The formation of dark greenish discoloration in hard eggs is due to iron and sulphur present in white and yolk. The percentage concentrations of yolk are 11mg of iron and 0.016 mg sulphur and of white are 0.9 mg iron 0.195 mg of sulphur. The sulphur is present mostly as cystine and methionine amino acids.
On heating hydrogen sulphide is liberated from sulphur containing proteins, which combines with the iron of the yolk and ferrous sulphide is formed. The amount of hydrogen sulphide evolved depends on time and temperature of heating and pH of the eggs.
The effect of temperature and period of cooking on the development of green color on the surface of the yolk is given in table
Even if the eggs are cooked for a long time there is no green color if cooked at low temperature.
When it is put in cold water, after cooking there is no green color due to the diffusion of hydrogen sulphide gas at the surface of the cooling.
If eggs are cooked for a long time and at a higher temperature ferrous sulphide, ring formation occurs, even if cooled in cold water immediately.
Eggs which have undergone deterioration during storage lose carbon dioxide, with the result there is increase in the alkalinity. High pH favours the production of hydrogen sulphide. In stale eggs there is more chance for ferrous sulphide ring formation.
To hard cook an egg with the minimum discoloration of yolk a fresh egg should be cooked in water at simmering temperature followed by prompt cooling.
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