14 BULLETII^ 1041, U. S. DEPAETMEIs^T OF AGKICULTUEE. 



water, potassium hydroxid, and sodium hydroxid all give a yellowish 

 coloration at first, which on oxidation turns brown. 



If the sweet potato is cooked in steam in such a way as to eliminate 

 the possibility of the introduction of iron, the discoloration on ex- 

 posure to air is small in extent. If to some of the material cooked 

 in this way there is added ferric chlorid, a greenish coloration is 

 obtained. If a quantity is mixed with iron filings and exposed to 

 the air the whole mass soon turns black. When a quantity is treated 

 in the same way with tin or zinc no effect is noted. 



Certain substances have been extracted from the sweet potato which 

 give reactions very similar to those above described. One of the 

 chief substances is soluble in acetone, glacial acetic acid, 70 per cent 

 ethyl alcohol, and in water. These substances appear to be hydroxy 

 compounds belonging to the aromatic series. 



The different varieties of sweet potatoes show considerable varia- 

 tion in their tendency to discolor. The Jersej^ group, including the 

 Gold Skin, Big-Stem Jersey, and Early Red Carolina, show it the 

 least of those tested, and members of the Spanish group, including 

 the Triumph and the deeply pigmented varieties, such as the Purple 

 " Yam," Japanese " Yam," and Dahomey, show it the most. It 

 would seem that there might be some correlation between this pig- 

 ment and the discoloration. All the varieties and strains here tested . 

 have shown these phenomena to a greater or less extent. In the light- 

 fleshed individuals the discoloration is more apparent than in the 

 more deeply colored varieties, though this may be due to the partial 

 masking of it by the deep -yellow color. 



If in packing sweet potatoes the cans are sealed without exhaust- 

 ing — that is, if air is left in the can — the product will darken. After 

 some time in storage the metal of the container becomes badly cor- 

 roded and the potato contained in it turns black. This darkening 

 begins at the top of the can ; that is, the portion exposed to the air 

 in the can turns brown and those portions exposed to both the air 

 and the metal of the can turn black. 



However, as the oxj^gen and the iron become diffused into the 

 material the whole becomes black. Those portions in actual contact 

 with the metal of the cans, if the air is excluded, remain bright 

 throughout. These findings are entirely contradictory to the report 

 of Kohman (14), in which it is stated that the darkening begins at 

 the bottom of the can where the material is in direct contact with 

 the metal of the container. If the can is filled quite full with the 

 potatoes at a temperature of 80° C. or above, sealed immediately, 

 and processed, very little action upon the metal of the container is 

 apparent and the material remains bright. The writers have kept 

 cans of sweet potatoes handled in this way for three years under 

 ordinary storage conditions with no discoloration taking place. 



