The composition of potatoes (Letherby) is as follows:
| Water | 75.00% | |
| Starch | 18.80% | |
| Nitrogenous matter | 2.00% | |
| Sugar | 3.00% | |
| Fat | .20% | |
| Salts | 1.00% | [36] |
From this we see that starch is the principal nutrient, therefore potatoes in use for food should be associated with nitrogenous substances, such as eggs, meat, fish, and milk. The potash salts which potatoes contain are very valuable. According to Letherby, an average of thirty-one analyses of the ash of potatoes gave 59.8 per cent. of potash, 19.1 per cent. of phosphoric acid, the other ingredients being in exceedingly small proportions. These salts are necessary to a healthy condition of the blood. Potatoes are a valuable antiscorbutic.
According to Mattieu Williams, scurvy prevailed in Norway to a very serious extent until the introduction of the potato; and Lang, with other good authorities, testifies that its disappearance is due to the use of potatoes by a people who formerly were insufficiently supplied with salts-giving vegetable food.
The salts of the potato are most abundant in or near the skin, and the decision of the question as to whether potatoes shall be pared or not before cooking is somewhat aided by this fact. For persons who eat but few other fresh vegetables by all means leave the skins on, but for those who have access to a good kitchen garden and have plenty of other vegetables and fruits from which to get their salts, it makes no important difference whether the skins are removed.
The potato is eminently a starch food, and this knowledge indicates the method of treatment in cooking. Since starch is its principal ingredient (the amount of nitrogenous matter being very small), if it is cooked with reference to that alone, it will be done in the best possible manner.
Starch, in order to be rendered most digestible and acceptable to the human system, must be subjected to a high temperature in the presence of some liquid. At 401° Fahr. (see [pages 33] and [34]) it is converted into dextrine. This change, if not performed outside the body, will be done in the ordinary processes of digestion after the starch is eaten; therefore the nearer we approach to it in cooking, the more perfectly is the food prepared which contains it.
Usually the first vegetable prescribed by the physician for a sick person who is beginning to use solids, is a baked potato. A baked potato, however, may be no better than a boiled potato unless it is cooked in so high a temperature that the starch is affected. Boiled potatoes cannot be subjected to a higher temperature than 212° Fahr. Baked potatoes may be done in such a way that they are but little better than boiled—for instance, done in a slow oven. On the other hand, if they are put into a temperature of 380° or 400° Fahr., or a hot oven, they will be done in such a manner that the conversion of starch will in a degree take place, and they will be consequently both palatable and easily digested.
Potatoes roasted in hot ashes or embers are delicious, and for the same reason. But it must not be understood that by cooking potatoes in a high temperature the starch which they contain is all changed into dextrine. This does not usually take place except in slight degree, but by the high temperature it is better prepared for this change in the processes of digestion. Probably what does take place is a sort of hydration of the starch, resulting in the complete swelling and final bursting of the granules, with possibly an intermediate change between this and dextrine. Just at the moment when potatoes are done they should be immediately taken from the fire and served at once. The potato is capable of being made into a variety of dishes, and when properly prepared has a delicate flavor which is very acceptable to most people. It is one of the most easily digested forms of starch-containing food.