Cooking eggs has been described as a culinary science. Cook the egg too long it becomes rubbery, if you don't cook it enough the egg has a texture similar to mucous. The structure of eggs changes when heated, beaten or emulsified with other ingredients. Proteins are responsible for the changes to an egg during cooking. The egg consists of a long chain of amino acids. When the egg cooks, the protein undergoes molecular changes. A number of variables affect the changes that occur in an egg during cooking. Among them include altitude, protein bonding temperature, bonding temperature of the egg yolk, how old the eggs are and the size of the pot in which they are cooked.
Temperature and Egg Whites
The temperature is what causes the proteins to bond, making the liquid proteins harden. The egg white changes from a cloudy, thick material to a white solid substance. Between 30 and 140 degrees, the proteins in the egg white begin to unfurl. The proteins bond above 140 degrees and solidify at 155 degrees although they remain moist. At 180 degrees the proteins bond firmly resulting in a firm, opaque egg white. If the egg is cooked slowly at a low temperature, the white will stay moist and not produce a rotten egg smell.
Egg Yolk Temperature
The egg yolk consists of fatty acids, cholesterol and some protein. It responds differently to heat than the egg whites, composed mostly of protein. Below 62.8 degrees C, the egg yolk will not experience changes. An egg cooked at this temperature will have a solid white but the liquid yolk will remain running. The yolk when cooked to 70 degrees C will turn into a firm liquid but have a bright orange hue. Cooked over 170 degrees, the yolk turns a pale yellow and will crumble. Above that temperature the yolk has a chalky texture and will release ferrous sulphide, which produces a rotten smell.
When cooking eggs, overcooking causes unpleasant culinary results in the egg's structure. If the egg is cooked to above 180 degrees, a hydrogen sulphide smell releases from the egg, which produces the unpleasant rotten egg aroma. The yolks turn a pale yellow colour and a green band forms around the yolk. When the egg becomes heated too rapidly in boiling water, the egg's shell cracks and the yolk and white leak out into the cooking water. Above 82.2 degrees C, the egg white becomes rubbery and tough.
Many baking recipes call for eggs. In a cake, the proteins in eggs help to hold a cake together. Gluten in the flour needs protein for the cake to bake until tall. Farm fresh eggs do not bake or cook well. The inner membrane of the egg sticks to the white.