1.1 Root and Tuber Crops Production
1.2 The Role of Roots and Tubers in Nutrition
1.3 Consumption Patterns for Roots and Tubers
1.4 General Characteristics of Root and Tuber Crops
The principal root and tuber crops of the tropics are cassava (Manihot esculenta Crantz), yam (Dioscorea spp.), sweet potato (Ipomoea batatas L.), potato (Solanum spp.) and edible aroids (Colocasia spp. and Xanthosoma sagittifolium). They are widely grown and consumed as subsistence staples in many parts of Africa, Latin America, the Pacific Islands and Asia.
The potential of these crops is particularly high in the humid tropics and those sub-humid tropics, which are not suitable for cereal production. Table 1.1 shows production data for root and tuber crops in developing countries where it is estimated about 300 million tons were produced in 1993. It is significant that cassava and potatoes are together almost 83% of the total.
Table 1.1: Production of root and tuber crops in developing countries in 1993 (million tons).
|
Crop |
Africa |
Latin America |
Asia |
Oceania |
Total |
% of total |
|
Cassava |
74.8 |
28.5 |
50.2 |
0.2 |
153.7 |
51.7 |
|
Potato |
7.5 |
12.9 |
70.0 |
1.48 |
91.8 |
30.9 |
|
Yam |
26.8 |
0.8 |
0.2 |
0.2 |
28.1 |
9.5 |
|
Sweet potato |
6.3 |
1.7 |
9.31 |
0.6 |
17.9 |
6.0 |
|
Taro |
3.5 |
0.04 |
1.8 |
0.3 |
5.6 |
1.9 |
|
Total |
118.9 |
43.9 |
131.5 |
2.8 |
297.1 |
100.0 |
1 The 105.2 million tons produced by China are not included in the total for Asia.
Source: Adapted from FAO Production Yearbook, Vol. 47, 1993.
Root and tuber crops are second only in importance to cereals as a global source of carbohydrates. They also provide some minerals and essential vitamins, although a proportion of the minerals and vitamins may be lost during processing as, for example, in the case of cassava. The quantity and quality of the protein in starchy staples are variable and relatively low on a fresh weight basis but compare favourably with some cereals on a dry weight basis (Appendix 1.1) In most traditional diets vegetable soups, meat, groundnuts, grain legumes and fish are good sources of protein and are frequently used to supplement root crops and compensate for their protein deficiencies. In some parts of Africa the diet is supplemented with the tender leaves of sweet potato, cassava and cocoyam which are rich sources of protein, minerals and vitamins (Hahn, 1984).
Table 1.2: Per capita daily consumption of food commodities in Africa as a percent of total consumption (Hahn, 1984)
|
|
Equatorial Africa |
Humid West Africa |
Semi-arid West Africa |
East Africa |
|
Root Crops |
41.4 |
29.6 |
19.1 |
18.6 |
|
Cereals |
26.7 |
38.9 |
49.0 |
48.5 |
|
Pulses |
4.9 |
1.5 |
3.5 |
3.8 |
|
Fruits & Vegetables |
6.2 |
7.7 |
2.0 |
4.4 |
|
Oil crops |
10.4 |
12.7 |
13.3 |
9.1 |
|
Livestock products |
3.2 |
3.7 |
4.5 |
6.4 |
|
Other products |
7.2 |
5.9 |
7.7 |
9.6 |
|
TOTAL |
100 |
100 |
100 |
100 |
There are two broadly based groups of consumers of roots and tubers:
· Rural consumers, who cultivate staple crops in a subsistence-oriented traditional production system and are largely self-sufficient; their choice of food is often determined by the opportunities for diversification of agricultural production in their area.· Urban consumers who, over time, have developed a preference for more convenient foods, partly determined by the availability and convenience of low-cost imports of wheat flour and rice as well as by an increased cash income, but probably and most significantly, by their improved purchasing power.
· The essential difference between these two groups is; in rural areas farmers produce for their own consumption and exchange any surplus with their neighbours or sell in their local market for extra income. The urban consumer buys most of his requirements and often considers roots and tubers difficult to store, sometimes wasteful because of spoilage and inconvenient to prepare and use when compared with other staples and the increasing prevalence of convenience foods (Aviles, 1987).
