When you hard-boil an egg, sometimes the yolk turns an unappetizing gray-green color. “The green-yolk phenomenon is actually very easy to explain,” says Kiasa Kuykendall, sales and marketing manager for Stiebrs Farms, an organic and free-range egg farm located in Washington state. “The longer you overcook it, [the more it] gets greener and turns gray.”
The discoloration is due to the formation of ferrous sulfide where the yolk and white meet, says Elisa Maloberti, the director of product marketing at the American Egg Board. It’s formed when “iron from the yolk reacts with hydrogen sulfide from the white.”
Discolored egg yolks are harmless, says Maloberti, but if you want to keep your yolks a more appetizing yellow, she offers the following guidelines:
• Keep the eggs in a single layer in the pan.
• Cover the eggs with water, bring the water to a boil over high heat, then turn off the flame.
• Let the eggs sit in the hot water for 15 minutes for large eggs (or 12 for medium, 18 for extralarge).
• Run cold water over the eggs after they’re done so they don’t overcook.