Yam tubers play very significant roles in the socioeconomic lives of the people of west Africa, the Caribbean, islands of the South Pacific, South-East Asia, India and parts of Brazil. Nigeria contributes significantly to the world’s total production of yam.
The composition of yam fibers varies according to species, culture, climate/ edaphic factors of the environment under which it was grown, its maturity at harvest and storage period.
Composition of some yam tubers.
Yam tubers have different species, and they vary in moisture content, carbohydrate, fat, crude protein, crude fibre and ash.
- Alata (purple yam) is a specie that has a moisture content of 73.83%. Carbohydrate, 83.33 %. Fat, 0.58%. Crude protein, 7.26%. Crude fibre, 2.29%. Ash, 5.16%.
- Cayenesis (commonly known as white yam) contains moisture content 83.40%. Carbohydrate, 87.82%. Fat, 0.39%. Crude protein, 6.15%. Crude fibre, 2.44%. Ash, 3.20%.
- Rotundata contains moisture content 75.90. Carbohydrate, 90.11%. Fat, 0.34. Crude protein, 4.42%. Crude fibre, 1.45%. Ash, 2.75%.
- Esculenta contains moisture content 81.15%. Carbohydrate, 88.62%. Fat, 0.05%. Crude protein, 7.73%. Crude fibre, 0.95%. Ash, 2.75%.
- Clumetorum contains moisture content 83.29%. Carbohydrate, 83.36%. Fat, 1.21%. Crude protein, 11.73%. Crude fibre, 1.02%. Ash, 2.68%.
The carbohydrate in yam is mainly starch, with only small amount of sugar. The starch is heat stable. When heated, the starch swells and becomes resistant to fragmentation and solubilisation and this accounts for the need to pound very well during pounded yam production. Amylopectin is the main constituent of the carbohydrate, with amylopectin accounting for 10-28% of the starch. Amylose particularly influences the properties of starch. Dioscorea rotundata is the most viscous of all the yam species and this may be the reason most people prefer it for producing Fufu or pounded yam.
Yam tubers are the most outstanding, among other roots and tubers, in terms of the relatively high protein content, which occurs as a mucin or gum that exudes readily from cut surfaces. The proteins are low in sulphur containing amino acids, methionine and cystine.
The vitamin c content of yam tubers is between 6-10mg/ 100g tuber tissue. Traces is vitamins A and B are also present. Calcium, iron and phosphorus are among the components of the mineral fraction of the tuner. Some yam species contain high quantity of polyphenolic compounds which undergo browning via enzymatic oxidation.
Yam is traditionally prepared into boiled yam, roasted yam, fried yam, baked yam and pounded yam. Innovatively, instant pounded yam flour is prepared by washing the yam tubers, peeling, dicing, sulpluting, steaming/ blanching, drying, grinding and packaging.