The legumes, to which belong peas, beans, and lentils, are usually classed among vegetables; but in composition they differ greatly from all other vegetable foods, being characterized by a very large percentage of the nitrogenous elements, by virtue of which they possess the highest nutritive value. Indeed, when mature, they contain a larger proportion of nitrogenous matter than any other food, either animal or vegetable. In their immature state, they more nearly resemble the vegetables. On account of the excess of nitrogenous elements in their composition, the mature legumes are well adapted to serve as a substitute for animal foods, and for use in association with articles in which starch or other non-nitrogenous elements are predominant; as, for example, beans or lentils with rice, which combinations constitute the staple food of large populations in India.
The nitrogenous matter of legumes is termed legumin, or vegetable casein, and its resemblance to the animal casein of milk is very marked. The Chinese make use of this fact, and manufacture cheese from peas and beans. The legumes were largely used as food by the ancient nations of the East. They were the “pulse” upon which the Hebrew children grew so fair and strong. According to Josephus, legumes also formed the chief diet of the builders of the pyramids. They are particularly valuable as strength producers, and frequently form a considerable portion of the diet of persons in training as athletes, at the present day. Being foods possessed of such high nutritive value, the legumes are deserving of a more extended use than is generally accorded them in this country. In their mature state they are, with the exception of beans, seldom found upon the ordinary bill of fare, and beans are too generally served in a form quite difficult of digestion, being combined with large quantities of fat, or otherwise improperly prepared. Peas and lentils are in some respects superior to beans, being less liable to disagree with persons of weak digestion, and for this reason better suited to form a staple article of diet.
All the legumes are covered with a tough skin, which is in itself indigestible, and which if not broken by the cooking process or by thorough mastication afterward, renders the entire seed liable to pass through the digestive tract undigested, since the digestive fluids cannot act upon the hard skin. Even when the skins are broken, if served with the pulp, much of the nutritive material of the legume is wasted, because it is impossible for the digestive processes to free it from the cellulose material of which the skins are composed. If, then, it be desirable to obtain from the legumes the largest amount of nutriment and in the most digestible form, they must be prepared in some manner so as to reject the skins. Persons unable to use the legumes when cooked in the ordinary way, usually experience no difficulty whatever in digesting them when divested of their skins. The hindrance which even the partially broken skins are to the complete digestion of the legume, is well illustrated by the personal experiments of Prof. Strümpell, a German scientist, who found that of beans boiled with the skins on he was able to digest only 60 per cent of the nitrogenous material they contained. When, however, he reduced the same quantity of beans to a fine powder previous to cooking, he was enabled to digest 91.8 per cent of it.
The fact that the mature legumes are more digestible when prepared in some manner in which the skins are rejected, was doubtless understood in early times, for we find in a recipe of the fourteenth century, directions given “to dry legumes in an oven and remove the skins away before using them.”
The green legumes which are more like a succulent vegetable are easily digested with the skins on, if the hulls are broken before being swallowed. There are also some kinds of beans which, in their mature state, from having thinner skins, are more readily digested, as the Haricot variety.