20 BULLEIIX 1041. U. S. DEPARTMENT OF AGRICULTURE. 
the residue was transferred to the extraction cup and the extraction 
completed in a Soxhlet. The greater part of the extraction, there- 
fore, was done in the cold, and the Soxhlet cup was used merely to 
remove the last traces of sugars. This procedure eliminated as far 
as possible any change during extraction. 
Determination of sugars in the extract was made according to the 
methods of the Association of Official Agricultural Chemists, and the 
total polysaccharids in the residue were also determined by these 
methods. 
It seemed desirable to know something of the nature of the poly- 
saccharid content. In preliminary tests the residues from the firm lots 
gave a blue color with iodin. while the residues from the soft samples 
gave a red color. This seemed to indicate the presence of dextrin in 
the latter samples and suggested that the plasticity of the canned 
sweet potato might depend on whether starch or dextrin was pres- 
ent. The observation appeared so significant that further tests were 
made to determine more definitely the identity of the substance. It 
was found to be precipitated by alcohol : upon hydrolysis it yielded 
reducing sugars : it was not precipitated by basic lead acetate, gave a 
red coloration with iodin. and possessed adhesive properties. It was 
evident, therefore, that during cooking the starch had been gelatinized 
and that insoluble starch, soluble starch, and dextrin possibly ex- 
isted in various proportions in the samples. An effort was then made 
to determine the amount of dextrin present. 
There was no very satisfactory method for the determination of 
dextrin in the presence of soluble starch, but the following method 
was finally adopted for its estimation : 1 gram of the dried residue- 
frorn the extraction was ground thoroughly with 10 to 15 c. c. of dis- 
tilled water. After grinding, more water was added and the whole 
transferred to a 100 c. c. volumetric flask. The volume was brought 
to about 90 c. c. and the solution allowed to stand for 30 minutes with 
frequent shaking. At the end of this time 2 c. c. of a basic lead-acetate 
solution was added to precipitate the soluble starch and the volume 
then made up to 100 c. c. After shaking thoroughly it was passed 
through a dry filter and the rotation of the filtrate taken. The deter- 
mination of dextrin from the polariscope readings was calculated by 
the formula given by Browne (S). It must be understood that the 
figures obtained are but rough approximations of the real dextrin 
content of the samples. For making the polariscope readings and 
assisting in the interpretation of the results the writers are indebted 
to Dr. S. F. Sherwood, of the Office of Sugar-Plant Investigations. 
Table 4 shows the results of the chemical analyses of samples of 
four varieties of sweet potatoes canned at intervals of W days during 
curing and storage. 
