22 bulleti:n' io41, xj. s. departmes^t of ageiculttjee. 



the percentage of dextrin is considerably higher than the figures 

 indicate. 



These figures are of especial significance when they are correlated 

 with those for the plasticity of samples given in Table 2. The marked 

 increase in dextrin content corresponds exactly in point of its ap- 

 pearance with the softening of the potato. In the ^ancy Hall and 

 Porto Eico this occurs after 10 days' curing of the potatoes, while 

 in the Southern Queen the marked increase is found in the lot canned 

 after 10 days' storage following curing. The marked firmness of 

 the Big-Stem Jersey canned at the end of 20 days of storage likewise 

 coincides with the low dextrin content found. Considering: the fact 

 that both in the determination of the plasticity and in the determina- 

 tion of the dextrin the methods used were crude, the relatively close 

 correlation of the two sets of figures appears significant. 



Samples of the Porto Eico variety canned immediately after dig- 

 ging gave a blue coloration with iodin, characteristic of starch, bat 

 the same variety after 10 days of curing gave a reddish color with 

 iodin. The Big-Stem Jersey, on the other hand, gave a blue colora- 

 tion with iodin at each of the stages of curing and storage. It 

 seems, therefare, that after cooking the soft potatoes contain dextrin 

 instead of starch as the chief polysaccharid, while the firmer ones 

 contain starch largely. It appears, therefore, that the plasticity of 

 the sweet potato after cooking is dependent upon the ratio of starch 

 to water present. During curing and storage, transformations take 

 place which on cooking result in the change of starch to variable 

 proportions of sugar, dextrin, and probably all the intermediate 

 products, depending upon the particular variety of sweet potato 

 used. 



This work suggests that the differences in the cooking quality of 

 varieties of the Irish potato may be due to causes of a similar nature. 



In making sweet-potato flour by the flake process Mangels and 

 Prescott {16) state that there developed hygroscopic and gummy 

 substances which interfered with the success of the process. These 

 workers used the Porto Eico, Nancy Hall, and Southern Queen 

 varieties, and the experiments were made in the late winter. Their 

 results are not surprising, therefore, for their material must certainly 

 have contained a large percentage of dextrin. The chances of suc- 

 cess would have been much better if the firm-fleshed varieties or 

 freshly dug potatoes had been used. 



It is shown by these tests that in some varieties the changes that 

 cause the loss of firmness occur rather quickly after digging. At 

 this point attention should be drawn perhaps to the fact brought 

 out in the work of some of the earlier investigators {12) that changes 

 occurring in sweet potatoes begin to some extent even before digging. 

 It is probable, therefore, that the firmness varies somewhat with 



