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Root crops (NRI, 1987, 308 p.)
Elephant yam (Amorphophallus spp.)
ELEPHANT YAM, Elephant bread, Elephant foot yam, Suran, Sweet yam.
Anto (Philipp.); Arsaghna, Balukund (Ind.); Chena (Mal.); Daga (Fiji); Ilis-ilis, Kand godda (Indon.); Karak-kavanai (Tamil); Karnai kilangu (Mal.); Kidaran (Ind.); Koe (Polyn.); Konjac, Konniaku, Konnyaku (Japan); Mo-yu (China); Ol (Assam); Ol kuchu (Bangl.); Oroy, Poï¿½gapong, Puï¿½gapung (Philipp.); Sooweg (Indon.); Suron (Fiji); Tamari (Sol. Is.); Telinga potato (Ind.); Teve (Tah.); Tigi (Philipp.); Waloor (Indon.); Zaminkund (Ind.).
A robust herbaceous plant, with an erect solitary stem usually 1-2.5 m in height and bearing at the top one or two tripartite leaves, each part of which is deeply dissected into numerous segments. Towards the end of the plant's cycle (usually 4-6 years) a large terminal inflorescence is produced, consisting of a short stalk and spathe and a spadix, which emits a malodorous smell, reminiscent of rotten meat. The corms are large globose depressed tubers, usually dull-yellow or brownish-yellow in colour, and these produce 5-10 cormels at the end of each growing season.
The genus Amorphophallus consists of about 90 species, but the most important and widespread in the tropics is Amorphophallus paeoniifolius (Dennst.) Nicolson (syn. A. campanulatus Decne) and according to some authorities this exists in two forms, the wild one, var. sylvestris, recognisable by its rough petioles, while the cultivated form, var. hortensis, has much smoother petioles. In Indonesia two closely related species A. oncophyllus Prain and A. variabilis Bl. occur and are utilised. In addition, A. konjac C. Koch (syn. A. rivieri Durien var. konjac (C. Koch) Engler) is cultivated and utilised in Japan and the warmer parts of China, and is also referred to as elephant yam in these areas.
Origin and distribution
The genus is indigenous to tropical Asia and Africa and A. paeoniifolius is found widely distributed in thickets and secondary growth forests at low and medium altitudes in the Philippines, Malaysia, Indonesia, Sri Lanka and the South-East Asia subcontinent. A. konjac originated in the Vietnam-southern China region and was introduced into Japan in the 10th century.
A. poeoniifolius is a tropical and subtropical crop and requires an average temperature of 25-35Â°C, preferably fairly equable during its growing period. The rainfall should be evenly distributed and between 100 and 150 cm, although the plant can be grown with a rainfall as low as 65 cm provided irrigation facilities are available. Warm humid conditions favour leaf growth and dry conditions favour the development of the corms. A. konjac requires cooler conditions with temperatures in the range 18-30Â°C, and is normally grown between latitudes 34Â°N and 43Â°N.
For optimum yields, deep loamy soils worked to a fine filth are necessary, preferably not alkaline. Good drainage is essential as the crop cannot stand waterlogging and heavy clays are therefore unsuitable. In India, it has been recommended that the crop should receive 25 t/ha of FYM, in addition to nitrogen 40 kg/ha, phosphorus 40 kg/ha and potassium 80 kg/ha. If no FYM is used additional nitrogen is advised, about 50 kg/ha in July and again in August for a fourth year crop.
Material - both A. paeoniifolius and A. konjac are propagated from small corms (cormels) or buds produced below ground from the base of the shoot; in A. paeoniifolius the cormels appear during the fourth year but in A. konjac they appear in the second or third year. A weight of 100-120 g per piece is usual for planting. Both species have a dormancy period of 2-3 months. A crop may be obtained in one year from A. paeoniifolius if a four year old corm is cut into sections, each of which weighs 1-1.2 kg and carries at least one cormel and planted. Viable seeds have been produced experimentally and may be used for breeding work.
