The gray, sometimes green ring you see around the yolk of a hard-boiled egg is no cause for concern. The off color is nothing more than a chemical reaction that occurs with the overcooking of hard-boiled eggs. The overcooking causes the sulfur in the egg white to react with the iron in the egg yolk. The result is ferrous sulfide, a chemical compound, which causes the discoloration.
The Perfect Hard Boil
Avoid the gray ring by cooking hard-boiled eggs no longer than necessary. For the perfect hard boil, set the eggs in a saucepan. Add just enough water to cover the eggs plus an additional inch. Set the saucepan over medium-high heat, removing it from the heat immediately when the water first begins to boil. Cover the saucepan with a matching lid and allow the eggs to sit in the hot water for 25 minutes.
Cooling the hard-boiled eggs after cooking is crucial to avoiding the gray ring around the yolk. Have a large bowl of ice water ready when your eggs are done cooking. Transfer the eggs from the hot water to the ice-water bath immediately after the 25-minute cooking period. The ice water halts the cooking, stopping the sulfur-iron reaction.
Refrigerate any hard-boiled eggs you are not eating right away. Eggs left at room temperature for too long develop dangerous bacteria that causes food-borne illnesses, some of which have the potential to be life-threatening. Do not hold hard-boiled eggs at temperatures above 40 degrees Fahrenheit for longer than two hours. At 90 degrees, hard-boiled eggs only remain safe to eat for one hour. Once refrigerated, hard-boiled eggs keep for seven to 10 days.