, 286)

Cassava is normally breed by stem cuttings. The shoot system emerges from the blossoms situated at the nodes of the cuttings. Apical control, parent plant effect, genotypes, stem width, planting direction and length of cutting are some of the feature which control the number of shoots that develop from a stem cutting. When planted straight, longer cuttings of certain genotype having more number of nodes and more number of shoots are produced. Cuttings planted straightly form shoots at all most all nodes while those planted in an inclined position form 1-2 shoots at the apical node. Shoots are not formed at the central nodes. Slim old or dry shoots produce lesser shoots than thick fresh cuttings. More shoots and more assimilate increase the number and size of the shoots. Just one shoot is formed in genotypes with a sturdy apical control. Planting methods also have an effect on apical control. Cassava stems are cylindrical with a width of 2 to 6 centimeters. Usually the stems grow up to 4 meters but in some genotypes they grow only up to 1 meter. The stems are usually woody with large pith and they differ in color ranging from whitish gray to brown or dark brown. The grownup parts of the stem exhibit prominent knob-like scars that are leaf scars at the nodes. (Centro Internacional de Agricultura Tropical (CIAT), p.12)

At the early stages of growth, each stem forms an average of one node per day and at the later stages one node per week. There are internodes and a node that subtends a leaf in every nodal unit. Many ecological factors like availability of light and moisture control the growth of internodes according to the genotypes and plant age. The internodes are unusually long under improper light, long under appropriate conditions and short under famine. The stem branches at different points in many genotypes. There are two types of branching methods like reproductive branching called 'forking' and side ways branching. Two or three secondary branches are formed from the main stem. In turn each secondary branch forms a set of tertiary branches and so on. When the apical meristem transforms from a vegetative to a reproductive state, reproductive branching is formed at the tip of the stem. An inflorescence is formed at that point of branching. Thus, flowering is the basis for reproductive branching. (Cock; Reyes, 542)

There is a variation in the number of branches and branching levels. The stems may sometimes have a sturdy apical dominance, which may restrain the side branching in the later level of growing stages. In any area of the stem, the side branching may occur. They may also grow from the axillary buds around the lower area of the stems. The side branches can grow at higher points on the plant after arid seasons. Normally, the side branches are very thin than the main stem and have smaller leaves and long internodes. Certain features like climatic factors, soil fertility, genotype, edaphic and cropping system influence the branching. Lamina and petiole are the parts of the Cassava leaves. Each leaf has got two stipules about 1cm long. The petioles ranges from green to purple and is 5 to 30 cm long. The lamina has even fringes and is extremely palmate or lobed. (Hillocks; Thresh, 24)