This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
The potato ranks first in importance among the class of tubers which serve man for food, both on account of its easy cultivation in a great variety of soils and on account of its digestibility when properly cooked. As an exclusive article of diet the potatoes too largely composed of starch to be of much nutritive value, and enormous quantities have to be eaten (several pounds a day) in order to supply enough nitrogen for the energy of the body. The potato, however, has less starch than rice, peas, or lentils. It also has less woody fibre than most underground vegetables. In Ireland this vegetable constitutes a greater proportion of the daily food than in almost any other country, and in periods of famine has been known to form four fifths of the entire food for a time, but of late years it has been largely supplemented by the cultivation of Indian corn and other products. The flavour and quality of the potato is influenced very much by the soil and climate in which it grows, a sandy soil being best.
König gives the percentage composition of the potato as water, 75.77; nitrogenous materials, 1.79 (others give 2.10); fat, 1.60; starch, 20.56; cellulose, 0.75-; ash, 0.97. It is thus seen to contain about one fourth solid matter.
Potato juice has a faintly acid reaction, and its vegetable acids are mainly combined with salts of potassium, but also with sodium and calcium. It also contains traces of iron, phosphoric and sulphuric acids, chlorine, silica, and magnesia. Owing to the large proportion - from 12 to 24 per cent - of nearly pure starch which is found in the potato, it is very extensively used in this country and elsewhere for the manufacture of laundry starch. Old potatoes, and those which have been long kept, show some alteration in the quantity of their starch, and a part is converted into sugar and gum.
Potato starch, as compared with other starches, is thoroughly digestible, but much depends upon the cooking. The starchy granules are tough and absorb water from the acid juices which surround them and from water added in cooking, and when properly prepared the potato becomes soft and mealy. When this is not the case, however, it remains hard and soggy, and is thoroughly indigestible.
The following statement in regard to the potato is made in a recent report of the British Commissioners of Prisons:
"Within and surrounding the cells is a fluid or juice the albuminous constituents of which are coagulated during the process of cooking. The watery part of this juice is absorbed by the starch granules, which swell up and distend the cells in which they are contained, so that they no longer adhere together, and the result is the loose flocculent mass which is described as a floury or mealy potato. Unless the potato be properly cooked, the fluid referred to is only partially absorbed, the cells do not become sufficiently distended and separated, and the potato is then described as 'waxy' and 'dense.' In this condition it is not digested, and consequently does not furnish to the system the antiscorbutic principle in which resides its chief value as an article of diet".
When potatoes are cooked in water, it is desirable not to remove their skins, for the latter prevent to a great extent the passage of the salts out into the fluid. The fact that potatoes will not decay if kept dry for a length of time makes them very useful vegetables upon sea voyages, when their antiscorbutic properties are especially serviceable.
Potatoes are more digestible when cooked by baking in their skins than by any other process. They then become mealy and their starch is digested with comparative ease by invalids. They are also quite digestible if steamed, or if boiled and mashed through a colander.
Potatoes must be avoided in all cases of feeble digestion unless they are perfectly mealy and crumble readily, and this quality depends not alone upon the method of cooking, but upon the nature of the potato itself, which varies according to the soil or the season of the year in which it has been grown. Very young potatoes are not mealy. They require more cooking and are less digestible than those of medium age. If too old, on the other hand, potatoes become waxy and equally undesirable. If they have been exposed to frost or have been cultivated in a damp, boggy soil, their digestibility is much impaired.