The purpose of cooking foods is threefold: to increase their nutritive value and render them more digestible; to develop their flavors; and to safeguard health by destroying germs and parasites.
Many vegetables are indigestible if uncooked; this is not only because their coarse cellular structure must be softened by cooking, but because most vegetables are made up largely of starch; and starch in its raw state cannot be appropriated by the body. Witness the pain caused by eating green apples, due to the presence of uncooked starch. In the case of the ripe fruit, the starch has been partially cooked by the heat of the sun's rays. Exposure to heat, as in boiling, causes the starch grains to swell and burst the hard outer coverings in which they are enclosed. The starches thus softened and released are converted during the digestive process into sugar, which the body so readily appropriates.
If the starch grains are subjected to a greater degree of heat, as in baking, a greater - a chemical change - takes place which approaches the changes made during the process of digestion. For this reason, toasted and twice-baked breads are easily assimilated.
It follows, therefore, that starchy foods require thorough cooking. This does not mean, however, that all foods should be made very soft, nor that they should be predigested. The digestive system of a healthy person likes to do its own work, and both desires and requires a certain amount of bulk. This much-needed bulk is largely supplied by the cellulose, or fibre-structure of vegetables, which has no nutritive value in itself, but has great mechanical value. In some forms it is not too coarse to be used uncooked, as in lettuce and celery; but as found in potatoes, the cereal grains, and in other combinations with starches, it needs to be softened by the cooking which renders the starch digestible.
Cooking develops and intensifies the flavors which render food more appetizing, and therefore more digestible. This is particularly true of meats. Cooking also serves to destroy germs and parasites; and, in the case of meats, to break down their heavy muscular fibres and tough connective tissues.
Albuminous Foods should be cooked at a low temperature. This refers principally to meats and eggs. Meats belong to the protein group of food, and contain much fibrin which closely resembles albumen. The simplest form of albumen is seen in the white of an egg. Heat hardens albumen, long exposure to intense heat rendering it almost insoluble, and therefore indigestible. For this reason eggs should be cooked at a low temperature; soups and broths likewise to extract the full flavor of the juices. Baked and roasted meats should first be exposed to intense heat, to sear the surface and thus prevent the rich juices from escaping. After a coating is formed, the balance of the cooking should be done at a low temperature.
These two principles, thorough cooking for starchy foods, and low temperature for albuminous foods, are a key to the reasons for the various methods given in the chapters that follow.