Yam
Plant Family | Dioscoreaceae
Chromosomes | 2n = 20
Flower | Dioecious
Origin | West Africa or Asia
Edible part | Tuber
Actualization | Rough, scaly
Shape | Long, cylindrical, some with "toes"
Taste | Starchy
Beta carotene | Usually very low
Annual, biennial, or perennial | Perennial
Plant | Monocot (a plant having one embryonic seed leaf).
Leaves | Leaves are veined with lengthy stems that are attached to the vines of the plant.
Root/Tuber description | Tuber can be cylindrical, curved or lobed, with brown, grey, black or pink skin and white, orange or purplish flesh.
Climate and weather | It is tolerant to frost conditions and can be grown in much cooler conditions as compared to other tubers.
Height | 1–3 m
Propagation | Tuber pieces
Diseases | Yam Anthracnose, Yam Mosaic Virus, Water yam virus, other foliage diseases
Harvesting | Harvesting is done before vines become dry and hard. After 7-12 months growth, tubers are harvested.
Elephant foot yam
Plant Family | Araceae
Chromosomes | 2n = 26
Flower | Monoecious
Origin | Southeast Asia
Edible part | Tuber
Actualization | Rough, thick skin
Shape | Large and round
Taste | Starchy
Beta carotene | Usually high
Annual, biennial, or perennial | Perennial
Plant | Tropical tuber crop, grown for its round corm. The stems can be 1–2 m tall.
Leaves | About 50 cm long and consist of several oval leaflets.
Root/Tuber description | Round corms are usually 3–9 kg, depending on the number of seasons that the crop is grown before harvest.
Climate and weather | It grows well in hot and humid climate. Well drained, fertile and sandy loam soil is ideal for its production. Stagnant water at any stage can affect its production.
Height | 1–2 m
Propagation | Small corms (cormels) or buds are used for this purpose. These are produced below ground level.
Diseases | Foot rot, Pythium root rot, Amorphophallus Mosaic and leaf blight
Harvesting | Corms can be dug up by hand. Take about 6–7 months to mature. Leaf yellowing and drying up of plants indicate that the crop is ready to harvest. Harvesting can begin after 5–6 months.
1.9 Anti-nutritional Factors in Roots and Tubers
Roots and tubers mostly contain variable amounts of anti-nutritional factors such as oxalates, phytates, amylase inhibitors, trypsin inhibitors, etc. The cultivated varieties of most of the edible roots and tubers, except cassava (which contains cyanogenic glycosides) do not possess any serious toxins, whereas the wild species may contain toxic principles, therefore must be correctly processed with appropriate methodology before consumption. However, some of these wild species serve as a useful reserve when food scarcity arises. The local people have developed suitable techniques to detoxify the roots and tubers before consumption (FAO, 1990). The various anti-nutritional factors present in roots and tubers along with their mode of elimination are presented in Table 1.8.
