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Root Crops: Intoxicating Yam (Dioscorea hispida)

By goGreen | October 3, 2011
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Common names

INTOXICATING YAM, Karukandu, Nam.

Botanical name

Dioscorea hispida Dennst. (Lasiophyton).

Family

Dioscoreaceae.

Other names

Gado(e)ng, Gadong mabok (Mal.); Kalt (Philipp.); Killoi, Koi (Thai.); Maranpash poll (Ind.); amo (Philipp.); Palidumpa, Pashpoli (Ind.).

Botany

D. hispida is a climber usually with a prickly stem, 6-10 mm in diameter, varying from glabrous to pubescent with fine white to brown hairs. Twining is clockwise (to the left). The leaves are trifoliate with oval to obovate leaflets, about 10 cm long by 8 cm broad, hairy, with small prickles on the underside of the main vein. Male flowers are in large, branched inflorescences; the female inflorescences are unbranched. The tubers are large, weighing 5-15 kg, roughly globose but deeply lobed, pale skinned, but covered with masses of fibrous roots: they are produced near the soil surface and are extremely poisonous.

Origin and distribution

This species grows wild in South-East Asia and Indonesia, and extends to Papua New Guinea and the Philippines and India. It is not cultivated to any great extent, though some cultivation is practiced in Java.

Cultivation conditions

D. hispida thrives in tropical rain forest conditions. It usually grows at relatively low elevations, less than 500 m, though it has been reported growing at altitudes up to 1 200 m in the Himalayas.

Planting procedure

As noted above, the plant is infrequently cultivated. When cultivation is practiced propagation is often by planting pieces of tuber in prepared mounds (see Yam).

Growth period

Maturity is normally reached in about 12 months.

Harvesting and handling

The tubers are usually lifted by hand with a digging stick or fork. It has been reported that if the tubers are exposed to temperatures below about 10°C, subsequent growth is adversely affected.

Primary product

Tubers-which have white or pale-yellow, starchy, highly toxic flesh.

Yields

In cultivation, yields of about 20 t/ha have been reported.

Main use

As a famine food-the tubers, growing near the surface, are easily accessible. Detoxification is essential and one method is to cut the tubers in pieces, cover the surface with wood ashes for 24 hours, then steep in sea water for several days, wash with fresh water, and dry. The process is repeated several times. Another method is to dry the slices mixed with ashes. A third is to salt the pieces of tuber and then press under water until no whitish sap remains. After detoxification the yams are usually tested by feeding to dogs or other domestic animals.

Subsidiary uses

The possibility of using the tubers as a source of starch has been considered, but so far appears not to have been commercially developed.

Secondary and waste products

The tubers are sometimes used to prepare poisons. The pounded tubers are also used in parts of Asia in local medicine for the treatment of open wounds. It has been suggested that the residue left after starch extraction could be used as an insecticide.

Special features

Tubers-an approximate analysis of the tubers has been given as: water 78 per cent; protein 1.81 per cent; fat 0.16 per cent; carbohydrate 18 per cent; fibre 0.93 per cent; ash 0.69 per cent. On a dry weight basis the tubers contained 0.2-0.7 per cent diosgenin and 0.044 per cent of the toxic alkaloid dioscorine.

Flour-the average composition of the flour extracted from the tubers was given as: protein 5.28 per cent; fat 0.23 per cent; starch 88.34 per cent; fibre 5.33 per cent; ash 0.66 per cent. It is suitable for both edible and industrial purposes and can be used for the manufacture of glucose. Starch granules from Indian tubers are non-stratified and oval-shaped, with an average longitudinal diameter of 35-40 microns and a gelatinisation temperature of 85°C. Starch from D. hispida differs from cassava and potato starches in that its viscosity does not fall appreciably after prolonged heating.

Processing

The following method is suggested for the preparation of flour or starch from D. hispida tubers.

(i) The tubers are thoroughly washed in clean water, either by hand or mechanically, to remove adhering soil, etc.

(ii) The tubers are mashed with water; a potato rasping machine is suitable for the preparation of flour, but for the production of starch, the tubers must be ground very finely in order to rupture the cell walls and liberate the starch granules.

(iii) In order to detoxify the material, the pulp is treated with lime water containing potassium permanganate; usually lime water equivalent to five times the weight of tubers and containing 0.005 per cent of potassium permanganate, is used. Any excess potassium permanganate is removed by treating the starch milk with sulphur dioxide.

(iv) The starch is allowed to settle out and is then washed and centrifuged as in the manufacture of sweet potato starch.

In the Philippines it has recently been suggested that starch or flour could be produced on a commercial scale by extracting the tubers with 95 per cent alcohol followed by treatment with 5 per cent sodium chloride or acidified water.

Source:

Tropical Development and Research Institute
127 Clerkenwell Road, London EC1R 5DB
Overseas Development Administration

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