8 BULLETIN 468, U. S. DEPARTMENT OF AGRICULTURE. 
form of dextrin by the heat, and part absorb water and swell. Pro- 
tein coagulates or hardens, much as the white of egg does when 
cooked, and at least a part of it is broken down into simpler bodies. 
The mineral salts are probably less affected, but some are broken 
down and form new compounds, at least a little sulphur being driven 
off in volatile ;£orm when the potatoes are boiled. As is the case with 
other vegetables, this sulphur no doubt contributes to the odor of 
the potatoes while cooking. It is the sum of these and minor changes 
which accounts for the difference of flavor and texture in a raw and 
a cooked potato. As may be seen from the figures in Table I (p. 5), 
the cooked potatoes, which contain no added fat or other materials, 
do not differ much from the raw as regards composition. The effects 
of cooking on the mechanical condition of the potato cells is shown 
in figure 2, page 3. 
The figures show the great changes in the mechanical condition of 
the potato flesh under the influence of heat, the broken cell walls 
and the increased bulk of the starch grains being particularly notice- 
able. The mealy, soft, porous mass of the cooked potato is in a fa- 
vorable condition for the action of the digestive juices, as moisture 
readily penetrates to all parts of it, while this is not the case with 
the rather tough flesh of the uncooked tuber. It is commonly said 
that raw starch, like that found in the potato, is not digestible, but 
investigations indicate that this is not the case and that the digestive 
juices will dissolve the starch, provided the cell walls are ruptured 
by chewing or in any other way so that the ferments may come in 
contact with it. 
One of the great advantages in cooking potatoes and similar vege- 
table foods is the improvement in flavor. This is due in part to the 
development of the cooked-starch taste, which is much more pleasant 
than that of raw starch. It is also due to changes in the flavor-yield- 
ing bodies. Raw potatoes, especially old ones, often have a decidedly 
bitter and disagreeable flavor (see p. 13), which is less marked after 
cooking, because some of the flavor-yielding bodies are removed. The 
reason why so many housekeepers consider boiling better for old po- 
tatoes than baking is that the boiling extracts more of the disagee- 
able elements. This also explains the common custom of soaking old 
potatoes before cooking and that of throwing away the water in which 
potatoes have been boiled. 
There are also disadvantages in the results of cooking, and, though 
they are less important than the advantages, they influence the food 
value somewhat. There is some reason for believing that the protein 
of the potato is stiffened by heat and becomes less readily digested 
than when raw. A much more serious matter is that considerable 
quantities of the nutrients may be lost during rooking (fig. 5). Sev- 
eral series of experiments have been made by investigators of the De- 
