Other related crops include cassava (Manihot esculenta), white-fleshed sweet potato (Ipomoea batatas L.), Potato (Solanum tuberosum L) and the edible aroids such as coco yam (Xanthosoma spp. Schott).
These crops grow well in the tropical and subtropical regions of the world, and Africa is home to many of the species. In Africa yams are grown mostly for household consumption and for small local commerce. Yam is a high quality food prepared for family and only for honoured guests. There is a demand for yams for food by people that have migrated to the cooler regions of the world. This demand has triggered the export of yams from the tropics, an act that has influenced prices and the elevation of yam commercial farming. Currently yams are preferably consumed fresh soon after harvest or after a brief period of storage. The 12 months maturity period for yams, compared to potatoes (3 months) or cassava (six months), makes the yam more expensive for the general processing into products like snack foods. In UK for example, fresh yam is more expensive than potatoes although it is consumed by only a small proportion of the population that comprises of the expatriate communities from Africa, Asia South America and the Caribbean. The reasons for the need of yam, despite the high costs will be the focus in this discussion
Characteristics and uses:
The food from yam is derived from the edible starchy tuberous root of Dioscorea species, a climbing tropical plant, exemplified best by the White African Yam (Dioscorea rotundata) that is widely exported worldwide. These herbaceous vines tend to give the high level of produce when grown with the support of trellis, usually made from bamboo poles. The tubers may weigh as much as 5Kg of edible matter that constitutes 20-30% carbohydrates and up to 15% proteins. There are, however, other chemicals, that may be significant nutritionally or medicinally. Many plants contain sap, in which are water-soluble glycosides or saponins. The saponins are chemically made up of carbohydrates group (usually polysaccharides, simple and complex sugars) and a non-carbohydrate moiety the sapogenins. The saponins are either steroids and have the characteristic 17 carbon atoms arranged in four rings, or may possess triterpenoid structures characterized by sweet tastes. These two basic structures, the steroid and the triterpenoid, explain respectively the anti arthritic properties in yams and the existence of sweet yams with extractable sugar levels. Both chemicals have been detected in both edible and toxic yams.