In tropical Africa root and tuber crops still constitute an important, often major, components in traditional diets. In rural areas there is usually an adequate supply to meet any local demand from the surplus production of subsistence farmers and by local trade. The demand for food in large urban areas has increased, and is continuing to increase due to the large population migration from the rural areas, which has continued for thirty years and shows no signs of abating. The migrating rural population takes with it its traditional heating habits, particularly until it becomes urbanised, despite the opportunities for diversification of eating habits with the choice of food available in the towns. Root crops, particularly cassava, remain in demand, but this demand is often unsatisfied because of the inherent limitations of the traditional production systems which impose serious constraints in marketing and processing. The general pattern of the supply of root crops from the surpluses of subsistence farming leads to high marketing costs and high consumer prices. As a consequence, in urban communities the consumption of root crops tends to be replaced by imported cereals, rice and wheat flour (FAO, 1987).
In some cases, this substitution can be on a large-scale and involve levels of cereal imports that cause grave concern. For example, in Central African urban centres (Douala in Cameroon, Brazzaville in the Congo and Libreville in Gabon) the consumption of bread has reached 80 kg per person per annum. If nothing is done to slow down or reverse the rate of substitution the increasing dependency on imported food products will lead to an increasing shift away from traditional foods.
In South America and the Caribbean, the overall per capita consumption of roots and tubers has declined, on average, by 2.5% per annum since 1970 while in the same period the consumption of cereals (often imported wheat and rice) has risen by about 1% per year (FAO, 1987). This reflects the rapid rate of urbanisation2 and the relatively low level of consumption of roots and tubers in the towns where they are rarely seen as staples. In Brazil, processed cassava as "farinha da mandioca" is a basic staple in many parts of the country, particularly in the northeast, but the principal market in the urban areas is for fresh cassava. There is scope for developing new processed products and the animal feed market is also quite large and has potential if the price of the processed product was to become competitive.
[2 At the beginning of the 1960's 30% of the population lived in the towns, by the mid 1980's 70% of the population was urban]
It is considered by many authorities that the increasing dependence in developing countries on imported cereals is unsustainable and the trend should be reversed by stimulating reliance on indigenous crops, in particular roots and tubers. The importance of these crops as a global source of food carbohydrates is well established. Regrettably, research and development on roots and tubers is limited and tends to be focused on pre-harvest production only, especially genetic improvement. What is needed is a well designed, integrated strategy of production, processing, and marketing to stimulate increased consumption and establish in developing countries the full potential of these crops, particularly with reference to their contribution to food self-sufficiency
1.4.1 Cassava (Manihot esculenta Crantz)
1.4.2 Yams (Dioscorea spp.)
1.4.3 Sweet potato (Ipomoea Batatas L.)
1.4.4 Potato (Solanum tuberosum L.)
1.4.5 The edible aroids
The importance of root and tuber crops as staple foods is because of their particular agronomic advantages:
· they are well adapted to diverse soil and environmental conditions and a wide variety of farming systems;· they are highly efficient of edible carbohydrates when compared to other food crops (Table 1.3).
Their more important limitations are their bulk, some tubers weigh over 5 kg, and perishability, moisture contents range from 60% to 90%. These are associated with high transport costs, a short shelf life and limited market margins, which impose serious constraints in the urban markets of developing countries.
Production patterns reflect the agro-climate of the area, traditional farming practices and often the local cultural heritage. With few exceptions roots and tubers are produced by small-scale farmers using traditional tools and without any inputs of fertilisers or chemicals for weed and pest control. Traditionally women have provided most of the labour for production and harvesting. Some form of sequential cropping practice is frequently followed together with intercropping by cereals, legumes and cash crops such as coffee and cocoa.
