SCALDING, PRECOOKING, AND CHILLING 29 
In cooking, the outer portion of the cortex separates from the 
deeper lying tissues, and the peel ordinarily may be removed readily 
by hand. The ease of peeling varies somewhat with different varie- 
ties. Usually the soft, moist types peel most readily. 
The length of the treatment for most satisfactory results varies 
with the size of the sweet potatoes. Peeling may be done easily be- 
fore the potatoes are completely cooked ; but, for other reasons, it is 
desirable that the heat penetrate to the center of the potatoes before 
they are removed for peeling. Treatment in flowing steam for 30 to 
35 minutes for sweet potatoes 2 to 3 inches in diameter has been 
found satisfactory. 
LOSSES OF DISSOLVED SUBSTANCES IN PRECOOKING SWEET POTATOES. 
When sweet potatoes are precooked in boiling water there is some 
loss due to substances going into* solution in the water, but this loss 
is too small to be of practical importance. With potatoes 2 to 2J 
inches in diameter when cooked in boiling distilled water for 30 
minutes, there was a loss of 0.33 per cent of the total dry matter. 
Examination of the water did not show the iodine reaction for starch, 
and the tests for sugars showed exceedingly small amounts. 
Gore (SI), in experiments on the amount of substances going into 
solution during scalding and cooking in boiling water, found in the 
water after 3 minutes of cooking 0.17 per cent of the total solids of 
the sweet potatoes; after a second treatment of the same duration, 
0.16 per cent ; and after a third treatment, 0.12 per cent. The water 
in which the same potatoes were boiled for 1 hour and 20 minutes 
longer was found to contain 2.14 per cent of the total solids of the 
potatoes. In the same paper, this worker reported a loss of 0.33 per 
cent of the total solids, when sweet potatoes that had been washed 
and trimmed were steamed for 1 hour and 20 minutes. 
It is seen, therefore, that even when precooked in boiling water, 
the losses from sweet potatoes are small, when the skins are left on. 
RELATION OF PRECOOKING TO QUALITY AND CLOSENESS OF PACK OF SWEET 
POTATOES. 
In a report of studies of the canning quality of different sweet- 
potato varieties, the writers (39) have shown that a close pack is very 
essential. Unless the material is packed very closely in the can, 
darkening and discoloration of the canned product occurs. When 
whole raw potatoes are packed into the cans, only 50 or 60 per cent 
as much material can be put in the can as may be done when the po- 
tatoes are cooked. This means that there is a very considerable 
amount of air in the cans. Under these conditions it is practically 
impossible to prevent discoloration of the product. 
When the raw sweet potatoes are passed through a food grinder, 
and then placed in the cans, underfilling may be avoided, but the ap- 
pearance of the finished product is not attractive. Also, the texture 
is granular. When canned in tin, too great strains are likewise de- 
veloped during the processing. 
Cooking and filling into the cans while still hot gives a closer pack, 
removes most of the air, and thus helps to make a satisfactory 
vacuum — essentials for best quality in canned sweet potatoes. 
During preliminary cooking, a pronounced intensification of the 
color occurs in the sweet potatoes. This is due, in part at least, to 
