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. STARCH DIGESTION--COOKED AND UNCOOKED [Sidenote: Comparative digestion of cooked and uncooked grain] The student will remember the reference made in Lesson V to experiments concerning the digestibility of starch when taken in various forms. In these experiments, though conducted for the purpose of demonstrating the supposed advantage of excessive cooking, the results showed that at the time the contents of the stomach were removed, all the proteids of the uncooked grain had been digested, while the percentage of proteid digested from the various forms of cooked grain grew less as the cooking was increased. As the chief function of the gastric juice is the digestion of proteids, the real significance of the above experiments was exactly the opposite from that which was intended to be proved. [Sidenote: Reasons given for cooking starch] The statement is frequently made that the starch of grain cannot be digested without cooking, because the cells enclosing the starch grains have indigestible or insoluble cellulose walls. The old theory is that cooking expands the starch and ruptures or tears down these walls, freeing the contents so that the digestive juices may act upon the enclosed starch granules. This is a theory unsupported by facts. The cell walls on the interior of the grain kernel are very filmy, and in the mature grain scarcely exist at all. The analysis of wheat flour shows only a trace of cellulose fiber. Were these cellulose walls within the wheat grain, as this theory commonly teaches, flour would show a liberal quantity of cellulose. The cellulose wall theory, as a necessity for cooking starch, is an excellent illustration of the ease with which a groundless statement or theory may be used to prove or to explain some popular prejudice.