Table 1.8 Anti-nutritional factors in roots and tubers and their mode of elimination
Roots/ tubers | Anti nutritional factor and their levels | Mode of Elimination | Reference
Raw Bitter Cassava | Saponin: 730 mg/kg • Oxalate: 49 mg/kg • Phytate: 12 320 mg/kg • cyanide: 14 300 mg/kg | Fermentation, pressing, frying, cooking or drying | Amira et al. (2014)
Dried Bitter Cassava | Saponin: 630 mg/kg • Oxalate: 32 mg/kg • Phytate: 8,770 mg/kg • Cyanide: 9,140 mg/kg | Fermentation, pressing, frying, cooking or drying | Amira et al. (2014)
Raw taro | Oxalate: 156.33 mg/100 g • Phytate: 85.47 mg/100 g | Soaking and boiling | Alcantara et al. (2013)
Yam: D. alata | Total free phenolics: 0.68 g/100 g • Tannins: 0.41 g/100 g • Total oxalate: 0.58 g/100 g • Hydrogen cyanide: 0.17 mg/100 g • Trypsin inhibitor: 3.65 TIU/mg • Amylase inhibitor: 6.21 AlU/mg soluble starch | Moist heat treatment (for amylase and trypsin inhibitor)Soaking followed by cooking before consumption (for phenolics, tannins, hydrogen cyanide and total oxalate) | Shajeela et al. (2011)
Yam: D. bulbifera var vera | Total free phenolics: 2.20 g/100 g • Tannins: 1.48 g/100 g • Total oxalate: 0.78 g/100 g • Hydrogen cyanide: 0.19 mg/100 g • Trypsin inhibitor: 1.21 TIU/mg • Amylase inhibitor: 1.36 AlU/mg soluble starch | Moist heat treatment (for amylase and trypsin inhibitor)Soaking followed by cooking before consumption (for phenolics, tannins, hydrogen cyanide and total oxalate) | Shajeela et al. (2011)
Elephant Foot Yam | Soluble oxalate: 13.53 mg/100 g | Soaking and boiling | NPARR (2010)
Boiled sweet potato | Phytate: 0.88 mg/100 g • Oxalate: 167.15 mg/100 g • Tannin: 0.68 mg/100 g | Cooking | Abubakar et al. (2010)
Tilclass="underline" Trypsin inhibitor unit, All): Amylase inhibitor unit
1.9.1 Cassava
The residual level of cyanogens in cassava products differ in different varieties, depending upon the nature and duration of the various processing techniques (Montagnac et al., 2008). Linamarin, a cyanogenic glycoside, occurs in varying amounts in different parts of the cassava plant (Obazu, 2008). It often co-exists as methyl-linamarin or lotaustralin. Linamarin may become converted into hydrocyanic acid or prussic acid when it comes into contact with an enzyme called linamarase, which is released on the rupturing of cassava cells. In the absence of this enzyme, linamarin is considered a stable compound which is not changed, even with boiling (FAO, 1990). If it is absorbed from the gut into the blood, it is probably excreted unchanged without causing any harm to the organism (Philbrick et al, 1977). Ingested linamarin can liberate cyanide into the gut during digestion process. However, proper processing and cooking methods can reduce the cyanide content to non-toxic levels. Sweet cassava roots contain less than 50 mg/kg HCN on a fresh weight basis, whereas the bitter variety may contain up to 400 mg/kg (Kwok, 2008). In dry tubers, cyanide residues can be in the range of 30-100 mg/kg (Agbidye, 1997). As per the African Organization for Standardization, cassava-based products, especially flour, should have the acceptable limit of cyanide content, 10 mg/kg. Simple boiling of fresh root pieces is not always reliable since the cyanide may only be partially liberated and only a part of linamarin may be extracted in the cooking water. The reduction of cyanides depends upon the treatment method. The cassava roots, when placed in cold water (27 °C) or boiling water (100 °C) for 30 min, has a reduced cyanide content of 8 % or 30 % of its initial value respectively (Essers, 1986).
Sun-drying processing techniques are not considered efficient for detoxification of cassava roots, because they do not effectively reduce cyanide content in a short interval of time. Sun-drying processing techniques reduce only 60–70 % of the total cyanide content present in the first two months of preservation (FAO, 1990). Fermentation is also considered an effective method of the detoxification process. The liberated cyanide is dissolved into the water when fermentation is effected by prolonged soaking and evaporates upon drying of the fermented cassava (FAO, 1990). Ighu (a processed cassava product) is processed manually using metallic shredding plates, which are moved vigorously by hand on the surface of peeled steamed cassava by reciprocating action. Ighu samples have lower HCN content, which makes this product safe for human consumption. The HCN content of the dry Ighu varies from 8.20-9.83 mg/kg (Iwe and Agiriga, 2013). The cyanide content of processed cassava tubers (garri) is significantly reduced after 48 hours of fermentation (Chikezie and Ojiako, 2013).