Table 1.5 Tropical roots and tubers: salient features
Taro
(i) Colocasia esculenta (L.) Schott var. esculenta
(ii) Colocasia esculenta (L.) Schott var. antiquorum
(iii) Xanthosoma sagittifolium
Family: Araceae
Scientific name: Colocasia esculenta C. esculenta var. esculenta: The variety (dasheen) has large cylindrical central corm.
C. esculenta var. antiquorum: This is a small globular central corm as compared to C. esculenta, with relatively large cormels arising from the corm itself. This variety is referred as the eddoe type of taro.
Xanthosoma sagittifolium: Popularly known as Macabo in Africa, has smaller edible cormels about the size of potatoes. Its corms and cormels are rich in starch.
Sweet potato
(i) Orange/copper skin with orange flesh
(ii) White/cream skin with white/cream flesh
(iii) Red/purple skin with cream/white flesh
Genus: Ipomoea
Family: Convolvulaceae
Scientific name: Ipomoea batatas
Orange/copper skin with orange flesh type: They have high beta-carotene content and are quick growers, which may become too big with longer growing periods.
White/cream skin with white/cream flesh type: White sweet potatoes are also called camote, batata or boniato. The outside skin of the white sweet potato is either a brownish-purple or a reddish-purple color, whereas the inside flesh is white or cream colored. It can produce good yield in a relatively short growing period (4 months), which is important for cold regions. Long and curved sweet potatoes are produced especially in sandy soils.
Red/purple skin with cream/white flesh type: It is mainly used in recipes that require mashed or grated sweet potatoes such as pies, breads and cakes, due to its high moisture content. It requires a growing period of 5 months to produce a good yield.
Yam
(i) White yam (Dioscorea rotundata Poir)
(ii) Yellow yam (Dioscorea cayenensis Lam.)
(iii) Water yam (Dioscorea alata L.)
(iv) Bitter yam (Dioscorea dumetorum)
Genus: Dioscorea
Family: Dioscoreaceae
Scientific name: Dioscorea spp.
White yam (Dioscorea rotundata Poir): This is cylindrical in shape, having smooth and brown skin with a white and firm flesh. It is widely grown and preferred yam species.
Yellow yam (Dioscorea cayenensis Lam.): The yellow yam has a longer vegetation period and a shorter dormancy as compared to white yam. It has acquired the name from its yellow flesh
Water yam (Dioscorea alata L.): This is the most widely spread out all over the globe. It is only second to the white yam in popularity in Africa. This tuber is cylindrical in shape, having white colored flesh and watery texture.
Bitter yam (Dioscorea dumetorum): This is also referred as the trifoliate yam because of its leaves. It has a bitter flavor and its flesh hardens if not cooked properly soon after harvesting. Some of its cultivars are highly poisonous.
Cassava
(i) Sweet and bitter cassava
(ii) Yellow cassava
Genus: Manihot
Family: Euphorbiaceae
Scientific Name: Manihot esculenta
Sweet and bitter cassava: Sweet cassava roots contain comparatively much lesser hydrogen cyanide as compared to bitter cassava. These varieties need to be detoxified before consumption through different types of treatments. Sweet cassava produces higher yields and requires lesser processing as compared to bitter cassava.
Yellow cassava: It is similar to ordinary varieties of cassava (Manihot esculenta), but it has yellow flesh inside the root. It does not need nutrient-rich soils or extensive land preparation and does not suffer during droughts.
Elephant foot yam
Genus: Amorphophallus
Family: Araceae
Scientific name: Amorphophallus paeoniifolius
The elephant foot yam originated in Southeast Asia. Amorphophallus species are herbs and only a single leaf emerges from the tuber, consisting of a vertical spotted petiole and a horizontal leaf-blade (lamina). Its popular varieties are Gajendra, Kusum and Sree Padma.
Giant taro
Genus: Alocasia
Family: Araceae
Scientific name: Alocasia macrorrhizos
The giant taro originates from rainforests of Malaysia to Queensland. The varieties recognized in Tahitiare the Ape oa, haparu, maota and uahea. It is edible, if cooked for adequate time, but its sap irritates the skin due to calcium oxalate crystals, or raphides, which are needle-like crystals.
1.7 Composition and Nutritional Value
Roots and tubers are one of the cheapest sources of dietary energy, in the form of carbohydrates. Their energy value is comparatively low when compared to cereals due to their higher amount of water. Because of the low energy content of roots and tubers as compared to cereals, it was earlier considered they were not suitable as baby foods. The nutritional composition of roots and tubers varies from place to place, depending on various factors such as climatic conditions, variety of crops and soils, etc. Carbohydrate is among the main nutrients, which dominate in roots and tubers. The protein content is low (1–2%) and in almost all root proteins, sulfur-containing amino acids are the limiting amino acids (FAO, 1990). Cassava, sweet potato and yam may contain little amounts of vitamin C. whereas yellow varieties of sweet potato, cassava and yam also contain β-carotene.
Vitamin C occurs in major and appropriate amounts in almost all tropical roots and tubers. The level may be reduced during cooking unless skins and cooking water are also used (Krieger, 2010). Most of the roots and tubers contain small amounts of the B complex vitamins, which act as a co-factor in the oxidation of food and production of energy. Sweet potato has high content of vitamins A, C and antioxidants that can help in preventing various diseases such as heart disease and cancer, enhance nutrient metabolism, bolster the immune system and even slow aging by promoting good vision and healthy skin. It is also an excellent source of manganese, copper, iron, potassium and vitamin B6 (IICA, 2013). Taro is a good source of potassium. The leaves of cassava and sweet potato can be cooked and eaten as a vegetable. The leaves contain appreciable amounts of functional constituents, vitamins and minerals such as β-carotene, folic acid and iron, which may provide protection against various diseases. The dry matter of roots is made up mainly of carbohydrate, usually 60–90 % (Ezeocha and Ojimelukwe, 2012).