Table 1.3: General characteristics of roots & tubers compared with cereals (FAO, 1983)
|
Cereals and oil seeds |
Roots and tubers |
|
Low moisture content, typically 10% to 15% |
High moisture content, typically 70% to 80% |
|
Small unit size, typically less than 1 gram |
Large unit size, typically 100 grams to 15 kg |
|
Very low respiration rate with very low generation of heat. Heat production is typically 0.05 megajoule/ton/day for dry grain |
High respiration rate. Heat production is typically 0.5 to 10 megajoules/ton/day at 0°C to 5 to 70 megajoules/ton/day at 20ºC |
|
Hard texture |
Soft texture, easily bruised |
|
Stable, natural shelf life is several years |
Perishable, natural shelf life is a few days to few months |
|
Losses usually caused by moulds, insects and rodents |
Losses usually caused by rotting (bacteria and fungi), senescence, sprouting and bruising |
Cassava is almost entirely produced and consumed in developing countries. It is highly productive, tolerant of poor soils, periods of drought and is relatively disease free and pest resistant. It provides a major source of energy for over 500 million people; the energy content of cassava in diets in the tropical areas of Africa, America and Asia has been estimated as 37%, 12% and 7% respectively. In recent years a substantial trade has developed in dehydrated cassava chips and pellets which are exported to Europe as a low-cost animal feed ingredient. The European Economic Community in 1975 is reported to have imported 2.3 million tons of dried cassava chips, mostly from Thailand.
1.4.1.1 General morphology and composition of the cassava root
The edible portion of cassava is a starchy root, which matures to harvest within 8 to 24 months of planting, depending on cultivar and climate. A mature cassava root may be anything from 15 to 100 cm in length and from 0.5 to 2.0 kg in weight, subject to variety and growing conditions. The root is circular in cross-section (Figure 1.1). It is generally fattest at the proximal end and tapers gently towards the distal end. Transversely a cassava root consists of three principal areas.
· The periderm. Comprises the outermost layer of the root. It is composed mostly of dead cork cells, which seal the surface of the root. The periderm is only a few layers of cells thick and as the root continues to increase in diameter, the outermost portions of it are sloughed off and replaced by new cork formations from the inside layers of the periderm.· The cortex. A layer 1 to 2 mm thick located immediately beneath the periderm.
· The starchy flesh. The central portion of the root, consisting mainly of parenchyma cells packed with starch grains.
Cassava contains about 1% protein and some 30-35% of amyloses and amylopectins on a dry weight basis; it is thus a predominantly starchy food. As a human food it has been criticised for its low and poor quality protein content, but the plant produces more weight of carbohydrate per unit area than other staple food crop under comparable agro-climatic conditions. The edible starchy flesh comprises some 80% to 90% of the root and includes:
|
water |
62% |
fibre |
1-2% |
|
carbohydrate |
35% |
minerals |
1% |
|
protein |
1-2% |
fat |
3% |
Figure 1.1 General view of a cassava root (Onwueme, 1983)
1.4.1.2 The problems of cyanogenic glycosides in cassava
Cassava roots and leaves contain cyanides in two different forms: i) the glycosides; linamarin and lotaustraline which are considered "bound" and ii) the non-glycosides; hydrogen cyanide (HCN) and cyanohydride which are considered "free". Free cyanide comprises 8%-12% of the total tuber cyanide. This cyanide can, under some circumstances, lead to human toxicity problems and cassava for food use has to be processed to remove cyanide-containing substances
There is remarkably little quantitative information on the effects of processing on cassava toxicity and its significance for humans. There is a great variation in toxicity between cultivars. A distinction is usually made between "sweet" cultivars with relatively low contents of cyanogenic glycosides (below 10mg/100g of fresh weight), and "bitter" cultivars with high cyanogenic glycoside content (above 20mg/100g fresh weight), although many intermediate forms exist. Traditionally the sweet cultivars were considered non-toxic while the bitter ones were considered toxic. Although the sweet cultivars are generally less toxic there is no direct correlation between toxicity and taste (Coursey and Haynes, 1970). Cyanide levels in the range 6 to 370 mg/kg have been found depending on the particular cultivar, growing conditions, (i.e. soil type, humidity, temperature) and the age of the plant. The highest proportion of HCN is found in the peels and the cortex layer immediately beneath the peels (Hahn, 1984; Onwueme, 1978). It is for this reason the cassava root is always peeled before being processed or consumed. Peeling removes the cortex and the outer periderm layer adhering to it. Peels can represent 10% to 20% of the fresh root weight, of which the periderm accounts for 0.5% to 2.0%.
