84. General Composition of Legumes.--Peas, beans, lentils, and peanuts
are the legumes most generally used for human food. As a class, they are
characterized by high protein content and a comparatively low per cent
of starch and carbohydrates. They contain the largest amount of
nitrogenous compounds of any of the vegetable foods, and hence are
particularly valuable in the human ration as a substitute for meats.[32]
For feeding animals the legumes are highly prized, particularly the
forage crops, clover and alfalfa. These secure their nitrogen, which is
the characteristic element of protein, from the free nitrogen of the
air, through the workings of bacterial organisms found in the nodules on
the roots of the plants. The legumes appear to be the only plants
capable of making use of the nitrogen of the air for food purposes.

85. Beans contain about 24 per cent of protein and but little fat,
less than is found in any of the grain or cereal products. The protein
of the bean differs from that of cereals in its general and structural
composition. It is a globulin known as legumin, and is acted upon
mainly by ferments working in alkaline solutions, as in the lower part
of the digestive tract. Beans have about the same amount of ash as the
cereals, but the ash is richer in potash and lime.

[Illustration: FIG. 19.--GRAPHIC COMPOSITION OF BEANS.
HACKED PART INDIGESTIBLE.]

86. Digestibility of Beans.--Beans are usually considered
indigestible, but experiments show they are quite completely digested,
although they require more work on the part of the digestive tract than
many other foods. The digestibility was found to vary with individuals,
86 per cent of the protein being digested in one case, and only 72 per
cent in another. The protein of beans is not as completely digested as
that of meats. When beans were combined with other foods, forming a part
of a ration, they were more completely digested than when used in large
amounts and with only a few other foods. The presence of the skin is in
part responsible for low digestibility. When in the preparation of beans
the skins, which contain a large amount of cellulose, are removed, the
beans are more completely digested. By cooking from twenty minutes to
half an hour in rapidly boiling water containing a small amount of soda,
the skins are softened and loosened and are then easily removed by
rubbing in cold water. Some of the soda enters into combination with the
legumin. Along