When an egg has been cooked in hot water for 15 minutes or longer a dark greenish color may be formed on the surface of the egg yolk. If the egg is immersed in cold water immediately after cooking, the green color is not produced or is less apparent than when the egg is left in the hot water to cool slowly.

The yolk contains most of the iron of the egg, about 85 times as much as the white. According to Sherman, the sulfur content of the white is slightly higher than that of the yolk, the white containing 0.214 per cent and the yolk 0.208 per cent. The sulfur of the white is more labile and more easily split off by heat than the sulfur of the yolk. Marlow and King say that the sulfur in egg whites and yolk is organically bound and that nearly all of it can be accounted for by the cystine and methionine sulfur.

Tinkler and Soar have shown that the green color is due to the formation of ferrous sulfide at the surface of the yolk. The white with prolonged heating at high temperatures evolves considerable quantities of hydrogen sulfide. The hydrogen sulfide combines with the iron of the yolk to form ferrous sulfide which produces the green color. The amount of hydrogen sulfide evolved depends on (1) the time of heating, (2) the temperature reached, and (3) the reaction of the egg. In a short cooking period little hydrogen sulfide is evolved. Likewise at a lower temperature a smaller amount of hydrogen sulfide is formed. Tinkler and Soar have also shown that the formation of ferrous sulfide takes place very, very slowly until the yolk reaches a temperature of 70°C. It is very seldom formed in eggs cooked for 1 to 1/4 hours at 70°C, or in eggs cooked 30 to 35 minutes at 85°C. Thus both the temperature attained by all or a portion of the egg white as well as the time held at this temperature influence the amount of hydrogen sulfide formed. In addition, as the reaction becomes more alkaline, the sulfur is split off more readily. Therefore, the extent of deterioration of the egg also affects the amount of hydrogen sulfide formed. This explains why some eggs have more ferrous sulfide formed than other eggs when cooked at the same time and cooled in the same manner. Tinkler and Soar have reported that the uncooked yolk is acid in reaction but upon being heated above 70° becomes alkaline.

Effect of rapid cooling upon formation of ferrous sulfide in cooked eggs. Hydrogen sulfide is a gas. When the egg is placed in cold water immediately after cooking the lowering of the temperature at the surface of the egg lowers the pressure there. One of the things learned in connection with the gas laws is that the pressure increases with increasing temperature. Since the white near the shell reaches a certain temperature more rapidly than the white near the yolk, more hydrogen sulfide is split off near the shell. But, since the temperature near the yolk is lower, hence has less pressure than near the shell, the hydrogen sulfide diffuses through the white towards the yolk. However, if the egg is placed immediately in cold water after cooking, the hydrogen sulfide diffuses to the surface of the egg, owing to the reduced pressure, but if left in hot water, or to cool in the air, the gas does not diffuse so quickly to the surface and ferrous sulfide is formed at the junction of the egg white and yolk. Tinkler and Soar have found that eggs cooked 15 minutes in boiling water and cooled slowly have some green color at the surface of the yolk; if cooled quickly, none or very little green color develops. Cooking in boiling water for 30 minutes gives a great deal of green color, no matter how the eggs are cooled. Since the yolk contains iron and a fairly large quantity of sulfur, they wondered why the entire yolk did not turn green when cooked as long as 7 hours. They found the sulfur compounds in the yolk were more stable and less easily broken down to form hydrogen sulfide. Thus even with long cooking the color of the interior of the yolk is not changed.