When you cut a hard-boiled egg, you might find a strange grayish-green ring around the yolk instead of the expected bright yellow color. This can make it seem like the egg is spoiled. However, the good news is that a green ring is usually the result of a cooking reaction, not a sign of mold or spoilage.
This reaction occurs between sulfur in the egg white and iron in the yolk. According to the University of Illinois Extension, overcooking eggs or cooking them at high temperatures can cause these ingredients to react, resulting in the greenish-gray color. Additionally, this color is more likely to appear if the eggs are cooled too slowly. So, the unusual ring often means the eggs were cooked too long or at too high a heat, rather than being bad.
Generally, a green ring is not a safety issue when it comes to food. It’s considered safe to eat. What do you think about this color change in hard-boiled eggs?
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Source: Kitchen, by recipes