What is primarily caused by overcooking eggs, resulting in a discoloration?

The phenomenon that primarily results from overcooking eggs, leading to a discoloration, is ferrous sulfide. When eggs are overcooked, particularly the yolks, the sulfur present in the egg white reacts with the iron in the yolk. This chemical reaction produces ferrous sulfide, which manifests as a gray-green discoloration around the yolk. This discoloration is not only visually unappealing but can also indicate a change in texture and flavor of the egg.

Understanding this process is crucial for anyone involved in cooking or food science, as it underscores the importance of cooking eggs just enough to avoid undesirable chemical reactions which can affect both appearance and taste. Other substances like chlorophyllin, pheophytin, and carotenoids are related to the discoloration of different foods, particularly when dealing with vegetables or fruits, and are not the primary cause of the issues seen with overcooked eggs.