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.
Albumin, or the "white " of an egg, is altered physically but not chemically by processes of\cooking. At about 1340 F. delicate fibrillar of coagulated albuminous material begin to stretch through the substance, and they increase with the temperature up to 1600 F. The fibrillae are so numerous that the entire mass is coagulated, but is still of a soft or gelatinous consistence.
It has been observed by Tarchnoff that the coagulum thus formed in the eggs of birds which, like the chick, are hatched with feathers, becomes white and opaque, whereas that of the eggs of birds which, like the plover, are hatched without feathers, is more transparent. If the coagulated albumin is heated still further, it becomes more and more dense, hard, dry, and brittle. When heated beyond the boiling point, or 2120 F., it forms a very tenacious, gluey substance, which can be used as a cement for mending broken china.
Eggs baked in puddings or in any other manner form one of the most insoluble varieties of albumin possible. A raw egg is ordinarily digested in the stomach in one and a half hour, but a baked egg requires from three and a half to four hours.
The principle involved in this account of the cooking of an egg is further illustrated by the process of overcooking beefsteak. When strong heat is too long applied in the process of broiling, the albumin of the meat becomes dried, shrivelled, and comparatively tasteless; and eggs cooked for persons with delicate digestions, instead of being "boiled" in water at 2120 F., should be placed in water at a temperature between 1700 and 1800 F., and immersed for fully ten minutes, at the end of which time they will be found of a uniform gelatinous consistence, very palatable, and not too tough to be readily acted upon by the gastric juice. If a cooking thermometer is not at hand, the water may be previously brought to the boiling point and then set aside, when in a moment or two it will cool to the proper temperature. This should be a little above the coagulation point of the egg albumin (1340 F.), because the process of raising the temperature of the egg is a slow one, and the water loses heat, in warming the egg (Williams). Eggs cooked in this manner are found to have the yolks more firmly coagulated than the white, which remains quite tender.
A practical way of attaining the above result is to pour a quart of recently boiled water over two eggs in a bowl and let them stand for ten or twelve minutes.
Another excellent way to cook an egg, as suggested by Henry, is to immerse a teacup in boiling water until it becomes thoroughly heated. It is then removed and the egg is broken and dropped into it, and the cup may be wrapped in a cloth. Sufficient heat is retained by it to cook the egg without water and remove any raw taste.
The yolk really coagulates at a lower temperature than the white, although as eggs are commonly cooked it does not have an opportunity to coagulate first. The former is composed of albumin and casein with fat (Lehmann). In the ordinary rapid cooking of eggs in boiling water the white is firmly set before there is time for the temperature of the interior of the egg to be thoroughly raised, and consequently the yolk is softer than the white. The shell of the egg facilitates the process of slow cooking of the albumin by protecting the interior and preventing the escape of the contents by solution, just as in the cooking of fish or flesh in water, the latter should be hot enough to immediately form an external coagulum of albumin sufficiently dense to prevent the diffusion of albuminous material and salts into the water.
In the making of omelettes and "scrambled" eggs the white is thoroughly mixed with the yolk, and the egg is more digestible than when fried or cooked so much that the albumin is hard.