When you heat a solid, such as ice, you transfer energy to the molecules, allowing them to break the chemical bonds that hold them in a solid state. Not all changes between solid and liquid are to do with melting or cooling, including congealing of cooked eggs. Eggs start as quite liquid in the fresh state and when they are heated, they go solid. When you heat a raw egg, an entirely different process takes place. This odd behavior is a result of the effect of the heat on the proteins they contain.
There are several different proteins in the egg white. Some are more jelled than others. If you crack an egg, you will see a runny, almost watery protein solution on the outside, and a firmer gel of egg white towards the center. The proteins in the yolk are the last to solidify, at the highest temperatures.
Eggs are rich in protein especially the egg whites. It’s this protein that causes eggs to become hard when boiled. Protein is a chain or string of amino acids. When you break those strings, by various methods, you are denaturing the protein.
The strands of protein in both the egg yolk and white are folded into precise shapes, each molecule forming a minute ball called a globular protein. But when these proteins are heated, some of the interactions holding the protein molecules into their precise globular shapes are broken and the molecules begin to unravel. This allows the separate protein chains to become entangled with one another and new and stronger interactions can form, which form a solid three-dimensional network of protein molecules, leading to a soft-boiled or eventually a hard-boiled egg. *This same protein denaturing keeps cookies or pan cakes fluffy happens when you add an egg.
Your digestive enzymes can break down such tangles more easily than the undenatured proteins.
You can denature a protein in several different ways. Here are two ways to do it with eggs:
Heat – When you heat an egg, the proteins gain energy and literally shake apart the bonds between the parts of the amino-acid strings, causing the proteins to unfold. As the temperature increases, the proteins gain enough energy to form new, stronger bonds (covalent) with other protein molecules. When you boil an egg, the heat first breaks (unfolds) the proteins, and then allows the proteins to link to other proteins. As the proteins form these new, strong bonds, the water that surrounded each protein molecule when the egg was liquid is forced out. That’s why the egg turns hard. (Heat affects all of the proteins in an egg, so it’s the best way to cook an egg.)
- Chemicals – You can also break the weak bonds between protein molecules with chemicals. According to an experiment done by the BBC Science Shack, if you put vodka on an egg white, the alcohol in it breaks weak hydrogen bonds in the protein. If you put vinegar on an egg white, the acid in it breaks ionic bonds. This works when you make mayonnaise. If you mix the two (vodka and vinegar), you break both types of bonds and quite effectively denature the protein. Lots of other chemicals will also break those weak bonds. This won’t work as well on the yolk, which, while also protein-rich, has lots of fats and other properties that make it more difficult for it to become hard.
- You can do something similar when you whisk egg whites: By exerting mechanical energy in the whisking process, you cause the protein bonds to break, and subsequently re-connect. Once these new, strong bonds are formed, the egg stays in that state. The proteins have formed a network of strong, permanent cross-links. A cooked, chemically-altered or well-beaten egg will never go back to its original state. Egg whites will however lose its stiffness and height. Egg whites at room temperature will beat stiffer and higher than egg white that are cold.
- When you whisk egg whites in a copper bowl, some copper ions migrate from the bowl into the egg whites. The copper ions form a yellow complex with one of the proteins in eggs, conalbumin. The conalbumin-copper complex is more stable than the conalbumin alone, so egg whites whipped in a copper bowl are less likely to denature (unfold). When glass or steel bowls are used, cream of tartar may be added to egg whites to stabilize the whites and increase the height.
When cooking eggs, use moderate heat. High heat causes the protein in eggs to become tough and rubbery.
When you use high heat to boil an egg, it causes a chemical reaction between the yolk and the white that leaves a green film around the yolk. That film is iron sulfide, caused by iron in the yolk reacting with hydrogen sulfide in the white. It won’t hurt you to eat it, and the egg will taste the same, but it sure looks as nice.
- Eggs contain all the essential protein, minerals and vitamins, except Vitamin C. And egg yolks are one of few foods that naturally contain Vitamin D.