The Science of Cooking an Egg

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.

The Yolk

Egg yolks are colored by xanthophylls, a yellow-orange pigment in green plants, yellow corn, and bugs. Yolk color is influenced by feed, exercise, and the lifestyle of the chicken.  Yolk color can be influenced by feed alone. In fact, a chicken running around in a yard, eating whatever it finds, is going to have varying yolk color intensity, depending on what is in season. Alfalfa creates a very light yellow yolk, whereas yellow corn can give a deeper yellow. More-intense-colored yolks are the result of feed with a heavy dosing of annatto or ample greens such as clover or kale; rye pasture; weeds such as mustard, pennycress, and shepherd’s purse; or feed that is high in beta-carotene vegetables such as carrots and beets. A quantity of red fruits can intensify the red-orange color of the yolk.

Exercise, pecking order, and bug eating contribute to the natural deep orange and/or red tones of a naturally colored yolk.

In the “modern world” we mess with mother nature.

Poultry raisers have long discussed influencing yolk color with various feed combinations to please their consumers. In 1919 a popular paper entitled “The influence of specific feeds and certain pigments on the color of the egg yolk and body fat of fowls,” by Leroy S. Palmer and Harry L. Kempster[1], was widely read. What the authors found was that “yellow corn is the best winter food for keeping up the coloring of adipose tissue during fattening” and that it was also what kept the egg yolks a nice sunny yellow color. Not too dark, not too light. People liked to purchase dressed chickens with a deep yellow skin color, and corn filled the bill. Corn, along with annatto (a derivative of the achiote tree, of tropical regions of the Americas, used in food dyes), is used heavily—to this day—in chicken feed to give a faux “healthy” yellow glow that normally could be found only in chickens raised in sunshiny fields.

A side note: Annatto has been linked with many cases of food-related allergies and is the only natural food coloring believed to cause as many allergic-type reactions as artificial food coloring. However, because it is not one of the “Big Eight” allergens (cow’s milk, eggs, peanuts, tree nuts, fish, shellfish, soy, and wheat) responsible for more than 90 percent of allergic reactions to food, its use is not regulated nor is a consumer able to avoid it easily (it’s rarely listed in labeling as anything other than “natural coloring”). Many people who consider themselves allergic to chicken eggs may, in fact, be allergic to the annatto feed additives.

Artificial egg coloring is still a very hot topic in poultry farming. Articles appear frequently in trade and scientific journals regarding the use of artificial and natural coloring agents in feed to intensify the yolk color of eggs and the skin color of chickens. One such article, “Formulation of Annatto Feed Concentrate for Layers and the Evaluation of Egg Yolk Color Preference of Consumers,”[1] in the Journal of Food Biochemistry (January 13, 2010), lays out the trickery that is rampant in the poultry industry:

Visual appearance, especially color, is one of the most important characteristics of foods and determines the acceptance or rejection of the product by the consumer. This statement is also true for poultry products, in which the color of skin, meat and egg yolk plays a fundamental role to some ethnic and regional consumers (Chichester, 1981; Hencken, 1992; Williams, 1992; Macdougall, 1994). The preference for well-pigmented poultry products is still evident in some markets, and thus, poultry producers add colorants to broiler and layer diets as a means of improving the attractiveness of these products (Klaui and Bauernfeind 1981; Hencken 1992; Liufa et al., 1997).

The interesting thing about the authors’ analysis is their assertion that the average consumer in the United States prefers a yolk that is a lighter yellow than what European consumers favor. This is presumably because those consumers have never eaten an egg from a chicken that pecks, scratches, chases bugs, and eats greens and weeds in a natural setting. The flavor and quality of eggs from a backyard chicken are vastly superior to what you get with commercially laid eggs.

[1] I. Ofosu, E. Appiah-Nkansah, L. Owusu, F. Apea-Bah, I. Oduro, I., and W. Ellis, “Formulation Of Annatto Feed Concentrate for Layers and the Evaluation of Egg Yolk Color Preference of Consumers,” 2010. Journal of Food Biochemistry, 34: 66–77. doi: 10.1111/j.1745-4514.2009.00264.x

[1] The Dairy Chemistry Laboratory and Department of Poultry Husbandry, University of Missouri, Columbia)