When an egg is boiled in boiling water for more than 10 minutes, a series of chemical changes will occur inside the egg. Protein contains methionine. After heating for a long time, sulfide decomposes and reacts with iron in egg yolk to produce iron sulfide and ferrous sulfide. If cooking time is too long, vitamin E, fat and cholesterol in eggs will be oxidized. The longer the cooking time, the higher the degree of oxidation of precious -3 unsaturated fatty acids in eggs. If you continue to cook, not only the nutrients in the eggs will be further oxidized and lost, but also a black film will be produced on the surface of the yolk, which is not good for health.