The roots and tubers used for food comprise the potato, turnip, carrot, parsnip, artichoke, onion, with the different plants yielding arrowroot, tapioca, and sago. The main nourishment of these plants is stored up almost entirely in the form of carbohydrates, chiefly starch. They contain very little protein and fat, and are therefore vastly inferior in nutritive value to the cereals and pulses. Some of the members of this group contain vegetable acid chiefly combined with potash, and these salts give them their recognised antiscorbutic properties. A considerable amount of the salts, however, is usually lost in the process of cooking.
The potato is the most important member of this group. About 95 per cent, of the potato consists of water and starch, the rest of the solids being made of fibre, sugar, vegetable jelly, and pectin. It is very poor in nitrogen, and contains practically no fat. The richness of the potato in starch - about 20 per cent. - is its most marked characteristic, starch being largely extracted from potatoes for commercial purposes. Of the total amount of nitrogen in potatoes only one-half is in the form of protein, the remaining half being in the form of ammonia compounds, e.g. asparagin, which are not available for food. Owing to this poverty in protein and fat, potatoes are not suitable as an exclusive food, but they are a valuable adjunct to other foods rich in nitrogen. In the process of cooking the albuminous juices are coagulated, the starch granules swell up and absorb the watery juices, and the potash assumes a mealy appearance. In this form it is easily digested. If the changes described do not take place, as happens with young potatoes, the potato has a more solid and waxy consistence, and in this form is much less digestible. As ordinarily cooked, some of the salts are lost, and to avoid this potatoes should be cooked in their jackets, or else steamed. The relatively large amount of alkaline salts in potatoes makes them specially useful in the treatment of scurvy.
The sweet potato is not used as an article of diet in this country, but is largely used in France, Spain, and the United States. It contains about 16 per cent, of starch, and 10 per cent, of sugar. It is too sweet to eat with meat as a vegetable. It is a most wholesome article of diet.
The yam is a tuber of a tropical climbing plant. It contains much starch. When cooked it becomes mealy and like the potato in flavour.
The nutritive value of the turnip is very small. It contains about 91 per cent, of water, and 5 to 6 per cent, of carbohydrates, chiefly in the form of pectin bodies, and about 1 per cent, of nitrogenous substances, most of this being in non-albuminoid form. Turnips have a marked tendency to cause flatulence.
Carrots are more nutritious than turnips, having about 8 to 10 per cent, of carbohydrates, chiefly in the form of sugar. They are not readily digested nor easily absorbed.
Parsnips closely resemble carrots in composition. They are rich in starch and sugar, the latter being largely lost in the process of cooking.
Jerusalem artichoke is a tuber. Like turnips, it contains no starch, but is rich in carbohydrates, chiefly a variety of sugar belonging to the gummy series. It has a sweet taste, and remains watery after cooking. It is of no great importance as a food, but is of value for making good soups and sauces.
Onions are of value both as flavouring agents and as vegetables. They impart a strong typical odour to the breath, clue to the presence of a pungent oil. The large Spanish onions are used as food. They have a moderate laxative action, due to their richness in cellulose.
Arrowroot is derived from the tuber of a West Indian plant (Maranta arundinaccd). The roots are dried and pulverised to a fine starchy flour, which consists of practically pure starch, with 15 to 20 per cent, of water. Its quality is judged by its whiteness, and by the ease from which there is made from it a firm, transparent, pleasant-tasting jelly, which remains firm and of good taste for three to four days.
It is easily digested and very completely absorbed, and is therefore of special value in the treatment of diarrhoea and some gastro-intestinal derangements. In cases where there is difficulty in digesting starches or sugars, arrowroot is an unsuitable food. It is also used to thicken clear meat soups and extracts.
Tapioca is also a pure starch obtained from the tuber of Manihot utilissima, cultivated in South America. Its grains are small. It contains over 85 per cent, of starch. It is not quite so easily digested and absorbed as arrowroot. It is useful for adding to soups, and for making into pudding with milk.
Sago is another starch obtained from the pith of the stems of several species of palm. In its composition and uses it is similar to tapioca.