The lethal dose of free HCN for an adult is 50-60 mg but the toxicity of bound HCN is less clearly understood. The glycosides are hydrolysed to HCN by the endogenous enzyme linamarase, which is present in the human digestive tract. All the traditional cassava processing methods reduce or remove the toxicity by releasing HCN from the glycosides. Since HCN is soluble in water and has a boiling point of 25°C it can be removed by soaking. Boiling fresh cassava has little effect on its toxicity as the glycoside limamatine is heat resistant and the enzyme linamarase is inactivated at 75°C.
While most processes rely on enzymatic hydrolysis to reduce the glycoside concentration, in practice the extent of glycoside breakdown is mainly controlled by fermentation time (Bruinsma et al. 1983). The so-called "sweet" types may be eaten raw or lightly boiled without harm. The "bitter" forms are traditionally processed by one or a combination of operations of peeling, grating, fermenting, dehydrating, sun drying, frying or boiling (Table 1.4). Hence, for example, fermentation before processing into products such as chikwangue or fufu eliminates almost all total and free HCN. The amount of total HCN is reduced by 83% to 96% in such products as gari and attieke for which the cassava roots are peeled and grated before processing. (See Chapter 5 Processing roots and tubers.)
The world production of yam was estimated at 28.1 million tons in 1993. 96% of this came from West Africa, the main producers being; Nigeria with 71% of world production; Côte d'Ivoire 8.1%; Benin 4.3% and Ghana 3.5% In the humid tropical countries of West Africa yams are one of the most highly regarded food products and are closely integrated into the social, cultural, economic and religious aspects of life. The ritual, ceremony and superstition often surrounding yam cultivation and utilisation in West Africa is a strong indication of the antiquity of use of this crop. Nigeria, the world's largest yam producer, considers it to be a "man's property" and traditional ceremonies still accompany yam production indicating the high status given to the plant.
There are many varieties of yam species widespread throughout the humid tropics but the edible yams are derived mainly from about ten. The most economically important species are:
· White yam (Dioscorea rotundata Poir). Originated in Africa and is the most widely grown and preferred yam species. The tuber is roughly cylindrical in shape, the skin is smooth and brown and the flesh usually white and firm. A large number of white yam cultivars exist with differences in their production and post-harvest characteristics.· Yellow yam (Dioscorea cayenensis Lam.). Derives its common name from its yellow flesh, which is caused by the presence of carotenoids. It is also native to West Africa and very similar to the white yam in appearance. Apart from some morphological differences (the tuber skin is firm and less extensively grooved), the yellow yam has a longer period of vegetation and a shorter dormancy than white yam.
· Water yam (Dioscorea alata L.) Originates from South East Asia, it is the species most widely spread throughout the world and in Africa is second only to white yam in popularity. The tuber shape is generally cylindrical, but can be extremely variable. Tuber flesh is white and "watery" in texture.
· Bitter yam (Dioscorea dumetorum). Also called trifoliate yam because of its leaves. Originates in Africa where wild cultivars also exist. One marked characteristic of the bitter yam is the bitter flavour of its tubers. Another undesired characteristic is that the flesh hardens if not cooked soon after harvest. Some wild cultivars are highly poisonous.
Table 1.4: Hydrogen Cyanide contents of some traditional African Cassava Products (IITA, 1982)
|
Product |
Total HCN mg/100 g of fresh weight |
Free HCN mg/100 g of fresh weight |
||
|
Mean |
% Reduction |
Mean |
% Reduction |
|
|
Peeled fresh roots |
15.9 |
-- |
0.65 |
-- |
|
Attieke |
0.65 |
95.9 |
0.07 |
89.2 |
|
Boiled tuberous roots |
4.79 |
69.9 |
0.70 |
+7.69 |
|
Chikwangue |
nil |
n/a |
Nil |
n/a |
|
Eba |
0.02 |
99.9 |
0.01 |
98.5 |
|
Fufu (Ghana) |
3.10 |
80.5 |
0.28 |
56.9 |
|
Fufu (Nigeria) |
0.72 |
95.5 |
0.35 |
46.2 |
|
Fufu (Zaire) |
0.02 |
99.8 |
Nil |
100 |
|
Gari (freshly fried) |
2.66 |
83.3 |
1.22 |
+87.7 |
|
Gari (after 4 months |
0.29 |
98.2 |
0.26 |
60.0 |
|
Konkonde |
0.72 |
95.5 |
0.09 |
86.2 |
|
Ntuka |
0.03 |
99.8 |
0.03 |
95.4 |
|
Oyoko |
1.46 |
90.8 |
0.68 |
+4.62 |
|
Plakali |
0.53 |
96.7 |
0.07 |
89.3 |
|
Fresh leaves |
201 |
N/A |
4.85 |
N/A |
|
Pondu (cooked leaves) |
8.53 |
95.8 |
Nil |
100 |
Depending on the species, yam grows for six to ten months and is dormant for two to four months, these two phases usually corresponding to the wet season and the dry season. For maximum yield the yam requires an annual rainfall of over 1,500 mm distributed uniformly throughout the growing season. White, yellow and water yams normally produce annually a single large tuber, often weighing 5-10 kg.
