Yam is composed mainly of starch (75–84 % of the dry weight) with small amounts of proteins, lipids and vitamins and is very rich in minerals (Shin et al., 2012). It is a good source of inulin, which is a form of sugar with a low calorific value with immense benefits to diabetics. Its phyto-nutritional profile comprises of dietary fiber and antioxidants, in addition to traces of minerals and vitamins (Slavin et al, 2011).
Plant carbohydrates mainly include celluloses, gums and starches. The properties of starch grains affect the processing qualities and digestibility of tropical roots/tubers. In addition to starch and sugar, root and tuber crops also contain some non-starch polysaccharides; such as celluloses, pectins and hemicelluloses, along with other associated structural proteins and lignins, which are collectively referred to as dietary fiber (FAO, 1990). The protein content and quality of tropical roots and tubers (Table 1.6) is variable, ranging from 1–2.7 %. Taro has the highest protein content (2.2 %) among the given roots and tubers (Table 1.6). However, the protein content is higher in the leaves (4.0 %) than the tubers. The comparison of nutritional profiles of various tropical roots and tubers is illustrated in Table 1.6.
Table 1.6 Comparison of nutritional profile of various tropical roots and tubers
Roots and tubers | Food energy (kilo-joule) | Moisture(%) | Protein(g) | Fat(g) | Fiber(g) | Total CHO and fiber (g) | Ash(g)
Cassava | 565 | 65.5 | 1.0 | 0.2 | 1.0 | 32.4 | 0.9
Sweet potatoes (white) | 452 | 72.3 | 1.0 | 0.3 | 0.8 | 25.1 | 0.7
Sweet potatoes (yellow) | 481 | 70.0 | 1.2 | 0.3 | 0.8 | 27.1 | 0.7
Yam | 452 | 71.8 | 2.0 | 0.1 | 0.5 | 25.1 | 1.0
Taro and tannia | 393 | 75.4 | 2.2 | 0.4 | 0.8 | 21.0 | 1.0
Giant taro | 255 | 83.0 | 0.6 | ― | ― | 14.8 | -
Elephant foot yam | 339 | 78.5 | 2.0 | ― | ― | 18.1 | -
Taro leaves | 255 | 81.4 | 4.0 | ― | ― | 11.9 | -
Sweet potato | tips | ― | 86.1 | 2.7 | ― | ― | ― | -
Source: FAO, (1972)
Tropical roots and tubers exhibit very low lipid content. The lipids are mainly structural lipids of the cell membrane, which enhance cellular integrity and help to reduce enzymatic browning (FAO, 1987). Most of the lipids present in tropical roots and tubers consist of equal amounts of unsaturated fatty acids, linoleic and linolenic acids and the saturated fatty acids, stearic acid and palmitic acid, etc.
Most of the roots and tubers are good sources of potassium and consist of lower amounts of sodium. This makes them particularly valuable and distinguishable in the diet of patients suffering from high blood pressure, who require limited sodium intake (Valli et al., 2013). In such cases, the high potassium to sodium ratio may provide an additional health benefit. Yam can supply a substantial portion of the manganese and phosphorus and to a lesser extent the copper and magnesium.
1.8 Characteristics of Tropical Roots and Tubers
High respiration rate, high moisture content (70–80 %) and larger unit size (100 g-15 kg) are the general characteristics of tropical roots and tubers. In addition, their soft texture and heat production rate of approximately 0.5-10 MJ/ton/day and 5-70 MJ/ton/day at 0 °C and 20 °C respectively are one of their distinct characteristics (FAO, 1993). These are perishable, having a limited shelf life of several days to fewer months, but have a better yield under adverse conditions as compared to other crops. The losses are usually caused by rotting (bacteria and fungi), senescence, sprouting and bruising (Atanda et al., 2011). The comparison of various tropical roots and tubers is given in Table 1.7.
Table 1.7 Comparison of various tropical roots and tubers
Cassava
Plant Family | Euphorbiaceae
Chromosomes | 2n = 36
Flower | Monoecious
Origin | Tropical America
Edible part | Root, leaves
Actualization | Firm
Shape | Large and irregular
Taste | Sweet or bitter
Beta carotene | Usually high in yellow cassava.
Annual, biennial, or perennial | Perennial
Plant | Woody plant with erect stems
Leaves | Simple lobed leaves up to 30 cm in length, but may reach 40 cm.
Root/Tuber description | Nutty flavored, starchy Root
Climate and weather | Survivor crop capable of withstanding long periods of dry weather.
Height | 1–2 m
Propagation | From stem cuttings
Diseases | Bacterial blight, cassava frogskin disease, Viral diseases, etc.
Harvesting | It is commonly harvested by separating the stem from the plant and then pulling out the roots from the ground.
Taro
Plant Family | Araceae
Chromosomes | 2n = 22, 26, 28, 38, and 42
Flower | Monoecious
Origin | Indo-Malayan
Edible part | Corm, cormels, leaves and petioles
Actualization | Rough, thick skin and doughy texture
Shape | Large, starchy, sphericalunderground tubers
Taste | Starchy
Beta carotene | Leaves contain high levels of beta carotene.
Annual, biennial, or perennial | Perennial
Plant | Large, starchy, spherical underground tubers. The large leaves of the taro are commonly stewed.
Leaves | Each leaf is made up of an erect petiole and a large lamina.
Root/Tuber description | Tubers are rounded, about the size of a tennis ball; each plant grows one large tuber, often surrounded by several smaller tubers.
Climate and weather | Can be grown in the fields where water is abundant.
Height | 1–2 m
Propagation | By offshoots from the mother corm
Diseases | Leaf blight, Erwinia soft rot, shot hole leaf disease
Harvesting | Taro tubers are harvested in nearly 200 days. The leaves can be picked after the first leaf is open.
Sweet potato
Plant Family | Convolvulaceae
Chromosomes | 2n = 90
Flower | Monoecious
Origin | Central or South America
Edible part | Root, leaves
Actualization | Smooth, with thin skin
Shape | Short, blocky, tapered ends
Taste | Sweet
Beta carotene | Usually high
Annual, biennial, or perennial | Perennial
Plant | Plant bears alternate heart-shaped or palmately-lobed leaves.
Leaves | Ovate-cordate, borne on long petioles, palmately veined, angular or lobed.
Root/Tuber description | The root is long and tapered, with a smooth skin, whose color may be yellow, orange, brown, red and purple. The flesh color ranges from white, pink, red, yellow, violet, orange and purple.
Climate and weather | The plant does not tolerate frost. It grows best at 24 °C in abundant sunshine and warm nights. Annual rainfalls of 750-1,000 mm are considered most appropriate.
Height | 0.30-0.46 m
Propagation | Transplants/vine cuttings
Diseases | Bacterial stem and root rot, bacterial wilt, soil rot
Harvesting | Harvested at any time after they have reached a suitable size (generally 3–4 months). Their flavor and quality will improve with colder weather. Can even wait until the frost has blackened all of the vines before harvesting.