Changes in Prepared Eggs

Effects of Temperature and Time. The key to cooking eggs is to keep the temperature low and/or the cooking time short. Heating eggs at high temperatures and/or for long periods of time diminishes the eggs’ texture, flavor, and color. Overheated proteins become tough and rubbery and shrink from dehydration, which is why overcooked scrambled eggs look curdled and feel dry and rubbery.

Egg whites and yolks coagulate at different temperatures. Egg whites first start to coagulate at about 140^oF (60^oC) and become completely coagulated at 149^o to 158^oF (65^oC to 70^oC). Slightly warmer temperatures of about 144^o to 158^oF (62^o to 70^oC) are needed for the egg yolks to start coagulating. This difference allows eggs to be cooked so their whites are firm but the yolks remain soft. An egg may be cooked at 142^oF(61^oC) for an hour and still have a soft yolk. Also, beaten eggs coagulate at a slightly higher temperature (about 156^oF/69^oC).

Effects of Added Ingredients. Adding other ingredients to eggs changes their coagulation temperature. For example, incorporating milk into whole eggs in a custard dish increases the coagulation temperature to about 175^oF (79^oC). Sugar also increases coagulation temperature, while the addition of salt and/or acid lowers it. Eggs can curdle if too much of an acid ingredient, such as tomato or vinegar, is added.

Color Changes. Undesirable color changes may occur during egg preparation. Sometimes when eggs are over-cooked or heated at too high a temperature, the sulfur in the egg white may combine with the iron in the yolk, the cooking water, or other iron sources to form ferrous sulfide, a green-colored compound with a strong odor and flavor. To eliminate the problem of “green yolk”, it is best to use stainless steel equipment and low cooking temperatures, to avoid overcooking, to cool hard-cooked eggs quickly in cold water, and to serve them immediately. Another change, which is more difficult to prevent in heated eggs, is the slight browning that results from the Maillard reaction, in which egg proteins react with the few carbohydrates that exist in an egg.