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close this bookTraditional Storage of Yams and Cassave and its Improvement (GTZ)
close this folder4 Yams
View the document(introduction...)
View the document4.1 The environmental requirements of yams
View the document4.2 The yam tuber
View the document4.3 Farm-economic aspects of yam production
View the document4.4 Yam harvesting
Open this folder and view contents4.5 Causes of storage losses for yams
Open this folder and view contents4.6 Traditional storage systems for fresh yams
Open this folder and view contents4.7 Measures to improve traditional yam storage

4.2 The yam tuber

Economically the most important part of the yam is its tuber This can vary greatly in shape and size and makes manual harvesting very difficult and has so far prevented any kind of mechanisation in harvesting. Cultivated forms of yam mostly produce cylindrical tubers which cam be very heterogeneous in size and weight.

The outer part of the tuber forms several layers of cork. These layers constitute effective protection from lesions, water loss and against the penetration of pathogens in the soil as well as in storage after the harvest The inner part of the tuber is formed by a tissue which is interwoven with vascular channels Carbohydrates, mainly in the form of starches, are stored in this tissue. Apart from the most important constituents of the tuber, water and carbohydrates, this also contains small quantities of proteins, fats and vitamins. As can be seen from Table 6, the tubers of various varieties of yam differ in the relative composition of their constituents.

Table 6: The composition of various species of yam tubers

Variety

Moisture

Carbohydrates

Fats

Crude protein

D. alata

65-73

22-29

0,1-0,3

1,1-2,8

D. rotundata

58 - 80

15 - 23

0,1 - 0,2

1,1 - 2,0

D. esculenta

67-81

17-25

0,1 -0,3

1,3- 1,9

D. bulbifera

63-67

27-33

0,1

1,1-1,5

NB. The figures have been rounded. The results for D. rotundata correspond to those for D. cayenensis which was not included in the table
Source: Coursey, 1967 (modified)

The yam tuber is primarily for vegetative propagation if complete tubers are used for propagation, germs will form in the region of the head. Also segments of the tuber can germinate as long as these include a piece of the outer surface of the tuber. The ability of the tuber to form germs at any point on its surface is made use of by the "Miniset Propagation Method" (INPT, 1988). Using this method, the plants required per hectare can be reduced from approximately two tons to approx. 400 kg