The major problems presently facing yam production are its high labour requirement, its low yield per hectare compared to crops such as cassava or sweet potato, the relatively large amount of planting material that is required and its long growing season. By far the most critical of these problems is labour requirement, which exceeds that of other comparable crops. For these reasons and problems of storing harvested yam, the costs of yam production are high and yam is slowly losing ground to cassava. It has been estimated that the cost per 1,000 calories of yam is four times greater than those of cassava. But, despite these high costs the nutritional value of yam is sufficiently high to justify further work into its general improvement.
1.4.2.1 General morphology and composition of the Yam tuber
The tuber shape and size can vary greatly due to genetic and environmental factors. However, cultivated forms of yam generally produce tubers that are more or less cylindrical in shape and 3-5 kg in weight. The yam tuber grows from a corm-like structure located at the base of the vine (Figure 1.2). Occasionally this corm remains attached to the tuber after harvest and sprouts will develop from it. When the corm separates from the tuber sprouting occurs from the tuber near to the point at which the corm was attached.
A transverse section of a mature yam tuber shows it to be composed of four concentric layers:
· Corky periderm. The outer portion of the yam tuber; it is a thick layer of cork cells, often cracked, but which provides an effective barrier against water loss and invasion by pathogens.· Cortex. A layer located immediately beneath the cork, comprising thin-walled cells with very little stored starch.
· Meristematic layer. Elongated thin-walled cells under the cortex. Sprouts are initiated from this layer.
· Ground tissue. The central portion of the tuber, composed of thick-walled starchy cells, with vascular bundles ramifying throughout the mass. Most yams are essentially composed of water, starch, small quantities of protein and other minor constituents (see Table 1.5).
Table 1.5: Composition of species of yams % wb. (Knoth, 1993)
|
Variety |
Moisture content |
Carbohydrate |
Fats |
Crude Protein |
|
D. alata |
65 - 73 |
22 - 29 |
0.1 - 0.3 |
1.1 -2.8 |
|
D. rotunda |
58 - 80 |
15 - 23 |
0.1 - 0.2 |
1.1 -2.0 |
|
D. esculenta |
67 - 81 |
17 - 25 |
0.1 - 0.3 |
1.3 - 1.9 |
|
D. bulbifera |
63 - 67 |
27 - 33 |
0.1 |
1.1 - 1.5 |
Figure 1.2 General Morphology and cross section of yam tuber (Omwueme, 1983)
Sweet potato is a crop with a significantly unrealised potential. It is capable of producing high yields of dry matter per unit area of land and labour and this potential can be achieved under a wide range of agro-climates and farming systems.
The crop originated in Central America but is now grown in many countries. Most of the world production is concentrated in 15 countries which account for almost 97% of total world output (Scott, 1992). China is the world's largest producer of sweet potato with 105 million tons in 1993, representing about 80% of total world production. A rapid growth in population during the 1980s, resulting in severe pressure on farmland, is considered to be a prime factor for the rapid expansion of production in many countries, in particular in Vietnam, Kenya, Rwanda, Burundi, North Korea and Madacasgar. The largest producers in Africa are: Uganda (1.9 million tons), Rwanda (0.7 million tons), Burundi (0.68 million tons) and Kenya (0.63 million tons).
