Boiling eggs can be tricky because you don't find out whether you've done it correctly until you cut open the end product. Since the white and yolk cook at different temperatures, it can be difficult to get a solid yolk without an overcooked, rubbery white, or a tender, solid white without a liquidy, raw yolk. The underlying process is more complicated than most people suspect, with a wide variety of factors influencing how the finished egg turns out, but the following steps will help you get the results you're looking for.
1. Choose eggs based on the desired result. Fresh eggs are less prone to cracking because they have a low pH in the white, causing it to adhere to the inner shell membrane, essentially "sealing" it together. But, this also makes fresh eggs more difficult to peel. Eggs which have been refrigerated for several days have higher pH and are more likely to crack, but they're easier to peel. Instead of waiting for fresh eggs to get old, you can add a teaspoon of baking soda to a quart of water when cooking (which raises the pH and reduces adhering) but it might make the eggs taste slightly more sulfuric. Or, just cook fresh eggs a little longer and allow the white to firm up in fridge before peeling. If you're going to be cutting the boiled eggs in half, you might want to use the freshest eggs you can find, since they tend to have a more centered yolk and less likelihood of greening.
2. Lay the eggs gently in an empty pot. Some sources recommend making a shallow hole with a pin at the flatter end so that it'll let the expanding air escape thus reducing the chance of cracking but studies have shown this isn't a reliable technique. Adding salt will make the eggs easy to peel. Salt or vinegar to the water may help the proteins in the white coagulate faster so any cracks in the shell quickly get plugged.
3. Cover the eggs with about 1" or 2.5cm of salty water. If the water is cold, the eggs will take longer to cook. If the water is hot, though, you may risk the water getting too hot too early and overcooking the eggs (i.e. exposing the eggs to boiling temperature for too long).
- French oeuf a la coque "from the shell" - cooked 2-3 minutes; remains semi-liquid throughout.
- Coddled or "soft-boiled" eggs - cooked 3-5 minutes; barely solid outer white; milky inner white; warm yolk; spooned from shell
- Mollet eggs - 5-6 minutes; semi-liquid yolk; outer white is firm enough for shell to be peeled
- Hard-boiled eggs - 10-15 minutes; 10 minutes for dark, yellow, moist and somewhat pasty yolk; 15 minutes for light yellow, dry, and granular yolk
5. Cool the eggs. To stop the cooking process, drain the hot water and replace with cold water. Add some ice cubes to the water and wait until the eggs are cool enough to peel and/or serve.
need a demonstration? here you go, hehehe~