The value of food for nutriment depends not only on the amount of nutrients it contains, but also on the amount of these the body can digest and use for its support.
Cooking changes the texture of food, making it in some cases more, and in others less, digestible; hence we should first ascertain whether cooking will improve the flavor and digestibility of the article of food. In general, the digestibility of animal foods is diminished by cooking, and that of vegetables increased, though there are exceptions. Vegetable foods are more difficult of digestion than animal foods, and their preparation is more complicated and thorough. The nutritive substances are inclosed in cells often with thick walls, and hence are not readily acted upon by the digestive fluids. When vegetables are boiled in water, the contents of the cells expand and burst through these walls. The fragrant and savory substances are set free with the other substances which were imprisoned in the cells, and their astringency and bitterness are tempered. Some of the constituents are dissolved by water, or suffer other changes. Starch, an important ingredient in many vegetable foods, such as potato, wheat, rice, etc., takes up water and assumes the soft pasty condition which is necessary for its transformation into soluble dextrine and sugar, which shows that the cooking of starchy foods is necessary.
The boiling of vegetables may, indeed, be termed a preparatory digestive process. Cells of other plants behave like the starch-bearing potato. The seeds of legumes, such as beans, peas, and lentils, are, in their natural state, difficult of digestion, because their starch granules lie closely packed within the indigestible cell walls. On boiling, the starch swells, the cells burst, and their contents are changed into a pulpy mass, - a very nutritious and digestible dish.
References: Art of Cookery - Ewing - p. 33; Elements of Cookery - Williams & Fisher - pp. 52, 53; The Chemistry of Cookery - Mattieu Williams - pp. 8-12.