Sweet potato has the shortest growing cycle of the root crops grown in the tropics. The crop is normally harvested when the vines and leaves have turned yellow, generally about 4 months after planting. In traditional farming systems, where the crop is mainly intended for consumption by subsistence farmers, harvesting may be spread over several months. The crop is lifted by hand; great care being taken to avoid damage to the tubers, with only the amount needed for immediate consumption being dug. If the crop is grown for sale or when there is a pronounced dry season, the whole crop may be lifted at once. On a larger or commercial scale effective harvesting machines have been developed; some consist simply of a plough that lifts the tubers to the surface. More sophisticated mechanical harvesters combine a vine cutter (rotary or flail type mower) to remove the vines incorporated with a plough to lift the tubers and a sorter and loading elevator.
Table 1.6: General chemical composition of Sweet Potato (Ipomea batatas L.)
|
Constituent |
Percent or (mg/100g) |
|
Moisture |
50 - 81 |
|
Protein |
1.0 - 2.4 |
|
Fat |
1.8 - 6.4 |
|
Starch |
8.0 - 29 |
|
Non-starch Carbohydrates |
0.5 - 7.5 |
|
Reducing Sugar |
0.5 - 7.5 |
|
Ash |
0.9 - 1.4 |
|
Carotene (average) |
4 mg /100 g |
|
Thiamine |
0.10 mg /100 g |
|
Ascorbic Acid |
25g /100 g |
|
Riboflavin |
0.06 mg /100 g |
1.4.3.1 General morphology and composition
There are many cultivars of sweet potato each with its own characteristics of size, shape, colour, storage life, levels of nutrition and suitability for processing. A single plant may produce 40 to 50 tubers ranging in length from a few to 30cm; they may be spindle-shaped or spherical and weigh from 100g to 1 kg. Tubers may have a smooth or irregular surface and the skin and the flesh may range from almost pure white through cream, yellow, orange and pink, to a very deep purple (Onwueme, 1978). The chemical composition of sweet potatoes varies greatly according to genetic and environmental factors. (See Table 1.6)
1.4.3.2 Sweet potato utilisation
Sweet potatoes are utilised as food as well as livestock feed all over the world. In the tropics the fresh roots are commonly boiled, fried or roasted and eaten as a carbohydrate constituent of the diet. Recent attention has been paid to the nutritional value of the leaves, which can contain as much as 27% protein (dm basis). In Africa, both the roots and the leaves are consumed. In parts of East Africa tubers are sometimes sliced and sun-dried to produce chips, which are later ground into flour. In Northern Cameroon sweet potato plays an important role in rural food security; dried chips are stored for use during the hungry period when the stocks of the staples sorghum and millet are depleted. In Asia, particularly Japan, Taiwan and South Korea, sweet potato is widely used as animal feed. In the USA, one third of sweet potato production is dehydrated and processed for animal feed.
The potato originated in highland tropical areas of South America from where it was introduced into Europe towards the end of the 16th century. There the potato developed as a temperate crop before it was later distributed throughout the world, largely as a consequence of the colonial expansion of European countries. Late maturing potato varieties from temperate zones can usually be grown successfully in the tropics at high altitude (1,200 m or more above sea level) down to areas at sea level where there is a marked cool season. It is now established as an important crop in a very wide range of climates and varieties tolerant of high temperatures and short days are being developed under the aegis of the Centro International de Papa (CIP).
In the tropics potatoes are harvested about four months after planting which results in higher yields, as compared to temperate climates where the main crop growing season can extend to six months. Main crop potatoes should not be harvested until they are fully mature, considered to be about two weeks after the tops have died off, at which stage the skin of the tuber is well set and be less prone to damage during harvesting. Early or "new" potatoes, which are harvested in an immature condition before the skins have set, can be easily damaged and do not store well or for long periods.
