A STUDY OF SWEET POTATO VARIETIES. 31 
terial, commonly known as " pie stock," is more uniform in color and 
texture and is equally as attractive. It is suitable for making pies, 
puddings, etc., and the firmer varieties may even be sliced and used in 
other ways. The material canned as whole potatoes retains to a 
greater degree the original form and shape of the potatoes, which 
seems to be the only possible advantage to the housewife. For the 
canner, packing whole may have a slight advantage in that the 
method is simpler, but canning as pie stock utilizes the entire crop, 
large potatoes as well as small. 
(4) The principal difficulty in canning sweet potatoes is due to the 
tendency of the cooked potato to darken on exposure to the air. When 
it is cooked in steam so as to exclude the air it assumes a clear bright 
color. On cooling in the air a darkening occurs, which is more 
marked in some varieties than in others. This discoloration disap- 
pears on reheating in steam but reappears on exposure to air. Metal- 
lic iron and iron salts accelerate and intensify this discoloration, and 
when large quantities are present and the material is exposed to the 
air for a considerable time it becomes black and the original bright- 
ness is not regained by reheating. 
In canning sweet potatoes if the air or oxygen is not excluded the 
mass darkens, the metal of the container is acted upon, and the ma- 
terial in time becomes black. If the air or oxygen is excluded the 
material remains bright. Filling the can at a temperature of 80° C. 
or above and sealing at once effect this more easily and completely 
than filling the can cold and then exhausting in the usual way. If 
the can is opened immediately after processing there is a slight ten- 
dency to darken on exposure to the air, but this tendency slowly dis- 
appears in storage. In these tests material kept in cans for one year 
showed no tendency to darken on exposure to the air. 
(5) The physical character of sweet potatoes is such that the 
penetration of heat into the mass is very slow. This so affects the 
length of the processing necessary to sterilize the product that the 
material should be filled into the can hot, sealed at a temperature not 
below 70° C, and processed immediately. If it becomes necessary 
to delay the work, the cans should be kept at the sealing temperature 
until they can be processed. The rate at which heat penetrates the 
can varies but little among the different varieties, and for practical 
purposes the variation is negligible. ^/ 
(6) The sweet potato has a high sug,§Ljrcontent ; hence, its flavor 
and quality are easily injured by long cooking at high temperatures. 
The time and temperature of processing J therefore, must be carefully 
adjusted. / 
(7) The plasticity of the sweet potato after cooking is due to the 
nature of its carbohydrate content. Tfrie potatoes which remain firm 
after cooking contain a high percentage of starch, while those which 
