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Curious Folks Ask 2: 188 Real Answers on Our Fellow Creatures, Our Planet, and Beyond by Sherry Seethaler

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Chapter 4. Funky phenomena

The green color is iron sulfide. It is produced when iron from proteins in the egg yolk reacts with hydrogen sulfide from proteins in the egg white.

Special proteins in the yolk store the iron because a developing chick needs it. Sulfur is found in certain amino acids—the building blocks of protein. Proteins in the egg white are particularly rich in sulfur-containing amino acids.

Cooking breaks down some of the proteins in the egg, causing the white to release hydrogen sulfide and the yolk to release iron. These two chemicals react where they meet. Using newer eggs and cooling the eggs rapidly after cooking can minimize greenness, ...

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