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Green eggs
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Why do the yolks of hard-boiled eggs go green around the edges and not the yolks of soft boiled eggs?
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For more on marking an answer as the "Best Answer", please visit our FAQ.The yellow comes from iron111 compounds. When eggs are boiled some of the proteins decompose to give hydrogen sulphide, which will reduce the iron 111 to iron11 which is green. This decomposition takes a long time to happen, and needs heat, so a hard boiled egg will do it but a soft one wont. Incidentally to stop it happening boil the eggs for no more than 8 minutes then plunge into ice water. The rapid cooling will prevent the greening.
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