If an egg is cooked in boiling water for more than 10 minutes, a series of chemical changes will occur inside. The protein structure becomes tighter and less likely to come into contact with protein-digesting enzymes in the gastric juices, so it is harder to digest. Egg protein contains more methionine, after a long time heating, it will decompose sulfide, it reacts with the iron in the yolk to form iron sulfide that is not easily absorbed by the human body, and the nutritional loss is more.