28 



AGRICULTURE HANDBOOK 495, U.S. DEPT. OF AGRICULTURE 



as follows: Southeast Asia, 12 to 

 25 metric tons; West Indies, 20 to 

 30 metric tons. These estimates ap- 

 pear too high. 



In summary, very excellent 

 yields can be obtained when cul- 

 tural conditions are appropriate, 

 and exceptional varieties of yams 

 can outyield most other farina- 

 ceous crops. 



CULINARY 

 CHARACTERISTICS 



Many varieties of the wing- 

 stemmed yam present problems in 

 the kitchen. First, one tuber is 

 usually too large for a single meal, 

 so a part is used and a part saved. 

 Since the cut surface is often 

 dirtied or dries and cracks, using 

 a tuber for several meals increases 

 wastage. Second, many tubers are 

 irregular in shape and are there- 

 fore hard to peel. Third, the bark 

 is sometimes thick and corky and 

 difficult to remove; this character- 

 istic is an advantage, however, for 

 it protects the tuber against 

 wounds. Cleaning and preparing 

 such yams for cooking can be an 

 arduous chore best avoided by use 

 of better varieties. Very few vari- 

 eties of D. (data produce small tu- 

 bers that can be boiled or baked 

 without peeling. On the other hand, 

 many good varieties yield tubers 

 of regular shape that are con- 

 venient to use. These, however, are 

 poorly distributed and not known 

 in much of the yam-growing world. 



When the tuber is cut open, 

 sticky proteinaceous gums exude. 

 The cut surface begins to change 



color with the oxidation of poly- 

 phenolic substances and may 

 become gray or ugly brown. Pro- 

 tection of the surface from oxygen 

 by cellophane or any impermeable 

 membrane decreases oxidation 

 tendency. Polyphenolic oxidation 

 is associated with off-flavors, in- 

 cluding bitterness, and should be 

 avoided. There is some suggestion 

 that spots that oxidize readily are 

 associated with virus infection. 

 The cut surface may sting human 

 flesh, especially tender areas such 

 as the inside of the wrist. This 

 sting, associated with calcium 

 oxalate crystals, usually disap- 

 pears in about 10 minutes. 



The flesh of the yam varies in 

 texture. Coarse texture, which 

 looks a little like soured milk, is 

 caused by the association of vessels 

 in thick bundles. In some varieties 

 the texture is very fine, and such 

 bundles cannot be seen. 



The flesh also varies in color. 

 Yellow, unusual but not unknown 

 in D. alata, is sometimes due to the 

 presence of still unidentified 

 carotenoids. Yellow color is dis- 

 tributed evenly and is usually 

 maintained during cooking. When 

 associated with polyphenolic oxi- 

 dation, however, the resulting 

 brownish gray is very unappetiz- 

 ing. The purple color of antho- 

 cyanin is usually not distributed 

 uniformally through the tissue, 

 and except to the eyes of those who 

 prefer such yams, the appearance 

 is not appealing. The flesh of the 

 best varieties is very white, 

 smooth, free of the appearance of 