Method - the soil should be well tilled. Frequently planting is on the flat, after paddy culture, but planting on ridges is also common. The corms are usually planted 10-15 cm deep. The crop often receives little cultural atten tion, although mulching or shading during the first 3-5 weeks of growth, followed by weeding and earthing up, is recommended. In India it is often grown mixed with other crops, such as arecanuts (Areca catechu), ginger (Zingiber officinale), methi (Trigonella foenum-graecum), cluster bean (Cyamopsis tetragonoloba) and bananas (Musa sp.). In parts of India, eg Bombay, the corms are usually dug at the end of each growing season, stored and then replanted, but in other areas, such as Japan, where A. konjac is grown, the corms are left in the ground for the whole growth cycle. It is essential for the corms of both species of Amorphophallus to be allowed a period of natural dormancy of 2-3 months. Amorphophallus has been recommended for intercropping with coconuts.
Field spacing - trials in India using cut pieces of four year old corms gave the highest yield per hectare when planted at 90x 120 cm. With seed pieces a spacing of 60x 120 cm has been recommended if accompanied by fertilisers.
Seed rate - approximately 1 600-2 000 kg/ha of corms or buds are required: when cut pieces of four year old corms are used the rate is about 10 000 kg/ha.
Pests and diseases
Reports from India indicate foot rot (caused by Rhizoctonia solani) as a serious problem. Drenching the soil around affected plants at monthly intervals with 0.2 per cent captan or 0.1 per cent quintozene was highly effective and led to almost double the tuber yields of untreated plants. Little has been reported from elsewhere.
The growth cycle of the corms normally takes about 8-12 months but the tubers are small and unmarketable after only one season and 3-4 seasons are required for an economic crop, except when planted from four year old corms as described.
Harvesting and handling
The corms are dug by hand when the leaves begin to wither and die, and weigh from 3 to 9 kg, depending upon the number of growing seasons. They are usually carefully cleaned and are stored in heaps preferably in well-ventilated sheds. Corms of A. paeoniifolius can lose as much as 25 per cent of their initial weight in the first month of storage, but can be successfully stored at 10Â°C for several months. Alternatively, they may be left in the ground until required, with a little irrigation if necessary. Corms dipped for I minute in a 4 per cent fungicidal emulsion can be stored at room temperature for about 2 months with minimal loss of weight or sprouting. In Japan corms of A. konjac that are to be replanted must be protected in store from low winter temperatures, since it has been found that if they are subjected to temperatures of - 5Â°C germination is affected.
Corms - the depressed globose corms often have a diameter of 30 cm or more, and under good cultural conditions can weigh 7-9 kg by the fourth season.
In India the average yield ranges between 12 and 22 t/ha; as an intercrop with coconuts 13 t/ha. However, under experimental conditions over 60 t/ha has been reported, and 36 t/ha in mixed cropping.
The corms and cormels of A. paeoniifolius are usually boiled or baked and eaten as a vegetable: in Japan A. konjac is mainly eaten as konnyaku, a gel-like food with an elastic texture made from konjac mannan flour (see Processing). The small one year old corms of A. konjac are considered to be a delicacy. Wild forms must be soaked in water for some time before cooking, and boiled for a lengthy period in order to remove the bitterness.
In the Philippines, the corms are sometimes boiled and fed to pigs. They may be used as a source of starch and alcohol and have been used to prepare a flour for breadmaking. In Indonesia, the species A. onchophyllus is used to produce flour for industrial purposes, and the Japanese species A. konjac is used as a commercial source of mannose. A glucomannan from species of Amorphophallus has been proposed for thickening food products such as ice cream or mayonnaise.
The konjac mannan flour from A. konjac is used to make paste that does not separate when frozen and thawed, and is not eaten by insects. It is also employed in the manufacture of paper, and in textiles it is used in the same manner as starch. Cotton and other fabrics are waterproofed by coatings based on konjac mannan flour. Cosmetics such as those for chapped skin, beauty creams and hair pomades may contain the flour.
Secondary and waste products
The young petioles and leaf blades may be boiled and eaten as a vegetable which is reported to resemble asparagus. Older, tougher petioles are used for livestock feeding, while the corms are reported to be used for medicinal purposes in parts of India.
The composition of the edible portion of the corms of A. paeoniifolius has been reported as: energy 330 kJ/100 g (approx); water 72-79 per cent; protein 1.7-5.1 per cent; fat 0.2-0.4 per cent; carbohydrate 18-24 per cent; fibre 0.ï¿½-0.8 per cent; ash 0.7-1.3 per cent; calcium 50-56 mg/100 g; iron 0.6-1.4 mg/100 g; phosphorus 20-53 mg/100 g; vitamin A 434 IU/100 g; thiamine 0.04-0.06 mg/100 g; riboflavin 0.05-0.08 mg/100 g; niacin 0.07-0.075 mg/100 g; ascorbic acid trace-3 mg/100 g. Most of the carbohydrate is starch (75-80 per cent); the starch granules vary in shape and size (about 5.5-19 microns).
