22 BULLEIIX 1041, U. S. DEPABIMEXT OF AGBICULTDBE. 
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 Xancy Hall and 
Porto Rico 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 Rico 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, therefore, 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 Rico, Xancy 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 
