SCALDING, PRECOOKING, AND CHILLING 33 
varies greatly with the time after digging when the cooking is done. 
This would seem to indicate that there is an increase in amount of 
diastase during storage. No determinations, however, were made 
on this point. 
The residues, after extraction with alcohol, contain various pro- 
portions of starch, dextrin, and intermediate products. In the un- 
cooked Nancy Hall variety the amount of dextrin was found to be 
negligible. When cooked immediately after digging, a dextrin con- 
tent of 0.21 per cent was found, and after storage the cooked potatoes 
showed 6.15 per cent. In the Big-Stem Jersey only small amounts 
of dextrin were found in the cooked potatoes, both immediately 
after digging and after curing and storage. Where a large amount 
of dextrin was found, the potatoes were soft and " moist " ; while in 
the varieties which remained firm after cooking, very little dextrin 
was found, but the polysaccharide was largely starch. 
It is apparent, therefore, that differences in sweetness and firm- 
ness of cooked sweet potatoes are due largely to the changes that 
occur during the cooking process. 
MICROSCOPIC APPEARANCE OF PRECOOKED SWEET POTATOES. 
Examining the cooked material under the microscope shows the 
cells to be more or less distorted in shape, and partly or entirely 
separated from each other. Very few broken or burst cells are seen. 
The heat seems to soften the pectic substances of the middle lamellae, 
or cause them to dissolve, so that the cells which they ordinarily 
hold together are allowed to separate. Differences in the amount 
and rate of this change may account to some extent for the differ- 
ences in the cooking quality of the various kinds of sweet potatoes. 
CONCLUSIONS REGARDING THE PRECOOKING OF SWEET POTATOES. 
Precooking of sweet potatoes, for a longer or shorter time, m 
either live steam or boiling water, is of great advantage when they 
are to be canned. It facilitates peeling and proper packing. By 
packing hot, the air is more completely eliminated from the can 
than can be accomplished in the ordinary exhaust, and discoloration 
of the product is reduced to a minimum. 
During the precooking, very significant chemical changes occur 
in the carbohydrates of the sweet potatoes. A large amount of 
starch is converted into sugar, dextrin, and intermediate products. 
The amount of the change and the character of the constituents 
formed vary with the conditions of storage, and with the variety 
of potatoes. 
In those cases where a very soft product is obtained, the starch is 
largely changed to sugar and dextrin: while with the varieties that 
yield a firm product after cooking, little dextrin is formed, and 
considerable amounts of starch remain unhydrolyzed. 
GENERAL DISCUSSION. 
In the presentation of the matter in hand, it is thoroughly 
recognized that considerations of very great importance, in con- 
nection with studies upon scalding and precooking, have not been 
