The egg white coagulates (turns from liquid and solidifies) at a temperature between 144 and 149 degrees Fahrenheit (62 and 65 Celsius). Egg yolk coagulates between 149 and 158 degrees Fahrenheit (65 and 70 Celsius). This difference in coagulation is why you can fry an egg and have the white fully solidify while the yolk remains runny or, for the sake of this chapter, how you can cook a soft-boiled egg so that the white is fully opaque while the yolk is still runny.
The coagulation is referred to as curdling in older recipes. It does describe the chemical reaction within the egg. When eggs are heated, the long chains of amino acids unfold and straighten out, and then as the temperature increases (to the ideal coagulation point), the proteins create stronger, firmer new bonds. The biological terminology for this is denaturing the protein. Denaturing is what happens when you cook an egg: in frying, when it forms a solid mass, and in scrambling, when it turns into soft lumps, or curds.
However, the current scientific definition of curdling is syneresis. (Syneresis is the separation of liquid from a coagulated mass, such as what happens in cheese-making when whey separates from the cheese curds.) In the case of eggs, their protein becomes overcoagulated and “breaks.” When this happens, liquid is forced out of the lumps of protein. Without liquid, the protein binds together more tightly. The result is a lumpy, almost granular texture.
Too high a heat or an overly long cooking time can cause this. When it happens, the cooking term (when one is talking about a sauce, for instance) is that it breaks. A break is considered to be a total and utter failure of the sauce, the custard, the emulsion.
Newer cookbooks and recipes on the Web try to slap on scientific names (used in other cooking science such as cheese-making) that are not a good fit. The point is that the modern definition that associates egg curds with syneresis (instead of denaturing) is confusing. Borrowed descriptive terms are not a good fit for cooked eggs.
Egg Proteins Altered
There are three ways to change the proteins in eggs: heat, beating, and chemical reaction.
1.) Heat causes the proteins to unfold and reconnect. Moderate heat (medium or medium-low) is better than high heat, which causes the bonds to connect too strongly, resulting in a rubbery, tough cooked egg. If you boil an egg on too high a heat, you may see a greenish tinge on the cooked egg yolk if the iron sulfide in the yolk reacts with the hydrogen sulfide in the white (This does not change the quality or the flavor of the egg, but it looks unattractive.)
2.) Beating or whisking egg whites causes the protein bonds to break and reconnect. The new bonds are stronger and cross-linked. Once an egg yolk is whipped into a foam, it will not return to a liquid.
3.) Chemicals that can denature, or break, the protein bonds in eggs include vinegar, lemon juice, cream of tartar, and alcohol. Numerous recipes call for the addition of an acid to egg whites for meringues, soufflés, sponge cakes, and the like, because it lengthens the protein strands and allows for stiffer egg white foam.
Egg whites are easier to alter than egg yolks, because the proteins in egg yolks are more resistant to change.