A. konjac is reported to have a higher water content (80-90 per cent), with only about 10 per cent of the carbohydrate as starch but up to 65 per cent as glucomannan.
A. paeoniifolius and A. konjac contain calcium oxalate crystals; the wild forms of A. paeoniifolius contain more than the cultivated and are strongly acrid.
No processing methods for A. paeoniifolius have been described, but A. konjac corms are commonly air-dried in the sun; the dried corms may be stored for long periods, or made into flour. The following procedures apply only to A. konjac.
Flour (konako or konjac mannan flour) - the corms are peeled, sliced and skewered onto bamboo sticks 60 - 90 cm long, 2 or 3 cm apart, and placed in the sun. After about one week the slices are crushed and then ground in a mortar and pestle run by a waterwheel. During the grinding a flap attached to the mortar blows away the fibre and cell debris (and some of the flour). The flour is known as konako: about 12 kg of konako is obtained from 100 kg of fresh tubers. The proximate composition of konako has been given as: water 17 per cent; fat 0.6 per cent; carbohydrate 68 per cent; fibre 2.3 per cent; ash 4.5 per cent. The flour is used in many recipes.
Konnyaku - konako flour is made into konnyaku by the following process:
(i) Water is stirred into the flour until it becomes uniformly soft and gelatinous.
(ii) The paste is allowed to stand, stirred, and allowed to stand for a further period of time.
(iii) A dilute, strained suspension of slaked lime is stirred into the gel, and mixed thoroughly until it thickens.
(iv) The mass is poured into shallow trays, allowed to stand until further thickening has developed, then cut into squares (often with about 10 cm sides).
(v) The squares are boiled in water for about 20 minutes, allowed to cool in the cooking water and then stored, refrigerated, in the same water. Konnyaku is reported to last indefinitely if the water is not changed.
Shirataki - the lime-treated gelatinous mass (step (iii) in the production of konnyaku) is pressed through a die (or sieve) before cooking, forming noodles called shirataki.
Konnyaku and shirataki are an important part of the Japanese diet and may be eaten fresh (or refrigerated), or canned. Published analytical figures for Hawaiian and Japanese konnyaku are:
Hawaiian konnyaku (fresh): energy 50 kJ/100 g; water 96.6 per cent; protein 0.09 per cent; carbohydrate 3.07 per cent; fibre 0.06 per cent; ash 0.24 per cent; calcium 63 mg/100 g; iron 0.3 mg/100 g; magnesium 7 mg/100 g; phosphorus 7 mg/100 g; potassium 10 mg/100 g; sodium 38 mg/100 g; thiamine 0.021 mg/100 g; niacin 0.02 mg/100 g; ascorbic acid 0.5 mg/100 g.
Japanese konnyaku (fresh): water 97.4 per cent; protein 0.1 per cent; carbohydrate 2.3 per cent; fibre 0.1 per cent, ash 0.2 per cent; calcium 43 mg/100 g; iron 0.4 mg/100 g; phosphorus 5 mg/100 g; sodium 10 mg/100 g.
Japanese konnyaku (canned): energy 54 kJ/100 g; water 96.48 per cent; protein 0.04 per cent; fat 0.01 per cent; carbohydrate 3.28 per cent; fibre 0.37 per cent, ash 0.19 per cent; calcium 63 mg/100 g; iron 0.28 mg/100 g; magnesium 3 mg/100 g; phosphorus 3 mg/100 g; potassium 18 mg/100 g; sodium 2 mg/100 g; ascorbic acid 0.1 mg/100 g.
The figures for shirataki would be similar.
Konnyaku in the diet is reported to lower plasma cholesterol.
Production and trade
Very little information is available: in Japan the area under A. konjac was estimated to be about 15 000 ha in 1979, with production of 90 000 t, and in India about 800 ha of A. paeoniifolius is reported.
Although the elephant yam continues to be a popular root crop in parts of India and eastern Asia, production is limited mainly because of the four year crop cycle. However, in Japan, breeding and selection aimed at improved disease resistance, higher yields, earlier maturity and higher mannan contents, are in progress.
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