38 BULLETIN 1265, U. S. DEPARTMENT OF AGRICULTURE 
Certain chemical changes occur during scalding and precooking 
which, with some vegetables, are very important. In most cases, the 
protein is altered somewhat, and the starch partially gelatinized. 
No significant changes occur in the sugars. With the sweet potato, 
however, marked transformations take place, in which much of the 
starch is converted into sugars, dextrin, and intermediate products, 
and the degree and character of these changes are responsible for 
the difference in consistency and in sweetness of the canned product. 
In the preparation of such materials as tomatoes and sweet pota- 
toes, scalding is an efficient method of loosening the skin, so that 
peeling is easily accomplished. In the case of tomatoes, spraying 
with cold water, or chilling promptly after scalding, is advantageous 
in hardening the pulp and thus making the fruit easier to handle. 
During scalding and the subsequent processing, the cells of the 
vegetable tissues tend to separate from each other, and the material 
becomes softened. This is due to the softening and partial dissolv- 
ing of the pectic substances of the middle lamellae which hold the 
cells together. Ammonia and the hydroxides of the alkali metals 
hasten this action, because the pectic materials are more soluble in 
these reagents. The alkaline earths, on the other hand, form insolu- 
ble compounds with the pectic substances, and the tissues are 
hardened rather than rendered more tender. For this reason, hard 
water used either in the scalding bath or in the liquor causes a 
hardening of the tissues and detracts from tenderness. 
When freshly scalded or precooked materials are packed imme- 
diately, the exhaust may be unnecessary. On the other hand, many 
of the advantages of scalding may be afforded by a good exhaust. 
Scalding, or scalding followed by chilling, does not reduce the 
time necessary for successful processing. Scalding assists in cleans- 
ing the raw materials when fresh, clean water is used, but may have 
the opposite effect when the scalding water is used repeatedly. 
From the standpoint of expense of canning, scalding and chilling 
should be avoided wherever possible, and, in choosing a procedure, 
care should be taken to make it simple and economical of time 
and labor. 
Chilling in cold water following scalding has no appreciable 
beneficial effect on the quality, flavor, or appearance of the finished 
product. It results in further losses of food material, makes sub- 
sequent exhaust necessary, and is therefore an expensive operation. 
The work of different investigators indicates that it has no justifica- 
tion from a bacteriological standpoint. The only value of chilling, 
so far as these investigations have indicated, is temporary hardening 
of the pulp of certain fruits and vegetables which must be peeled 
by hand. 
Some form of scalding is desirable in the preparation of most 
vegetables, especially when they are to be canned in tin. Live steam. 
if available, is preferable to boiling water, since it produces all 
the desirable effects of scalding without causing appreciable losses 
of nutrient materials. 
