When raw eggs are boiled in boiling water for more than 10 minutes, a series of chemical reactions will occur inside raw eggs. Protein's function will be more and more inseparable, and it is not easy to get in touch with protein-pepsin in gastric acid, so it cannot be digested and absorbed. Protein contains methionine. After long-term heating, sulfate dissolves and reacts with iron in egg yolk to produce iron sulfide and ferrous sulfide. This is also the reason why the surface layer of egg yolk will cause dark green.