Also called trifoliate yam as a result of its leaves. It originates in Africa where wild cultivars also exist. One noticeable characteristic of this species is the bitter flavour of its tubers. Another undesired characteristic is that the flesh hardens if not cooked soon after harvest. Some wild cultivars are very poisonous. Dumentorum has cylinder-shaped stems with digitate compound leaves of 3, 5, or 7 leaflets. Twining is clockwise. Tubers are bunched and the flesh colour varies from white to creamy white or yellow. Tubers may be bitter and occasionally they are soaked for about three days before being prepared. Tubers reach maturity in about 10 months (Dumont & Vernier, 2000).
45 2.5.7 Storage of yams
A conservative estimate indicates that about 15 percent of yams produced annually never reach the market, mainly because of post-harvest losses. Losses are due to lack of appropriate storage facilities, a variety of disease, pest and sprouting, even in storage, losses still occur, over one million tons of yam tubers are lost annually in storage in West Africa (Coursey, 1967). Sources of storage losses are rotting, pest respiration and sprouting. Sprouting causes a reduction in the food reserves by translating carbohydrates from the tuber into the sprouts for metabolic
processes. There are various methods of yam storage:
i. Scientific methods
Sprouting of D. rotunda tuber can be delayed by non-lethal dose of gamma – irradiation (Adesuyi & Mackenzie, 1973). Rotting causes the greatest amount of loss of dry matter in yam storage, it is due mostly to the effects of fungi and bacteria notably Serratia been implicated (Okafor, 1966). To prevent rotting storage, wounding of the tubers during harvesting and handling must be avoided. Curing of yam tubers at 25% and low humidity (55 – 62 percent), for 5 days before storage prevents to a certain degree of storage rot.
Respiration is a serious source of storage loss in yams since harvested tubers are living, they are respiring and the substrate for the respiration is the dry matter which is stored in the tubers. One of the effective ways of reducing respiratory loss is to keep the temperature low, but not below 160C.
ii. Traditional methods
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Storage of yams by the indigenous peoples of the tropics and subtropics is carried out mostly by traditional methods. These traditional methods which are employed vary from country to country and from region to region. The variations are often related to climate but local natural resources and customs also influence the choice of storage method used. Some traditional methods used by people are:
iii. Burying (clamp) or pit storage
This involves the digging of holes or pit about 1m in diameter in the ground and leaving them with dried leaves and grasses. After selecting good quality yam tubers, they are placed in the pit, alternating each layer of tubers with dried grasses. They are finally covered with a layer of dried grasses and a top layer of soil. This method of yam storage is not very effective due to the lack of sufficient ventilation and the difficulty of inspecting the tubers.
iv. Field storage
Leaving the yams in the field is the simplest method of storage. Tubers are stacked in piles and covered with grasses, sorghum or millet stalks.
v. Barn storage
In the humid areas of West Africa, Yams are often kept in barns. Basically, barns consist of a vertical wooden framework. To this framework, the yam tubers are tied individually by means of tie vines or strings, or they can be laid on simple open work shelving inside the barn.
In some cases, the barns are covered with thatched roofs to protect the tubers from rain and the heat of the sun. Placing yams on the shelves require less time and labour than tying. Shelving
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has the advantage that insect infested and rotting yams can be seen easily and removed. The disadvantage with shelving lies in the fact that it is easier for rats to hide amongst the yams.
Losses due to delay in some cases can amount to 40 – 50 percent (Olorunda & Adesugi, 1973).
Losses can be reduced by protecting the barn from rats by fixing a barrier made of iron sheeting around the barn.
vi. Compound storage
Outside the forest zone, for example in the savanna regions of West Africa, yams can be stored either in the field or in the farm compound. Yams stored in the farm compound are stacked in cribs like those used for maize. The cribs are raised well off the ground with rat guards fitted to the legs.
vii. Cold storage of yam tubers
The application of low temperature storage to yams is limited by the fact that yams are susceptible to low temperature injury at a temperature of 10-120C or less (Coursey, 1967). The temperature of 16-170C has been successfully employed to prolong the storage time of yams (Passam, 1977). At 160C the dormancy and hence the storage life of water yam tubers may be extended by as much as four months, particularly if the tubers are cured prior to storage so as to reduce infection by wound pathogens (Gonzalez & Collazo 1972). The storage of yam tubers at lowered temperature has the advantage of reducing the major sources of storage loss (respiration, sprouting and rotting).
48 2.5.8 General economic importance of yam
Fresh yam can be eaten fried, boiled, or roasted like potatoes (Wanasundera & Ravindran 1994).
Different people prepare yam differently. Fufu is prepared from cassava in combination with plantain or cocoyam but in yam producing zones or during the scarcity of plantain and cocoyam, Fufu is prepared from boiled yam and cassava. In Benin and Western parts of Nigeria, yam tubers are processed into slightly fermented flour called elubo for a product called amala.
Yam tubers can be processed into yam chips and pellets, which are milled to produce yam flour.
Mestres et al. (2002) reported that there has been an increase in production and marketing of yam chips in Nigeria, Benin, and Togo. In the industry, yam chips can be milled and used for various food products for example biscuit and weaning food. Attempts to manufacture fried yam chips, similar to French fried potatoes have been reported from Puerto Rico and the potential for its production on a commercial level has been highlighted (Abasse, et al., 2003). Yam tubers have also been processed into starch or into poultry and livestock feed just as cassava (Opara, 1999). Yam starch is used in the production of all-purpose adhesives. Producers of cartons, packaging companies, leather and shoes use the adhesives for their products. Recently, several beneficial properties of yams were reported, and the extracts of yam showed antioxidant activity and possessed scavenging properties against free radicals (Farombi, et al., 2000).