We usually speak of softening the cellulose by means of cooking. Apparently what we really do is to dissolve the intercellular substances that bind the walls together, and thus make it possible for the cell walls to be mechanically ruptured, either in the process of cookery or by the pressure exerted in the mouth. Part at least of this intercellular substance belongs to the pectin group that causes the jelling of fruit juices.
SWELLING OF THE STARCH.
The first process in rendering the starch of the vegetable digestible is one of hydration. It is important, therefore, that an abundance of water be present when starch is cooked. Some vegetables like the potato contain so much water that the necessary amount for the starch is supplied within the vegetable itself. The grains and other dry vegetables need to have a large amount supplied. The swelling of the starch grains upon hydration is probably an important agent in the rupturing of the cellulose cell wall already referred to.
THE CELL WALLS RUPTURED.
Sugar is the soluble carbohydrate of the vegetable, as starch is the insoluble form in which this nutriment is stored. Some vegetables, such as carrots, show large amounts of sugar, while starch is absent from this part of the plant. Other typical vegetables containing a large amount of sugar are beets, parsnips, artichokes, sweet potato. Onions, cabbage, and some varieties of peas, string beans, squash and sweet corn all contain a fair amount.
Vegetables containing a large amount of starch are represented by potato, sweet potato, rice, peas, beans and lentils. Some vegetables containing a large amount of cellulose are squash, potato, beet, celery, cabbage.
COMPOSITION OF THE CARROT AND TURNIP.
As a rule, we do not look to the vegetable world for our main supply of proteid, yet some of our vegetables, notably the legumes, do contain an abundant supply of this food principle. Whether this is as available for use in the body as the proteid in meat is often questioned. With ordinary cooking processes it evidently is not, but with long continued heat the matter is different. That there is no inherent difference between vegetable and animal proterd, so far as its digestibility is concerned, would seem to be indicated by the fact that when the vegetable is finely divided, as in the case of some of the vegetable meals, it is absorbed to a greater extent than in its ordinary form. It is said, for instance, that when lentils are soaked and boiled until soft, 60 per cent of their proteid is absorbed, while in the lentil meal 90 per cent is utilized by the body. No care-ful experiments have been made to see what proportion of the boiled lentils would have been absorbed if the cooking had been continued for several hours. There is every reason, however, to think that the percentage would be increased. Anyone who has compared dry peas or beans cooked two hours, or until they have just become soft, with those cooked from eight to twelve hours will realize the difference in the result.
COMPOSITION OF THE CABBAGE. Blackened portions represent amount dissolved in cooking.
COMPOSITION OF THE POTATO.