A STUDY OF SWEET POTATO VAFdETIES. 13 



Home Economics, and the Office of Horticultural and Pomological 

 Investigations of the Department of Agriculture. The results of 

 this exhibit were entirely similar to those of 1919. The Gold Skin 

 was again unanimously awarded first place, and the Porto Kico, 

 Nancy Hall, and Vineless Pumpkin " Yam " of the moist-fleshed 

 group and the Big-Stem Jersey, Improved Big Stem, Yellow Stras- 

 burg, and Triumph of the firmer fleshed types, in about the order 

 given, received favorable comment. It is almost certain that dif- 

 ferent conditions as regards time of digging, curing, and storage 

 would have altered the results somewhat, but it is of interest to note 

 that out of the first dozen selected as best varieties for canning six 

 w^ere selected both seasons. 



Differences in the quality of the whole-potato product as com- 

 pared with the pie stock were too small to be significant. This ex- 

 hibit demonstrated again that a highly desirable canned product of 

 either the dr}^ firm tj^pe or the moist -type may be secured even in 

 the cured potatoes by the selection of the proper varieties. 



DISCOLORATION. 



The greatest difficulty encountered in the canning of sweet pota- 

 toes is to overcome the tendency of the product to discolor or darken 

 •when exposed to the air. When the raw potatoes are peeled by hand 

 they turn brownish or dark-colored irregularly OA^er the surface. 

 This discoloration is much more pronounced in the region of the 

 cortex, but it is apparent to a lesser extent throughout the potato. 

 When the potatoes are cooked and then exposed to the air they 

 oxidize somewhat and become darker. When exposed to the air for 

 a few hours and then reheated in the absence of oxygen, this dis- 

 coloration almost entirely disappears, but it promptly reappears on 

 exposure to the air. There is an oxidase present in the sweet potato 

 which would explain its behavior in the raw state, but this enzym 

 does not account for the discoloration after cooking, since the dark- 

 ening takes place even after the exposure of the potato to a tempera- 

 ture of 116° C. for one hour in the autoclave. Oxygen appears to be 

 necessary, for this darkening does not occur in cans of sweet potatoes 

 which have been properly exhausted. 



The substance which is first formed in this discoloration seems to 

 be very unstable. It is destroyed or changed on reheating in steam, 

 but forms again in air. 



Iron or iron salts have a very marked effect upon the discoloration, 

 causing an intensification of it and rendering it very much more per- 

 manent. Acids tend to inhibit it and alkalis to intensify it. If sec- 

 tions of raw sweet potato are placed in ammonia a yellowish color 

 at first appears, which on standing becomes green. This occurs first 

 in the cortex and may appear throughout the entire section. Lime 



