18 



BULLETIN 468, U. S. DEPARTMENT OF AGRICULTURE. 



of the sweeter and more juicy kinds quite generally preferred in the 

 South, as distinguished from the dry, starchy varieties which are 

 preferred in northern markets. 



The edible portion of the sweet potato is not a tuber like the white 

 potato, but a true root. Its internal structure is more uniform than 

 that of the white-potato tuber, but its role in the life history of the 

 plant is much the same, that is, to act as a storehouse of plant food 

 for the growth and early development of a new crop of plants. 

 Above ground the plant is a vine which occasionally produces flowers 

 (and in warm countries seeds) resembling somewhat those of its 

 relatives, the morning-glories. The first touch of frost is fatal to the 

 vines of most varieties, so the cultivation is limited to warm countries 

 where the plants are perennials and the growth is practically con- 

 tinuous, or to regions where the summer is long enough to insure the 

 ripening of a crop. Some of the earlier maturing sorts are exten- 

 sively grown as far north as New Jersey. 



The color of the sweet-potato skin ranges from light tan to dark 

 brown, or red and purplish tones, and the flesh from almost white 

 or pale lemon yellow to a deep reddish orange. The medium and 

 lighter shades are most frequently seen in the northern markets. The 

 weight of the roots also varies considerably, but those which are of 

 medium size and of regular shape are to be preferred for the table. 



Although the bulk of the sweet-potato crop is used for human food, 

 some of the coarser kinds are fed to stock, and a small part is used 

 for the manufacture of starch. 



COMPOSITION AND NUTRITIVE VALUE. 



In general chemical composition the sweet-potato root resembles 

 the tuber of the white potato, although there are important differ- 

 ences between them. The average composition of sweet potatoes 

 raw and cooked is given in Table II, together with similar figures 

 for the white potato. 



Table II. — Average composition of sweet and white potatoes. 



Kind of potato. 



Ref- 

 use. 



Water. 



Pro- 

 tein. 



Fat. 



Carbohydrates. 



Sugar, 



starch, 



etc. 



Crude 

 fiber. 



Ash. 



Fuel 

 value per 

 pound. 



Per ct. 



Sweet potato (edible portion) 



Sweet potato (as purchased) 



Sweet potato (cooked) 



Sweet potato (canned) 



White potato for comparison (edi- 

 ble portion) 



White potato for comparison (as 

 purchased) 



20.0 



20.0 



Per ct. 

 69.0 

 S5.2 

 51.9 

 55.2 



78.3 



62.6 



Per ct. 

 1.8 

 1.4 

 3.0 

 1.9 



2.2 



1.8 



Per ct. 

 0.7 



.6 

 2.1 



.4 



.1 



.1 



Per ct. Per ct. 

 26. 1 1. 3 



21.9 



42.1 

 40.6 



18.0 



14.7 



Per ct. 

 1.1 



1.0 



.8 



Calories. 

 560 

 450 

 905 

 800 



375 



305 



