Besides the substances already mentioned, there is also a trace of solanin, a poisonous substance which may occur in greater or less amounts and which is said to give the characteristic flavor to the potato. This trace of solanin is supposed to be volatilized during the cooking of the vegetable, and so it is improbable that we ever eat it in any large amounts. If the potato is old and has been allowed to sprout, if it is unripe, or if it has been grown too near the surface and so has a decidedly green color, it may contain sufficient solanin to cause some digestive disturbance. Instances of this, however, are probably very rare. A fear of it makes us careful to cut away the flesh immediately around the sprout in an old potato. Care should also be taken to prevent sprouting, not only for this reason, but because the sprouts use up the food material in the tuber. Potatoes, then, should be stored in a dark, dry, cool place, and should be protected against freezing. A potato that has been frozen has a sweetish taste and is never so mealy as a good potato.
Potatoes are distinguished as mealy, soggy, and waxy. Most people prefer a mealy potato. This quality in the vegetable is supposed to be due to the amount and distribution of the starch. If, however, in cooking, the steam in a potato is allowed to condense to water, the potato becomes soggy. For this reason potatoes should never be allowed to cease boiling while they are cooking; they should be dried out as completely as possible when they are done, and served in an uncovered dish. Baked potatoes should be pricked with a fork or opened at once when they are done. Some potatoes are naturally soggy, but a good potato can be made so by poor handling in its preparation for the table. New potatoes are much more waxy than older ones, owing, perhaps, to the larger amount of protein present.
Potatoes are sold both by measure and by weight, but in many places dealers are now required to sell by weight, because that gives a more uniform amount to the customer. Potatoes should run fifteen pounds to a peck. In selecting, those of medium size and with a smooth skin should be chosen. A large potato is more liable to break up in cooking, and a small one means too much trouble in preparation if it is to be pared.
In preparing potatoes for the table, they should first be washed and then scrubbed with a small brush. If they are to be boiled, they may or may not be pared before cooking. If they are pared and then exposed to the air for any length of time they will turn dark, owing to the action of oxygen, together with a ferment which is found in the potato. This can be prevented by dropping the potatoes into cold water, which excludes the air. Soaking, however, should be avoided, for it removes some of the food material, which means loss of nutriment, and is only permissible if the potato is rather old, wizened, or inferior. In that case, the product is so much improved by the soaking that we are justified, even though some food value is lost. Since the cortical layer contains a higher percentage of both the protein and mineral salts than the rest of the potato, unless paring is carefully done we lose a large part of the most valuable ingredients. If much fruit and salad vegetables are included in the diet, it may not be necessary to consider the loss of mineral salts; but if it is desired to preserve them, the potato should be cooked in its jacket. This means that the potato is not quite so white, but there is no special reason why a perfectly white potato should be demanded. If potatoes are put on in cold water to boil, the same effect as soaking is obtained. Most of the mineral matter and protein, and some of the starch are lost. If, instead, the potatoes are placed in boiling water, the protein is coagulated quickly and less of it escapes. Most of the mineral salts are still dissolved by the water and so lost, since potato water has rather too strong and disagreeable a flavor to be palatable and is usually thrown away. Potatoes may be steamed with little loss of nutriment, or baked, in which case practically nothing is lost but water. Potatoes are cooked partly to hydrate the starch, and partly because the expansion of water into steam means the breaking of the cellulose walls of the cells, whereby the contents become more readily digestible. Probably the chief reason is the improvement of flavor.
Since potatoes contain a small amount of cellulose, compared with most other vegetables, they are digestible, and there is comparatively little difference in their digestibility as a result of different ways of cooking. A mealy potato seems to be more digestible than a soggy or waxy one, probably because it is better broken up, and so the digestive juices can get at it better. Potatoes have long been classed as a starchy food, and most books state that there is so little protein present that it need not be taken into account. Max Rubner, in a recent paper, states that the protein present is of such a character and amount as to form a balanced ration, if it were possible to consume the necessary bulk to supply the needed energy. Potatoes are so bulky, on account of the large amount of water present, that they cannot serve as a sole food.
Sweet potatoes differ botanically from white in that they are thickened roots instead of stems. Chemically, they contain about nine per cent less water, and more carbo-hydrate. Most of this additional carbohydrate is sugar, which accounts for the sweet taste. Sweet potatoes grown in different regions vary greatly in the amount of sugar, those grown in the south containing a larger percentage than those in the north. There is so little difference in food value between sweet and white potatoes that they may be substituted for one another in the diet.
U. S. Dept. of Agriculture. Farmers' Bulletin No. 244. "Cooking Quality of Potatoes." Farmers' Bulletin No. 256. "Preparation of Vegetables for the
Table." Farmers' Bulletin No. 295. "Potatoes and other Root Crops as Food." Office of Exp. Station Bulletin No. 43. "Losses in Cooking Vegetables." "Comparison of the Digestibility of Potatoes and Eggs."
1. Why should not potatoes be tightly covered while boiling?
2. How should they be cared for when done?
3. Why are new potatoes more often cooked in their skins or jackets than old potatoes?
4. When do new potatoes come into market?
5. What is the average cost of potatoes?
6. Is it fairer to sell potatoes by weight or measure? Would a bushel of very large potatoes or of very small potatoes give the purchaser most for his money?
7. How should potatoes be kept to prevent sprouting? What harm does the sprout do the potato?
8. Are old or new potatoes considered more digestible? Why?
9. If you are going to use the potato mashed, what is the advantage of cutting the potato into slices before cooking? What is the disadvantage?
10. Why should potatoes be pared as thinly as possible without too great waste of time? Where do the mineral salts in potatoes lie?
11. If the potatoes you wish to boil together are not all one size, what will you do?
12. Compare the temperature you obtained for boiling water with the temperatures to be obtained at sea level, and on high mountains. Explain the variations.