14 BTLLEIIX 1041. U. S. DEPABTMEXT OF AGRICULTURE. 
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 Jersey group, including the 
Gold Skin, Big-Stem Jersey, and Early Bed 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 oxygen 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. 
