The reason is as follows: 1. Expired pure milk is prone to breeding pathogenic microorganisms, and if consumed, the expired pure milk containing pathogenic microorganisms will easily lead to intestinal infectious diseases, causing abdominal pain, bloating, diarrhea and other discomforts. 2. The nutrients in expired milk have been decomposed and consumed, so if consumed again, it will not play a role in supplementing the body’s nutrition. 3. The allergenic substances are often contained in the milk, especially the decomposition of the proteins in pure milk can easily lead to the high incidence of allergic diseases, which can cause itching and rashes in mild cases, or asthma in severe cases, and endanger health.