When you put an ice on a frying pan and heat it up, the ice will turn from solid into liquid, and then become gas. When you put a solid butter on a pan and heat it up, the butter will melt into liquid. But when you boil an egg, or fry it, the egg will go from liquid into solid state, why do eggs go the other way round?
Generally when you heat up a solid matter, its molecules will gain energy and vibrate stronger, stronger vibration will overcome the attraction between molecules and achieve more freedom of movement, and finally solid will turn into liquid and gas. But when you heat up an egg, there is a very interesting chemistry activity that happens inside the egg. Let me do some simple explanations without deep chemistry vocabulary. Hope you enjoy it.
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What egg is made of?
Egg is a food that is rich in protein, and also contains a lot of water. Protein is made of amino acids and there are 20 types of amino acids. That is the amino acid that made the egg special.
Protein is a chain of amino acids, each amino acid attached to another amino acid and forms a long string structure. The amino acids are attached by strong bonds (peptide linkage) between atoms. Those amino acid strings are not likely to break while you’re heating an egg.
In the raw eggs, there are many strings of amino acids, each long string of amino acids is in folded form, globular like a ball shape in three dimensional structure. We also call this folded protein.
Why raw egg in liquid form?
Why the protein (amino acids) string in folded form? It is because there is some weak bonding between the different parts of the amino-acid string. In each protein string, there are amino acids that love water, called hydrophilic. And there are amino acids that hate water, called hydrophobia. The water loving parts will be on the outside of the folded protein, and the water hating parts will stay inside of the folded protein. So the weak bonding, hydrophilic, and hydrophobic properties of the amino acids string make the protein in folded ball shape.
When the egg is not cooked, each folded string of protein can move freely inside the water as the hydrophilic parts of protein are attracted with water molecules, the protein is dissolved in the solvent, this is why raw egg in the liquid form. The interaction of the protein with the water determines the raw egg in liquid form.
What happened when the egg heated up?
When the egg is heated up, all the weak bonds between different parts of the amino acids string will break and unfold. This is called denaturation. The change of the protein structure will change the egg from their original state.
Folded protein with globular structure is weak in nature and easily breaks down when higher temperature applied, but the strong bond (peptide linkage) between amino acids will remain strong unaffected, long string structure still remain. As the temperature increases, the protein string molecule gains more energy to form new, stronger bonds with other strings of protein molecules.
The protein strings form a mesh and the water no longer surrounds the protein and will trap inside the space of the protein strings mesh. When more and more protein strings unfold and bond with each other, the mesh gets stronger and finally the egg becomes solid. The process of forming the mesh is just like hand knitting of yarn, the yarn thread is like a protein string, after the knitting process, the yarn threads cross each other and firmly hold together.
Other egg denaturation ways
Besides heat denaturation, there is a chemical way of denaturation. When you put alcohol, or vinegar on an egg white, the weak bonds (hydrogen & ionic bonds) between the different parts of the folded protein string will break. Another denaturation way is to beat the egg white ( when you want to make a cake), the beating process causes the weak bonds to break, until the egg white gets stiff.