Once an egg is laid, its yolk and albumen (egg white) cool and, as a result, contract. This shrinkage creates an air pocket at the egg's larger (nontapered) end. The air in the pocket can cause the shell to crack when you place the egg in simmering water (or, if the cook insists, in boiling water). The reason is that the heat of the water expands the trapped gas (air) in the pocket, creating an atmospheric pressure many times greater than exists in either the water or your kitchen. This built-up pressure easily cracks the fragile shell, releasing the trapped gas and, unfortunately, allowing some of the albumen to ooze into the water.
The problem can be overcome simply by piercing the egg's larger end with an ordinary pushpin. Its metal pinpoint is short enough not to rupture the membrane that separates the trapped air from the albumen but thick enough to make a convenient hole for the air to escape through as it expands while being heated in the simmering water. Result: no cracked shell.
So from now on, remove your egg from the refrigerator, pierce it, and gently ease the cold egg into a waiting pot of boiling water. (Adjust heat and simmer for 5 to 7 minutes for soft-cooked eggs and 12 to 15 minutes for hard-cooked eggs, depending on preference. However, always use the 15-minute timing if you plan to slice the eggs for use as a garnish). If you do not have a pushpin or other suitable piercing device, then you must bring your eggs to room temperature before adding them to the hot water. This technique is not as foolproof as the pushpin method, but it is more reliable than the begin-in-cold-water method.
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