This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Effect of coagulation at low and high temperatures. Eggs cooked in water held at 70°C. are not firm like those that are cooked in boiling water. The white cooked at 70°C. is very soft and jelly-like in consistency. The yolk held at this temperature for an hour or longer has the appearance of an uncooked yolk, but is more viscous, more waxy, thicker, and does not flow like the uncooked yolk. The color also remains more like that of the uncooked yolk, a deeper orange, instead of the yellow which is developed in eggs cooked at higher temperatures. The white of an egg cooked at least 12 minutes in boiling water is rather firm and may be tough. Eggs cooked at temperatures between 70° and 100°C. have textures intermediate between the ones described above, i.e., they are firmer than the former and usually more tender than the latter.
By cooking for a short time at a higher temperature the outer edge of the white may be firm to a depth depending on time of cooking, a portion near the yolk may be unchanged, and the yolk may be unchanged or the outer portion may be slightly cooked.
The softer coagulum at the lower cooking temperature has been explained as due to the effect of low temperature. Robertson suggests that it may be due to only partial coagulation of the protein at the low temperature.
The white of some eggs is often more tender than that of other eggs cooked in the same lot. Perhaps this may be explained by the reaction, those which are more alkaline coagulating less readily. There is also the possibility that the white of eggs cooked an extremely long time may become more tender.
Barmore mixed egg white thoroughly and placed it in a cement briquette mold. The mold was immersed in water of the desired temperature and left 40 minutes. The tensile strength and the depth a steel ball and rod would penetrate were determined. Below 77.5°C. the samples were too tender to handle. The three tests all indicated a decided increase in tenderness as the temperature of coagulation was reduced with the exception of the steel ball at 93° and 101 °C. Here the data indicated that the change may have decreased instead of increasing the toughness. The other tests gave opposite results at these temperatures.
Rate of heat penetration. The higher the temperature of the cooking water, the more rapid is the rate of heat penetration. As the temperature of the egg and water become more nearly equal the rate of heat transference is very much slower, but the temperature at the center of the yolk even after cooking in boiling water for 4 hours or longer is never quite as high as that of the water, but is a fraction of a degree lower.
Time required for cooking eggs in water. Since coagulation takes place at a definite rate, which increases with a rise in temperature, both the length of time the egg is left in the water and the temperature of the water affect the coagulation rate, and thus the time necessary for cooking. At a temperature just a little above the temperature at which coagulation of the white occurs, 60° to 65°C, a very long time is required to coagulate the mass of the egg white. If the cooking temperature is 70° or a little higher, more than an hour is required to coagulate the white and yolk. At a cooking temperature of 85° to 90°C. from 25 to 35 minutes are necessary to have the yolk the same consistency throughout. A shorter time leaves a portion of the egg yolk an orange color, instead of the uniform, powdery yellow. At the temperature of boiling water about 12 minutes are required to complete the cooking of the yolk uniformly. With increase in height above sea level these times are lengthened.
The exact time of cooking at any given temperature depends upon the temperature of the egg when placed in the water, the quantity of water in relation to the size of the egg, and the rate of heating the water.