What is responsible for the dark ring around cooked egg yolks?

The dark ring around cooked egg yolks is called the "vitelline membrane". It's not actually a ring, but a thin membrane that surrounds the yolk.

Here's why it appears darker after cooking:

* Protein Denaturation: When an egg is heated, the proteins in the yolk and white denature and coagulate. This process causes the vitelline membrane to become more visible and slightly darker.

* Iron Concentration: The yolk contains iron, and when it's cooked, the iron reacts with other compounds in the egg, making it appear darker.

* Heat Transfer: The vitelline membrane is more susceptible to heat than the yolk itself. As the yolk cooks, the membrane absorbs heat and contracts, creating a darker appearance.

The color and prominence of the ring can vary depending on:

* Cooking temperature: Higher temperatures will cause a more pronounced ring.

* Cooking time: Longer cooking times will also result in a darker ring.

* Egg freshness: Older eggs may have a thicker vitelline membrane, resulting in a more noticeable ring.

It's important to note that the dark ring around cooked egg yolks is perfectly normal and safe to eat. It's simply a result of the cooking process and doesn't affect the taste or texture of the yolk.