The yolk of a hardboiled egg often has a green tinge right at the interface to the white. Besides not looking very good, I think it also maybe contributes a sulfurous odor. What causes this green coloration and how can I prevent it?
This is caused by overcooking the egg. The green color is a result of overheating causing the iron and sulfur compounds in the egg to express. You can prevent it by gently boiling the egg, and plunging it into an ice bath when it is done. This stops the carry-over heat from continuing to cook the egg.
Some interesting, relevant excerpts:
The Temperature Timeline of Boiling an Egg
Egg yolks, on the other hand, follow a different set of temperatures:
- At 145 degrees: They begin to thicken and set up.
- At 158 degrees: They become totally firm, but are still bright orange and shiny.
- At 170 degrees: They become pale yellow and start to turn crumbly.
- 170 degrees-plus: They dry out and turn chalky. The sulfur in the whites rapidly reacts with the iron in the yolks, creating ferrous sulfide, and tinging the yolks.
So long as your water never come above 180 degrees—at sea level, that's the quivering stage just below a simmer—you have no chance of overcooking
Also the comments are quite interesting. Just search for "green". Not sure if this works, but one commenter suggests a method for avoiding the green coloration:
I watched Chef Pepin's cookshow once he taught the audience an important trick: to poke a tiny hole at the broad end of the egg (using a push pin or similar) before boiling. With this you can effectively eliminate the sulfuric smell and taste of the egg yolk, and simultaneously remove the greyish green "rim" around the yolk (which you can see between the egg white and the egg yolk from the pictures above).
This method has work perfectly for me! And these small details are what make a perfect egg!