1.4.4.1 General morphology and composition of the potato tuber
The potato is basically a swollen stem mainly composed of water (80%) making it a bulky commodity and one which responds strongly to its prevailing environment. In cross section there are four clearly distinguishable areas (Figure 1.3):
· Skin or Periderm. A ring of six to ten suberized cell layers, usually thicker at the stem than at the bud end although the total skin thickness can vary substantially depending on variety and growing conditions. The skin can easily be removed in immature tubers but not when the tubers have reached full maturity. If the tuber tissue is wounded, the tuber is able to form a new layer of suberized cells, known as wound periderm. Lenticels, which are a circular group of suberized cells, are formed in the periderm and are essential for the respiration of the tuber since the skin is almost impermeable to CO2 or O2. Potato eyes (effectively buds on the stem), the bud and stem ends are also present on the periderm surface.· Parenchyma tissue. Composed of cells of the cortex and the perimedullary zone. It represents the major part of the tuber and contains starch grains as reserve material.
· The ring of vascular bundles. When the tuber is cut lengthwise part of the vascular tissue is revealed as a ring, known as the xylem, (Figure 1.3).
· The medullar rays and medulla. Also known as the pith.
The chemical composition of potatoes is very variable and is greatly influenced by variety, environment and farming practices. Starch constitutes 65% to 80% of the dry weight of the tuber. Potatoes are also an important source of protein, iron, riboflavin and ascorbic acid. An average range of composition is given in Table 1.7.
Figure 1.3: Diagram of a longitudinal section of a potato tuber (Rastovski et. al., 1981)
1.4.4.2 Glycoalkaloids in potato (Solanin)
When potato tubers are exposed to light during harvesting, handling and marketing a green colour will develop in the periderm and in the outer parenchyma cells of the cortex caused by the synthesis of chlorophyll which causes a bitter taste. Also bitter-tasting and resulting from exposure is a substance which is attributed to the formation of glycoalkaloids but the formation of chlorophyll and glycoalkaloids are independent processes. The glycoalkaloids content of potato tuber varies between 210mg/100g (fresh weight) (Rastovski et al, 1981). The highest levels of glycoalkaloids are found in the periderm and cortex; hence about 60% will be removed by peeling. Potatoes containing amounts greater than 1mg/100g are generally considered unsuitable for human consumption.
Colocasia esculenta (commonly known as taro) and Xanthosoma sagittifolium (Tania) are the most important of the edible aroids. Colocasia esculenta is considered by most botanists to be a polymorphic species with several botanical varieties. The two main varieties are, esculenta var. esculenta which produces a large edible main corm and few lateral cormels (commonly between 4 and 10) and C. esculenta var. antiquorum which produces a small or medium size corm and a large number of edible cormels (15-40).
Table 1.7: Average constituents of Potato (Solanum tuberosum L.)
|
Constituents |
Percentage (wb) |
|
Moisture |
50 - 81 |
|
Protein |
1.0 - 2.4 |
|
Fat |
1.8 - 6.4 |
|
Starch |
8 - 29 |
|
Non-starch Carbohydrates |
0.5 - 7.5 |
|
Reducing Sugar |
0.5 - 2.5 |
|
Ash |
0.9 -1.4 |
|
Carotene (average) |
4 mg /100 g |
|
Thiamine |
0.10 mg /100 g |
|
Riboflavin |
0.06 mg /100 g |
|
Ascorbic Acid |
12 mg /100 g |
Figure 1.4: General view of a taro corm (FAO, 1987)
Table 1.8: The chemical composition of Taro and Tania (% wb)
|
Constituent |
Taro |
Tania |
|
Moisture |
63 - 85 |
70 - 77 |
|
Carbohydrates |
13 - 29 |
17 - 26 |
|
Protein |
1.2 - 3.0 |
1.3 - 3.7 |
|
Fat |
0.16 - 1.18 |
0.2- 0.4 |
|
Crude fibre |
6.60 - 1.18 |
0.6 - 1.9 |
|
Ash |
0.60 -1.3 |
0.6 -1 .3 |
1.4.5.1 Tania
Tania originally came from the tropical Americas and is often confused with taro as the corms have the appearance of rather large taro tubers. The leaves of tania are arrow-shaped and 45 to 90 cm long. Each plant has a central corm surrounded by lateral cormels which can vary in size between 12 and 25 cm in length and 12 to 15 cm in diameter. The flesh may be white, yellow or sometimes pink. When cooked tania is more nutritious than tare but is rather indigestible because of its larger starch grains. The main corms usually contain oxalic acid and for this reason usually only the cormels are eaten. The chemical composition of tania is indicated in Table 1.8.
