What happens to egg when it is boiled?

Some people while boiling the eggs put the eggs in cold water while some others heat the water to boiling, then put the egg. Both methods have advantages and disadvantages. When you drop an egg in boiling water, you heat it up quickly. When you start with cold water, you heat it slowly. And the difference in heating makes a difference in the cooked egg white. An egg white is about 10% protein and 90% water. It’s the proteins that cause the egg white to solidify when it is cooked. Egg white proteins are long chains of amino acids. In a raw egg, these proteins are curled and folded to form a compact ball. Weak bonds between amino acids hold the proteins in this shape until they are not heated. When heated, the weak bonds break and the protein unfolds. Then its amino acids form weak bonds with the amino acids of other proteins, a process called coagulation.

The resulting network of proteins captures water, making a soft, digestible gel. If you keep the heat turned up too high or too long when you cook an egg, the proteins in the egg white form more and more bonds, squeezing some of the water out of the protein network and making the egg white rubbery. Also if the eggs are cooked for long time, a greenish-gray ring may appear around a hard-cooked egg yolk. The ring is caused by a chemical reaction between sulfur (from the egg white) and iron (from the egg yolk), which react to form ferrous sulfide. The reaction is usually caused by overcooking. What you need to do is to avoid overcooking the eggs. Also make sure that you have a bowl filled with chilled water in which you place the eggs after they are boiled. This will avoid carry over cooking of the egg and formation of grayish ring. Carryover cooking is cooking of a food item because of its internal heat even after you remove it from the pan. Refrigerate the eggs in their shells until you're ready to use them. Hard-cooked eggs in the shell can be refrigerated up to one week. Starting with cold water lets you heat the egg more slowly, which keeps the whites from getting rubbery. But this method takes longer and gives you less control over the cooking time. Starting with boiling water offers more control over timing but this may cook the whites into a rubbery state. And it has another disadvantage: The egg is more likely to crack because the air in the egg has less time to escape as the egg heats up.