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 object of cooking vegetables, as in the case of cooking meat, is to render them more digestible, to give variety, to modify their flavour, and in some cases to preserve them.
Some coarse vegetables, such as turnips, carrots, beets, and potatoes, while they make good raw food for animals, are unpalatable and indigestible for man, and require softening and alteration by prolonged boiling in soft water or by some other form of cooking.
The cooking of vegetables macerates the cellular fibres or walls of the cells and softens their contents, rendering the vegetable much more easy of mastication, while the effect of the heat and moisture is to cause swelling and rupture of the starch granules, in which condition they are more easily and promptly acted upon by the starch-digesting ferments contained in the saliva and pancreatic juice.
Many vegetables contain albuminous substances, besides sugars, gums, and starches. The albuminous material is coagulated by appropriate temperature and the other matters are more or less soluble in water.
Most vegetables contain a very large percentage of water in their natural state, but in many of the cooking processes to which they are subjected, excepting perhaps in baking potatoes and the like, water is always added in considerable quantity, either hot or cold.