The question whether foods should be eaten cooked or uncooked can best be answered by examining the chemical and mechanical changes produced in the process of cooking, and their consequent physiological effects.
Cooking may be divided into two classes, namely, Moist Heat and Dry Heat. To illustrate:
Effect of heat on sugars
Sugars are not chemically affected by boiling with water, while starch, cooked with boiling water, or steam, absorbs from three to five times its bulk of moisture, and changes into a soft, pasty, or semi-dissolved mass. Under dry heat, sugars are converted into a brown substance, known as caramel, while starch cooked under a temperature of 300° to 400° of dry heat, is changed into a dextrin, of which toast and zwieback are examples.
Effect of heat on fats
Fats are not changed chemically by moist heat; that is, by being boiled in water, but the globules are melted and the hot fat spreads in a film over other material which may be present. In dry heat, fats are chemically decomposed, forming irritating vapors. The odors of frying fat are due to the presence of small quantities of these decomposition products. In larger quantities, and with greater heat, these substances are exceedingly irritating to the mucous membrane of the stomach and the intestines.
Effect of heat on proteids
The chemical changes produced by heating proteids are of much more importance than are those which take place in other foods. Simple proteids, such as albumin and globulin, are coagulated at a temperature of about 160°. This change is familiar in the coagulation of egg whites under low temperature. Other proteids undergo similar changes, governed by the degree and kind of heat (dry or moist), to which they are subjected. This change in proteid material continues with the application of prolonged heat, until the proteid, under dry heat, is converted into a dark brittle mass, wholly insoluble and indigestible.
If the student will take the white of an egg, and bake it for some time in an oven, he will observe the coagulation or hardening of the proteid. The chemical nature of this change is one of great complexity. The molecules combine with each other, forming almost indestructible substances. The combined or coagulated forms of proteid are represented in nature by horns, hoofs, finger nails, and hair.