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



among varieties. Probably none 

 approximates the viscosity of the 

 starches of the African species D. 

 rotundata. The starch gel is 

 opaque. 



The properties of D. alata 

 starches need more study in refer- 

 ence to industrial use. Sufficient 

 differences have been found among 

 varieties to suggest that selection 

 for particular starch types might 

 be possible. Yam starches would 

 probably be useful in a number of 

 industrial processes, but their cost 

 is high compared to the cost of 

 corn and cassava starches. 



D. alata tubers contain surpris- 

 ingly large amounts of protein 

 About 85 to 95 percent of 

 the nitrogen can be recovered as 

 amino acids. Varieties that contain 

 only 6 percent protein, a low fig- 

 ure for yams, thus contain three to 

 four times more protein than their 

 rival staple, cassava. Moreover, 

 many varieties contain 8 percent 



Table 4. 



protein and as much as 15 percent 

 has been recorded. 



The protein of D. alata is not 

 well balanced (table 4) . However, 

 usually only the sulfur-bearing 

 amino acids methionine and 

 cystine are particularly short. The 

 percentage of these desirable 

 amino acids has not been carefully 

 estimated but varies from 20 to 50. 

 It should be possible to select vari- 

 eties with fairly balanced protein, 

 for strong varietal differences 

 exist, not only in protein content 

 but also in sulfur-bearing amino 

 acid content (table 5) . In the ab- 

 sence of selected varieties, pro- 

 teins high in methionine should 

 supplement staple yam diets. 

 Lysine is often present in greater- 

 than-normal quantities in D. alata 

 tubers. 



The red and purple pigments of 

 D. alata are anthocyanins of 

 cyanidin. Purple forms occur in 

 the parenchyma of the tuber tissue. 



— Essential amino acid content of D. alata cultivars compared 

 with FAO reference protein 



[Grams amino acid per 100 grams protein] 



* . FAO Cultivar 



Ammo ^ 



_ J reference . , , 'White , , .t^ i , , 'Smooth 



^''^ proteini '^l^^^^o' Lisbon' 'Forastero' 'Barbados' g,^,.^, 



Valine 4.2 4.9 4.8 5.2 4.7 4.8 



Isoleucine 4.2 4.2 4.2 4.7 4.0 4.1 



Leucine 4.8 7.7 7.8 7.4 8.1 8.1 



Threonine 2.8 4.2 3.9 7.1 3.9 4.2 



Methionine 2.2 1.6 1.5 1.6 1.5 1.7 



Cystine 2.0 .4 .4 .2 .4 .3 



Phenylalanine •• 2.8 5.8 5.8 5.9 6.1 6.1 



Tyrosine 2.8 2.8 2.7 2.2 3.1 3.0 



Lysine 4.2 5.1 5.0 4.8 5.0 5.4 



1 FAO Nutritional Studies 24 {12). 



