This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The theories for heat coagulation have been considered in Chapter I (The Relation Of Cookery To Colloid Chemistry). But, however the process of coagulation is brought about, the coagulation temperature, the time required for coagulation, and the factors that cause variation in coagulation temperature are of interest in egg cookery, because they determine the temperature to which certain dishes, such as custards and cooked salad dressings, may be heated. If heated beyond this point, separation into solid and liquid may result and curdling occurs, or the salad dressing may become thinner.
Coagulation temperature of egg white. Undiluted egg white coagulates at about 60°C. or becomes jelly-like at this temperature. Coagulation may start at a slightly lower temperature, but the amount coagulated at the lower temperature depends on how long the egg is held at that point. In coagulating, the egg white changes from a clear, transparent mass to a white and opaque one. If the egg white is heated slowly, a point is reached - about 62°C. - at which it will not flow in a test tube. It is still firmer at 64° to 65 °C. In cooking, the temperature is seldom held a long time at a definite degree but may rise gradually and often rapidly.
Coagulation temperature of the egg yolk. The egg yolk requires a higher temperature for coagulation than the egg white. It begins to thicken at about 65 °C. but does not reach a stage at which it does not flow until about 70°. Since the yolk does not change color during coagulation it is more difficult to determine when it is coagulated.
When the white and yolk are mixed by beating and are heated slowly the mass begins to thicken at 65°C. and becomes stirrer not far from 70°.
Factors affecting heat coagulation of egg proteins. The following factors affect the coagulation temperature of the egg proteins: (1) temperature, (2) time, (3) concentration of protein, (4) salt content and its concentration, (5) reaction of the egg solution or mixture, and (6) sugar.
Temperature and time. The rate of coagulation increases with increasing temperature. At high temperatures it is so rapid that it seems nearly instantaneous. Eggs cooked in boiling water will cook at a much faster rate or in a shorter time than those cooked in water at 70°C. Chick and Martin have found that "heat coagulation is a reaction with a high temperature coefficient, the reaction velocity of which varies considerably with different proteins and according to the acidity and saline content of the solution." They have reported that the temperature coefficient for crystalline egg albumin in water solution and 1 per cent concentration is 1.9 times for 1°C. rise in temperature. The temperature coefficient is greater than 2 for the results of Lepeschkin given below. In cooking eggs or foods containing large proportions of egg, the amount of coagulum formed at a definite temperature depends upon the length of time the food is held at the specified temperature and the number of degrees the specified temperature is above the point at which coagulation may begin. Using filtered egg white, Lepeschkin found that, with water at a definite temperature, a longer time was required before the egg appeared turbid at lower temperatures than at higher temperatures. His findings may be summarized:
Seconds required for coagulation
As the temperature of milk custards is elevated the firmness of the custard is increased. At a definite temperature, depending on the rate of heating, an optimum consistency is obtained. Heating to a temperature higher than this increases syneresis and porosity of the custard.
Woodruff and Meyer found that by increasing the temperature from 79° to 91° by 4° intervals the strength, when tested by a gel tester of distilled-water-egg custard gels with 0.2 M and 0.05 M sodium chloride and 0.2 M magnesium chloride plus 23 per cent sugar, was increased. But at the higher temperatures the gels were porous and undesirable. The concentration of egg used was approximately 2 to a cup of liquid.
Concentration. Robertson states, "The concentration of the protein, and especially the presence of other substances in the solution, very markedly affects the coagulation temperature." This is illustrated in egg cookery. The whole egg coagulates at a temperature not far from 70°C. When 1 to 2 tablespoons of milk or water are added, as in the making of plain omelets, coagulation occurs at a temperature above 70°C. When 1 egg is added to a cup of milk the coagulation temperature is much higher than that of egg alone. It is about 80°C, the amount coagulated at a definite temperature depending upon the rate of heating. If 2 eggs are added to a cup of milk the concentration of egg is greater than when 1 is used, and coagulation under the same conditions for heating occurs at a slightly lower temperature.
Salts. The salt content of the egg or of the material with which the egg is combined affects the coagulation of the egg proteins. Lepeschkin has shown that, if egg albumin is dialyzed so that the mineral content is lowered, the albumin does not coagulate on heating. He has also shown that coagulation varies with the salt concentration, that some concentrations cause coagulation and others do not. However, if, to the heated egg albumin that has been dialyzed, salts are added, coagulation takes place, but this often requires a definite concentration for maximum coagulation, higher or lower concentrations not being so effective or failing to bring about coagulation.
The effect of the salt content on coagulation can be shown by combining egg as for custard, but substituting distilled water for the milk, which when heated to 83° to 86°C. does not gel. If to this distilled-water custard a definite concentration of a salt is added, coagulation will occur on heating. Salts which will bring about coagulation are iron lactate, ferric chloride, calcium chloride, sodium chloride, aluminum chloride, aluminum sulfate, sodium sulfate, magnesium sulfate, sodium acetate, potassium tartrate, sodium potassium tartrate, calcium phosphate (monobasic and secondary), sodium and potassium phosphate. However, some produce a firmer coagulum than others, for with each salt a definite concentration brings about optimum coagulation. If the distilled-water custard is heated to 83° - 86° before the salt is added, and the salt is then added, coagulation occurs; but when the mixture is stirred, curdling occurs to a greater or lesser extent. A milk-salt mixture, such as the Sherman or Steenbock formulas used in animal-feeding experiments, in the right concentration forms a coagulum in distilled-water custards that is similar in consistency to that produced by milk custards.