20 BULLETIlSr 1041, U. S. DEPABTMENT 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 apj^eared 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 gelatinizedi 

 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 

 from 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 (<?). 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 10 days during 

 curing and storage. 



